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diff --git a/noao/digiphot/daophot/doc/userdocs/daophot.usr.tex b/noao/digiphot/daophot/doc/userdocs/daophot.usr.tex new file mode 100644 index 00000000..ce4ed623 --- /dev/null +++ b/noao/digiphot/daophot/doc/userdocs/daophot.usr.tex @@ -0,0 +1,2005 @@ +%de 21 (summer solstice, midnight, la serena) (this is IT, Phil!) +%more done on Dec 27th or so +%typos fixed 28 Dec 1pm +%more done on Dec 28th (1st 4-m night morning) +%copy xfered from Chile on Jan 1, 1990 +%modifications made after the AAS meeting, Jan 15 1990: +% end itemize problems fixed +% buku aperture photometry stuff added Jan 15/16 +% started to make Lindsey's changes Jan 28th +% Next set of changes made Feb. 18th when we SHOULD have been +% off with marcia having a good time. +% more modifications Monday Feb 19 +% march 20/21 mods made in Boulder----Lindsey's comments +% march 20/21 mods made in Boulder---beginning of JB's comments +% march 27 mods back in Tucson +% may 11th, fixed the sumed/average read-noise problem! +\documentstyle[11pt,moretext]{article} +\begin{document} +\title{A User's Guide to Stellar CCD Photometry with IRAF} +\author{Philip Massey \and Lindsey E. Davis} +\date{March 29, 1990} +\maketitle +\begin{abstract} +This document is intended to guide you through the steps for obtaining +stellar photometry from CCD data using IRAF. It deals both with the +case that the frames are relatively uncrowded (in which case simple +aperture photometry may suffice) and with the case that the frames +are crowded and require more sophisticated point-spread-function +fitting methods (i.e., {\bf daophot}). In addition we show how one +goes about obtaining photometric solutions for the standard stars, and +applying these transformations to instrumental magnitudes. +\end{abstract} +\tableofcontents +\eject +\section{Introduction} +This user's guide deals with both the ``relatively simple" case of +isolated +stars on a CCD frame (standard stars, say, or uncrowded program stars) +and the horrendously more complicated case of crowded field +photometry. We describe here all the steps needed to obtain instrumental +magnitudes and to do the transformation to the standard system. There +are, of course, many possible paths to this goal, and IRAF provides no +lack of options. We have chosen to illuminate a straight road, but many +side trails are yours for the taking, and we will occasionally point +these out (``let many flowers bloom"). This Guide is {\it not} intended +as a reference manual; for that, you have available (a) the various +``help pages" for the routines described herein, (b) ``A User's Guide +to the IRAF APPHOT Package" by Lindsey Davis, and (c) ``A Reference +Guide to the IRAF/DAOPHOT Package" by +Lindsey Davis. (For the ``philosophy" and algorithms of DAOPHOT, see +Stetson 1987 {\it PASP} {\bf 99}, 111.) +What {\it this} manual is intended to be is a real +``user's guide", in which we go through all of the steps necessary to go +from CCD frames to publishable photometry. (N.B.: as of this writing +the IRAF routines for determining the standard transformations and +applying those transformations are still being written.) We outline +a temporary kludge that will work with Peter Stetson's CCDRED VMS +Fortran package. Hopefully the PHOTRED package currently under +development at Cerro Tololo will be available by Spring 1990, and +this manual will then be revised. + +The general steps involved are as follows: (1) fixing the header +information to reflect accurate exposure times and airmasses, +(2) determining and cataloging the characteristics of your data (e.g., +noise, seeing, etc.), +(3) obtaining instrumental magnitudes for all the standard stars +using aperture photometry, (4) obtaining instrumental magnitudes for +your program stars using IRAF/daophot, (5) determining the aperture +correction for your program stars, (6) computing the transformation +equations for the standard star photometry, and (7) applying these +transformations to your program photometry. We choose to illustrate +these reductions using {\it UBV} CCD data obtained with an RCA chip on the 0.9-m +telescope at Cerro Tololo, but the techniques are applicable to data +taken with any detector whose noise characteristics mimic those of a +CCD. + +If you are a brand-new IRAF user we strongly recommend first reading the +document ``A User's Introduction to the IRAF +Command Language" by Shames and Tody, which can be found in Volume 1A +of the 4 blue binders that compose the IRAF documentation. (Actually +if you are a brand-new IRAF user one of us recommends that you find +some simpler task to work on before you tackle digital stellar photometry!) +The procedures described here will work on any system supported by IRAF; +for the purposes of discussion, however, we will assume that you are +using the image display capabilities of a SUN. If this is true you then +may also want to familiarize yourself with the ins and outs of using +the SUN Imtool window; the best description is to be found in ``IRAF +on the SUN". + +We assume that your data has been read onto disk, and that the basic +instrumental signature has been removed; i.e., that you are ready +to do some photometry. If you haven't processed your +data this far yet, we +refer you to ``A User's Guide to Reducing CCD Data with IRAF" by Phil +Massey. + +\section{Getting Started} + +\subsection{Fixing your headers} +You're going to have to this some time or another; why not now? There +are two specific things we may need to fix at this point: (a) Add any +missing header words if you are reducing non-NOAO data, (b) correct the +exposure time for any shutter opening/closing time, and (c) correct the +airmass to the effective middle of the exposure. + +Two things that will be useful to have in your headers are the exposure +time and the airmass. If you are reducing NOAO data then you will +already have the exposure time (although this may need to be corrected +as described in the next paragraph) and enough information for the {\bf +setairmass} task described below to compute the effective airmass of the +observation. You can skip to the ``Correcting the exposure time" +section below. +If +you are reducing non-NOAO data you should examine your header with a + +\centerline{ {\bf imhead} imagename{\bf l+ $|$ page} } + +\noindent +and see exactly what information {\it is} there. If you are lacking the +exposure time you can add this by doing an + +\centerline{ {\bf hedit} imagename{\bf ``ITIME"} value {\bf add+ up+ +ver- show+} } + +\noindent +If you know the effective airmasses you can add an ``AIRMASS" keyword in +the same manner, or if you want to compute the effective airmass +(corrected to mid-exposure) using {\bf setairmass} as described below, +you will need to have the celestial coordinates key words ``RA" and +``DEC", as well as the siderial-time (``ST"), +and preferably the coordinate ``EPOCH" and the date-of-observation +(``DATE-OBS"), all of which should have the form shown in Fig.~\ref{header}. + +You may want to take this opportunity to review the filter numbers in the +headers, and fix any that are wrong. If you are lacking filter numbers +you may want to add them at this point. + +\subsubsection{Correcting the exposure time} +The CTIO 0.9-m has an effective exposure time that is +25-30ms longer than the requested exposure time (Massey et al. 1989 {\it +A.J.} {\bf 97}, 107; Walker 1988 {\it NOAO Newsletter} {\bf No. 13}, +20). First see what "keyword" in your header gives the exposure time: + +\centerline{ +{\bf imhead} imagename{\bf.imh l+ $|$ page} } + +\noindent +will produce a listing such as +given in Figure~\ref{header}. +\begin{figure} +\vspace{3.2in} +\caption{\label{header}Header information for image n602alu.imh}. +\end{figure} +The exposure time keyword in this header is ``ITIME". In this case +we wish to add a new exposure time to each of the headers; we will call +this corrected exposure time +EXPTIME, and make it 25 ms larger than whatever value is listed as +ITIME. To do this we use the {\bf hedit} command as follows: + +\centerline{ +{\bf hedit *.imh EXPTIME ``(ITIME+0.025)" ver- show+ add+}.} + +\noindent +An inspection of the headers will now show a new keyword EXPTIME. +(Walker lists a similar correction for the CTIO 1.5-m shutter, but the +CTIO 4-m P/F shutters have a negligible correction. +The direct CCD shutters on the Kitt Peak CCD cameras give +an additional 3.5ms of integration on the edges but 13.0ms in the +center [e.g., Massey 1985 {\it KPNO Newsletter} {\bf 36}, p. 6]; +if you have any 1 second exposures you had best correct these by +10ms or so if you are interested in 1\% photometry.) + +\subsubsection{Computing the effective airmass} +The task {\bf setairmass} in the {\bf astutil} package will compute +the effective airmass of your exposure, using the header values of RA, +DEC, ST, EPOCH, and DATE-OBS, and whatever you specify for the observatory +latitude. An example is shown in Fig.~\ref{setairmass}. +\begin{figure} +\vspace{2.5in} +\caption{\label{setairmass} The parameter file for {\bf setairmass}.} +\end{figure} +The default for the latitude is usually the IRAF +variable {\bf observatory.latitude}. To by-pass this ``feature", simply +put the correct latitude in the parameter file +(e.g., $-30.1652$ for CTIO, +$+31.963$ for KPNO; $+19.827$ for Mauna Kea.). + +\subsection{{\bf imexamine:} A Useful Tool} + +The {\bf proto} package task {\bf imexamine} is a powerful and versatile task +which can be used to interactively examine image data at all stages of +the photometric reduction process. In this section we discuss and +illustrate those aspects of {\bf imexamine} which are most useful to +photometrists with emphasis on three different applications of the task: +1) examining the image, for example plotting lines and columns +2) deriving image characteristics, for example computing the +FWHM of the point-spread function 3) comparing the same region +in different images. + +The task +{\bf imexamine} lives within the {\bf proto} package, and you will also need +to load {\bf images} and {\bf tv}. Then +{\bf display} the image, and type {\bf imexamine}. +When the task is ready to accept input the image cursor will begin blinking +in the display window, and the user can begin executing various keystroke +and colon commands. The most useful data examining commands are summarized +below. The column, contour, histogram, line and surface plotting commands +each have their own parameter sets which set the region to be plotted and +control the various plotting parameters. All can be examined and edited +interactively from within the {\bf imexamine} task using the +appropriate {\bf :epar} command. + +\begin{description} + \item[c] - Plot the column nearest the image cursor + \item[e] - Make a contour plot of a region around the image cursor + \item[h] - Plot the histogram of a region around the image cursor + \item[l] - Plot the line nearest the image cursor + \item[s] - Make a surface plot of a region around the image cursor + \item[:c N] - Plot column N + \item[:l N] - Plot line N + \item[x] - Print the x, y, z values of the pixel nearest the image cursor + \item[z] - Print a 10 by 10 grid of pixels around the image cursor + \item[o] - Overplot + \item[g] - Activate the graphics cursor + \item[i] - Activate the image cursor + \item[?] - Print help + \item[q] - Quit {\bf imexamine} + \item[:epar c] - Edit the column plot parameters + \item[:epar e] - Edit the contour plot parameters + \item[:epar h] - Edit the histogram plot parameters + \item[:epar l] - Edit the line plot parameters + \item[:epar s] - Edit the surface plot parameters + +\end{description} + + +Example 1 below shows how a user can interactively +make and make hardcopies of image line plots using {\bf imexamine} and at the same time +illustrates many of the general features of the task. + +The {\bf imexamine} task also has some elementary image analysis capability, including +the capacity to do simple aperture photometry, compute image statistics +and fit radial profiles. The most useful image analysis commands are +listed below. + +\begin{description} +\item[h] - Plot the histogram of a region around the cursor +\item[r] - Plot the radial profile of a region around the cursor +\item[m] - Plot the statistics of a region around the cursor +\item[:epar h] - Edit the histogram parameters +\item[:epar r] - Edit the radial profile fitting parameters +\end{description} + +Example 2 shows how a photometrist might use {\bf imexamine} +and the above commands to estimate the following image characteristics: +1) the full width at +half maximum (FWHM) of the point-spread function, 2) the background sky level +3) the standard deviation of the background level 4) and the radius at which +the light from the brightest star of interest disappears into the noise +(this will be used to specify the size of the point-spread-function, +e.g.,PSFRAD). + +Finally {\bf imexamine} can be used to compare images. Example 3 +shows how to compare regions in the original image and in the +same image with all the fitted stars subtracted out. The example +assumes that the target image display device supports multiple frame buffers, +i.e. the user can load at +least two images into the display device at once. + +The {\bf imexamine} task offers even more features than are discussed here and the +user should refer to the manual page for more details. + +\vspace{12pt} +{\bf Example 1:} Plot and make hardcopies of image lines within {\bf imexamine}. + +\begin{itemize} +\item {\bf display} the image and then type {\bf imexamine}. +\item move the image cursor to a star and tap {\bf l} to plot the image +line nearest the cursor +\item tap the {\bf g} key to activate the graphics cursor +\item type {\bf :.snap} to make a hardcopy of the plot on your default device +\item expand a region of interest by first moving the graphics +cursor to the lower left corner of the region and typing {\bf E}, +and then moving the graphics cursor to the upper right corner +of the region and typing anything +\item type {\bf :.snap} to make a hardcopy of the new plot +\item tap the {\bf i} key to return to the image cursor menu +\item type {\bf :epar l} to enter the line plot parameter set, change the +value of the logy parameter to yes and type {\bf CNTL-Z} to exit and +save the change +\item repeat the previous line plotting commands +\item type {\bf q} to quit {\bf imexamine} +\end{itemize} + +{\bf Example 2:} Compute some elementary image characteristics using +{\bf imexamine}. + +\begin{itemize} +\item {\bf display} the image and then type {\bf imexamine}. +\item move to a bright star and tap the {\bf r} key +\item examine the resulting radial profile plot and note the final +number on the status line which is the FWHM of the best fitting +Gaussian +\item repeat this procedure for several stars to estimate a good +average value for the FWHM +\item set the parameters of the statistics box ncstat and nlstat +from 5 and 5 to 21 and 21 with {\bf :ncstat 21} and {\bf :nlstat 21} +commands so that the sizes of the statistics and histogram +regions will be identical +\item move to a region of blank sky and tap the {\bf m} key to get an +estimate of the mean, median and standard deviation of the +sky pixels in a region 21 by 21 pixels in size around the +image cursor +\item leave the cursor at the same position and tap the {\bf h} key to +get a plot of the histogram of the pixels in the same region +\item tap the {\bf g} key to activate the graphics cursor, move the +cursor to the peak of the histogram and type {\bf C} to print out +the cursor's value. The ``x" value then gives you a good estimate of +the sky. Similarly, you can move the cursor to the +half-power point of +the histogram and type {\bf C} to estimate the standard deviation +of the sky pixels. Tap the {\bf i} key to return to the +image cursor menu +\item compare the results of the h and m keys +\item repeat the measurements for several blank sky regions and note +the results +\item move to a bright unsaturated star and turn up the zoom and + contrast of the display device as much as possible +\item using the {\bf x} key mark the point on either side of the center +where the light from the star disappears into the noise +and estimate PSFRAD +\item type {\bf :epar r} to edit the radial profile fitting parameters +and set rplot to something a few pixels larger than PSFRAD +and tap the {\bf r} key +\item note the radius where the light levels off and compare with +the eyeball estimate +\item repeat for a few stars to check for consistency +\item type {\bf q} to quit imexamine +\end{itemize} + +\noindent +{\bf Example 3:} Overplot lines from two different images. + +\begin{itemize} +\item {\bf imexamine image1,image2} +\item move the image cursor to a star and type {\bf z} to print the +pixel values near the cursor +\item tap the {\bf n} key to display the second image followed by {\bf z} +to look at the values of the same pixels in the second +image +\item tap the {\bf p} key to return to the first image +\item tap {\bf l} to plot a line near the center of the star and tap +the {\bf o} key to overlay the next plot +\item tap the {\bf p} key to return to the second image and without +moving the image cursor tap the l key again to overplot +the line +\item type {\bf q} to quit imexamine +\end{itemize} + +\subsection{Dealing with Parameter Files (Wheels within Wheels)} + +The {\bf daophot} (and {\bf apphot}) packages are unique in IRAF in that +they obtain +pertinent information out of separate ``parameter files" that can be +shared between tasks. As anyone that +has used IRAF knows, each IRAF command has its own parameter file that +can +be viewed by doing an {\bf lpar} {\it command} or edited by doing an +{\bf epar} {\it command}. +However, in {\bf daophot} and {\bf apphot} there are ``wheels within +wheels"---some of the parameters are in fact parameter files themselves. +For instance, the aperture photometry routine {\bf phot} does not +explicitly +show you the methods and details of +the sky fitting in its parameter file. +However, if you do an {\bf lpar phot} +you will see a parameter +called ``fitskypars" which +contains, among many other things, the radii of the annulus to be used +in determining the sky value. +You will also find listed ``datapars" (which specifies the properties +of your data, such as photons per ADU and read-noise), ``centerpars" +(which +specifies the centering algorithm to be used), and ``photpars" (which gives +the +size of the digital apertures and the zero-point magnitude). +The contents of any of these parameter files can be altered either by +{\bf epar}ing them on their own or by typing a ``:e" while on that +line of the main parameter file. If you do the latter, a control-z +or a ``:q" will bring you back. +For example, to examine or edit {\bf fitskypars}, you can +do an explicit {\bf lpar fitskypars} +or {\bf epar fitskypars}, or you can do an {\bf epar phot}, move the +cursor down to the ``fitskypars" line, and then type a {\bf :e} to edit +(see Fig.~\ref{wheels}). +\begin{figure} +\vspace{4.2in} +\caption{\label{wheels}Changing the Sky Annulus in {\bf fitskypars}.} +\end{figure} +Confusing? You bet! +But once you are used to it, it is a convenient and powerful way to +specify a whole bunch of things that are used by several different +commands---i.e., you are guaranteed of using the same parameters in +several different tasks. If there is only one thing that you want to +change in +a parameter file you {\it can} enter it on the command line when +you run the command, just as if it were a ``normal" (hidden) parameter, +i.e., {\bf phot imagename dannulus=8.} does the same as +running {\bf epar fitskypars} and changing the ``width of sky annulus" +{\bf dannulus} to 8.0. + +Mostly these things are kept out of the way (``very hidden" parameters) +because you {\it don't} want to be changing them, once you have set them +up for your data. There are exceptions, such as changing the PSF radius +in making a point-spread function in a crowded field (Sec. 4.6). +However, +you are well protected here if you leave the {\bf verify} switch on. +A task will then give you an opportunity to take one last look at +anything +that you really care about when you run the task. For instance, if we +had simply run {\bf phot} on an image (we'll see how to do this shortly) +it would have said ``Width of sky annulus (10.)", at which point we +could +either have hit [CR] to have accepted the 10., or we could have +entered a new value. + + +\section{Aperture Photometry on your Standards} + +Standard stars provide a good example of relatively uncrowded +photometry, +and in this section we will describe how to obtain instrumental +magnitudes for your standards using {\bf phot}. +The basic steps are +\begin{itemize} + \item Decide what aperture size you wish to use for measuring your + standards {\bf (this should be the same for all the frames).} At the + same time we will pick a sky annulus. + \item Set up the various parameter files ({\bf datapars, + centerpars, fitskypars, photpars}) to have the correct values. + \item For each frame: + \begin{enumerate} + \item Identify the standard star(s) either + interactively using a cursor + or by using the automatic star finding algorithm + {\bf daofind}. + \item Run the aperture photometry program {\bf phot} + on each of your standard star frames. +\end{enumerate} +\end{itemize} +Although the routines you will need to use are available both in the +{\bf daophot} and {\bf apphot} packages, we strongly advise you to run +them from the {\bf daophot} package: the default setup is somewhat different, +and the two packages each have their own data parameter files. + +\subsection{Picking an Aperture Size} +Unfortunately, there are not good tools available with IRAF to do this +yet, and we will restrict our discussion here to some of the +considerations before telling you to just go ahead and use a radius that +is something like 4 or 5 times the FWHM of a stellar image; e.g., +12 or 15 +pixels as a radius, assuming you have the usual sort of ``nearly +undersampled" FWHM$\approx3$ data. +You might naively expect (as I did) that you wish to pick an aperture +size +that will ``contain all the light" from your standard stars, but in fact +this is impossible: the wings of a star's profile extend much further +than you imagine at a ``significant" level. King (1971 {\it Publ. +A.S.P.} {\bf 83}, 199) and Kormendy (1973 {\it A.J.} {\bf 78}, 255) +discuss the fact that on photographic plates the profile of a star +extends out to {\it arcminutes} at an intensity level far exceeding the +diffraction profile; Kormendy attributes this to scattering off of dust +and surface irregularities on the optical surfaces. +Massey {\it et al}.\ (1989 {\bf 97}, 107) discusses +this in regards to CCD's and standard star solutions using the very data +we are using here as an example (which is not exactly a coincidence). +Although the intensity profile falls off rapidly, the increase in area +with radius increases rapidly, and in practical terms Massey {\it et +al.} +found that in cases where the FWHM was typically small (2.5-3 pixels), +increasing the digital aperture size from a diameter of 18 pixels to +one of 20 pixels resulted in an additional 1-2\% increase in light +for a well-exposed star, and that this increase continues +for larger apertures until masked by the photometric errors. + +Given that you presumably want 1\% photometry or better, what should you +do? +Well, the fact that photoelectric photometery through fixed apertures +in fact does +work suggests that there is some radius beyond which the same fraction +of +light is excluded, despite variations in the seeing and guiding. You do +not want to choose a gigantic aperture ($>$ 20 pixels, say) because the +probability of your having a bad pixel or two goes up with the area. +But you do not want to choose too small an aperture ($<$10 pixels, say) +or you will find yourself at the mercy of the seeing and guiding. Most +photoelectric photometrists will use an aperture of at least 10 +arcseconds in diameter, but remember you have one advantage over them: +you are not sensitive to centering errors, since any digital aperture can +be exactly centered. +If you +have enough standard star observations (I used about 300 obtained over a +10 night run) you can +compute magnitude differences between a large aperture (20 pixels), +and a series of smaller apertures (8, 10, 12, 15, 18) for each filter, +and then see for which radius the difference (in magnitudes) becomes +constant. Unfortunately, there are no tools currently available within +IRAF for taking the differences between two apertures, or for conveniently +plotting these differences, so you are on your own. My recommendation +would be that if you have typical data with a +FWHM of $\leq 4$ pixels, that you use something like an aperture of 12 to 15 +pixels in radius for your standard stars. {\bf You can save yourself a lot +of trouble if you simply adopt a single radius for all the standards +from all the nights for all filters.} + +\subsection{Setting Things Up} + +As discussed in ``Dealing with Parameter Files" (Section 2.1) we must +setup the parameter files from which {\bf phot} will get the details of +what it is going to do. The easiest way to do this is to simply +{\bf epar phot}, and on each of the four parameter lists to do a +{\bf :e}. Mostly we will leave the defaults alone, but in fact you will +have to change at least one thing in each of the four files. + +\begin{figure} +\vspace{3.5in} +\caption{\label{photdatapars} Parameters for {\bf datapars}.} +\end{figure} +In {\bf datapars} (Fig.~\ref{photdatapars}) we need to specify both +the FWHM +of a star image ({\it fwhmpsf}) and the +threshold value above sky ({\it threshold}) if we are going to use the +automatic star-finding routine {\bf daofind}; the choices for these +are discussed further below. In order to have +realistic +error estimates for our aperture photometry we need to specify +the CCD readnoise {\it readnoise} in electrons and the +gain (photons per ADU) for the CCD {\it epadu}. +In order to +correct the results for the exposure time we need the exposure time +keyword {\it +exposure}. Do an + +\centerline{{\bf imhead} {\it imagename} {\bf l+ $|$ page}} + +\noindent +to see a +listing of all the header information (Fig.~\ref{phothead}). +\begin{figure} +\vspace{4.0in} +\caption{\label{phothead} Header information for std159.imh} +\end{figure} +By specifying the (effective) airmass and filter keywords, +these can be carried along in the photometry file for use when we do +the standards solution ({\it airmass} and {\it filter}). Finally we use +{\it datamin} and {\it datamax} so we will know if we exceeded the +linearity of the CCD in the exposure, or whether there is some anomalously +low valued pixel on which our star is sitting. +Since the value of the sky on our standard exposures is +probably nearly zero, {\it datamin} should be set to a negative value +about three times the size of the readnoise in {\it ADU's}; e.g., $-3 \times +65. \div 2.25 \approx -90$ in this example. Note that although we will +later argue that the shape of the PSF changes a little about 20000 +ADU's (presumably due to some sort of charge-transfer problem), +for the purposes of simple aperture photometry we are happy +using 32000 ADU's as the maximum good data value. (We do not really +want to use 32767 as afterall the overscan bias was probably at a +level of several hundred.) + +\begin{figure} +\vspace{3.0in} +\caption{\label{photcenterpars} Parameters for {\bf centerpars}.} +\end{figure} +In {\bf centerpars} (Fig.~\ref{photcenterpars}) we need to +change the centering algorithm {\it calgorithm} +from the default value of ``none" to +``centroid". If the FWHM of your frames are unusually large ($>4$, say, +you would also do well to up the size of {\bf cbox} to assure that the +centering works well; make it something like twice the FWHM. In this +case the FWHM is 3 pixels or a bit smaller, and we are content to leave +it a the default setting of 5 pixels. + +\begin{figure} +\vspace{2.7in} +\caption{\label{photfitskypars} Parameters for {\bf fitskypars}.} +\end{figure} +In {\bf fitskypars} (Fig.~\ref{photfitskypars}) +the only things we must specify are the size and +location of the annulus in which the modal value of the sky will be +determined. If you are going to use a value of 15 for your photometry +aperture, you probably want to start the sky around pixel 20. Keeping +the width of the +annulus large (5 pixels is plenty) assures you of good sampling, but +making it too large increases the chances of getting some bad pixels in +the sky. + +\begin{figure} +\vspace{2.7in} +\caption{\label{photphotpars} Parameters for {\bf photpars}.} +\end{figure} +In {\bf photpars} (Fig.~\ref{photphotpars}) +we merely need to specify the size (radius) of the +aperture we wish to use in measuring our standards. + +\subsection{Doing It} +There are basically two ways of proceeding in running photometry on the +standard stars, depending upon how you are going to identify the +relevant star(s) on each frame. If you have only one (or two) +standard stars on each frame, and it is always one of the brightest +stars present, then you can avoid a lot of the work and use the +automatic star-finding program {\bf daofind} to find all your standards +and the whole thing can be done fairly non-interactively. However, +if you are one of the believers in cluster field standards, then you +may actually want to identify the standards in each field using the +cursor on the image display so that the numbering scheme makes sense. +We describe below each of the two methods. + +\subsubsection{Automatic star finding} +First let's put the name of each frame containing standard stars into +a file; if you've put the standard star exposures into a separate +directory this can be done simply by a {\bf files *.imh $>$ stands}. +This will leave us with funny default output file +names for a while (we advise against +including the ``.imh" extension when we discuss crowded field photometry +in the next section), but this will only be true for a short +intermediate +stage. + +We want to run {\bf daofind} in such a way that it finds only the +brightest +star or two (presumably your standard was one of the brightest stars +in the field; +if not, you are going to have to do this stuff as outlined below in +the ``Photometry by eye" section). We will delve more fully into the +nitty-gritty of {\bf daofind} in the crowded-field photometry section, +but here we are content if we can simply find the brightest few stars. +Thus the choice of the detection +threshold is a critical one. If you make it too low you will find all +sorts of junk; if you make it too high then you may not find any stars. +You may need to run {\bf imexamine} on a few of your images: first +{\bf display} the image, and then {\bf imexamine}, using the ``r" cursor +key to produce a radial profile plot. Things to note are the +typical full-width-half-maximum and the peak value. If your sky is +really around zero for your standard exposures, then using a value +that is, say, twenty times the readnoise (in ADU's) is nearly guaranteed to +find only the brightest few stars; do your radial plots in {\bf +imexamine} show this to be a reasonable value? In the example here we +have decided to use 500 ADUs as the threshold ($20 \times 65 \div 2.25 +\approx 500$). + +Now {\bf epar daofind} so it resembles that of Fig.~\ref{photdaofind}. +\begin{figure} +\vspace{3.5in} +\caption{\label{photdaofind} Parameter file for {\bf daofind}.} +\end{figure} +Go ahead and execute it (Fig. ~\ref{daoout}). +\begin{figure} +\vspace{3.5in} +\caption{\label{daoout} Screen output from a {\bf daofind} run.} +\end{figure} +Note that since {\it verify} is on that you +will be given a chance to revise the FWHM and detection threshold. By +turning verbose on you will see how many stars are detected on each +frame. +%Probably the best way of doing this is to write the output from +%{\bf daofind} into a file; do a +% +%\centerline{ {\bf daofind @stands $|$ tee starsfound} } +% +%\noindent +%to put the output into the file ``starsfound" as well as on the screen. +Make a note of any cases where no stars were found; these you will have +to +go back and do with a lower threshold. + +The run of {\bf daofind} produced one output file named {\it +imagename.imh.coo.1} for each input file. If you {\bf page} one of +these you will find that it resembles that of Fig.~\ref{photcooout}. +\begin{figure} +\vspace{3.7in} +\caption{\label{photcooout} Output file from {\bf daofind}.} +\end{figure} +The file contains many lines of header, followed by the {\it x} and {\it +y} center values, the magnitudes above the threshold value, the ``sharpness" +and ``roundness" values, and finally an ID number. +In the example shown +here in Fig.~\ref{photcooout} two stars were found: one 2.9 mags +brighter than our detection threshold, and one about 0.4 mag brighter +than our detection threshold. + +In a few cases we doubtlessly found more than one star; this is a good +time to get rid of the uninteresting non-standards in each field. +If things went by too fast on the screen for you to take careful notes +while running {\bf daofind} we can find these cases now: do a + +\centerline{ {\bf txdump *coo* image,id,x,y yes }} + + +\noindent +to get a listing of the location and number of stars found on each image. +If you have cases where there were lots of +detections (a dozen, say) you may find it easier to first {\bf sort +*.coo* mag} in order to resort the stars in each file by how bright they +are. Of course, your standard may not be the brightest star in each +field; you may want to keep an eye on the {\it x} and {\it y} values to +see if it is the star you thought you were putting in the middle! +To get rid of the spurious stars you will need to {\bf edit} each of the +output files (e.g., {\bf edit std148.imh.coo.1} ) and simply delete the +extras. + +Finally we can run aperture photometry on these frames, using the +``.coo" files to locate the standard star in each frame. {\bf epar +phot} until it resembles that of Fig.~\ref{photphot}. +\begin{figure} +\vspace{3.5in} +\caption{\label{photphot} The parameter file for a run of {\bf phot}.} +\end{figure} +Note that we are specifying a {\it single} output file name +(``standstuff" in this example); {\it all} the photometry output will be +dumped into this single file, including things like the airmass and filter +number. Go ahead and execute {\bf phot}. +You should see something much like that of Fig.~\ref{photrun} on the +screen. +\begin{figure} +\vspace{5.5in} +\caption{\label{photrun} Running {\bf phot} non-interactively +on the standard stars.} +\end{figure} +We will discuss the output below under ``Examining the results". + +\subsubsection{Photometry by Eye} +In this section we will discuss the case of selecting stars {\it +without} +running the automatic star-finding program, using the image display +window and the cursor. The first step is to {\bf epar phot} so it +resembles that of Fig.~\ref{photeye}. +\begin{figure} +\vspace{3.5in} +\caption{\label{photeye} Parameter file for {\bf phot} when stars will +be selected interactively.} +\end{figure} +Note that we have replaced the {\bf coords} coordinate list with the +null string (two adjacent double-quotes) and turned ``interactive" on. + +We need to display the frame we are going to work on in the imtool +window: + +\centerline { {\bf display std145 1} } + +\noindent +will display image {\bf std145.imh} in the first frame buffer. + +Now let's run {\bf phot}. We are not likely to be {\it too} accurate +with where we place the cursor, so to be generous we will increase the +allowable center shift to 3 pixels; otherwise we will get error messages +saying that the ``shift was too large": + +\centerline{ {\bf phot std145 maxshift=3.} } + +\noindent +(Note that even though {\bf maxshift} is a parameter of {\bf centerpars} +we can change it on the command line for {\bf phot}.) Also note that we +left off the ``{\bf .imh}" extension for a reason: we are going to take +the default names for the output files, and they will be given names +such as {\bf std145.mag.1} and so on. If we had included the {\bf .imh} +extension would would now be getting {\bf std145.imh.mag.1} names. + +At this point I get a flashing circle in my {\bf imtool} window; I don't +know what you get (it depends upon how your defaults are set up) but +there should be some sort of obvious marker on top of your image. +Put it on the first star you wish to measure and hit the space bar. The +coordinates and magnitude should appear in the {\bf gterm} window, and +you are ready to measure the next star on this frame. Proceed until all +the stars on this frame are measured, and then type a ``q" followed by +another ``q". Display the next frame, and run {\bf phot} on it. + +When you get done you will have kerjillions of files. + +\subsection{Examining the Results: the power of {\bf txdump }} + +Depending upon which of the two methods you selected you will either +have a single file {\bf standstuff} containing the results of all your +aperture photometry, or you will have a file for each frame ({\bf +stand145.mag.1}, {\bf stand146.mag.1} \ldots)containing the stars +on each frame. In either event the file will pretty much resemble that +shown in Fig.~\ref{photphotout}. +\begin{figure} +\vspace{7.5in} +\caption{\label{photphotout} Output file from {\bf phot}.} +\end{figure} +The file begins with a large header describing the parameters in +force at the time that {\bf phot} was run. There is, however, a real +subtlety to this statement. If you had changed a parameter in {\bf +datapars}, say, (or any of the other parameters) between running {\bf +daofind} and {\bf phot}, the header in {\bf phot} will reflect only the +setting that was in force at the time that {\bf phot} was run---in other +words, it does not take the values of what was used for the {\bf +threshold} from the coordinate file and retain these, but instead simply +copies what value of {\bf thresh} happens to be in {\bf datapars} at the +time that {\bf phot} is run. To those used to the +``self-documenting" feature of VMS DAOPHOT this is a major change! + +Once we get past the header information we find that there are 5 lines +per star measured. The ``key" to these five lines of information are +found directly above the measurement of the first star. On the first +line we have ``general information" such as the +image name, the beginning x and y values, the id, +and the coordinate file. On the next line we have all the centering +information: the computed x and y centers, +the x and y shift, and any centering errors. On the third line of the +file we have information about the sky. On the fourth line we have some +information out of the image header: what was the integration time, what +was the airmass, and what was the filter. Note +that {\bf phot} has used that integration time in producing the +magnitude---the exposures are now normalized to a 1.0 sec exposure. +The fifth line gives the actual photometry, including the size of the +measuring aperture, the total number of counts within the aperture, the +area of the aperture, and the output magnitude, photometric error, and +any problems encountered (such as a bad pixel within the aperture). + +We can extract particular fields from this file (or files) by using the +{\bf txdump} command. For instance, are there any cases where there +there were problems in the photometry? We can see those by saying + +\centerline{\bf txdump standstuff image,id,perror} + +\noindent +(If you did ``Photometry by eye" you can substitute {\bf *mag*} for {\bf +standstuff}.) +When it queries you for the ``boolean expression" type + +\centerline{ {\bf perror!$=$"No\_error"} } + +\noindent +The ``!$=$" construction is IRAF-ese for "not equal to"; therefore, this +will select out anything for which there was some problem in the +photometry. + +We can create a single file at this point containing just the +interesting results from the photometry file(s): do a + +\centerline{ {\bf txdump standstuff +image,id,ifilt,xair,mag,merr yes $>$ standsout} } + +\noindent +to dump the image name, id-number, filter, airmass, magnitude, +and magnitude error into a file {\bf standsout}. (Again, if you did +``Photometry by Eye" substitute {\bf *mag*} for {\bf standstuff}). +Unfortunately, what you do with this file is up to you right now until +the standard reductions routines become available. In the example shown +here we have selected the fields in the same order as used in Peter +Stetson's VMS CCDCAL software, and at the end of this manual we will +describe a (painful) kludge that nevertheless {\it will} let you use +these numbers with that software. + +\section{Crowded Field Photometry: IRAF/daophot} +\subsection{Historical Summary} + +In the beginning (roughly 1979) astronomers +interested in obtaining photometry from stars in ``relatively" crowded fields +would make the journey to Tucson in order to use Doug Tody's RICHFLD +program which ran on the IPPS display system. +RICHFLD allowed the user to define a +point-spread-function (PSF), and then fit this PSF to the brightest star +in a group, subtract off this star, and then proceed to the next +brightest star, etc. This represented a giant qualitative improvement +over the possibilities of aperture photometry, and allowed stars +separated by a few FWHM's to be accurately measured. + +Beginning in 1983, a group of RICHFLD users at the DAO (including +Ed Olszewski and Linda Stryker) began modifications to the ``poorman" +program of Jeremy Mould. This was largely motivated by the +implementation of the ``Kitt Peak CCD" at the prime-focus of the Tololo +4-m, and the idea was to design a crowded-field +photometry +program that (a) allowed simultaneous PSF-fitting, (b) made +use of the {\it known noise characteristics of a CCD} to do the fitting +in a +statistically correct manner (i.e., to make ``optimal" use of the data), +and (c) to be largely batch oriented. +In mid-1983 Peter Stetson arrived at the DAO, and took over +the effort. The result was +DAOPHOT, which did all these things and more. +By 1986 DAOPHOT was well distributed within the astronomical community. +The basic algorithms and philosophy can be found in Stetson 1987 (PASP +{\bf 99}, 111). + +DAOPHOT (and its companion program ALLSTAR) were not part of a +photometry +package; they were instead stand-alone Fortran +programs which did not deal in any way with the issue of image display +or what to do with the instrumental magnitudes once you had them. They +were also only supported on VMS, although several ``frozen" versions +were translated into UNIX by interested parties around the country. +There was therefore +much to be gained from integrating the algorithms of daophot +with IRAF in order to make use of +the image display capabilities and general tools for manipulating +images. Also, since many astronomers were now reducing their CCD data +with IRAF, it avoided the necessity of translating the IRAF files into +the special format needed by VMS DAOPHOT. Dennis Crabtree began this +translation program while at the DAO; it was taken over by Lindsey Davis +of the IRAF group in early 1989, and taken to completion in early 1990. +Pedro Gigoux of CTIO is currently hard at work on the photometry +reduction package, scheduled for completion sometime during the spring. + +\subsection{{\bf daophot} +Overview} +The steps involved in running daophot are certainly more involved than +in simple aperture photometry, but they are relatively straightforward. +The following sections will lead you through the necessary procedures. +Alternative routes will be noted at some points, and more may be gleaned +from reading the various "help" pages. A general outline is given here +so that you have some overview in mind; a detailed step-by-step summary +is provided at the end of this section. + +\begin{itemize} +\item Before you reduce the first frame, {\bf imexamine} your data to +determine FWHM's and the radius at which the brightest star you wish to +reduce blends into the sky. Run {\bf imhead} to find the ``key-words" +in your data headers for exposure times, filter number, and airmass. +Enter these, along with the characteristics of your chip (read-noise, +photons per ADU, maximum good data value) +into the parameter sets {\bf datapars} and {\bf +daopars}. +\item Use {\bf daofind} and {\bf tvmark} +to produce a list of x and y positions of most +stars on the frame. +\item Use {\bf phot} to perform aperture photometry on the identified +stars. This photometry will be the basis of the zero-point of +your frame via the PSF stars. This is also the only point where sky +values are determined for your stars. +\item Use {\bf psf} to define the PSF for your frame. If your PSF stars are crowded this +will require some iteration using the routines {\bf nstar} and {\bf +substar}. +\item Use {\bf allstar} to do simultaneous PSF-fitting for all the stars +found on your frame, and to produce a subtracted frame. +\item Use {\bf +daofind} on the subtracted frame to identify stars that had been +previously hidden. +\item Run {\bf phot} {\it on the original frame} to obtain aperture photometry +and sky values for the stars on the new list. +\item Use {\bf append} to merge the two aperture photometry lists. +\item Run {\bf allstar} again on the merged list. +\end{itemize} +When you have done this for your {\it U, B,} and {\it V} frames it is +then time to +\begin{itemize} +\item Use {\bf txdump}, {\bf tvmark}, and the image display +capabilities to come up with a consistent matching between the frames. +If there are additions or deletions then you will need to re-run +{\bf phot} and {\bf allstar} one more time. +\end{itemize} +Finally you will need to +\begin{itemize} +\item Determine the aperture correction for each frame by subtracting +all but the brightest few isolated stars on your frames and then running +{\bf phot} to determine the light lost between your zero-point aperture +and the large aperture you used on your standard stars. +\end{itemize} + +\subsection{How Big Is A Star: A Few Useful Definitions} + +The parameter files {\bf datapars} and {\bf daopars} contain three +``size-like" variables, and although this document is not intended as +a reference guide, there is bound to be confusion over these three +parameters, particularly among those new to DAOPHOT. In the hopes +of un-muddying the waters, we present the following. + +\begin{description} +\item[fwhmpsf] This is the full-width at half-maximum of a stellar object +(point-spread function, or psf). The value for {\bf fwhmpsf} gets used +only by the automatic star-finding algorithm {\bf daophot}, unless you +do something very bad like setting {\bf scale} to non-unity. + +\item[psfrad] This is the ``radius" of the PSF. When you construct a PSF, +the PSF will consist of an array that is +$$(2 \times psfrad +1) \times +(2 \times psfrad + 1)$$ +on a side. The idea here is that ``nearly all" of the light of the brightest +star you care about will be contained within this box. If you were to construct +a PSF with some large value of {\bf psfrad} and then run {\bf nstar} or +{\bf allstar} +specifying +a smaller value of {\bf psfrad}, the smaller value would be used. Making +the {\bf psfrad} big enough is necessary to insure that the wings of some +nearby bright star are properly accounted for when fitting a faint star. + +\item[fitrad] This is how much of the psf is used in making the fit +to a star. The ``best" photometry will be obtained (under most circumstances) +if this radius is set to something like the value for the fwhm. + +\end{description} + +\subsection{Setting up the parameter files ``daopars" and ``datapars" } + +The first step in using IRAF/daophot is to determine and store the +characteristics of your data in two parameter files called ``datapars" +and ``daopars"; these will be used by the various daophot commands. +In Section 1 we discussed how to deal with parameter files, and +in Section 2 we went through setting up ``datapars" for the standard +star solutions; at the risk of repeating ourselves, we will go through +this again as the emphasis is now a little different. + + +First inspect your headers by doing an {\bf imhead} imagename {\bf long+ +$|$ page}. +This will produce a listing similar to that shown in Fig.~\ref{newhead}. +\begin{figure} +\vspace{3.0in} +\caption{\label{newhead}Header for image n602alu.imh.} +\end{figure} +The things to note here are (a) what the filter keyword is (we can +see from Fig.~\ref{newhead} that the answer is F1POS; while there is +an F2POS also listed, the second filter bolt was not used and was always +in position ``zero"), +(b) what the effective exposure +time keyword is (EXPTIME in this example), and (c) what the effective +airmass keyword is (AIRMASS in this example). + +Next you need to examine some ``typical" frames in order to determine +the FWHM ({\bf fwhmpsf}) and the radius of the brightest star for which +you plan to do photometry ({\bf psfrad}). +First {\bf display} an image, and use the +middle button of the mouse (or whatever you need to do on your image +display) to zoom on a few bright stars. On the SUN the "F6" key will +let you see x and y values. The ``default" PSF radius is 11 pixels: +are your stars bigger than 23 pixels($23=2 \times 11 + 1$) +pixels from one side to the other? The FWHM is undoubtably variable +from frame to frame, but unless these change by drastic amounts (factors +of two, say) using a ``typical" value will doubtless suffice. You can +use the {\bf imexamine} routine to get some idea of the FWHM; do +{\bf imexamine} filename and then strike the ``r" key (for radial +profile) after centering the cursor on a bright (but unsaturated) star. +The last number on the plot is the FWHM of the best-fit Gaussian. + +We are now ready to do an {\bf epar datapars}. This parameter file +contains information which is data-specific. We set {\bf fwhmpsf} to the FWHM +determined above, and we enter the names of the keywords determined from +the header inspection above. The ``gain" and ``read-noise" are values +you have either determined at the telescope (using the Tololo routines) +or which are carved in stone for your chip. Choosing the value +for datamax, the ``Maximum good data value", +(in ADU's, NOT electrons) is a little bit trickier. In the case of +aperture photometry we were satisfied to take the nominal value for +the chip, but point-spread-function fitting is a bit more demanding +in what's ``linear". The data obtained +here was taken with an RCA chip, and we all know that RCA chips are +linear well past 100,000 e-. Thus, naively, we would expect that +with a gain of 2.25 that the chip was still linear when we hit the +digitization limit of 32,767 ADU's. Subtract off 500 for the likely +bias, and we {\it might} think that we were safe up to 32,200. However, +we would be wrong. Experience with PSF fitting on these data shows that +something (presumably in those little silver VEB's) has resulted in +these data being non-linear above 20,000 ADU's. My suggestion here is +to start with the nominal value but be prepared to lower it if the +residuals from PSF fitting appear to be magnitude dependent (more on this +later). The value for +{\bf datamin}, the +``Minimum good +data value", will be different for each frame (depending what the sky +level is) and there is not much point in entering a value for that yet. +Similarly the value we will use for threshold will change +from frame to frame depending upon what the sky level is. +When you are done your {\bf datapars} should resemble that of +Fig.~\ref{datapars}. +\begin{figure} +\vspace{2.7in} +\caption{\label{datapars} A sample {\bf datapars} is shown.} +\end{figure} + +Next we will {\bf epar daopars}. This parameter file contains +information specific to what you want {\bf daophot} to do. The only things here +we might want to change at this point are the ``Radius of the psf" {\bf psfrad} +(if your experiment above showed it should be increased somewhat), and +you might want to change the fitting radius {\bf fitrad}. Leaving the fitting +radius to ``something like" the FWHM results in the best SNR (you can +work this out for yourself for a few different regimes if you like to +do integrals). The ``standard values" are shown in Fig.~\ref{daopars}. +\begin{figure} +\vspace{2.7in} +\caption{\label{daopars} A sample {\bf daopars} is shown.} +\end{figure}. + +\subsection{Finding stars: {\bf daofind} and {\bf tvmark} } +The automatic star finder {\bf daofind} convolves a Gaussian of +width FWHM with the image, and looks for peaks greater than some +threshold in the smoothed image. It then keeps only the ones that are +within certain roundness and sharpness criteria in order to reject +non-stellar objects (cosmic rays, background galaxies, bad columns, +fingerprints). We have already entered a reasonable value for the FWHM +into {\bf datapars}, but what should we use as a threshold? We expect +some random fluctuations due to the photon statistics of the sky +and to the read-noise of the chip. You can calculate this easily by +first +measuring the sky value on your frame by +using {\bf imexamine} and the ``h" key to produce a histogram of +the data ({\bf implot} and the ``s" key is another way). In the example +shown in Fig~\ref{hist} we see that the sky value is roughly 150. +\begin{figure} +\vspace{3.6in} +\caption{\label{hist} The {\bf imexamine} histogram (``h" key) indicates +that the sky value is roughly 150.} +\end{figure} +In general, if $s$ is the sky value in ADU, $p$ is the number of +photons per ADU, and $r$ is the read-noise in units of electrons, +then the expected $1\sigma$ variance in the sky +will be +$$\left(\sqrt{s\times p + r^2}\right)/p$$ +in units of ADU's. For the example here we expect +$1\sigma=\left(\sqrt{150.\times 2.25 + 65^2}\right)/2.25=30$ ADU's. +Of course, if you have averaged N frames in producing your image, +then you should be using +$N\times p$ as the gain both here and in the value entered in +{\bf datapars}; similarly the readnoise is really just $r \times \sqrt{N}$. +If instead you summed N frames then the gain is just {\it p} and the +readnoise is still $r\times \sqrt{N}$. + +In the example shown here the expected $1\sigma$ variation of the sky is +30 ADU's; we might therefore want to set our star detection threshold to +3.5 times that amount. That won't guarantee that every last star we +find is real, nor will it find every last real star, but it should do +pretty close to that! + +We should use this opportunity to set datamin in {\bf +datapars} to some value like $s-3\sigma$. In this case we will set it +to 60. This is not currently used by {\bf daofind} but will be used +by all the photometry routines. Fig.~\ref{ndatapars} shows the data +parameters with the appropriate values of threshold and datamin now +entered. +\begin{figure} +\vspace{3.0in} +\caption{\label{ndatapars} Datapars with {\bf threshold} and {\bf datamin} +entered.} +\end{figure} + +We now can {\bf epar daofind} so it resembles that of +Fig.~\ref{daofind}. +\begin{figure} +\vspace{3.0in} +\caption{\label{daofind} Parameters for {\bf daofind}.} +\end{figure} +Note that although nothing appears to be listed under {\bf datapars} the +default name is ``datapars"; you could instead have created a separate +data parameter file for each ``type" of data you have and have called +them separate names (you could do this by doing an {\bf epar datapars} +and then exiting with a ``:w newnamepar"). This might be handy if +all your {\it U} frames were averages, say, but your {\it B} and {\it V} +frames were +single exposures; that way you could keep track of the separate +effective gain and readnoise values. In that case you would enter the +appropriate data parameter name under {\bf datapars}. As explained earlier, +you could also do a +``:e" on the {\bf datapars} line and essentially do the {\bf epar datapars} from +within the {\bf epar daofind}. +For normal star images, the +various numerical values listed are best kept exactly the way they are; +if you have only football shaped images, then read the help page for +{\bf daofind} for hints how best to find footballs. + +We can now run {\bf daofind} by simply typing {\bf daofind}. +As shown in Fig.~\ref{daofind} that we were asked for the FWHM and threshold +values; this is a due to having turned ``verify" on in the parameter +set. This safeguards to a large extent over having forgotten to set +something correctly. A [CR] simply takes the default value listed. + +Running {\bf daofind} produced an output file with the (default) +filename of {\bf n602csb.coo.1}. +(Do {\it not} give the {\bf .imh} extension +when specifying the image name, or the default naming +process will get very confused!) We can page +through that and see the x and y centers, the number of magnitudes +brighter than the cutoff, the sharpness and roundness values, and the +star number. However, of more immediate use is to use this file +to mark the found stars on the image display and see how we did. +If we have already displayed the frame in frame 1, then we can {\bf epar +tvmark} to make it resemble Fig.~\ref{tvmark}. +\begin{figure} +\vspace{2.7in} +\caption{\label{tvmark} Parameter file for {\bf tvmark}.} +\end{figure} +This will put red dots on top of each star found. + +We can see from Fig.~\ref{dots} that {\bf daofind} did a pretty nice +\begin{figure} +\vspace{7.0in} +\caption{\label{dots} Stars found with {\bf daofind} and marked with +{\bf tvmark}.} +\end{figure} +job. If we didn't like what we saw at this point we could rerun +{\bf daofind} with a slightly higher or slightly lower threshold---try +varying the threshold by half a sigma or so if you are almost right. +As you may have guessed, subsequent runs will produce output files with +the names n602csb.coo.2, n602csb.coo.3,... +If you are using a very slow computer, or are exceedingly impatient, + you could have saved some +time by putting a ``c" (say) under ``convolv" in your first run of +{\bf daofind}---this would have saved the +smoothed image as cn602csb.imh, and would drastically reduce +the number of cpu cycles needed to rerun {\bf daofind} with +a different threshold value. +If you really very happy with what {\bf daofind} did but you +just want to add one or two stars at this point, you +can in fact do that quite readily using {\bf tvmark}. Set the +parameters as in Fig.~\ref{tvmark}, but turn interactive on. +Position the cursor on top of the star you wish to add and strike +the ``a" key. Note that this will ``disturb" the format of the file, +but we really don't care; it will still work just fine as the input to +{\bf phot}. + +Note that it is fairly important that you do a good job at this stage. +If you have used too low a threshold, and have a lot of junk marked as +stars, these fictitious objects are likely to wander around during the +PSF-fittings until they find something to latch onto---{\it not} a good +idea. However, you also do not want the threshold to be so high that +you are missing faint stars. Even if you are not planning to publish +photometry of these faint guys, you need to have included them in the +list of objects if they are near enough to affect the photometry of +stars for which you do have some interest. If you find that varying the +threshold level does not result in a good list, then something is +wrong---probably you have badly over- or under-estimated the FWHM. +When you are close to the ``perfect" value of the threshold, +changing its value by as little as half a sigma will make a substantial +difference between getting junk and real stars. + +\subsection{Aperture Photometry with {\bf phot} } +The next step is to do simple aperture photometry for each of the stars +that have been found. These values will be used as starting points in +doing the PSF fitting, and this is the only time that sky values will be +determined. + +{\bf One of the few ways of ``crash landing" in the current +implementation of the software is to forget to reset ``datamin" in the +datapars file before running phot on a new frame. It is the only +critical parameter which is not queried when verify is turned on. Therefore, +this is a good time to check to see that ``datamin" is really set to +several sigma lower than the sky value of this particular frame.} + +The aperture photometry routine {\bf phot} has more parameters than all +the others put together: there are the parameter files +{\bf centerpars}, {\bf fitskypars}, and {\bf photpars}. +Fortunately the ``verify" +option frees you from having to look at these, and helps prevent you +from making a mistake. If this is your first pass through DAOPHOT it is +worth your while to do the following: + +\centerline{ {\bf unlearn centerpars} } + +\centerline{ {\bf unlearn fitskypars} } + +\centerline{ {\bf unlearn photpars} } + +\noindent +If you have used {\bf phot} for measuring standard stars, then this will +reset the defaults to reasonable values for crowded-field photometry; +in particular, we want to make sure that the centering +algorithm in {\bf centerpars} is set to ``none". +Do an {\bf epar phot} and make it look like that of Fig.~\ref{phot}. +Since we have the ``verify" switch turned on, we can be happy, not +worry, and simply type {\bf phot}. +{\bf phot} will then prompt you as shown in +Fig.~\ref{phot}. +\begin{figure} +\vspace{7.0in} +\caption{\label{phot} Questions and answers with {\bf phot}.} +\end{figure} +Note that the answers were particularly simple: we told it the name of +the frame we wished to work with, we accepted the default for the coordinate +list (it will take the highest ``version" of image.coo.NUMBER) and the +default for the output photometry list (n602csb.mag.1 will be produced +in this case.) We accepted the centers from {\bf daofind} as being +``good enough" to not have to recenter (they are good to about one-third +of a pixel, plenty good enough for aperture sizes of 2.5 pixels and +bigger; when we run this routine later on the second pass we would make +a Big Mistake by turning centering on here, so leave it off). +The sky +values will be taken from an annulus extending from a radius of 10 +pixels to a radius of 20 pixels, and it will determine the standard +deviation of the sky from the actual data. Note that this is probably a +lot closer in than you used on your standard stars; in crowded regions +of variable background keeping this annulus relatively close in will +help. +Finally, we used a measuring +aperture of 3 pixels. The number of counts within this aperture will be +what defines the zero-point of your frame, as we will see in Section 4.9, +and keeping this value {\it fixed} to some value like your typical FWHM +will keep you safe. + +\subsection{Making the PSF with {\bf psf} } + +If you are used to the VMS version of DAOPHOT, you are in for a pleasant +surprise when it comes to making a PSF within the IRAF version. +Nevertheless, just because it's easy doesn't mean that you shouldn't be +careful. + +What constitutes a good PSF star? Stetson recommends that a good PSF +star meets the following criteria: +\begin{enumerate} +\item No other star at all contributes any light within one fitting +radius of the center of the candidate star. (The fitting radius will be +something like the FWHM.) +\item Such stars as lie near the candidate star are significantly +fainter. (``Near" being defined as, say, 1.5 times the radius of the +brightest star you are going to measure.) +\item There are no bad columns or rows near the candidate star; there +should also be no bad pixels near the candidate star. +\end{enumerate} + + +In making a PSF, you wish to +construct a PSF which is free from bumps and wiggles (unless those +bumps and wiggles are really what a single isolated star would look like.) +First off, does it matter if we get the PSF ``right"? If we had +only isolated stars, then the answer would be no---any +old approximation to the PSF would give you +good relative magnitudes, and there are programs in the literature +which do exactly this. However, if your stars are relatively isolated +you are not going to gain anything by PSF-fitting over aperture photometry +anyway, so why bother? If you are dealing with crowded images, then the +PSF has to be right {\it even in the wings}, and for that reason we +construct a PSF empirically using the brightest and least crowded stars +in our frame. +If you are very, very +lucky you will find that your brightest, unsaturated star is well +isolated, and has no neighbors about it---if that's the case, use that +one and forget about the rest. Usually, however, you will find that +it isn't quite that easy, and it will be necessary to construct the PSF +interatively. The steps involved will be +\begin{enumerate} + \item Select the brightest, least-crowded stars for the zeroth-order + PSF. + \item Decrease the size of the PSF radius and fit these stars + with their neighbors using {\bf nstar}. + \item Subtract off the PSF stars and their neighbors using + {\bf substar} to see + if any of the PSF stars are ``funny"; if so, go back to + the step 1 and start over. + \item Edit the {\bf nstar} results file ({\bf imagename.nst.N}) + and delete the entries for the PSF stars. You are left + with a file containing the magnitudes and positions of just + the neighbors. + \item Subtract off just the neighbors using this file as input + to {\bf substar}. Display + the results, and examine the region around each PSF star. + Are the neighbors cleanly removed? + \item Increase the PSF radius back to the original value. + Construct an improved PSF using the new frame (the one with the + neighbors gone.) + \item Run {\bf nstar} on the PSF stars and their neighbors again, and + again subtract these using {\bf substar}. Examine the results. + If you are happy, proceed; otherwise, if the neighbors need + to be removed a bit more cleanly go back to step 4. +\end{enumerate} + +First {\bf display} the frame, and put dots on all the stars you've found +using {\bf tvmark} as discussed above. Next {\bf epar psf} and make sure +it looks like that of Fig.~\ref{psfparams}. +\begin{figure} +\vspace{2.5in} +\caption{\label{psfparams} Parameter file for {\bf psf}} +\end{figure} +We have set this up so we can choose the stars interactively from the +display window. + +Next run {\bf psf}. The defaults that you will be asked to {\bf verify} +are probably fine, but pay particular attention to {\bf psf radius} +and {\bf fitting radius}. The {\bf psf radius} should be as large +as you determined above (11 usually works well on ``typical" CCD +frames whose star images have FWHM's $\approx 3$). The ``fitting radius" +should be relatively generous here---maybe even larger than what you +want to use on your program stars. A reasonable choice is approximately +that of the FWHM. + +You will find that the cursor has turned into a circle and is sitting +on your image in the display window. Position it on a likely looking +PSF star, and strike the ``a" key. You will be confronted with a mesh +plot that shows the star and it surroundings. To find out more +about the star (such as what the peak data value is you can type +an ``s" while looking at the mesh plot. To reject the star type an +``x", to accept the star type an ``o". In the latter case, you will +next see a mesh plot that +shows you the star with a two-dimensional Gaussian fit removed from the +star. +Again, exit this with a ``o". If you don't find these mesh +plots particularly useful, you can avoid them by setting {\bf showplot=no} +in the {\bf psf} parameters (see Fig.~\ref{psfparams}). +At this point you will be told what the star number was, what the +magnitude was, and what the minimum and maximum data values within +the PSF were. (If you picked a star whose peak intensity was greater +than ``datamax" it will tell you this and not let you use this star.) +When you are done selecting stars, type a ``w" (to write the PSF to +disk) followed by a ``q". + +If in making the PSF you noticed that there were stars you could have +used but didn't because they had faint neighbors not found in the earlier +step of star finding, you can add these by hand by simply +running {\bf tvmark} interactively and marking the extra stars. First +{\bf epar tvmark} so it resembles that of Fig.~\ref{tvmark}. Then: + +\centerline{ {\bf display n602csb 1} } + +\centerline{ {\bf tvmark 1 n602csb.coo.1 interactive+} } + +\noindent + +Striking the ``l" key will mark the stars it already knows about onto +the display (as red dots this time around); positioning the cursor on the +first star you wish to add and type an ``a". When you are done adding +stars exit with a ``q" and re-run {\bf phot}. + +Now that you have made your preliminary PSF, do a {\bf directory}. You'll +notice that in addition to the image {\bf n602csb.psf.1.imh} that the +{\bf psf} routine has also added a text file {\bf n602csb.psg.1}. If +you {\bf page} this file you will see something like that of Fig.~\ref{psg}. +\begin{figure} +\vspace{3.5in} +\caption{\label{psg} The ``point spread function group" file +{\bf n602csb.psg.1}} +\end{figure} +This contains the aperture photometry of each PSF star plus its neighbors, +with each set constituting a ``group". Running the psf-fitting photometry +routine {\bf nstar} will fit PSF's to each of the stars within a group +simultaneously. + +Before we run {\bf nstar}, however, we must decide what psf radius to use. +Why not simply keep it set to the value found above (e.g., something like 11 +pixels)? The answer to this is a bit subtle, but understanding it will +help you diagnose what is going wrong when you find a PSF going awry (and +don't worry, you will). Let's consider the case that you construct a PSF +from a single star with one neighbor whose center is 12 pixels away from +the center of the PSF star, and let's have the PSF radius be 11 and the PSF +fitting radius be 3. The PSF looks something like that of Fig.~\ref{bump}. +\begin{figure} +\vspace{5.0in} +\caption{\label{bump} The zeroth order PSF of a star with a neighbor 12 pixels +away.} +\end{figure} +The light from the neighbor star ``spills +over" into the PSF. + +What happens when you try to fit two PSF's simultaneously? The bump from the +PSF of the brighter star sits within the fitting radius of the fainter star, +and it is the sum of the PSF's which are being fit to each star (that's +what ``simultaneous" means). Thus there is an ``implicit subtraction" of +the fainter star simply from fitting the bumpy PSF to the brighter star, +and the brightness of the fainter star will be underestimated. The way +to avoid this is to see that the PSF of the brighter star does not come +within the fitting radius of the fainter star, and {\it that} we can +accomplish easily by truncating the PSF size to something like the separation +of the two stars minus the fitting radius. Thus in the example here +we would want to fit the two stars using PSF's that were only ($12-3=9$) +pixels in radius. It's true that there may still be light of the PSF +star beyond this radius, but that will matter only if the PSF star is still +going strong when you get within the {\it fitting radius} of the fainter +star. + +Now that we understand all that, run {\bf nstar}. Specify the appropriate +image name for ``image corresponding to photometry" and give it +the ``.psg" file {\bf n602csb.psg.1} for the ``input group file". +Remember to decrease +the {\bf psf radius} when it tries to verify that number. {\bf nstar} +will produce a photometry output file {\bf n60csb.nst.1}. +You can +subtract the fitted PSF's from these stars now by running {\bf substar}. +Again, {\bf verify} the PSF radius to the smaller value. When the routine +finishes, {\bf display} the resultant frame {\bf n60csb.sub.1.imh} and +take a look at the PSF stars...or rather, where the PSF stars (and their +neighbors) were. Are they subtracted cleanly? Does one of the PSF +stars have residuals that look the reverse of the residuals of the others? +If so, it would be best to reconstruct the PSF at this point throwing out +that star---possibly it has a neighbor hidden underneath it, or has something +else wrong with it. Are the variations in the cores of the subtracted image +consistent with photon statistics? To answer this you may want to play +around with {\bf imexamine} on both the original and subtracted images, +but if the stars have cleanly disappeared and you can't even tell where +they were, you are doing fine. + +The worst thing to find at this point +is that there is a systematic pattern with position on the chip. This +would indicate that the PSF is variable. There is the option for making +a variable PSF, but the assumption is that the PSF varies smoothly in x +and +y; usually this is not the case. (In the case of the non-flat TI chips +the variations are due to the potato-chip like shape.) If you {\it do} +decide the PSF is variable, be sure to use plenty of stars in making the +PSF. As it says in the ``help page", +twenty-five to thirty is then not an unreasonable number. If that +doesn't scare you off, nothing will. + +If the brightest stars have residuals that are systematically different than +those of the fainter stars, maybe that chip wasn't quite as linear as you +thought, or perhaps there are charge transfer problems. This proved to +be the case for the RCA CCD data being reduced here. In Fig.~\ref{yuko} +we show the residuals that result when we based our PSF on a star whose +peak counts were 30000 ADUs. +Empirically we found that stars with peaks of 18K ADUs (a mere 40K electrons) +were safe to use, with the result that the dynamic range of our data +was simply not quite as advertised. Although the PSF function broke down +above 18K, the chip remained ``linear" in the sense that aperture photometry +continued to give good results---the total number of counts continued to +scale right up to the A/D limit of 32,767 ADUs (72K electrons after bias +is allowed for). This appears to be a subtle charge transfer +\begin{figure} +\vspace{7.0in} +\caption{\label{yuko} A ``before" and ``after" pair of images, where the +PSF was constructed with a star that was too bright. Note the systematic +residuals for the two bright stars. A ``bad" PSF star would result in a +similar effect; however, in these data we found that there was always a +systematic effect if the PSF stars were about 18000 ADU.} +\end{figure} +problem. + +We will assume that you have gotten the PSF to the point where +the cores of the stars disappear cleanly, although there may be residuals +present due to the neighbors. Our next step is to get rid of these neighbors +so that you can make a cleaner PSF. Edit the {\bf nstar} output file +{\bf n602csb.nst.1} and delete the lines associated with the PSF stars, +leaving only the neighbors behind. You can recognize the PSF stars, as +they are the first entry in each group. When you are done with this +editing job, re-run {\bf substar}, using the edited ``.nst" file as the +photometry file. Again in running {\bf substar} make sure you {\bf verify} +the PSF radius to the smaller value you decided above. Examine the results +on the image display. Now the PSF stars should be there but the neighbors +should be cleanly subtracted. Are they? If so, you are ready to proceed. +If not, re-read the above and keep at it until you get those neighbors +reasonably well out of the frame. + +We can now run {\bf psf} on the subtracted frame---the one with only the +neighbors gone. We have added some noise by doing the subtraction, and +so we should reset {\bf datamin} to several sigma below the previously +used +value. We are going to have to do more typing this time when +we run it, as the defaults for things will get very confused when we +tell it that the ``Image for which to build PSF" is actually +{\bf n60csb.sub.1}. For the ``Aperture photometry file" we can tell +it the original photometry file {\bf n602csb.mag.1} if we want, or +even the old ``.psg" file {\bf n602csb.psg.1} since every star that +we are concerned about (PSF star plus neighbor) is there. Go ahead +and give it the next `version" number for the ``Output psf image" +{\bf n602csb.psf.2} and for the ``Output psf group file" +{\bf n602csb.psg.2}. +We can of course do this all on the command line: + +\centerline{ {\bf psf n602csb.sub.1 n602csb.mag.1 n602csb.psf.2 +n602csb.psg.2 datamin=-150.} } + +\noindent +An example is shown in Fig.~\ref{psf1}. +{\it This time make sure you take the +large psf radius.} +\begin{figure} +\vspace{7.0in} +\caption{\label{psf1} Making the first revision PSF using the frames with the +neighbors subtracted. Compare this to Fig. 23, which shows the +same region before the neighbors have been removed.} +\end{figure} +Make a new PSF using the cursor as before. + +How good is this revised PSF? There's only one way to find out: run +{\bf nstar} on the original frame, this time keeping the psf radius large. +Then do {\bf substar} and examine the frame with both the PSF stars and +neighbors subtracted. Does this show a substantial improvement over the +first version? Now that you have a cleaner PSF it may be necessary to repeat +this procedure (edit the {\bf n602csb.nst.2} file, remove the PSF stars, +run {\bf substar} using this edited file to produce a frame with the +just the neighbors subtracted this time using a better PSF, run {\bf psf} +on this improved subtracted frame) but probably not. + +\subsection{Doing the psf-fitting: {\bf allstar}.} +The next step is to go ahead and run simultaneous PSF-fitting on all +your stars, and produce a subtracted frame with these stars removed. +To do both these things you need only run {\bf allstar}. The defaults +are likely to be right: see Fig.~\ref{allstar}. +\begin{figure} +\vspace{3.5in} +\caption{\label{allstar} Running {\bf allstar}.} +\end{figure} +As you may imagine, {\bf allstar} produces a photometry file +{\bf n602csb.als.1}, and another subtracted image: {\bf imagename.sub.N}. + +Display the subtracted frame, and blink it against the original. Has +IRAF/daophot done a nice job? If the stars are clearly gone with a few +hidden ones now revealed, you can be proud of yourself---if the results +are disappointing, there is only one place to look, and that is in the +making of the PSF. Assuming that all is well, it is now time to +add those previously hidden stars into the photometry. +The easiest way to do this is to run {\bf daofind} on the subtracted +image. +Set the value of {\bf datamin} to a value several sigma lower +than what you had used earlier in case the subtraction process generated +some spuriously small values, and you will want to {\it increase} the +value of threshold by 1 or 2 sigma above what you used previously. +Why? Because the subtraction process has certainly added noise to the +frame, and if you don't do this you will be mainly adding spurious +detections. Use {\bf tvmark} as before to examine the results of {\bf +daofind}; remember that the coordinate file name will be +{\bf imagename.sub.N.coo.1} this time around. If you are really close, +but want to add a couple of stars, re-run {\bf tvmark} on this file +using +{\bf interactive+}; this will allow you to add (and delete) coordinates +from the file. + +Now run {\bf phot} using this new coordinate file as the input list. +However, you do want to use the {\it original} frame for this photometry; +otherwise the sky values for the newly found stars will be very messed +up owing to the many subtracted images. A new aperture photometry file +{\bf n602csb.mag.2} will have been produced. Use {\bf append} to +concatenate these two files: {\bf append n602csb.mag.1,n602csb.mag.2 +n602csb.mag.3}. You can now re-run {\bf allstar} using this combined +photometry file as the input. + +\subsection{Matching the frames} +In the example here we have been reducing the {\it B} frame of +a set of {\it UBV}. Once all three frames have been reduced it is often +necessary to do a little fiddling. Have the same stars been identified +in each group? In many cases you don't want the same stars to have been +identified in each clump---afterall, some stars are red, some are blue +(that's presumably why you are doing this afterall, right?), but in some +cases you may find that a clump was identified as three objects on the +{\it U} and the {\it V} frames and clearly should have been three on the +{\it B} frame but instead is four or two. What to do? + +Using {\bf tvmark} it is relatively easy to set this right. First we +need to use {\bf txdump} to produce a file for each frame that can be +displayed. Do something like an + +\centerline{ {\bf txdump n602csu.als.2 $>$ tvu}} + +\noindent +followed by an + +\centerline{ {\bf txdump n602csb.als.2 $>$ +tvb}} + +\noindent +and a + +\centerline{ {\bf +txdump n602csv.als.2 $>$ tvv}} + +\noindent +In each case select {\bf xc,yc} and use +{\bf MAG!=INDEF} as a selection criteria. Thus you will then have three text +files that contain only the x's and y's of the stars with photometry. + +Next display the three frames ({\bf display n602csu 1}, {\bf display +n602csb 2}, {\bf display n602csv 3}) and put colored dots up to denote +the different allstar stars: + +\centerline{ {\bf tvmark 1 tvu color=204 inter-},} + +\centerline{ +{\bf tvmark 2 tvb color=205 inter-},} + +\noindent +and + +\centerline{ {\bf tvmark 3 tvv color=206 +inter-}} + +\noindent +will give pleasing results. Zoom, pan, register, and blink +around the frames until you are convinced that you really do want to +add or delete a star here or there. If you want to add or delete a star to the +{\it U} frame list, do a + +\centerline{ {\bf tvmark 1 tvu color=203 inter+}} + +\noindent +You are +now in interactive mode, and centering the cursor on the star you want +to add and striking the ``a" key will append the x and y value of the +cursor the tvu list. Similarly, striking the ``u" key +will delete a star from the list if you are using IRAF v2.9 or later. +(For earlier versions you are just going to have to do a little +editing by hand, good luck!) The star you add or delete will have +a white dot appear on top of it. +If you need to switch to a different coordinate file, simply exit the +interactive {\bf tvmark} with a ``q" and re-execute it specifying, for +example, {\bf tvmark 3 tvv color=203 inter+}. + +When you are done with adding and deleting stars, then it is time to +redo the photometry. Do a {\bf phot n602csu coords=tvv datamin=100} +in order to generate new aperture photometry and sky values. These +can then be run through {\bf allstar}, and the procedure repeated for +each +of the frames. + +\subsection{Determining the Aperture Correction} + +The zero-point of your magnitudes have been set as follows. When you +ran {\bf phot} using a small aperture (3 pixels in the example above) +magnitudes were defined as -2.5 * log{(Counts above sky)/(Exposure +time)} + Const. +(The constant Const was hidden away in {\bf photpars} and is the +magnitude assigned to a star that had a total of one ADU per second +within the measuring aperture you used.) When you defined your PSF the +magnitudes of the PSF stars determined from the aperture photometry were +then used to set the zero-point of the PSF. However, your standard +stars were presumably measured (if you did things right) through a much +larger aperture, and what we must do now is measure how much brighter +the PSF would have been had its zero-point been tied to the same size +aperture used for the standard stars. + +We need to determine the aperture correction from the brightest, +unsaturated stars (so there will still be reasonable signal above sky +at the size of the large aperture); if you can pick out stars that are +reasonably well isolated, so much the better. If this sounds vaguely +familiar to you, you're right---this is basically what you did for +selecting PSF stars, and these would be a good starting point for +selecting stars for determining the aperture correction. Ideally you +would like to use at least five such stars, but since when is data +reduction ideal? Nevertheless, it is in the determination of the +aperture correction the largest uncertainty enters in doing CCD +photometry on crowded fields. + +We will first need to pick out the brightest, isolated stars and then +to subtract off any stars that might affect their being measured through +the large ``standard star" aperture (e.g., something like 15 pixels). +To do this we need good photometry of any of these neighbor stars, and +we describe two ways to do this (1) the very long complicated way, and +(2) the very short easy way: + +\begin{enumerate} + +\item {\bf Method 1: Using the image display} +We can also use {\bf tvmark} to mark the stars that we wish to use for +aperture photometry. First we should remind ourselves what are multiple +stars and what aren't: {\bf display} the image, and then use {\bf +tvmark} to mark the stars with {\bf allstar} photometry: + +\centerline{ {\bf display n602csb 1} } + +\centerline{ {\bf txdump n602csb.als.2 xc,yc yes $>$ tvb} } + +\centerline{ {\bf tvmark 1 tvb color=204 interact-} } + +\noindent +Now go through and mark the stars you want to use as the aperture +correction stars {\it plus any neighbors that might contribute light +to a large aperture centered on the bright stars:} + +\centerline{ {\bf tvmark 1 bapstars color=203 interact+ }} + +\noindent +Use the ``a" key to generate a list ({\bf bapstars}) of the approximate +{\it x} and {\it y} positions of these stars. Next run this list +through {\bf phot} to generate improved centers and good sky values: + +\centerline{ {\bf phot n602csb bapstars bapphot calgor=``centroid" } } + +\noindent +Next run the photometry output file {\bf bapphot} through {\bf group}: + +\centerline{ {\bf group n602csb bapphot default default crit=0.2} } + +\noindent +This will have generated a ``group" file {\bf n602csb.grp.1}. + +\noindent +Finally (!) run this group file through {\bf nstar}: + +\centerline{ {\bf nstar n602csb default default default} } + +\item {\bf Method 2: Using the ``.psg" files} +If you used a goodly number ($>3-5$, say) stars in +making the PSF, then we will simply use these stars as the aperture +correction stars. Your last {\bf nstar} run should have produced an +``{\bf .nst}" file that contains good photometry for the PSF stars {\it +and} their neighbors. (If you don't remember if you did this, run {\bf +nstar} using the ``{\bf .psg}" as the input group file.) Note that this +method relies upon the assumption that the sum of the psf radius and psf +fitting radius is about as large as the size of the large aperture you +will use, so that all the important neighbors have been included in the +point-spread-function group, but this is probably a reasonable +assumption. + +\end{enumerate} + +Now that we are done with the preliminaries (!!), +we now want to produce two files: one of them containing only the +neighbors that we wish to subtract off, and another containing only the +bright isolated stars which we want to use in computing the aperture +correction. To do this we will use {\bf group} to divide up the ``{\bf +.nst}" file (we could simply use the editor but that would be a lot of +work). First we will use {\bf txdump} on the {\bf nstar} file to see the magnitude +range covered by the PSF stars and their neighbors: hopefully there +won't be any overlap. To do this try + +\centerline{ {\bf txdump n602csb.nst.3 id,group,mag yes} } + +\noindent +In the example shown in Fig.~\ref{grouping} we see that the PSF stars +\begin{figure} +\vspace{2.0in} +\caption{\label{grouping} The three PSF stars and their groups.} +\end{figure} +have magnitudes of 13.9, 15.0, and 16.5 in the three groups; all the +neighbor stars are fainter than 17.0. Thus we can use {\bf select} +to create a file containing the +photometry of the faint stars: + +\centerline{ {\bf select n602csb.nst.3 n602csbsub} } + +\noindent +and answer {\bf MAG$>$17.0} when you are queried for the ``Boolean +expression". This will put the photometry of the stars you wish to get +rid of into the file {\bf n602csbsub}. Next do an + +\centerline{ {\bf txdump n602csb.nst.3 xc,yc $>$ n602csbap} } + +\noindent +and answer {\bf MAG$<$17.0} in response to ``Boolean expression". This +will put the {\it x} and {\it y} values of the stars we wish to use for +the aperture correction into the file +{\bf n602csbap}. Next subtract the stars in the first file: + +\centerline{ {\bf substar n602csb n602csbsub} } + +\noindent and accept the defaults. This will result in the subtracted +image {\bf n602csb.sub.N}. It is this file on which we wish to run +the aperture photometry to determine the aperture correction: + +\centerline{ +{\bf phot n602csb.sub.N n602csbap n602csbapresults apertures=3.,15. annulus=20. dannu=5.} } + +\noindent +You will see something like Fig.~\ref{apcor1} on your terminal. +In this example we've made the assumption that the aperture size that +set your zero-point in making the PSF was 3 pixels (i.e., what you used +with {\bf phot} Way Back When), and that the aperture size used on your +standard stars was 15 pixels. +\begin{figure} +\vspace{3.0in} +\caption{\label{apcor1} The aperture correction run of {\bf phot}.} +\end{figure} +It is time to drag out your hand calculator. Using all three stars we +find an average aperture correction of $-0.371$ with a standard +deviation of the mean of 0.012 mag; given the large range in magnitude, +I might have been tempted to ignore the two fainter stars and keep the +aperture correction based only upon the brightest star (the frame is +sparsely populated, and there isn't a whole heck of a lot else we can +do). By an amazing coincidence, the aperture correction based just on +the brightest star is also $-0.371$. + + +\subsection{{\bf daophot} summary} +\begin{itemize} +\item Set up {\bf datapars} and {\bf daopars}. + \begin{enumerate} + \item Do an {\bf imhead} on some image and note the keywords for the + filter position, the effective exposure time, and the effective + airmass. + \item Use {\bf display} and {\bf imexamine} on a few frames to + determine the typical full-width-half-max + of stars and what would be a good + value to use for the radius of the psf (i.e., what radius will + contain the brightest star for which you wish to do photometry.) + \item Enter these into {\bf daopars} (psfrad) and {\bf datapars} + (header key words, fwhm). Also check that the correct values + are entered in {\bf datapars} for the gain (photons per ADU) + and read-noise (in electrons), as well as the ``maximum good data + value". + \end{enumerate} +\item Find stars. + \begin {enumerate} + \item Do an {\bf implot} or {\bf imexamine} to determine the sky + level on your frame. Calculate the expected $1\sigma$ error. + \item Enter the sky value minus 3$\sigma$ as your value for + {\bf datamin} in {\bf datapars}. + \item Run {\bf daofind} using as a threshold value 3 to 5 $\sigma$. + \item Use {\bf tvmark} to mark the stars found ({\bf imagename.coo.1}). + If you need to, rerun {\bf daofind} with a larger or small + threshold. + \end {enumerate} +\item Run aperture photometry using {\bf phot}. +\item Generate a PSF. Run {\bf psf} and add stars using the ``a" key. Try + to select bright, uncrowded stars. Then: + \begin {enumerate} + \item Run {\bf nstar} using the file {\bf imagename.psg.1} as the + ``input photometry group" file. If there are neighbors, be sure + to decrease the psf radius as explained above. + Run {\bf substar} (also using the smaller sized psf radius) + and display the + resultant subtracted frame {\bf imagename.sub.1}. Do the residuals + of the PSF stars look consistent, or is one of them funny? If need + be, start over. + \item Remove any neighbor stars by editing the PSF stars out of the + ``.nst" file, and rerunning {\bf substar}. Run + {\bf psf} on the subtracted file, using the normal psf radius again. + You will have to over-ride the defaults for the input and output file + names now that you are using the subtracted image. Rerun {\bf nstar} + on the original frame using the normal psf radius and the revised + PSF. Run {\bf substar} and display the results. Are the PSF stars + nicely removed, and do the areas around the PSF stars look clean? + It may be necessary to remove neighbors again using this revised + PSF. + \end {enumerate} +\item Run {\bf allstar}. Display the subtracted frame and see if your stars + have been nicely subtracted off. +\item Run {\bf daofind} on the subtracted frame, using a value for + {\bf threshold} which is another $\sigma$ or two larger than before, + and a value for {\bf datamin} which is several $\sigma$ lower than + before. Use {\bf tvmark} to examine the results, and if need be + run {\bf tvmark} interactively so that you may add any extra stars. +\item Run aperture photometry using {\bf phot} {\it on the original frame}, + using the new coordinate list produced above. +\item {\bf append} the two aperture photometry files. +\item Run {\bf allstar} using the combine photometry file. +\item Repeat all of the above for each frame in your ``set" (e.g., all short + and long exposures in each filter of a single field, say. +\item Use {\bf txdump} to select the stars from the allstar files which + have magnitudes not equal to ``INDEF". Mark these stars using + {\bf tvmark}, and then use the capabilities of the image display + and {\bf tvmark} to match stars consistently from frame to frame. + Rerun {\bf phot} and {\bf allstar} on the final coordinate lists. +\item Determine the aperture corrections. +\item Transform + to the standard system (see the next section) and then + publish the results. +\end{itemize} +\section{Transforming to the Standard System} + +This section will eventually tell you how to easily and painless obtain +the transformation equations for going from your instrumental magnitudes +to the standard system, and how to apply these transformation equations +to your program fields. Unfortunately, the IRAF routines for doing this +are still under construction. +In the meanwhile, we are providing here a kludge solution that can be +used by initiates of Stetson's VMS CCDCAL routines. If you haven't been +made a member of the club yet, and don't feel like waiting until the +IRAF routines are become available before you get results, then I would +recommend getting a hold of the good Dr. Stetson and bribing him until he +offers to send you a copy of CCDCAL. There is an excellent manual that +comes along with it, and we will not attempt to repeat any of that +material here. + +\subsection{Standard Star Solution} +First we will describe how to get output good enough to fool +the CCDCAL software into believing the photometry was produced by CCDOBS +(for the standard magnitudes), and what modifications need to be made +to CCDSTD.FOR + +On the standard file do a {\bf txdump standstuff lid,ifilt,xair,mag,merr +$>$ foolit} to dump the star number, filter number, airmass, and +instrumental magnitudes and errors into the file {\bf foolit}. +Unfortunately, you are now going to have to edit this file and stick in +the star name (in what ever form you have it in creating the library of +standard stars with CCDLIB) in place of the image name and star ID. +(These were simply placed in the file to help guide you). While you are +at it, line up the filter numbers, airmasses, and magnitudes into nice, +neat columns. When you get done, stick in a line at the top that gives +the number of instrumental magnitudes and their names, using a +i1,13x,n(6x,a6) format. For instance, in the case shown here there +are 3 instrumental magnitudes, U, B, and V. Finally, the filter numbers +have to be edited so they agree with these (e.g., they must denote +instrumental magnitude 1, 2, and 3...now aren't you sorry you didn't +decide to wait until the IRAF routines were finished?). In +Fig~\ref{groan} we show an example of the ``before" and ``after" file. +\begin{figure} +\vspace{3.5in} +\caption{\label{groan}The output of {\bf txdump} and the final file +ready for {\bf ccdstd}. Note the switching of the filter number ``5" +with ``1".} +\end{figure} + +CCDOBS.FOR itself now needs to be modified. Search for line statement +``1120" (which will say JSTAR=JSTAR+1). Add a line that sets the +integration time to 1 (tint=1.). Modify the READ statement as shown +in Fig.~\ref{ccdobs}, and finally modify the 213 FORMAT statement +so it actually matches your data file. +\begin{figure} +\vspace{2.5in} +\caption{\label{ccdobs} Modifications to CCDOBS.FOR} +\end{figure} +You should now be able to compile, link, and run this modified +version of CCDOBS and have it work on your standard star data. + +\subsection{Program Stars} +The work required for faking ``CCDCAL" is actually a lot less. The data +files are easily produced. Do a + +\centerline{{\bf txdump n602csu.als.2 +id,xc,yc,mag,merr,nit,chi $>$ csu} } + +\centerline{{\bf txdump n602csb.als.2 id,xc,yc,mag,merr,nit,chi $>$ +csb}} + +\centerline{{\bf txdump n602csv.als.2 id,xc,yc,mag,merr,nit,chi $>$ +csv}} + +\noindent +answering {\bf MAG!=INDEF} to ``boolean expression" each time. +These three files ({\bf csu}, {\bf csb}, {\bf csv} can be used +with CCDCAL once a single modification is made to CCDCAL.FOR: on +statement number 2020 change the format to ``free format", e.g., +2020 IF(NL(IOBS).NE.2) READ(2,*,END=2040). When CCDCAL queries +you for an integration time, be sure to tell it 1.0, as your data have +already been corrected for exposure times. + +\section{Acknowledgements} +We are grateful to Jeannette Barnes and Carol Neese for critical +readings of this document, although final blame for style and content +of course rests with the authors. +\end{document} diff --git a/noao/digiphot/daophot/doc/userdocs/daoref.ms b/noao/digiphot/daophot/doc/userdocs/daoref.ms new file mode 100644 index 00000000..6ef3d1f6 --- /dev/null +++ b/noao/digiphot/daophot/doc/userdocs/daoref.ms @@ -0,0 +1,6290 @@ +.LP +\0 +.de XS +.DS +.ps -1 +.vs -1p +.ft CB +.. +.de XE +.DE +.ft R +.ps +.vs +.. +.de YS +.nf +.ps -1 +.vs -1p +.ft CB +.. +.de YE +.fi +.ft R +.ps +.vs +.. +.RP +.TL +A Reference Guide to the IRAF/DAOPHOT Package +.AU +Lindsey E. Davis +.AI +IRAF Programming Group +.K2 +.ce +.TU +.br +.ce +January 1994 +.AB +.PP +DAOPHOT is a software package for doing stellar photometry in crowded stellar +fields +developed by Peter Stetson (1987) of the Dominion Astrophysical +Observatory. IRAF/DAOPHOT uses the task structure and +algorithms of DAOPHOT to do crowded-field stellar photometry within the +IRAF data reduction and analysis environment. +.PP +This document briefly describes the principal similarities and differences +between DAOPHOT and IRAF/DAOPHOT, the data preparation required to +successfully use IRAF/DAOPHOT, how to examine and edit the IRAF/DAOPHOT +algorithm parameters, how to run the IRAF/DAOPHOT package tasks interactively, +non-interactively, or in the background, and how to examine +and perform simple database operations on the output photometry files. +.PP +This document is +intended as a reference guide to the details of using and +interpreting the results of IRAF/DAOPHOT not a user's cookbook or a general +guide to doing photometry in IRAF. Its goal is to take the user +from a fully reduced image of a crowded stellar field to aperture +corrected instrumental magnitudes using a small artificial image as a +sample data set. +First time IRAF/DAOPHOT users +should consult \fIA User's Guide to Stellar Photometry With IRAF\fR, by +Phil Massey and Lindsey Davis. Detailed descriptions of the DAOPHOT photometry +algorithms can be found in Stetson (1987, 1990, 1992). +.AE +.ds CH +.bp +\0 +.bp +.PP +.na +.LP +\fBContents\fP +.sp 1 +1.\h'|0.4i'\fBIntroduction\fP\l'|5.6i.'\0\01 +.sp +2.\h'|0.4i'\fBDAOPHOT and IRAF/DAOPHOT\fP\l'|5.6i.'\0\01 +.sp +3.\h'|0.4i'\fBPreparing Data for DAOPHOT\fP\l'|5.6i.'\0\03 +.sp +4.\h'|0.4i'\fBSome IRAF Basics for New IRAF and DAOPHOT Users\fP\l'|5.6i.'\0\04 +.br +.sp +\h'|0.4i'4.1.\h'|0.9i'\fBPre-loaded Packages\fP\l'|5.6i.'\0\04 +.br +\h'|0.9i'4.1.1.\h'|1.5i'The DATAIO Package\l'|5.6i.'\0\05 +.br +\h'|0.9i'4.1.2.\h'|1.5i'The PLOT Package\l'|5.6i.'\0\05 +.br +\h'|0.9i'4.1.3.\h'|1.5i'The IMAGES Package\l'|5.6i.'\0\05 +.br +\h'|0.9i'4.1.4.\h'|1.5i'The TV Package\l'|5.6i.'\0\05 +.br +\h'|0.4i'4.2.\h'|0.9i'\fBOther Useful Packages and Tasks\fP\l'|5.6i.'\0\05 +.br +\h'|0.4i'4.3.\h'|0.9i'\fBImage Types, Image Directories, and Image Headers\fP\l'|5.6i.'\0\05 +.br +\h'|0.4i'4.4.\h'|0.9i'\fBThe Image Display and Image Cursor\fP\l'|5.6i.'\0\06 +.br +\h'|0.4i'4.5.\h'|0.9i'\fBThe Graphics Device and Graphics Cursor\fP\l'|5.6i.'\0\07 +.sp +5.\h'|0.4i'\fBSome DAOPHOT Basics for New DAOPHOT Users\fP\l'|5.6i.'\0\08 +.br +.sp +\h'|0.4i'5.1.\h'|0.9i'\fBLoading the DAOPHOT Package\fP\l'|5.6i.'\0\08 +.br +\h'|0.4i'5.2.\h'|0.9i'\fBLoading the TABLES Package\fP\l'|5.6i.'\0\08 +.br +\h'|0.4i'5.3.\h'|0.9i'\fBRunning the Test Script\fP\l'|5.6i.'\0\08 +.br +\h'|0.4i'5.4.\h'|0.9i'\fBOn-line Help\fP\l'|5.6i.'\0\09 +.br +\h'|0.4i'5.5.\h'|0.9i'\fBEditing the Package Parameters\fP\l'|5.6i.'\010 +.br +\h'|0.4i'5.6.\h'|0.9i'\fBEditing the Task Parameters\fP\l'|5.6i.'\011 +.br +\h'|0.4i'5.7.\h'|0.9i'\fBInput and Output Image Names\fP\l'|5.6i.'\011 +.br +\h'|0.4i'5.8.\h'|0.9i'\fBInput and Output File Names\fP\l'|5.6i.'\012 +.br +\h'|0.4i'5.9.\h'|0.9i'\fBAlgorithm Parameter Sets\fP \l'|5.6i.'\012 +.br +\h'|0.4i'5.10.\h'|0.9i'\fBInteractive Mode and Non-Interactive Mode\fP \l'|5.6i.'\014 +.br +\h'|0.4i'5.11.\h'|0.9i'\fBImage and Graphics Cursor Input\fP\l'|5.6i.'\014 +.br +\h'|0.4i'5.12.\h'|0.9i'\fBGraphics Output\fP\l'|5.6i.'\015 +.br +\h'|0.4i'5.13.\h'|0.9i'\fBVerify, Update, and Verbose\fP\l'|5.6i.'\015 +.br +\h'|0.4i'5.14.\h'|0.9i'\fBBackground Jobs\fP\l'|5.6i.'\015 +.br +\h'|0.4i'5.15.\h'|0.9i'\fBTiming Tests\fP\l'|5.6i.'\016 +.sp +6.\h'|0.4i'\fBDoing Photometry with DAOPHOT\fP\l'|5.6i.'\016 +.br +.sp +\h'|0.4i'6.1.\h'|0.9i'\fBThe Test Image\fP\l'|5.6i.'\016 +.br +\h'|0.4i'6.2.\h'|0.9i'\fBTypical Analysis Sequence\fP\l'|5.6i.'\017 +.br +\h'|0.4i'6.3.\h'|0.9i'\fBCreating and Organizing an Analysis Directory\fP\l'|5.6i.'\019 +.br +\h'|0.4i'6.4.\h'|0.9i'\fBReading the Data\fP \l'|5.6i.'\019 +.br +\h'|0.4i'6.5.\h'|0.9i'\fBEditing the Image Headers\fP\l'|5.6i.'\019 +.br +\h'|0.9i'6.5.1.\h'|1.5i'The Minimum Image Header Requirements\l'|5.6i.'\019 +.br +\h'|0.9i'6.5.2.\h'|1.5i'The Effective Gain and Readout Noise\l'|5.6i.'\019 +.br +\h'|0.9i'6.5.3.\h'|1.5i'The Maximum Good Data Value\l'|5.6i.'\021 +.br +\h'|0.9i'6.5.4.\h'|1.5i'The Effective Exposure Time\l'|5.6i.'\022 +.br +\h'|0.9i'6.5.5.\h'|1.5i'The Airmass, Filter Id, and Time of Observation\l'|5.6i.'\022 +.br +\h'|0.9i'6.5.6.\h'|1.5i'Batch Header Editing\l'|5.6i.'\024 +.br +\h'|0.4i'6.6.\h'|0.9i'\fBEditing, Checking, and Storing the Algorithm Parameters\fP\l'|5.6i.'\024 +.br +\h'|0.9i'6.6.1.\h'|1.5i'The Critical Algorithm Parameters\l'|5.6i.'\024 +.br +\h'|0.9i'6.6.2.\h'|1.5i'Editing the Algorithm Parameters Interactively with Daoedit \l'|5.6i.'\024 +.br +\h'|1.5i'6.6.2.1.\h'|2.2i'The Data Dependent Algorithm Parameters \l'|5.6i.'\025 +.br +\h'|1.5i'6.6.2.2.\h'|2.2i'The Centering Algorithm Parameters\l'|5.6i.'\028 +.br +\h'|1.5i'6.6.2.3.\h'|2.2i'The Sky Fitting Algorithm Parameters\l'|5.6i.'\029 +.br +\h'|1.5i'6.6.2.4.\h'|2.2i'The Aperture Photometry Parameters\l'|5.6i.'\029 +.br +\h'|1.5i'6.6.2.5.\h'|2.2i'The Psf Modeling and Fitting Parameters\l'|5.6i.'\030 +.br +\h'|1.5i'6.6.2.6.\h'|2.2i'Setting the Algorithm Parameters Graphically\l'|5.6i.'\031 +.br +\h'|0.9i'6.6.3.\h'|1.5i'Checking the Algorithm Parameters with Daoedit\l'|5.6i.'\031 +.br +\h'|0.9i'6.6.4.\h'|1.5i'Storing the Algorithm Parameter Values with Setimpars\l'|5.6i.'\032 +.br +\h'|0.9i'6.6.5.\h'|1.5i'Restoring the Algorithm Parameter Values with Setimpars\l'|5.6i.'\032 +.br +\h'|0.4i'6.7.\h'|0.9i'\fBCreating a Star List\fP\l'|5.6i.'\032 +.br +\h'|0.9i'6.7.1.\h'|1.5i'The Daofind Task\l'|5.6i.'\033 +.br +\h'|1.5i'6.7.1.1.\h'|2.2i'The Daofind Algorithm\l'|5.6i.'\033 +.br +\h'|1.5i'6.7.1.2.\h'|2.2i'The Daofind Algorithm Parameters\l'|5.6i.'\033 +.br +\h'|1.5i'6.7.1.3.\h'|2.2i'Running Daofind Non-Interactively\l'|5.6i.'\034 +.br +\h'|1.5i'6.7.1.4.\h'|2.2i'Running Daofind Interactively\l'|5.6i.'\034 +.br +\h'|1.5i'6.7.1.5.\h'|2.2i'The Daofind Output\l'|5.6i.'\036 +.br +\h'|1.5i'6.7.1.6.\h'|2.2i'Examining the Daofind Output\l'|5.6i.'\037 +.br +\h'|0.9i'6.7.2.\h'|1.5i'Rgcursor and Rimcursor\l'|5.6i.'\038 +.br +\h'|0.9i'6.7.3.\h'|1.5i'User Program\l'|5.6i.'\039 +.br +\h'|0.9i'6.7.4.\h'|1.5i'Modifying an Existing Coordinate List\l'|5.6i.'\039 +.br +\h'|0.4i'6.8.\h'|0.9i'\fBInitializing the Photometry with Phot\fP\l'|5.6i.'\039 +.br +\h'|0.9i'6.8.1.\h'|1.5i'The Phot Algorithm\l'|5.6i.'\039 +.br +\h'|0.9i'6.8.2.\h'|1.5i'The Phot Algorithm Parameters\l'|5.6i.'\040 +.br +\h'|0.9i'6.8.3.\h'|1.5i'Running Phot Non-interactively\l'|5.6i.'\040 +.br +\h'|0.9i'6.8.4.\h'|1.5i'Running Phot Interactively\l'|5.6i.'\042 +.br +\h'|0.9i'6.8.5.\h'|1.5i'The Phot Output\l'|5.6i.'\043 +.br +\h'|0.9i'6.8.6.\h'|1.5i'Examining the Results of Phot\l'|5.6i.'\044 +.br +\h'|0.4i'6.9.\h'|0.9i'\fBCreating a Psf Star List with Pstselect\fP\l'|5.6i.'\044 +.br +\h'|0.9i'6.9.1.\h'|1.5i'The Pstselect Algorithm\l'|5.6i.'\045 +.br +\h'|0.9i'6.9.2.\h'|1.5i'The Pstselect Algorithm Parameters\l'|5.6i.'\045 +.br +\h'|0.9i'6.9.3.\h'|1.5i'How Many Psf Stars Should Be Selected ?\l'|5.6i.'\046 +.br +\h'|0.9i'6.9.4.\h'|1.5i'Running Pstselect Non-interactively\l'|5.6i.'\047 +.br +\h'|0.9i'6.9.5.\h'|1.5i'Running Pstselect Interactively\l'|5.6i.'\047 +.br +\h'|0.9i'6.9.6.\h'|1.5i'The Pstselect Output\l'|5.6i.'\048 +.br +\h'|0.9i'6.9.7.\h'|1.5i'Examining and/or Editing the Results of Pstselect\l'|5.6i.'\048 +.br +\h'|0.4i'6.10.\h'|0.9i'\fBComputing the Psf Model with Psf\fP\l'|5.6i.'\049 +.br +\h'|0.9i'6.10.1.\h'|1.5i'The Psf Algorithm\l'|5.6i.'\049 +.br +\h'|0.9i'6.10.2.\h'|1.5i'Choosing the Appropriate Analytic Function\l'|5.6i.'\050 +.br +\h'|0.9i'6.10.3.\h'|1.5i'The Analytic Psf Model\l'|5.6i.'\050 +.br +\h'|0.9i'6.10.4.\h'|1.5i'The Empirical Constant Psf Model\l'|5.6i.'\051 +.br +\h'|0.9i'6.10.5.\h'|1.5i'The Empirical Variable Psf Model\l'|5.6i.'\051 +.br +\h'|0.9i'6.10.6.\h'|1.5i'Rejecting Bad Data from the Psf Model\l'|5.6i.'\051 +.br +\h'|0.9i'6.10.7.\h'|1.5i'The Model Psf Psfrad and Fitrad\l'|5.6i.'\052 +.br +\h'|0.9i'6.10.8.\h'|1.5i'Modeling the Psf Interactively Without a Psf Star List\l'|5.6i.'\052 +.br +\h'|0.9i'6.10.9.\h'|1.5i'Fitting the Psf Model Interactively Using an Initial Psf Star List\l'|5.6i.'\054 +.br +\h'|0.9i'6.10.10.\h'|1.5i'Fitting the Psf Model Interactively Without an Image Display\l'|5.6i.'\055 +.br +\h'|0.9i'6.10.11.\h'|1.5i'Fitting the Psf Model Non-interactively\l'|5.6i.'\056 +.br +\h'|0.9i'6.10.12.\h'|1.5i'The Output of Psf\l'|5.6i.'\057 +.br +\h'|0.9i'6.10.13.\h'|1.5i'Checking the Psf Model\l'|5.6i.'\059 +.br +\h'|0.9i'6.10.14.\h'|1.5i'Removing Bad Stars from the Psf Model\l'|5.6i.'\062 +.br +\h'|0.9i'6.10.15.\h'|1.5i'Adding New Stars to a Psf Star Group\l'|5.6i.'\062 +.br +\h'|0.9i'6.10.16.\h'|1.5i'Refitting the Psf Model With the New Psf Star Groups\l'|5.6i.'\062 +.br +\h'|0.9i'6.10.17.\h'|1.5i'Computing the Final Psf Model\l'|5.6i.'\063 +.br +\h'|0.9i'6.10.18.\h'|1.5i'Visualizing the Psf Model with the Seepsf Task\l'|5.6i.'\063 +.br +\h'|0.9i'6.10.19.\h'|1.5i'Problems Computing the Psf Model\l'|5.6i.'\064 +.br +\h'|0.4i'6.11.\h'|0.9i'\fBDoing Psf Fitting Photometry with Peak, Nstar, or Allstar\fP \l'|5.6i.'\065 +.br +\h'|0.9i'6.11.1.\h'|1.5i'Fitting Single Stars with Peak\l'|5.6i.'\065 +.br +\h'|1.5i'6.11.1.1.\h'|2.2i'The Peak Algorithm\l'|5.6i.'\065 +.br +\h'|1.5i'6.11.1.2.\h'|2.2i'Running Peak \l'|5.6i.'\065 +.br +\h'|1.5i'6.11.1.3.\h'|2.2i'The Peak Output\l'|5.6i.'\066 +.br +\h'|0.9i'6.11.2.\h'|1.5i'Fitting Stars with Group, Grpselect, Nstar and Substar\l'|5.6i.'\067 +.br +\h'|1.5i'6.11.2.1.\h'|2.2i'The Group and Nstar Algorithms\l'|5.6i.'\067 +.br +\h'|1.5i'6.11.2.2.\h'|2.2i'Running Group, Grpselect, and Nstar\l'|5.6i.'\068 +.br +\h'|1.5i'6.11.2.3.\h'|2.2i'The Nstar Output\l'|5.6i.'\070 +.br +\h'|0.9i'6.11.3.\h'|1.5i'Fitting Stars With Allstar\l'|5.6i.'\071 +.br +\h'|1.5i'6.11.3.1.\h'|2.2i'The Allstar Algorithm\l'|5.6i.'\071 +.br +\h'|1.5i'6.11.3.2.\h'|2.2i'Running Allstar\l'|5.6i.'\072 +.br +\h'|1.5i'6.11.3.3.\h'|2.2i'The Allstar Output\l'|5.6i.'\073 +.br +\h'|0.4i'6.12.\h'|0.9i'\fBExamining the Output Photometry Files\fP\l'|5.6i.'\073 +.br +\h'|0.4i'6.13.\h'|0.9i'\fBProblems with the Photometry\fP\l'|5.6i.'\074 +.br +\h'|0.4i'6.14.\h'|0.9i'\fBDetecting Stars Missed By Daofind\fP\l'|5.6i.'\075 +.br +\h'|0.4i'6.15.\h'|0.9i'\fBInitializing the Missing Star Photometry with Phot\fP\l'|5.6i.'\075 +.br +\h'|0.4i'6.16.\h'|0.9i'\fBMerging Photometry Files with Pfmerge\fP\l'|5.6i.'\076 +.br +\h'|0.4i'6.17.\h'|0.9i'\fBRefitting the Stars with Allstar\fP\l'|5.6i.'\076 +.br +\h'|0.4i'6.18.\h'|0.9i'\fBExamining the Subtracted Image\fP\l'|5.6i.'\076 +.br +\h'|0.4i'6.19.\h'|0.9i'\fBComputing an Aperture Correction\fP\l'|5.6i.'\076 +.sp +7.\h'|0.4i'\fBReferences\fP\l'|5.6i.'\077 +.sp +8.\h'|0.4i'\fBAppendices\fP\l'|5.6i.'\077 +.br +.sp +\h'|0.4i'8.1.\h'|0.9i'\fBThe Instrumental Magnitude Scale\fP\l'|5.6i.'\077 +.br +\h'|0.4i'8.2.\h'|0.9i'\fBThe Analytic Psf Models\fP\l'|5.6i.'\077 +.br +\h'|0.4i'8.3.\h'|0.9i'\fBThe Error Model\fP\l'|5.6i.'\078 +.br +\h'|0.4i'8.4.\h'|0.9i'\fBThe Radial Weighting Function\fP\l'|5.6i.'\078 +.br +\h'|0.4i'8.5.\h'|0.9i'\fBTotal Weights\fP\l'|5.6i.'\078 +.br +\h'|0.4i'8.6.\h'|0.9i'\fBBad Data Detection\fP\l'|5.6i.'\078 +.br +\h'|0.4i'8.7.\h'|0.9i'\fBStellar Mergers\fP\l'|5.6i.'\079 +.br +\h'|0.4i'8.8.\h'|0.9i'\fBFaint Stars\fP\l'|5.6i.'\079 +.br +.bp +\0 +.ds CH - % - +.bp 1 +\0 + +.TL +A Reference Guide to the IRAF/DAOPHOT Package +.AU +Lindsey E. Davis +.AI +IRAF Programming Group +.K2 +.ce +.TU +.br +.ce +January 1994 + +.NH +Introduction + +.PP +DAOPHOT is a software package for doing stellar photometry +in crowded fields developed by Peter Stetson of the DAO (1987, 1990, 1992). +The IRAF/DAOPHOT package uses the task structure and algorithms of DAOPHOT +to do crowded field photometry within the IRAF data reduction and +analysis environment. +.PP +Input to IRAF/DAOPHOT consists of an IRAF image file, numerous parameters +controlling the analysis algorithms and, optionally, graphics cursor and/or +image display cursor input. IRAF/DAOPHOT produces output photometry files +in either text format or STSDAS binary table format. Some IRAF/DAOPHOT tasks +also produce image output and graphics output in the form of plot metacode +files. +.PP +Separate tasks are provided for examining, editing, storing, and recalling +the analysis parameters, creating and editing star +lists, computing accurate centers, sky values and initial magnitudes +for the stars in the list, computing the point-spread function, +grouping the stars into physical associations, fitting the stars either +singly or in groups, subtracting the fitted stars from the original image, +and adding artificial test stars to the original image. A set of tools are +also provided for examining and editing the output photometry files. + +.NH +DAOPHOT and IRAF/DAOPHOT + +.PP +The principal similarities and differences between DAOPHOT and IRAF/DAOPHOT +are summarized below. +.IP [1] +The structure of IRAF/DAOPHOT is very similar to the +structure of DAOPHOT. All the DAOPHOT photometry tasks and many of +the utilities tasks are present in +IRAF/DAOPHOT and in many cases the DAOPHOT task names have been preserved. +A listing of the DAOPHOT photometry tasks and their closest IRAF/DAOPHOT +equivalents is shown below. + +.TS +l l +l l +l l. +DAOPHOT\tIRAF/DAOPHOT +TASK\tEQUIVALENT + +add*\taddstar +allstar\tallstar +attach\tN/A +append\tpfmerge,pconcat +find\tdaofind +group\tgroup +monitor\tdaophot.verbose=yes +nomonitor\tdaophot.verbose=no +nstar\tnstar +offset\tpcalc +options\tdaoedit +peak\tpeak +photometry\tphot +pick\tpstselect +psf\tpsf +select\tgrpselect +sort\tpsort,prenumber +sub*\tsubstar +.TE + +.IP [2] +Some DAOPHOT utilities tasks are missing from IRAF/DAOPHOT. +The DAOPHOT tasks \fBdump\fR, \fBexit\fR, \fBfudge\fR, +\fBhelp\fR, \fBlist\fR, and \fBsky\fR +have been replaced with general IRAF tasks, or with IRAF system facilities +that perform the equivalent function. The missing DAOPHOT utilities tasks +and their IRAF equivalents are shown below. + +.TS +l l +l l +l l. +DAOPHOT\tIRAF/DAOPHOT +TASK\tEQUIVALENT + +dump\tlistpixels,imexamine +exit\tbye +fudge\timreplace,fixpix,imedit +help\thelp daophot +list\timheader +sky\timstatistics,phistogram,imexamine +.TE + +.IP [3] +The IRAF/DAOPHOT default algorithms are the DAOPHOT II algorithms +(Stetson 1992). +.IP [4] +Users have more choice of and control over the algorithms +in IRAF/DAOPHOT than they do in DAOPHOT. For example the +IRAF/DAOPHOT aperture photometry task \fBphot\fR offers several +sky fitting algorithms besides the default "mode" algorithm, +and full control over the sky fitting algorithm parameters. +.IP [5] +The algorithm parameters in IRAF/DAOPHOT are grouped by function into +six parameter sets or psets rather than three as in DAOPHOT. +The six IRAF/DAOPHOT parameter sets with their DAOPHOT equivalents +in brackets are: +1) \fBdatapars\fR, the data definition parameters (daophot.opt), +2) \fBfindpars\fR, the detection algorithm parameters (daophot.opt), +3) \fBcenterpars\fR, the aperture photometry centering algorithm parameters +(no equivalent), +4) \fBfitskypars\fR, the aperture photometry sky fitting parameters (photo.opt), +5) \fBphotpars\fR, the aperture photometry parameters (photo.opt), +6) \fBdaopars\fR, the IRAF/DAOPHOT psf fitting parameters (daophot.opt, +allstar.opt). +.IP [6] +The IRAF/DAOPHOT algorithm parameter sets unlike the DAOPHOT parameter sets +can be interactively examined, +edited and saved with the \fBdaoedit\fR task using the image display +and radial profile plots. +.IP [7] +The IRAF/DAOPHOT algorithm parameter sets unlike the DAOPHOT parameter sets +can be saved and restored as a function of image using the \fBsetimpars\fR task. +.IP [8] +Memory allocation in IRAF/DAOPHOT is dynamic not static as in +DAOPHOT. IRAF/DAOPHOT allocates and frees memory as required +at run-time subject to the physical memory and swap space limitations of +the host computer. +.IP [9] +The IRAF/DAOPHOT point-spread function look-up table is stored in an +IRAF image not an ASCII table as in DAOPHOT. +.IP [10] +Unlike DAOPHOT, the IRAF/DAOPHOT tasks \fBdaofind\fR, \fBphot\fR, +\fBpstselect\fR +and \fBpsf\fR can be run interactively using the image display and graphics +window or non-interactively. Display and graphics capabilities were +deliberately omitted from DAOPHOT to minimize portability problems. +.IP [11] +The IRAF/DAOPHOT output photometry files can be written in either text +format as in DAOPHOT or STSDAS binary table format. +.IP [12] +Unlike DAOPHOT, fields or columns in both IRAF/DAOPHOT text and +STSDAS binary table photometry files are identified +by name and have an associated units and format specifier. +The IRAF/DAOPHOT photometry file input routines search for column +names, for example "GROUP,ID,XCENTER,YCENTER,MAG,MSKY" as +appropriate but are independent +of their placement in the input file. +.IP [13] +Several general purpose IRAF/DAOPHOT tasks are available for performing +operations on the final output photometry catalogs. In addition to +\fBpcalc\fR, \fBpconcat\fR, \fBpfmerge\fR, \fBprenumber\fR, +and \fBpsort\fR which are +also available in DAOPHOT, there are three photometry file editing tasks which +have no analog in DAOPHOT \fBpdump\fR, \fBpexamine\fR, and \fBpselect\fR. +All these tasks work on IRAF/DAOPHOT output text files or STSDAS binary +tables. An IRAF/DAOPHOT task is supplied for converting output text files to +STSDAS binary tables so as to make use of the even more general STSDAS +tables manipulation tools in the TABLES package. +.IP [14] +The IRAF/DAOPHOT output files are self-documenting. +All the information required to comprehend the history of or decode the +output photometry file is in the file itself, including the IRAF version +number, host computer, date, time, and names of all the +input and output files and the values of all the parameters. +.PP +For the remainder of this document IRAF/DAOPHOT will be referred to +as DAOPHOT. + +.NH +Preparing Data for DAOPHOT + +.IP [1] +DAOPHOT assumes that the images to be analyzed exist on disk in IRAF +image format. DAOPHOT can read and write old IRAF format ".imh" images +and ST IRAF format ".hhh" images. +When the IRAF FITS kernel becomes available DAOPHOT will be able +to read FITS images on disk as well. +QPOE IRAF format ".qp" images must be rasterized before they can +be input to DAOPHOT. +.IP [2] +All internal DAOPHOT calculations are done in real precision. +The pixel type of the image data on disk may be any of the following +data types: short integer, unsigned short integer, integer, long integer, +real or double. Users should realize that the extra precision in +images of type double will not be used by DAOPHOT. +.IP [3] +The instrumental signature must be removed from the input images +prior to running DAOPHOT. All CCD images should be overscan +corrected, bias corrected, dark current corrected and flat-fielded. +Users should be aware of the IRAF CCDRED package for reducing CCD data. +.IP [4] +DAOPHOT assumes that the input pixel data is linear. +If the data is non-linear over a large fraction of its total dynamic range, +the data must be linearized before running DAOPHOT. +.IP [5] +Saturated pixels or pixels distinguishable from good data by intensity, +do not need to be removed from the image prior to running DAOPHOT. +For example if the data +is non-linear only above 25000 counts, DAOPHOT can be instructed to +ignore pixels above 25000 counts. +.IP [6] +Extreme-valued pixels should be removed from the images prior to running +DAOPHOT. Extreme-valued pixels include those with values at or near +the floating point limits of the host machine and host machine special +numbers produced by operations like divide by zero, floating point +underflows and overflows, etc. The latter category of extreme-valued +pixels should not be produced by IRAF software, but may be produced by +user programs including imfort programs. +Floating point operations involving such numbers will frequently cause +arithmetic exception errors, since for efficiency and portability reasons +the DAOPHOT package and most IRAF tasks do not test for +their presence. +The \fBimreplace\fR task in the PROTO package can be used to remove extreme- +valued pixels. +.IP [7] +The background sky value should NOT be subtracted from the image prior +to entering the DAOPHOT package. The DAOPHOT fitting routines use an optimal +weighting scheme which depends on the readout noise, the gain, and the +true counts in the pixels. If the mean sky has been subtracted +then the counts in the image are not the true counts and the computed weights +will be incorrect. For similar reasons users should not attempt to +correct their magnitudes for exposure time by dividing their images +by the exposure time. +.IP [8] +Cosmic ray and bad pixel removal programs should be used with caution. If the +data and parameter values are set such that the cosmic ray and bad pixel +detection and +removal algorithms have difficulty distinguishing between stars and bad +pixels or cosmic rays, +the peaks of the stars may be clipped, altering the point-spread function +and introducing errors into the photometry. +.IP [9] +DAOPHOT assumes that the local sky background is approximately flat in the +vicinity of the object being measured. This assumption is equivalent to +requiring that the local sky region have a unique mode. Variations +in the sky background which occur on the same scale as the size of the +local sky region will introduce errors into the photometry. +.IP [10] +The point spread function must be constant or smoothly +varying with position over the entire image. This is the fundamental +assumption +underlying all of DAOPHOT. All stars in the image must be indistinguishable +except for position and magnitude. The variable point spread function +option is capable of handling second order variability as a function of +position in the image. +.IP [11] +The input images should not have undergone any operations which fundamentally +alter the image point spread function or the image statistics in a non-linear +way. For example, non-linear image restoration tasks must not be run on +the image to prior to running DAOPHOT. +.IP [12] +The gain, readout noise, exposure time, +airmass, filter, and observing time should be present and correct in the +image headers before DAOPHOT reductions are begun. +DAOPHOT tasks can extract this information from the image headers, use it +in the computations, and/or store +it in the output photometry files, greatly simplifying the analysis +and subsequent calibration procedures. +.fi + +.NH +Some IRAF Basics for New IRAF and DAOPHOT Users + +.NH 2 +Pre-loaded Packages + +.PP +Under IRAF versions 2.10 and later the DATAIO, PLOT, IMAGES, TV and NOAO +packages are pre-loaded so that all the tasks directly under them are +available when +IRAF is started. Each of these packages contains tasks which are useful +to DAOPHOT users for various reasons, and each is discussed briefly below. + +.NH 3 +The DATAIO Package + +.PP +DAOPHOT users should be aware of the DATAIO \fBrfits\fR and \fBwfits\fR tasks +which are used to transport data into and out of IRAF. Any input +and output images, including point-spread function look-up table images, +should normally be archived with \fBwfits\fR. +The cardimage reader and writer tasks for archiving text files, +\fBrcardimage\fR and \fBwcardimage\fR, are also located here. + +.NH 3 +The PLOT Package + +.PP +Various general purpose image and file plotting utilities can be found +in the PLOT packages. DAOPHOT users should be aware of the interactive image +row and column plotting task \fBimplot\fR, the image contour plotting task +\fBcontour\fR, the image surface plotting task \fBsurface\fR, image +histogram plotting task \fBphistogram\fR, the image radial profile +plotting task \fBpradprof\fR, and the general purpose graphing tool +\fBgraph\fR. The tasks \fBgkidir\fR and \fBgkiextract\fR are also useful +for extracting individual plots from the plot metacode files which may +be produced by some DAOPHOT tasks. + +.NH 3 +The IMAGES Package + +.PP +The IMAGES package contains a set of general purpose image operators. DAOPHOT +users +should be aware of the image header examining tasks \fBimheader\fR and +\fBhselect\fR, the header editing task \fBhedit\fR, the coordinate and +pixel value dumping task \fBlistpixels\fR, and the image statistics +task \fBimstatistics\fR. + +.NH 3 +The TV Package + +.PP +The TV package contains tasks which interact with the image display including +the all important \fBdisplay\fR task for displaying images, the +interactive image examining task \fBimexamine\fR, and the \fBtvmark\fR task +for marking objects on the image display. DAOPHOT users should become +familiar with all three of these tasks. + +.NH 2 +Other Useful Packages and Tasks + +.PP +The NPROTO package contains two useful tasks, \fBfindgain\fR, +for computing the gain and readout noise of a CCD +from a pair of biases and flats, and \fBfindthresh\fR for computing +the standard deviation of the background in a CCD frame given the +readout noise and gain. The ASTUTIL package contains the \fBsetairmass\fR +task for computing and/or correcting the airmass given the appropriate +input data. +Users might also wish to experiment with the tasks in the artificial +data package ARTDATA, and run the resulting images through DAOPHOT. + +.NH 2 +Image Types, Image Directories, and Image Headers + +.PP +The IRAF image environment is controlled by several +environment variables. The most important of these for DAOPHOT users +are: \fBimtype\fR the disk image format, \fBimdir\fR the default pixel +directory, and \fBmin_lenuserarea\fR the maximum length of the image header. +The values of these environment variables can be listed +as shown below. + +.YS +cl> show imtype +imh +cl> show imdir +/data/davis/pixels/ +cl> show min_lenuserarea +24000 +.YE + +.PP +\fB"imh"\fR is the default image format for most IRAF users, \fB"hhh"\fR the +default image format for ST users, and \fB"qp"\fR the photon counting format +used for photon counting data. DAOPHOT will work transparently on +"imh" and "hhh" images. "qp" event lists must be rasterized prior to using +DAOPHOT. When IRAF supports FITS images on disk, image format "fits", DAOPHOT +will be able to work directly on FITS images as well. IRAF uses the +image name extension, e.g. "imh" to automatically sense the image +disk format on input. The output disk format is set by: 1) the +extension of the output image name if present e.g. "imh", 2) the cl +environment variable \fBimtype\fR if the output image is opened as a new +image, e.g. the output of the \fBrfits\fR task, 3) the type of the input +image if the output image is opened as a new copy of an existing image, +e.g. the output of the \fBimcopy\fR task. +.PP +\fBimdir\fR specifies the default image pixel directory for "imh" format +files. The image header files are written to the current directory +and the pixel files are written to imdir. imdir can be set +to an existing directory on a scratch disk, the current +directory "HDR$", or the subdirectory pixels under the current +directory "HDR$pixels/". DAOPHOT users should keep both the intrinsic +speed of a disk and its network configuration in mind when setting +imdir. +.PP +\fBmin_lenuserarea\fR is the size of the image header area reserved +in memory when a new or existing image is opened. +The current default value of 24000 corresponds to space for approximately +300 keywords. +If an image on disk has a header larger than this the image header will +be truncated when it is read. +For most DAOPHOT users the default value is sufficient. However users whose +images have large headers or who are +creating a point-spread function using more than ~70 stars should set +min_lenuserarea to a larger value, e.g. 40000. +.PP +The following example shows how to change the default pixel directory to +HDR$pixels/ and set min_lenuserarea to 40000. To avoid redefining these +quantities for every session, users should enter the redefinitions into +their login.cl or loginuser.cl files. + + +.YS +cl> reset imdir = "HDR$pixels/" +cl> reset min_lenuserarea = 40000 +.YE + +.NH 2 +The Image Display and Image Cursor + +.PP +Several DAOPHOT tasks are interactive tasks or have an interactive as well +as a non-interactive mode. In interactive mode these tasks must be able to +read the image cursor on a displayed image and perform various +actions depending on the position of the image cursor and the keystroke +command typed. +.PP +DAOPHOT will work with the display servers Imtool, Saoimage, and Ximtool. +DAOPHOT users should be aware that both Imtool and Ximtool support multiple +frame buffers while SAOimage does not. Multiple frame buffers are an +important feature for users who wish to compare their original +images with the DAOPHOT output images from which all the fitted +stars have been subtracted. Users running DAOPHOT on a remote machine, e.g. +one with lots of memory and/or disk space, but displaying on their local +machine also need to set the \fBnode\fR environment variable to +the name of the local machine. + +.YS +cl> show node +ERROR: No such environment variable + show (node) +cl> set node = mymachine +.YE + +.PP +The maximum size of the display server frame buffer is defined by the +environment variable \fBstdimage\fR whose value can be printed as +shown below. + +.YS +cl> show stdimage +imt512 +.YE + +In the previous example the default frames buffers are 512 pixels square. +A user whose images are 2K square will want to reset the default frame +buffer size as shown below. + +.YS +cl> reset stdimage = imt2048 +cl> show stdimage +imt2048 +.YE + +.PP +In order for image cursor read-back to function correctly the environment +variable \fBstdimcur\fR must be set to "stdimage" as shown below. + +.YS +cl> show stdimcur +stdimage +.YE + +To check that image cursor read-back is functioning correctly the user +should display an image and try to bring up the image display cursor +as shown below. + +.YS +cl> display image 1 +cl> =imcur +.YE + +The image cursor should appear on the image display reading the correct +image pixel coordinates and ready to accept a +keystroke command. Any keystroke will terminate the cursor read. + +.NH 2 +The Graphics Device and Graphics Cursor + +.PP +Some interactive DAOPHOT tasks have graphics submenus which require +them to be able to read the graphics cursor on for example a radial +profile plot and perform various +actions based on the position of the graphics cursor in the +plot and the keystroke +command issued. The default graphics device is determined by +the \fBstdgraph\fR environment variable as shown below. + +.YS +cl> show stdgraph +xgterm +.YE + +To check that graphics cursor read-back is functioning correctly the user +should draw a plot and try to bring up the graphics cursor as +shown below. + +.YS +cl> contour image +cl> =gcur +.YE + +The graphics cursor should appear in the graphics window ready to accept a +keystroke command. Any keystroke will terminate the cursor read. + +.NH +Some DAOPHOT Basics for New DAOPHOT Users + +.NH 2 +Loading the DAOPHOT Package + +.PP +The DAOPHOT package is located in the digital stellar photometry package +DIGIPHOT. To load DIGIPHOT and DAOPHOT the user types the package names +in sequence as shown below, + +.YS +cl> digiphot +di> daophot +.YE + +after which the following menu of tasks appears. + +.YS +addstar daotest nstar pexamine psf +allstar datapars@ pcalc pfmerge psort +centerpars@ findpars@ pconcat phot pstselect +daoedit fitskypars@ pconvert photpars@ seepsf +daofind group pdump prenumber setimpars +daopars@ grpselect peak pselect substar +.YE + +Task names with a trailing "@" are parameter set tasks. +The remaining tasks are script and/or compiled tasks. +After the DAOPHOT package is loaded the user can redisplay +the package menu at any time with the command. + +.YS +da> ? daophot +.YE + +.NH 2 +Loading the TABLES Package + +.PP +The DAOPHOT photometry tasks write their output photometry files in +either text format (the default) or ST binary tables format. Users wishing +to use the ST binary tables format should acquire and install +the ST TABLES external package. Without the TABLES package the DAOPHOT +photometry tasks will read and write ST binary tables, but DAOPHOT +utilities like \fBpsort\fR which call TABLES package +tasks will not run on ST binary tables. +.PP +When DAOPHOT is loaded, it checks to see if the TABLES package is defined, +and if so loads it. A warning message is issued if the TABLES package is +undefined. The TABLES package tasks can be listed at any time after DAOPHOT +is loaded with the following command. + +.YS +da> ? tables +.YE + +.NH 2 +Running the Test Script + +.PP +The DAOPHOT package includes a script task \fBdaotest\fR which +executes each of the core DAOPHOT photometry tasks in turn using a test +image stored +in FITS format in the DAOPHOT test directory. \fBDaotest\fR is run as +shown below. + +.YS +da> daotest + +DAOTEST INITIALIZES THE DAOPHOT TASK PARAMETERS +TYPE 'q' or 'Q' TO QUIT, ANY OTHER KEY TO PROCEED + +Name of the output test image: test + +INITIALIZE THE DAOPHOT PACKAGE + +TESTING THE DAOFIND TASK +TESTING THE PHOT TASK +TESTING THE PSTSELECT TASK +TESTING THE PSF TASK +TESTING THE PEAK TASK +TESTING THE GROUP TASK +TESTING THE GRPSELECT TASK +TESTING THE NSTAR TASK +TESTING THE ALLSTAR TASK (CACHE=YES) +TESTING THE ALLSTAR TASK (CACHE=NO) +TESTING THE SUBSTAR TASK +TESTING THE ADDSTAR TASK + +DAOPHOT PACKAGE TESTS COMPLETED +.YE + +On task completion the user will find the input image in +test.imh, the psf image in test.psf.1.imh, the subtracted image produced +by \fBallstar\fR in test.sub.1.imh, the input image with artificial stars +added in test.add.1.imh, copies of all the output photometry files in +test.log, and copies of the plots produced by the \fBpsf\fR task +in test.plot on disk. +.PP +Users should be aware that the \fBdaotest\fR task will reset the DAOPHOT +task and algorithm parameters to their default values before and after it +is executed. + +.NH 2 +On-line Help + +.PP +A one-line description of each DAOPHOT task can be obtained by typing +the following command, + +.YS +da> help daophot\fR +.YE + +upon which the following package menu appears. + +.YS +digiphot.daophot: + addstar - Add stars to an image using the computed psf + allstar - Group and fit psf to multiple stars simultaneously +centerpars - Edit the centering algorithm parameters + daoedit - Review/edit algorithm parameters interactively + daofind - Find stars in an image using the DAO algorithm + daopars - Edit the daophot algorithms parameter set + daotest - Run basic tests on the daophot package tasks + datapars - Edit the image data dependent parameters + findpars - Edit the star detection parameters +fitskypars - Edit the sky fitting algorithm parameters + group - Group stars based on position and signal/noise + nstar - Fit the psf to predefined groups of stars + peak - Fit the psf to single stars + phot - Compute skies and initial magnitudes for a star list + photpars - Edit the aperture photometry parameters + psf - Compute the point spread function + seepsf - Compute an image from the point spread function + setimpars - Save/restore parameter sets for a particular image + substar - Subtract the fitted stars from the original image + + pcalc - Do arithmetic operations on list of daophot databases + pconcat - Concatenate a list of daophot databases + pconvert - Convert a text database to a tables database + pdump - Print selected fields from daophot databases + pfmerge - Merge a list of photometry databases + pstselect - Select candidate psf stars based on proximity + grpselect - Select groups from a daophot database + pexamine - Interactively examine and edit a daophot database + prenumber - Renumber stars in a daophot database + pselect - Select records from a daophot database + psort - Sort a daophot database\fR +.YE + +.PP +All the DAOPHOT tasks have on-line manual pages which can be +listed on the terminal. The following command lists the help for the +\fBphot\fR task on the terminal. + +.YS +da> phelp phot\fR +.YE + +Any section of the manual pages can be listed individually. +For example the examples section of the \fBphot\fR manual page can be +listed as follows. + +.YS +da> phelp phot sections=examples\fR +.YE + +The help page for \fBphot\fR can be piped to the local default printer as +follows. + +.YS +da> phelp phot | lprint\fR +.YE + +Finally the manual pages for the whole DAOPHOT package can be printed +by typing. + +.YS +da> phelp daophot.* | lprint\fR +.YE + + +.NH 2 +Editing the Package Parameters + +.PP +DAOPHOT has a package parameter set which defines the DAOPHOT +package environment. The DAOPHOT package parameters can edited +with epar as shown below. + +.YS +da> epar daophot +.YE + +.YS +Image Reduction and Analysis Facility + PACKAGE = digiphot + TASK = daophot + (version = "Dec92") + (text = yes) Text file on output ? + (verify = yes) Verify critical parameters ? + (update = no) Update critical parameters ? + (verbose = yes) Print verbose output ? +(graphics = "stdgraph") Default graphics device + (display = "stdimage") Default display device + (mode = "ql") +.YE + +To edit a parameter simply move the cursor to the parameter in question, +enter the new value, type return, and finally type \fB:wq\fR to quit and +update the parameter set. Package parameters can also be edited on the +command line as shown below. + +.YS +da> daophot.text = yes +.YE + +.PP +The DAOPHOT package parameters control the operation of the DAOPHOT package +as a whole. For example the \fBtext\fR parameter specifies whether the +output photometry files will be written in text or STSDAS binary tables format, +the parameters \fBverify\fR, \fBupdate\fR, and \fBverbose\fR determine +the default mode of operation of the DAOPHOT package tasks, and the parameters +\fBgraphics\fR and \fBdisplay\fR determine the default graphics and display +devices for the entire package. + +.NH 2 +Editing the Task Parameters + +.PP +The DAOPHOT task level parameters specify the input and output images and +files, the algorithm parameter sets, the graphics and image display input and +output devices, and the mode of operation of each DAOPHOT task. +.PP +To enter and edit the parameter set for the DAOPHOT \fBphot\fR task +the user types the following command, + +.YS +cl> epar phot +.YE + +after which the parameter set for the \fBphot\fR task appears on the +terminal ready for editing as shown below. + +.YS +Image Reduction and Analysis Facility +PACKAGE = daophot + TASK = phot + +image = Input image(s) +coords = default Input coordinate list(s) +output = default Output photometry file(s) +skyfile = Input sky value file(s) +(plotfil= ) Output plot metacode file +(datapar= ) Data dependent parameters +(centerp= ) Centering parameters +(fitskyp= ) Sky fitting parameters +(photpar= ) Photometry parameters +(interac= no) Interactive mode ? +(radplot= no) Plot the radial profiles? +(verify = )_.verify) Verify critical phot parameters ? +(update = )_.update) Update critical phot parameters ? +(verbose= )_.verbose) Print phot messages ? +(graphic= )_.graphics) Graphics device +(display= )_.display) Display device +(icomman= ) Image cursor: [x y wcs] key [cmd] +(gcomman= ) Graphics cursor: [x y wcs] key [cmd] +(mode = ql) +.YE + +The \fBphot\fR parameters can be edited by moving +the cursor to the line opposite the parameter name, entering the new value +followed by a carriage return, and typing \fB:wq\fR to exit the +\fBepar\fR task and update the parameters. +.PP +In the following sections the \fBphot\fR task is used to illustrate +some general features of the DAOPHOT package. + +.NH 2 +Input and Output Image Names + +.PP +The \fBphot\fR parameter \fIimage\fR +defines the image to be analyzed. The +root image name, the value of \fIimage\fR +stripped of directory and section information, +sets up the default input and output image naming convention for the task. +Users should avoid appending the ".imh" or ".hhh" extension +to their image name specification as these extensions are not required by IRAF +image i/o and become part of the default output image names. +.PP +The \fBphot\fR task does not create an output image but DAOPHOT tasks +which do, will by default create an output image name of the form +"image.extension.?" where image is the input image name +stripped of directory +and section information, extension is an id appropriate +to the task, and ? is the next available version number. +For example the first run of the \fBsubstar\fR task on the image "image" +will create an image called "image.sub.1", the second an image +called "image.sub.2", and so on. The default output image naming convention +can always be overridden by the user in any task. + +.NH 2 +Input and Output File Names + +.PP +DAOPHOT uses a default input and output file naming convention based on the +root image name or the input image name with the directory and +section specification removed. Users should avoid appending the ".imh" or +".hhh" extension to their input image name specification as these extensions +are not required by IRAF image i/o and become part of the default input +and output file names. +.PP +If a DAOPHOT task expects its input to have been written +by another DAOPHOT task, and the input file parameter value is "default", +the task will search for an existing +file called "image.extension.?" where image is the root image +name, extension identifies the task expected to have written the file, +and version is the highest version number for that file. For example, +if the user sets the \fBphot\fR parameters \fIimage\fR and +\fIcoords\fR to "m92b" and "default", \fBphot\fR will search +for a coordinate file called "m92b.coo.#" written by the +\fBdaofind\fR task. The default input file naming convention +can be over-ridden by the user at any point. +.PP +The output file naming convention works +in an identical manner to the input file naming convention, +although in this situation ? is the next available +version number. For example if the user sets the \fBphot\fR task +parameter \fIoutput\fR to "default", the output photometry file name +will be "image.mag.?" +where ? is 1 for the first run of \fBphot\fR, 2 for the second run, and so +on. The default output file naming convention can be over-ridden +by the user at any point. + +.NH 2 +Algorithm Parameter Sets + +.PP +The DAOPHOT parameters have been grouped together into parameter sets +or psets. +The use of psets encourages the logical grouping of parameters, permits +the various DAOPHOT tasks to share common parameters, and +permits the user to optionally store the DAOPHOT algorithm parameters +with the data rather than in the default uparm directory. +.PP +Six DAOPHOT psets, \fBdatapars\fR, \fBfindpars\fR, \fBcenterpars\fR, +\fBfitskypars\fR, \fBphotpars\fR and \fBdaopars\fR +control the DAOPHOT algorithm parameters. The \fBphot\fR task +uses four of them, \fBdatapars\fR which specifies data dependent +parameters like \fIfwhmpsf\fR (the full-width half-maximum of the psf), +\fIsigma\fR (the standard deviation of +the sky background), \fIepadu\fR and \fIreadout noise\fR +(the gain and readout noise of the detector), +and the \fBcenterpars\fR, \fBfitskypars\fR and \fBphotpars\fR parameter +sets which define the centering algorithm, sky fitting algorithm +and aperture photometry algorithm parameters respectively, +used by phot to compute initial centers, sky values, +and initial magnitudes for the stars to be analyzed. The \fBfindpars\fR pset +controls the star detection algorithm parameters used by the \fBdaofind\fR +task. The \fBdaopars\fR pset defines the psf model fitting +and evaluation parameters including the radius of the psf, the fitting radius, +and the grouping parameters used by all the psf fitting tasks. +.PP +By default the pset parameters can be examined, edited and stored +in the user's uparm directory, in the same manner as the task level +parameters. For example to list the current \fBdatapars\fR +pset the user types. + +.YS +da> lpar datapars +.YE + +To edit the \fBdatapars\fR parameter set, the user types either + +.YS +da> epar datapars + +or + +da> datapars +.YE + +and edits the parameter set in the usual manner with \fBepar\fR. +All the DAOPHOT tasks which reference this +pset will pick up the changes from the uparm directory, assuming +that the \fIdatapars\fR parameter is specified as "" in the calling task. +The user can also edit the \fBdatapars\fR +pset from within the \fBphot\fR +task or any other task which calls it as shown below. + +.YS +da> epar phot +.YE + +Move the cursor to the \fBdatapars\fR parameter line and type \fB:e\fR. +The menu for the +\fBdatapars\fR pset will appear ready for editing. Edit the desired +parameters and type \fB:wq\fR. \fBEpar\fR will return to the main +\fBphot\fR parameter set after which other psets or the main task parameters +can be edited. +.PP +Psets may also be stored in user files providing +a mechanism for saving a particular pset +with the data. +The example below shows how to store a pset in a file in the same directory +as the data and recall it for use by the \fBphot\fR task. The user types + +.YS +da> epar phot +.YE + +as before, enters the \fBdatapars\fR menu with \fB:e\fR and edits the +parameters. The command \fB:w data1.par\fR +writes the parameter set to a file called "data1.par" and a \fB:q\fR +returns to the main task menu. +A file called "data1.par" containing the new \fBdatapars\fR parameters +is written in the current directory. At this point the user is still in the +\fBphot\fR parameter set at the line opposite \fBdatapars\fR. He/she +enters "data1.par" on the line opposite this parameter. +The next time \fBphot\fR is run the parameters will +be read from "data1.par" not from the pset in the uparm directory. +The new parameter set can be edited in the usual way by typing + +.YS +da> epar data1.par + +or + +da> epar phot +.YE + +Users should be sure to append a .par extension to any pset files they +create as IRAF needs this extension to identify the file as a pset. +.PP +It is possible to develop quite efficient and creative schemes for using psets. +For example a user might choose to copy each crowded stellar field +image to its own directory, copy the default psets \fBdatapars\fR, +\fBfindpars\fR, \fBcenterpars\fR, \fBfitskypars\fR, \fBphotpars\fR +and \fBdaopars\fR to the files "datapars.par", "findpars.par", +"centerpars.par", "fitskypars.par", "photpars.par" and "daopars.par" in +each image directory, and then edit +the parameter sets of the top level tasks to look for psets with those names. +Once this is done the psets in each directory can be edited at will +without ever needing to edit the names of the psets in the top +level tasks. +.PP +The individual pset parameters themselves have the same attributes as +task level parameters. Hidden pset parameters may be altered on the +command line in the same way as task parameters. +The only distinction between task level parameters and pset parameters +is that the latter may be stored in or read from a user defined file. + +.NH 2 +Interactive Mode and Non-Interactive Mode + +.PP +The \fBphot\fR task's \fIinteractive\fR parameter +switches the task between interactive and non-interactive mode. +.PP +In interactive mode user instructions in the form of single keystroke +commands or colon commands are read from the image cursor. +For example the \fBphot\fR task \fB'i'\fR keystroke command enters the +interactive setup menu and the \fB'v'\fR keystroke command verifies the +current parameters. The colon commands are used to show or set any parameter. +For example, if the user does not like the fact that the full-width +half-maximum of a star +as measured with the cursor is 2.5368945 he/she can set it to 2.54 by +typing \fB:fw 2.54\fR. +.PP +In non-interactive mode the input files and images are read, +the parameters are read from the psets, +and the output files are written, +all, with the exception of an optional verification step, without the +intervention of the user. +.PP +The DAOPHOT parameter editing task \fBdaoedit\fR and the photometry catalog +examining task \fBpexamine\fR are interactive tasks. +Four other DAOPHOT tasks, \fBdaofind\fR, \fBphot\fR, \fBpstselect\fR, +and \fBpsf\fR +have an interactive and a non-interactive mode. The default mode for +\fBdaofind\fR, \fBphot\fR, and \fBpstselect\fR is non-interactive while +for \fBpsf\fR +it is interactive. +The remaining DAOPHOT tasks are currently non-interactive tasks. + +.NH 2 +Image and Graphics Cursor Input + +.PP +All tasks which can be run interactively accept commands from the logical image +cursor parameter \fIicommands\fR. Logical image cursor commands can +read from the logical image cursor, \fIicommands\fR = "" or a file, +\fIicommands\fR = "filename". The logical image cursor is normally +the physical image cursor and the value of the IRAF environment +variable \fBstdimcur\fR is normally "stdimage". In cases where the image +display device is non-existent or cursor read-back is not implemented for +a particular device the logical image cursor may be reassigned globally to the +the graphics cursor or the standard input +by setting the IRAF environment variable \fBstdimcur\fR as follows. + +.YS +da> set stdimcur = "stdimage" (image cursor default) + +da> set stdimcur = stdgraph (graphics cursor) + +da> set stdimcur = "text" (standard input) +.YE + +If logical image cursor commands are read from the standard input or a +file, the commands must have the following format + +.YS +[x y wcs] key [cmd]\fR +.YE + +where x and y stand for the x and y position of the image cursor, wcs defines +the world coordinate system, key is +a keystroke command, and cmd is an optional user command. +Quantities in square brackets are optional. The necessity for their +presence is dictated by the nature of the keystroke command. In the +case of the \fBphot "i"\fR keystroke described above they are required, whereas +in the case of the \fBphot "v"\fR keystroke they are not. +.PP +Some interactive commands require input from the logical graphics cursor +parameter \fIgcommands\fR which may be the logical graphics cursor, +\fIgcommands\fR = "", or a file of graphics cursor commands, +\fIgcommands\fR = "filename". +In DAOPHOT the logical graphics cursor must be set to the physical +graphics cursor and the value of the IRAF environment variable +\fBstdgcur\fR should be "stdgraph". + +.NH 2 +Graphics Output + +.PP +The \fBphot\fR parameters \fIgraphics\fR and \fIdisplay\fR specify the +default vector graphics and image display graphics devices. +Vector graphics output is written to the user's +graphics window, and image +graphics is overlaid on the user's image display. window +All interactive vector graphics output is written to +the device specified by \fIgraphics\fR. An example of this type of graphics +output is the +radial profile plot of a star plotted by the \fBphot\fR interactive +setup menu. +Image graphics is written to the image display device +specified by \fIdisplay\fR. +Examples of this type of output are the optional crosses +which mark the centers of the stars being measured by \fBphot\fR. +\fBIRAF does not currently support writing interactive graphics +to the image display device +so the display marking features of DAOPHOT are not supported\fR. +The single exception occurs in the situation +where the user is running interactively +off a contour plot as described in the \fBphot\fR help documentation. +In this case marking will work if +the parameter \fIdisplay\fR is set to "stdgraph". +DAOPHOT tasks which reference \fIgraphics\fR or \fIdisplay\fR will, in +interactive mode, issue +a warning if they cannot open either or both of these devices, +and continue execution. +.PP +Some DAOPHOT tasks permit the user to save plots of the results +for each measured star in a plot metacode file. +For example. if the \fBphot\fR task parameter \fIplotfile\fR is defined, +then for each star written to \fIoutput\fR +a radial profile plot is written to the plot metacode file \fIplotfile\fR. +\fIPlotfile\fR is opened in append mode and succeeding executions +of \fBphot\fR will write to the end of the same file. +Users should be aware plotfile can become very large and +that writing radial profile plots +to \fIplotfile\fR will greatly slow the execution of \fBphot\fR or any +other task. + +.NH 2 +Verify, Update, and Verbose + +.PP +In non-interactive mode the algorithm parameter values are read from the psets, +critical parameters are verified if the \fIverify\fR switch is on, and +updated if both the \fIverify\fR and \fIupdate\fR switches are on. +The \fIverify\fR and \fIupdate\fR options are also available as +separate keystroke commands in interactive mode. +Users must remember to turn off the +\fIverify\fR switch if they submit a task to the background or the task +will pause and wait indefinitely for input from the terminal. +.PP +In interactive or non-interactive mode a results summary and/or +error messages are written +to the standard output if the \fIverbose\fR switch is on. +Users must remember to redirect +any verbose output to a file if they submit the task to the background or +it will be lost. + +.NH 2 +Background Jobs + +.PP +Any DAOPHOT task can be run in background by appending an ampersand +to the end of the command. For example the \fBphot\fR task can be run +as a background job as shown below. + +.YS +da> phot image image.coo.1 image.mag.1 verbose- verify- & +.YE + +The user must be sure to turn off verbose mode +and set the verify switch to no. VMS users may have to append a queue +name after the trailing ampersand. +If verbose output is desired it can be captured in a file as shown +in the example below below. The & after the > will ensure that any error +output is also captured. + +.YS +da> phot image image.coo.1 image.mag.1 verbose+ inter- verify- \\ + >& listing & +.YE + +.NH 2 +Timing Tests + +.PP +Any DAOPHOT or IRAF task can be timed by prepending a $ sign to the +command as shown below. + +.YS +da> $phot image image.coo.1 image.mag.1 inter- verify- verbose- & +.YE + +At task termination the computer will print the cpu and elapsed +time on the terminal. +.PP +Care must be taken in using this feature +to make timing comparisons between hosts or even between runs on the same host, +as factors like which queue a task is submitted to (VMS), which version of +the OS the host is running, which version of the compiler +two programs were compiled under, +whether the disks are local or networked, and the number of users on the +machine will effect the elapsed time and/or the cpu time. + +.NH +Doing Photometry with DAOPHOT + +.NH 2 +The Test Image + +.PP +Each of the DAOPHOT analysis steps summarized in the following section +and discussed in detail in succeeding +sections uses the artificial image stored in fits format in +the file "daophot$test/fits3.fits" as test data. This image is small, +51 by 51 pixels, contains 10 stars whose coordinates and magnitudes +are listed below, has, a mean background level of ~100, poisson noise +statistics, a gain of 1.0, and a readout noise of 0.0. + +.YS +# Artificial Stars for Image Test + + 41.0 4.0 17.268 + 23.0 7.0 17.600 + 18.0 8.0 17.596 + 26.0 22.0 16.777 + 36.0 22.0 16.317 + 8.0 23.0 16.631 + 31.0 25.0 16.990 + 21.0 26.0 19.462 + 29.0 34.0 17.606 + 36.0 42.0 16.544 +.YE + +.PP +Results for this test image are used to illustrate the text. It is hoped +that users so inclined will be able to mimic the reductions on +their host machine. The fact that the image is small, means that +the tasks execute quickly, it is possible to display all the +important results in the manual, and it is possible for +the user to track and examine all the important numbers, something not +easy with larger images. Users are encouraged to construct more +challenging artificial images with the ARTDATA package, and to run +them through DAOPHOT. +.PP +All the examples in the following text were run +under IRAF 2.10.3 on a SPARCstation IPX. Users with different hardware +may see minor deviations from the output shown here due to machine +precision differences. + +.NH 2 +Typical Analysis Sequence + +.PP +The following sequence of operations summarizes the steps required to analyze +a crowded stellar field with DAOPHOT. +.IP [1] +Create a directory in which to analyze the image and make it the current +working directory. By default all output photometry and image files +will be written there. +.IP [2] +Read the reduced image into the working directory with the DATAIO package +task \fBrfits\fR. +.IP [3] +Check that the correct exposure time, airmass, filter id, time of +observation, gain, and readout noise are present and correct +in the image header with the \fBhselect\fR task. Enter / edit them +with the \fBhedit\fR task if they are not. Correct the exposure time for +shutter error, the airmass to mid-exposure, and the gain +and readout noise to the effective gain and readout noise, using the +\fBhedit\fR and/or \fBsetairmass\fR tasks. +.IP [4] +Edit the DAOPHOT algorithm psets with the interactive \fBdaoedit\fR +task. The parameters that require editing at this point are: +1) the numerical parameters +\fIfwhmpsf\fR (full-width at half-maximum of the point-spread function), +\fIsigma\fR (standard deviation of the background in counts), \fIdatamin\fR +(the minimum good data value in counts), \fIdatamax\fR (the maximum good +data value in counts), and the image header keyword parameters +\fIccdread\fR, \fIgain\fR, \fIexposure\fR, \fIairmass\fR, \fIfilter\fR, +and \fIobstimes\fR in the \fBdatapars\fR parameter set, +2) \fIcbox\fR (the centering box width) in the \fBcenterpars\fR parameter set, +3) \fIannulus\fR (inner radius of the sky annulus) and +\fIdannulus\fR (width of the sky annulus) in the \fBfitskypars\fR parameter set, +4) \fIapertures\fR (radii of the photometry apertures) in the \fBphotpars\fR +parameter set, and 5) \fIpsfrad\fR (maximum radius of the psf model) +and \fIfitrad\fR (psf model fitting radius) in the \fBdaopars\fR parameter set. +.IP [5] +Create an initial star list using the \fBdaofind\fR task. +Mark the detected stars on the image display with the \fBtvmark\fR task +and adjust the \fBfindpars\fR parameter \fIthreshold\fR until +a satisfactory star list is created. +.IP [6] +Compute sky background values and initial magnitudes for +the detected stars using the \fBphot\fR task and the star +list written by the \fBdaofind\fR task in step [5]. +.IP [7] +Create a psf star list using the \fBpstselect\fR task +and the photometry file written by \fBphot\fR in step [6]. Mark +the coordinates of the psf stars on the image display with the +\fBtvmark\fR task +and edit out any non-stellar objects, stars with +neighbors within \fIfitrad\fR pixels, or stars with obvious +cosmetic blemishes, using the \fBpexamine\fR task. +.IP [8] +Compute the current psf model using the +\fBpsf\fR task, the input photometry file written by the \fBphot\fR task +in step [6], and the psf star list written by the \fBpstselect\fR task +in step [7]. +.IP [9] +Fit the current psf model to the psf stars and their neighbors +using the \fBnstar\fR task, the psf star group photometry file +written by the \fBpsf\fR task in step [8] or created by the user in step [11], +and the current psf model written by the \fBpsf\fR task in steps [8] or [13]. +Subtract the fitted psf stars +and their neighbors from the original image using the \fBsubstar\fR task, +the photometry file written by the \fBnstar\fR task, and the current +psf model. +Display the subtracted image, mark the psf stars and their neighbors +on the display with the \fBtvmark\fR task, +and examine the \fBnstar\fR photometry +file and the subtracted image with the \fBpexamine\fR task. +If all the psf stars subtract out cleanly and none of them have any +significant neighbors, skip directly to step [14]. If all the psf stars +and their neighbors subtract out cleanly, and one or more of the psf +stars do have significant neighbors, skip directly to step [13]. +.IP [10] +Reexamine the subtracted image written in step [9]. Remove any psf stars +revealed by the subtraction to be non-stellar, multiple, or to contain +cosmetic blemishes, +from the psf star list written by the \fBpsf\fR task in step +[8] using the \fBpexamine\fR task. +If any bad psf stars are detected recompute the psf model by returning to +step [8] using the newly edited psf star list in place +of the one written by the previous execution of the \fBpsf\fR task in step [8]. +.IP [11] +Add any psf star neighbors too faint to be detected by the \fBdaofind\fR +task in step [5] but bright enough to effect the computation of the +psf model, to the original psf star group photometry file written +by the \fBpsf\fR task in step [8], +by estimating their positions, sky values, and magnitudes interactively +with the \fBphot\fR task, merging the results with the original psf star group +photometry file +using the \fBpfmerge\fR task, and regrouping the stars with the \fBgroup\fR +task. Refit the newly grouped psf stars and their neighbors using +the current psf model by returning to step [9], +replacing the original input group photometry file with the one +including the new psf star neighbors. +.IP [12] +Using the subtracted image written by the \fBsubstar\fR task in step [9], +note any systematic patterns in the psf star residuals with distance from +the star (\fIthese indicate a poorly chosen value for the annulus, +dannulus, function, or psfrad parameters), +position in the image (\fIthese suggest that the psf is variable +and that the value of the varorder parameter should be increased\fR), +or intensity (\fIthis suggests problems with the image data itself, e.g. +non-linearity\fR). If the problem is in the sky fitting parameters +edit the appropriate algorithm parameters and return to step [6]. If +the problem is in the psf modeling and fitting parameters, edit the +appropriate algorithm parameters and return to step [7]. I the problem +appears to be in the data or the data reduction procedures, review the +data taking and reduction history of the image before proceeding. +.IP [13] +Subtract the psf star neighbors but not the psf stars from the original +image using the \fBsubstar\fR task, +the photometry file written by the \fBnstar\fR task +in step [9], and the psf star list and current psf model written by +the \fBpsf\fR task in step [8]. +Recompute the current psf model using +the psf neighbor star subtracted image, the psf star group photometry file +written by the \fBpsf\fR task in step [8] or created by the user in step [11], +and the psf star list written in step [8]. +If the \fIvarorder\fR parameter was changed +return to step [9]. +Otherwise save the psf star neighbor subtracted image as it may be +required for computing the image +aperture correction in step [20], and proceed to step [14]. +.IP [14] +Fit the final psf model computed in steps [8] or [13] +to the stars in the photometry file written in +step [6] using the \fBallstar\fR task. +.IP [15] +Run \fBdaofind\fR on the subtracted image produced by \fBallstar\fR in step +[14] in order to pick up stars missed by the first pass of \fBdaofind\fR in +step [5]. +.IP [16] +Run \fBphot\fR on the original image using the new star list produced by +\fBdaofind\fR in step [15] and the \fBphot\fR algorithm parameters used +in step [6]. +.IP [17] +Merge the photometry file produced by \fBallstar\fR in step [14] with +the one produced by \fBphot\fR in step [16] using the \fBpfmerge\fR +task. +.IP [18] +Rerun \fBallstar\fR on the original image using the merged photometry file +created in step [17] and the psf model created in steps [8] or [13]. +.IP [19] +Repeat steps [15]-[18] as required, remembering to run \fBdaofind\fR +on the subtracted image produced by \fBallstar\fR and \fBphot\fR on the +original image. +.IP [20] +If the psf model is constant, compute the aperture correction for the +image using the original image and a sample of bright well-isolated stars +if possible, or the image with +the psf neighbor stars subtracted if necessary, the +\fBphot\fR task, and the PHOTCAL package \fBmkapfile\fR task. +If the psf model is variable, compute the aperture correction by calculating +the mean magnitude difference, for the psf stars with any +the neighbors subtracted, between the psf model fitted magnitudes computed +by the \fBnstar\fR task, and large aperture photometry magnitudes computed +with the \fBphot\fR task. +.IP [21] +Archive the algorithm parameters for the image with the \fBsetimpars\fR task +and proceed to the next image. + + +.NH 2 +Creating and Organizing an Analysis Directory + +.PP +By default DAOPHOT reads and writes data from and to the current working +directory. To create and set a new working directory the user must +execute the commands \fBmkdir\fR and \fBchdir\fR as shown below. + +.YS +da> mkdir testim +da> chdir testim +.YE + +.PP +DAOPHOT can in the course of reducing a single image, +generate a large number of photometry catalogs and output images. +Users should take a moment to consider how they wish to organize their data +directories before beginning any DAOPHOT analysis. Some possibilities for data +directory organization are: 1) by night of observation for standard star fields, +2) by star field for multi-filter observations of a crowded field, or +3) by individual image for single filter observations of several fields, +or any combination of the above. + +.NH 2 +Reading the Data + +.PP +DAOPHOT input images are normally read into IRAF from FITS files with +the DATAIO package task \fBrfits\fR. The following example shows how to +read the DAOPHOT test image stored in the FITS file "daophot$test/fits3.fits" +into the IRAF image test.imh. + +.YS +da> rfits daophot$test/fits3.fits 1 test +File: test Artificial Starfield Size = 51 x 51 +.YE + +When IRAF supports FITS format images on disk this step will no longer be +necessary, although for some images it may still be desirable for +image i/o efficiency reasons. + +.NH 2 +Editing the Image Headers + +.NH 3 +The Minimum Image Header Requirements + +.PP +Before beginning DAOPHOT reductions the user must gather +all the data required to determine the following quantities: +1) the effective readout noise of the detector in electrons, 2) the effective +gain of the detector in electrons per count, 3) the maximum good data value +of the detector in counts, 4) the effective exposure time in any units +as long as these units are identical for all the images to be analyzed +together, +5) the filter id, 6) the effective airmass of the observation at mid-exposure, +and 7) the time of the observation. + +.NH 3 +The Effective Gain and Readout Noise + +.PP +The DAOPHOT package tasks require correct effective +gain and readout noise values for: +1) the computation of the magnitude errors in the \fBphot\fR (gain only +required), \fBpeak\fR, \fBnstar\fR and \fBallstar\fR tasks, +2) the computation of the optimal weights used by the non-linear +least-squares fitting code in the \fBpeak\fR, \fBnstar\fR, and +\fBallstar\fR tasks, +3) the computation of the predicted signal-to-noise +ratios in the \fBgroup\fR task, +4) the computation of the sharpness and chi statistics in the \fBpeak\fR, +\fBnstar\fR, and \fBallstar\fR tasks, and 5) the correct computation of +the poisson noise (gain only required) in the \fBaddstar\fR task. +.PP +Nominal gain and readout noise values for a single image +should be obtained from the instrument +scientist. These values should also be determined/checked empirically with the +PROTO package task \fBfindgain\fR using bias and flat-field frames that +are unprocessed and uncoadded so that the noise characteristics of the +original data are preserved. +.PP +If the input image is the sum or average of several frames +the gain and readout noise values in the image headers must be edited +from single frame to effective gain and readout noise values +as shown below. In the following examples +gain and effective gain are in electrons / ADU, +readout noise and effective readout noise are in electrons, and N is the +number of individual frames which +have been summed, averaged, or medianed to create the input image. + +.nf + [1]. The image is the sum of N frames + + effective gain = gain + effective readout noise = sqrt (N) * readout noise + + [2]. The image is the average of N frames + + effective gain = N * gain + effective readout noise = sqrt (N) * readout noise + + [3]. The image is the median of N frames + + effective gain = 2.0 * N * gain / 3 + effective readout noise = sqrt (2 * N / 3) * readout noise +.fi + +.PP +The following example shows how to add the correct values of gain and +readout noise, which in this very artificial example are 1.0 and 0.0 +respectively, to the header of the test image with the \fBhedit\fR task. + +.YS +da> imheader test l+ +test[51,51][real]: Artificial Starfield with Noise + No bad pixels, no histogram, min=71.00896, max=535.1335 + Line storage mode, physdim [51,51], length of user area 163 s.u. + Created Mon 09:59:00 17-May-93, Last modified Mon 09:59:00 17-May-93 + Pixel file 'tucana!/d0/iraf/davis/test.pix' [ok] + 'KPNO-IRAF' / + '10-05-93' / + IRAF-MAX= 5.351335E2 / DATA MAX + IRAF-MIN= 7.100896E1 / DATA MIN + IRAF-BPX= 32 / DATA BITS/PIXEL + IRAFTYPE= 'REAL ' / PIXEL TYPE +da> hedit test gain 1.0 add+ verify- +add test,gain = 1. +test updated +da> hedit test rdnoise 0.0 add+ verify- +add test,rdnoise = 0. +test updated +da> imheader test l+ +test[51,51][real]: Artificial Starfield with Noise + No bad pixels, no histogram, min=71.00896, max=535.1335 + Line storage mode, physdim [51,51], length of user area 244 s.u. + Created Mon 09:59:00 17-May-93, Last modified Mon 09:59:00 17-May-93 + Pixel file 'tucana!/d0/iraf/davis/test.pix' [ok] + 'KPNO-IRAF' / + '10-05-93' / + IRAF-MAX= 5.351335E2 / DATA MAX + IRAF-MIN= 7.100896E1 / DATA MIN + IRAF-BPX= 32 / DATA BITS/PIXEL + IRAFTYPE= 'REAL ' / PIXEL TYPE + GAIN = 1. + RDNOISE = 0.\fR +.YE + +.PP +The following example shows how to correct the single frame +values of gain and readout noise, already present in the input image +header, to account for the fact that the input image is actually the +average of three frames (note that the frames are NOT actually independent +in this example!). + +.YS +da> imsum test,test,test testav3 option=average +da> hedit testav3 gain "(3.0*gain)" verify- +testav3,GAIN: 1. -> 3. +testav3 updated +da> hedit testav3 rdnoise "(rdnoise*sqrt(3.0))" verify- +testav3,RDNOISE: 0. -> 0. +testav3 updated +da> imheader testav3 l+ +testav3.imh[51,51][real]: Artificial Starfield with Noise + No bad pixels, no histogram, min=unknown, max=unknown + Line storage mode, physdim [51,51], length of user area 244 s.u. + Created Mon 11:02:22 17-May-93, Last modified Mon 11:02:22 17-May-93 + Pixel file 'tucana!/d0/iraf/davis/testav3.pix' [ok] + 'KPNO-IRAF' / + '10-05-93' / + New copy of test + IRAF-MAX= 5.351335E2 / DATA MAX + IRAF-MIN= 7.100896E1 / DATA MIN + IRAF-BPX= 32 / DATA BITS/PIXEL + IRAFTYPE= 'REAL ' / PIXEL TYPE + GAIN = 3. + RDNOISE = 0. +.YE + +.NH 3 +The Maximum Good Data Value + +.PP +Datamax is the maximum good data value in counts. Datamax +is the count level at which the detector saturates or the count +level at which it becomes non-linear, whichever is lower. DAOPHOT requires +a correct value of datamax to: 1) identify bad data in the \fBdaofind\fR, +\fBphot\fR, \fBpsf\fR, \fBpeak\fR, \fBgroup\fR, \fBnstar\fR, +and \fBallstar\fR tasks, and 2) identify saturated stars in the \fBphot\fR, +\fBpsf\fR, and \fBsubstar\fR tasks. +.PP +Users should be sure to allow adequate leeway for the detector bias level +in their determination of datamax. Test is an artificial image +linear over its entire data range. However as an example assume that it was +actually observed with a detector which is linear from 0 to 25000 counts +at a gain setting of 1.0, and that the mean bias level that was subtracted +from the raw data was ~400 counts. +In that case the user should set datamax to something like 24500 not 25000 +counts. +.PP +Datamax may be stored in the image header with \fBhedit\fR +as shown below. The use of the header keyword gdatamax instead of +datamax avoids any confusion with the reserved FITS keywords +datamin and datamax should they already be present in the image header, +or the IRAF keywords iraf-max and iraf-min which have the same meaning. + +.YS +da> hedit test gdatamax 24500 add+ verify- +add test,gdatamax = 24500 +test updated +.YE + +.NH 3 +The Effective Exposure Time + +.PP +The exposure time is used by the \fBphot\fR task to normalize the computed +initial magnitudes to an effective exposure time of one time unit. The +magnitude scale established in \fBphot\fR is preserved +in all the subsequent DAOPHOT analysis. Setting the correct exposure +time in the image headers before beginning DAOPHOT reductions will +simplify the book-keeping required in the later calibration step +significantly. +.PP +Exposure times should also be corrected +for any timing errors in the instrument shutter, although this is normally +important only for short exposure observations of standard stars. +.PP +The following example shows how to add the exposure time in seconds +to the image header, and how to correct it for a known shutter error +of 13 milli-seconds. Note that rather than overwrite the nominal exposure time +exptime, the user has chosen to store the corrected exposure time in +a new keyword cexptime. + +.YS +da> hedit test exptime 1.0 add+ verify- +add test,exptime = 1. +test updated +da> hedit test cexptime "(exptime+.013)" add+ verify- +add test,cexptime = 1.013 +test updated +da> imheader test l+ +test[51,51][real]: Artificial Starfield with Noise + No bad pixels, no histogram, min=71.00896, max=535.1335 + Line storage mode, physdim [51,51], length of user area 365 s.u. + Created Mon 09:59:00 17-May-93, Last modified Mon 09:59:00 17-May-93 + Pixel file 'tucana!/d0/iraf/davis/test.pix' [ok] + 'KPNO-IRAF' / + '10-05-93' / + IRAF-MAX= 5.351335E2 / DATA MAX + IRAF-MIN= 7.100896E1 / DATA MIN + IRAF-BPX= 32 / DATA BITS/PIXEL + IRAFTYPE= 'REAL ' / PIXEL TYPE + GAIN = 1. + RDNOISE = 0. + GDATAMAX= 24500 + EXPTIME = 1. + CEXPTIME= 1.013 +.YE + +.NH 3 +The Airmass, Filter Id, and Time of Observation + +.PP +The airmass, filter id, and time of observation are not used directly by +any of the DAOPHOT tasks. They are read from the image header and recorded +in the output photometry files. Correctly setting the airmass, +filter id, and the time of observation in the image headers before running +any DAOPHOT tasks will however significantly reduce the book-keeping +required in the subsequent calibration step. +.PP +The airmass can be computed and/or corrected to mid-exposure with the +ASTUTIL package task \fBsetairmass\fR. By default \fBsetairmass\fR requires +that the name of the observatory, date of observation, ra and dec, epoch of +the ra and dec, sidereal time, and exposure time be recorded +in the image header in the appropriate units in the keywords +observat, date-obs, ra, dec, epoch, st, and exptime. Hopefully most or +all of this information is already in the image header but in case +it is not, the following example shows how to edit it in and run +\fBsetairmass\fR. + +.YS +da> hedit test observat "CTIO" add+ verify- show- +da> hedit test "date-obs" "12/10/88" add+ verify- show- +da> hedit test ra "(str('21:51:59.0'))" add+ verify- show- +da> hedit test dec "(str('02:33:31.0'))" add+ verify- show- +da> hedit test epoch 1985.0 add+ verify- show- +da> hedit test st "(str('20:47:55.0'))" add+ verify- show- +da> setairmass test show- +da> imheader test l+ +test[51,51][real]: Artificial Starfield with Noise + No bad pixels, no histogram, min=71.00896, max=535.1335 + Line storage mode, physdim [51,51], length of user area 649 s.u. + Created Mon 09:59:00 17-May-93, Last modified Mon 09:59:00 17-May-93 + Pixel file 'tucana!/d0/iraf/davis/test.pix' [ok] + 'KPNO-IRAF' / + '10-05-93' / + IRAF-MAX= 5.351335E2 / DATA MAX + IRAF-MIN= 7.100896E1 / DATA MIN + IRAF-BPX= 32 / DATA BITS/PIXEL + IRAFTYPE= 'REAL ' / PIXEL TYPE + GAIN = 1. + RDNOISE = 0. + GDATAMAX= 24500 + EXPTIME = 1. + CEXPTIME= 1.013 + OBSERVAT= 'CTIO ' + DATE-OBS= '12/10/88' + RA = '21:51:59.0' + DEC = '02:33:31.0' + EPOCH = 1985. + ST = '20:47:55.0' + AIRMASS = 1.238106 +.YE + +The tortuous syntax required to enter the ra, dec, and st keywords is +necessary in order to avoid \fBhedit\fR turning strings like +"21:51:59.0" into numbers, +e.g. 21.86639. \fBSetairmass\fR permits the user to change the +default names for the date-obs and exptime image header keywords but +not those of observat, ra, dec, epoch or st. +To list the observatories in the IRAF observatory database and/or to find out +how to deal with the case of data taken at an observatory not in the +observatory database, the user should consult the help page for the +\fBobservatory\fR task. +.PP +The filter id is a string defining the filter used to take the observations. +It can be easily edited into the image header as shown below. + +.YS +da> hedit test filters V add+ verify- show- +.YE + +Users should be aware that any embedded blanks will be removed from the +filter id after it is read from the image header, but before it is +recorded in the photometry files. For example a filter id of "V band" +in the image header will become "Vband" in the photometry file. +.PP +The time of observation is a string defining the time at which the +observation was taken. The time of observation may be ut or local +standard time. If the time of observation is not already recorded in +the image header it can be entered in the usual fashion as shown below. + +.YS +da> hedit test ut "(str('00:07:59.0'))" add+ verify- show- +.YE + +.PP +After editing the "final" image header should look something like the +following. + +.YS +da> imheader test l+ +test[51,51][real]: Artificial Starfield with Noise + No bad pixels, no histogram, min=71.00896, max=535.1335 + Line storage mode, physdim [51,51], length of user area 730 s.u. + Created Mon 09:59:00 17-May-93, Last modified Mon 09:59:00 17-May-93 + Pixel file 'tucana!/d0/iraf/davis/test.pix' [ok] + 'KPNO-IRAF' / + '10-05-93' / + IRAF-MAX= 5.351335E2 / DATA MAX + IRAF-MIN= 7.100896E1 / DATA MIN + IRAF-BPX= 32 / DATA BITS/PIXEL + IRAFTYPE= 'REAL ' / PIXEL TYPE + GAIN = 1. + RDNOISE = 0. + GDATAMAX= 24500 + EXPTIME = 1. + CEXPTIME= 1.013 + OBSERVAT= 'CTIO ' + DATE-OBS= '12/10/88' + RA = '21:51:59.0' + DEC = '02:33:31.0' + EPOCH = 1985. + ST = '20:47:55.0' + AIRMASS = 1.238106 + FILTER = 'V ' + UT = '00:07:59.0'\fR +.YE + + +.NH 3 +Batch Header Editing + +.PP +The previous examples described in detail how to enter each of the required +keyword and value pairs into the image header using the \fBhedit\fR task. +Users with large number of header keywords to enter should consider using the +more batch oriented alternative task \fBasthedit\fR. + + +.NH 2 +Editing, Checking, and Storing the Algorithm Parameters + +.NH 3 +The Critical Algorithm Parameters + +.PP +The critical DAOPHOT algorithm parameters that should be set +before beginning any DAOPHOT analysis are: +1) the +full-width at half-maximum of the psf \fIfwhmpsf\fR, the standard +deviation of the sky background in counts \fIsigma\fR, the minimum and +maximum good data values \fIdatamin\fR and \fIdatamax\fR, and the image +header keyword parameters +\fIccdread\fR, \fIgain\fR, \fIexposure\fR, \fIairmass\fR, \fIfilter\fR, +and \fIobstimes\fR in the \fBdatapars\fR parameter set, +2) the default centering algorithm \fIcalgorithm\fR and centering box +\fIcbox\fR parameters in the \fBcenterpars\fR parameter set, 3) the sky fitting +algorithm \fIsalgorithm\fR, and the sky annulus \fIannulus\fR and +\fIdannulus\fR parameters in the \fBfitskypars\fR parameter set, +4) the \fIapertures\fR parameter in the \fBphotpars\fR parameter set, +and 5) the psf radius \fIpsfrad\fR +and fitting radius \fIfitrad\fR parameters in the \fBdaopars\fR parameter set. +The reamining parameters should be left at their default values, at least +initially. + +.NH 3 +Editing the Algorithm Parameters Interactively with Daoedit + +.PP +The DAOPHOT algorithm parameter editing task is \fBdaoedit\fR. \fBDaoedit\fR +permits +the user to edit all the algorithm parameter sets at once. It offers all the +capabilities of the IRAF parameter editing task \fBepar\fR, plus the +ability to set parameters using the displayed image and radial +profile plots of isolated stars. +.PP +To run \fBdaoedit\fR the user displays the image, types \fBdaoedit\fR, and waits +for the image cursor to appear ready to accept user commands. The following +example summarizes a typical \fBdaoedit\fR parameter editing session. + +.YS +da> display test 1 fi+ +da> daoedit test +.YE + +.IP ... +Execute the command \fB":epar datapars"\fR and enter the correct +values for the \fIdatamax\fR parameter, and the image header +keyword parameters \fIccdread\fR, \fIgain\fR, \fIexposure\fR, \fIairmass\fR, +\fIfilter\fR, and \fIobstime\fR. +.IP ... +Choose a bright isolated star and execute the \fBr\fR cursor +keystroke command to plot its radial profile. +.IP ... +From the information in the radial plot header and the plot +itself estimate reasonable values for the full-width at +half-maximum of the psf, the sky level, and the standard +deviation of the sky level in the image. +.IP ... +Repeat the previous step for several stars in order to +confirm that the original estimated values are reasonable. +.IP ... +Execute the \fB":epar datapars"\fR command once more and enter +the estimated values of the full-width at half-maximum of the psf and +the standard deviation of the sky background in the \fIfwhmpsf\fR +and \fIsigma\fR parameters respectively. +.IP ... +Set the \fIdatamin\fR parameter to the estimated sky background level +minus k times the standard deviation of the sky background, where +k is a number between 5.0 and 7.0. + +.IP +then + +.IP ... +Execute the command \fB":epar centerpars"\fR and set the \fIcbox\fR +parameter to 5 pixels or ~ 2 * \fIfwhmpsf\fR whichever is +greater. +.IP ... +Execute the command \fB":epar fitskypars"\fR and set the \fIannulus\fR +parameter to ~ 4 * \fIfwhmpsf\fR and the \fIdannulus\fR parameter to a +number between 2.5 * \fIfwhmpsf\fR and 4.0 * \fIfwhmpsf\fR. +.IP ... +Execute the command \fB":epar photpars"\fR and set the apertures +parameter to ~ 1.0 * fwhmpsf or 3 pixels whichever is greater. +.IP ... +Execute the command \fB":epar daopars"\fR and set the \fIpsfrad\fR +parameter to ~ 4 * \fIfwhmpsf\fR + 1 and the \fIfitrad\fR parameter to +~ 1.0 * \fIfwhmpsf\fR or 3 pixels whichever is greater. + +.IP +or alternatively + +.IP ... +Move to a bright star and execute the \fBi\fR cursor keystroke +command to enter the interactive setup menu. +.IP ... +Mark the \fIfwhmpsf\fR, \fIcbox\fR, \fIannulus\fR, \fIdannulus\fR, +\fIapertures\fR, \fIpsfrad\fR, and \fIfitrad\fR parameters with the +graphics cursor on the displayed radial profile plot, and verify and/or +roundoff the marked values. +.PP +The following sections discuss in detail how to edit each of the +parameter sets using the test image as a specific example. + +.NH 4 +The Data Dependent Algorithm Parameters + +.PP +A subset of the datapars parameters are used to specify the +characteristics of the detector, including the saturation or linearity +limit (\fIdatamax\fR) and noise model (\fIccdread\fR +and \fIgain\fR), and the parameters of the observation, including +exposure time (\fIexposure\fR), +airmass (\fIairmass\fR), filter (\fIfilter\fR), and time of observation +(\fIobstime\fR). +.PP +To edit the \fBdatapars\fR algorithm parameter set +from within the \fBdaoedit\fR task the user enters the command +\fB":epar datapars"\fR to invoke the \fBepar\fR task and edits the +parameters in the usual manner. +Editing is terminated with the usual \fB":wq"\fR command which returns the +user to the main \fBdaoedit\fR command loop. +.PP +After the appropriate \fIdatamax\fR, \fIccdread\fR, \fIgain\fR, +\fIexposure\fR, \fIairmass\fR, +\fIfilter\fR, and \fIobstime\fR parameter values for the +test image are entered, the \fBdatapars\fR +parameter should look as follows. + +.YS +Image Reduction and Analysis Facility +PACKAGE = daophot + TASK = datapars + +(scale = 1.) Image scale in units per pixel +(fwhmpsf= 2.5) FWHM of the PSF in scale units +(emissio= yes) Features are positive ? +(sigma = 0.) Standard deviation of background in counts +(datamin= INDEF) Minimum good data value +(datamax= 24500) Maximum good data value +(noise = poisson) Noise model +(ccdread= rdnoise) CCD readout noise image header keyword +(gain = gain) CCD gain image header keyword +(readnoi= 0.) CCD readout noise in electrons +(epadu = 1.) Gain in electrons per count +(exposur= cexptime) Exposure time image header keyword +(airmass= airmass) Airmass image header keyword +(filter = filter) Filter image header keyword +(obstime= ut) Time of observation image header keyword +(itime = 1.) Exposure time +(xairmas= INDEF) Airmass +(ifilter= INDEF) Filter +(otime = INDEF) Time of observation +(mode = ql) +.YE + +.PP +Users should realize that the values of the parameters \fIreadnoise\fR +and \fIepadu\fR will be used for the gain and readout noise if the image +header keywords specified by \fIccdread\fR and \fIgain\fR are not found +in the image header or cannot be correctly decoded. Similarly the values of +the \fIitime\fR, +\fIxairmass\fR, \fIifilter\fR, and \fIotime\fR parameters will be used +for the exposure time, airmass, filter id, and time of observation if +the image header keywords specified by \fIexposure\fR, \fIairmass\fR, +\fIfilter\fR, and \fIobstime\fR are not found in the image header +or cannot be correctly decoded. +.PP +The \fBdatapars\fR parameters \fIfwhmpsf\fR, \fIsigma\fR, and \fIdatamin\fR +are used to: 1) determine the size of star for which the \fBdaofind\fR star +detection algorithm is optimized (fwhmpsf), 2) define the \fBdaofind\fR +algorithm detection threshold for faint objects (sigma), +3) define the fwhm of the psf for the \fBphot\fR task centering algorithms +"gauss" and "ofilter" (fwhmpsf), +4) supply a first guess for the true fwhm of the psf to the psf +function fitting task \fBpsf\fR (fwhmpsf), 5) determine the +minimum good data value +in the \fBdaofind\fR, \fBphot\fR, \fBpsf\fR, \fBpeak\fR, \fBgroup\fR, +\fBnstar\fR, and \fBallstar\fR tasks (datamin). +.PP +Reasonable values for these parameters can be obtained by examining the +radial profile plots of several isolated stars from within the +\fBdaoedit\fR task as outlined below: + +.IP ... +Move the image cursor on the displayed image to a +reasonably bright isolated star (a good candidate is +the star at pixel 8,23 in the test image) and execute +the \fBr\fR keystroke command. +A radial and integrated profile plot of the selected +star will appear on the screen with the largest photometry aperture radius, +inner and outer radii of the sky annulus, and median sky level in the sky +annulus marked on the plot. +.IP ... +Assuming that the plot is normal, note the computed +fwhmpsf (2.6 rounded to the nearest tenth of a pixel for +the star at 8,23), median sky value (100 counts rounded +to the nearest count for the star at 8,23), and standard +deviation of the sky values (10 counts rounded to the +nearest count for the star at 8,23) written in the plot header. +These numbers suggest a value of ~50 for datamin (50 is ~5 +standard deviations of the background counts below the +background count estimate) +.IP ... +Edit the estimated values into the datapars pset by +typing the command \fB":epar datapars"\fR, entering the values, +and typing \fB":wq"\fR to update the parameter set. + +.IP +or + +.IP ... +Enter them individually using the daoedit colon commands, +e.g. \fB":fwhmpsf 2.5"\fR, \fB":sigma 10.0"\fR, and \fB":datamin 50.0"\fR. +.IP ... +Check the new values of \fIfwhmpsf\fR, \fIsigma\fR, and \fIdatamin\fR +by doing radial profile plots of several +other isolated stars (the stars at 36,42 and 41,4 in +the test image are good test stars). +.IP ... +On the basis of the estimated \fIfwhmpsf\fR of these stars change the +fwhmpsf parameter back to 2.5 with the command +\fB":fwhmpsf 2.5"\fR. +.IP ... +Check that the observed standard deviation of the sky +background, sigma, agrees reasonably well with the +predicted value, psigma, based on the median sky level, +and the effective gain and readout noise of the image. +For the test image these numbers are related as shown below. + +.nf + psigma = sqrt (median sky / effective gain + + (effective rdnoise / effective gain) ** 2) + ~ sqrt (100.0 / 1.0 + (0. / 1.0) ** 2) + ~ 10.0 + ~ sigma +.fi + +.IP ... +If psigma and sigma are significantly different check +that the sky region is uncrowded, that the effective +gain and readout noise values are correct, and that +earlier reduction procedures have not altered the image +statistics in some fundamental manner + +.PP +The \fIemission\fR parameter must be left at "yes", +since DAOPHOT assumes that stars are local maxima not local minima. +.PP +The \fInoise\fR parameter must be left at "poisson" since poisson noise +statistics are assumed throughout the DAOPHOT package. +.PP +The \fIscale\fR parameter defines the units in which radial distances +in the image will be measured. For example if the image scale +is 0.25 "/pixel, users can set \fIscale\fR to 0.25 if they wish +to define the \fIfwhmpsf\fR, \fIcbox\fR, \fIannulus\fR, \fIdannulus\fR, +\fIapertures\fR, \fIpsfrad\fR, \fIfitrad\fR and all the other algorithm +parameters which are defined in terms of a radial distance in arc-seconds. +For simplicity most users choose to leave scale set to 1.0 and +work in pixels. +.PP +The final version of the \fBdatapars\fR parameter set should look something +like the following. + +.YS +Image Reduction and Analysis Facility +PACKAGE = daophot + TASK = datapars + +(scale = 1.) Image scale in units per pixel +(fwhmpsf= 2.5) FWHM of the PSF in scale units +(emissio= yes) Features are positive ? +(sigma = 10.) Standard deviation of background in counts +(datamin= 50.) Minimum good data value +(datamax= 24500) Maximum good data value +(noise = poisson) Noise model +(ccdread= rdnoise) CCD readout noise image header keyword +(gain = gain) CCD gain image header keyword +(readnoi= 0.) CCD readout noise in electrons +(epadu = 1.) Gain in electrons per count +(exposur= cexptime) Exposure time image header keyword +(airmass= airmass) Airmass image header keyword +(filter = filter) Filter image header keyword +(obstime= obstime) Time of observation image header keyword +(itime = 1.0) Exposure time +(xairmas= INDEF) Airmass +(ifilter= INDEF) Filter +(otime = INDEF) Time of observation +(mode = ql) +.YE + + +.NH 4 +The Centering Algorithm Parameters + +.PP +The \fBcenterpars\fR parameter set controls the centering algorithms used by +the \fBphot\fR aperture photometry task. DAOPHOT users should concern +themselves with only two of these parameters, \fIcalgorithm\fR and \fIcbox\fR, +and leave the remaining \fBcenterpars\fR parameters at their default values. +.PP +\fICalgorithm\fR specifies the default \fBphot\fR centering algorithm. Its value +should be "none" if the input coordinate list is the output of the +\fBdaofind\fR task, or "centroid", "gauss", or "ofilter" if the input +coordinate list was +created with the image or graphics cursor list tasks \fBrimcursor\fR +or \fBrgcursor\fR or the coordinates are +read from the image cursor in interactive mode. The choice of centering +algorithm is not critical since the centers are recomputed using accurate +non-linear least-squares fitting techniques during the psf fitting +process. The most efficient and simplest choice is "centroid", although +more accurate results may be obtained with "gauss" which is +very similar to the centering algorithm used in \fBdaofind\fR. +.PP +The \fIcbox\fR +parameter determines the width in scale units of the data used to compute +the center if \fIcalgorithm \fR is not "none". +For reasonable results \fIcbox\fR should be set to the equivalent of 5 or +~ 2 * \fIfwhmpsf\fR in pixels whichever is larger. +.PP +\fBCenterpars\fR can be edited from within the \fBdaoedit\fR task +with the command \fB":epar centerpars"\fR. After editing, the \fBcenterpars\fR +parameter set should look like the example below. Note that for the test +image \fIfwhmpsf\fR is ~2.5 pixels so \fIcbox\fR is left at 5.0. + +.YS +PACKAGE = daophot +TASK = centerpars + +(calgori= none) Centering algorithm +(cbox = 5.) Centering box width in scale units +(cthresh= 0.) Centering threshold in sigma above background +(minsnra= 1.) Minimum signal-to-noise ratio +(cmaxite= 10) Maximum iterations +(maxshif= 1.) Maximum center shift in scale units +(clean = no) Symmetry clean before centering +(rclean = 1.) Cleaning radius in scale units +(rclip = 2.) Clipping radius in scale units +(kclean = 3.) K-sigma rejection criterion in skysigma +(mkcente= no) Mark the computed center +(mode = ql) +.YE + +.NH 4 +The Sky Fitting Algorithm Parameters + +.PP +The \fBfitskypars\fR parameter set controls the sky fitting algorithm +parameters used by the \fBphot\fR task. At this point DAOPHOT users should +concern themselves with only three of these parameters: \fIsalgorithm\fR, +\fIannulus\fR, and \fIdannulus\fR. +.PP +Users should realize that the \fBphot\fR task computes sky values +for the individual stars, and that these values are +used in the \fBpsf\fR task to compute the psf, averaged to form a group sky +value in the \fBpeak\fR, \fBnstar\fR and \fBallstar\fR tasks if sky refitting +is disabled (the default) or an initial sky value if sky refitting +is enabled, and used +to compute the predicted signal-to-noise ratios in the \fBgroup\fR task. +Although the option to refit the skies at a later stage of analysis exists, +there are difficulties associated with this choice. It is +in the user's best interest to determine the skies as accurately as +possible as early as possible, since sky determination will probably be +the single most important factor in doing good photometry. +.PP +In cases where contamination of the sky region is mostly due +to crowding by neighboring stars users should use the default sky fitting +algorithm "mode"; if the variations in the background are due instead +to nebulosity or large contaminating objects so that the sky statistics are +confused +"median", "centroid", or "crosscor" might be a better choice; in cases +where the sky statistics +are so poor that the histogram is aliased, undersampled, or sparse such +as might be the case with +very low sky backgrounds "mean" might be the best choice. +When in doubt about the correct choice the user should leave \fIsalgorithm\fR +at "mode" but examine the results carefully for accuracy at each step. +.PP +A good starting value for the inner radius of the sky annulus is ~ 4 * +\fIfwhmpsf\fR +or ~ 10 pixels for the test image. The width of the sky annulus should be +sufficient to give a reasonable sample of sky pixels, >= 5 pixels. We have +chosen a dannulus of ~4 * \fIfwhmpsf\fR or 10 pixels for the test image. +.PP +\fBFitskypars\fR can be edited from within the \fBdaoedit\fR task +with the command \fB":epar fitskypars"\fR. After editing the \fBfitskypars\fR +parameter set should look like the example below. + +.YS +PACKAGE = daophot +TASK = fitskypars + +(salgori= mode) Sky fitting algorithm +(annulus= 10.) Inner radius of sky annulus in scale units +(dannulu= 10.) Width of sky annulus in scale units +(skyvalu= 0.) User sky value +(smaxite= 10) Maximum number of sky fitting iterations +(sloclip= 0.) Lower clipping factor in percent +(shiclip= 0.) Upper clipping factor in percent +(snrejec= 50) Maximum number of sky fitting rejection iteratio +(sloreje= 3.) Lower K-sigma rejection limit in sky sigma +(shireje= 3.) Upper K-sigma rejection limit in sky sigma +(khist = 3.) Half width of histogram in sky sigma +(binsize= 0.1) Binsize of histogram in sky sigma +(smooth = no) Boxcar smooth the histogram +(rgrow = 0.) Region growing radius in scale units +(mksky = no) Mark sky annuli on the display +(mode = ql) +.YE + +.NH 4 +The Aperture Photometry Parameters + +.PP +The \fBphotpars\fR parameter set controls the aperture photometry algorithm +parameters used by the \fBphot\fR task. At this point DAOPHOT users should +concern themselves with only one of these, \fIapertures\fR, the radius +of the aperture through which the initial magnitudes will be computed. +A good rule of thumb is to set the aperture radius to the maximum +of 3 pixels or 1.0 * \fIfwhmpsf\fR pixels. Although magnitudes can be measured +through more than one aperture at a time, it is the magnitude of the +smallest aperture radius along with \fIzmag\fR and the exposure time +which set the DAOPHOT instrumental magnitude scale, and +the magnitudes through the other apertures contribute nothing to the +DAOPHOT analysis until it comes time to compute accurate aperture +corrections. Therefore it is in the user's best interest to set \fIapertures\fR +to a single value at this point and carefully record it. +.PP +\fBPhotpars\fR can be edited from within the \fBdaoedit\fR task +with the command \fB":epar photpars"\fR. After editing the \fBphotpars\fR +parameter set should look like the example below. Note that in this example +\fIfwhmpsf\fR is ~2.5 pixels so \fIapertures\fR is left at 3.0. + +.YS +PACKAGE = daophot +TASK = photpars + +(weighti= constant) Photometric weighting scheme +(apertur= 3.0) List of aperture radii in scale units +(zmag = 25.) Zero point of magnitude scale +(mkapert= no) Draw apertures on the display +(mode = ql) +.YE + +.NH 4 +The Psf Modeling and Fitting Parameters + +.PP +The \fBdaopars\fR parameter set controls the psf computation, star grouping, +and psf fitting +parameters used by the \fBpstselect\fR, \fBpsf\fR, \fBpeak\fR, +\fBgroup\fR, \fBnstar\fR, +\fBallstar\fR, \fBsubstar\fR, and \fBaddstar\fR tasks. At this point +DAOPHOT users should +concern themselves with only two of these parameters \fIpsfrad\fR, the radius +over which the psf will be defined, and \fIfitrad\fR, the radius +over which the psf will be fit to the individual stars. A good rule of thumb is +to set \fIpsfrad\fR to the radius at which the radial profile of the brightest +star of interest disappears into the noise plus 1, something like +~ 4 * \fIfwhmpsf\fR + 1, and +to set \fIfitrad\fR to the maximum of 3 pixels or ~ 1 * \fIfwhmpsf\fR in pixels. + +.PP +\fBDaopars\fR can be edited from within the daoedit task +with the command \fB":epar daopars"\fR. After editing the \fBdaopars\fR +parameter set should look something like the example below for the +test image. + +.YS +PACKAGE = daophot +TASK = daopars + +(functio= gauss) Analytic component of psf +(varorde= 0) Order of psf variation +(nclean = 0) Number of cleaning passes +(saturat= no) Use wings of saturated stars +(matchra= 3.) Matching radius in scale units +(psfrad = 11.) Radius of psf in scale units +(fitrad = 3.) Fitting radius in scale units +(recente= yes) Recenter stars during fit +(fitsky = no) Recompute group sky value during fit +(sannulu= 0.) Inner radius of sky annulus in scale units +(wsannul= 11.) Width of sky annulus in scale units +(flaterr= 0.75) Flat field error in percent +(proferr= 5.) Profile error in percent +(maxiter= 50) Maximum number of iterations +(clipexp= 6) Data clipping exponent +(clipran= 2.5) Data clipping range in sigma +(critove= 1.) Critical overlap group for membership +(maxnsta= 10000) Maximum number of stars to fit +(maxgrou= 60) Maximum number of stars to fit per group +(mode = ql) +.YE + +.NH 4 +Setting the Algorithm Parameters Graphically + +.PP +Each of the radial distance dependent parameters \fIfwhmpsf\fR, +\fIcbox\fR, \fIannulus\fR, \fIdannulus\fR, \fIapertures,\fR, +\fIpsfrad\fR, \fIfitrad\fR can be edited +individually and interactively by marking the current radial profile +plot with the +graphics cursor after executing the appropriate keystroke command. +For example the \fBf\fR keystroke command will prompt the user to +mark the fwhm of +the psf on the current radial profile plot, +verify the marked value, and update the \fIfwhmpsf\fR parameter. +.PP +All the radial distance dependent parameters listed above +can be edited at once my moving the +image cursor to a bright star, typing the \fB daoedit i\fR keystroke command +to invoke the interactive graphics setup menu. +The size of the radial profile plot and the sky regions +are set by the \fIscale\fR, \fIannulus\fR, and \fIdannulus\fR parameters. +The centering algorithm +used is always "centroid" regardless of the value of the \fIcalgorithm\fR +parameter, \fIcbox\fR and \fIscale\fR determine the centering box size, +and the photometry is computed inside the largest aperture specified by +the \fIapertures\fR parameter. After the user finishes marking all the +parameters on the plot +he/she is given an opportunity to verify or edit the results, e.g., change the +value for fwhmpsf from 2.536 as read from the graphics cursor to 2.5. + +.NH 3 +Checking the Algorithm Parameters with Daoedit + +.PP +The purpose of setting all the critical algorithm parameters to reasonable +values before beginning any DAOPHOT analysis, is to ensure that the user +gets off to a good start. Although setting the parameters to unreasonable +values often results in bizarre results which are immediately obvious, +e.g., the detection of thousands of +noise spikes, the problems can sometimes be more subtle. +For example, a sky annulus that is too close to the star will result in +measured sky values which are too high and poor subtractions of +the fitted stars which may not be discovered until the user has +become thoroughly exasperated trying to produce good fits to the psf +stars. +.PP +The current DAOPHOT algorithm parameters can be checked at any time with +the \fBdaoedit\fR task and the \fB":lpar"\fR command. For example the +\fBdatapars\fR parameters set can be listed with the \fBdaoedit +":lpar datapars"\fR command. The remaining parameters sets \fBfindpars\fR, +\fBcenterpars\fR, \fBfitskypars\fR, \fBphotpars\fR, and \fBdaopars\fR +may be listed in the same way. +.PP +When listing the algorithm parameters users should check that: +.IP [1] +the \fBdatapars\fR image header keyword parameters \fIccdread\fR, \fIgain\fR, +\fIexposure\fR, \fIairmass\fR, \fIfilter\fR, and \fIobstime\fR are +properly set. +.IP [2] +the \fBdatapars\fR \fIfwhmpsf\fR, \fIsigma\fR, \fIdatamin\fR, and \fIdatamax\fR parameters +are appropriate for the image. Be especially careful of datamin as the +correct value for this parameter varies with the mean sky. +.IP [3] +the \fBdatapars\fR parameter \fIscale\fR is 1.0 unless the user is +thoroughly aware of the meaning of this parameter and the consequences +of setting it to something other than 1.0, and \fIemission\fR is "yes". +.IP [4] +the \fBcenterpars\fR \fIcbox\fR parameter is +appropriate for the image and the remaining \fBcenterpars\fR parameters +are at their default values unless the user +really understands the consequences of altering these parameters. +.IP [5] +the \fBfitskypars\fR \fIannulus\fR, and \fIdannulus\fR +parameters are appropriate for the image and the remaining \fBfitkskypars\fR +parameters are at their default values unless the user really +understands the consequences of altering these parameters. +.IP [6] +the \fBphotpars\fR \fIapertures\fR parameter is appropriate for the image +and the remaining parameters are at their default values unless the user +really understands the consequences of altering these parameters. +.IP [6] +the \fBdaopars\fR \fIpsfrad\fR and \fIfitrad\fR parameters are appropriate +for the image and all the remaining \fBdaopars\fR parameters are at their +default values unless the user really understands the consequences of +altering these parameters. + +.NH 3 +Storing the Algorithm Parameter Values with Setimpars + +.PP +The current values of all the algorithm parameters for a particular +image may be saved in a file on disk at any +point in the reduction sequence by executing the \fBsetimpars\fR task. +The following command saves the current values of the parameters for +the test image in a file called "test.pars". + +.YS +da> setimpars test no yes +.YE + +Repeating the previous command at any point in the reduction sequence will +replace the stored parameter values with the current parameter values. + + +.NH 3 +Restoring the Algorithm Parameter Values with Setimpars + +.PP +At some point the user may wish to interrupt work on a particular image and +begin work on a different image. This should be no problem as long as the +user remembers to save the algorithm parameter sets with \fBsetimpars\fR +as described in the previous section. +.PP +The command to restore the algorithm parameter sets for the test image is: + +.YS +da> setimpars test yes no +.YE + +or + +.YS +da> setimpars test yes no parfile=test.pars +.YE + + +.NH 2 +Creating a Star List + +.PP +The initial input to the DAOPHOT package is a star list. +Star lists may be created with the DAOPHOT package task \fBdaofind\fR, +interactively with the image or graphics cursor (the +\fBrimcursor\fR and \fBrgcursor\fR tasks), by another IRAF task, or by +any user program which writes a text file in the correct format. +.PP +Legal star lists are text files containing a list of stars, one star +per line with the x and y coordinates in columns one and two. +Blank lines, lines beginning with "#", and lines containing anything other +than numbers in columns one and two are ignored. +A sample DAOPHOT star list is shown below. + +.YS +# Artificial Stars for Image Test + + 41.0 4.0 17.268 + 23.0 7.0 17.600 + 18.0 8.0 17.596 + 26.0 22.0 16.777 + 36.0 22.0 16.317 + 8.0 23.0 16.631 + 31.0 25.0 16.990 + 21.0 26.0 19.462 + 29.0 34.0 17.606 + 36.0 42.0 16.544 +.YE + +.NH 3 +The Daofind Task + +.PP +The \fBdaofind\fR task, searches for point sources +in an image whose peak intensities are above some user-defined threshold, +computes approximate centers, magnitudes, and +shape characteristics for all the detected objects, and writes the results +to the output star list. + +.NH 4 +The Daofind Algorithm + +.PP +By default the \fBdaofind\fR algorithm performs the following steps: +.IP[1] +reads the \fBdaofind\fR task parameters, including the input image +and output star list names and the \fBdatapars\fR and \fBfindpars\fR +algorithm parameters, and asks the user to verify the +\fIfwhmpsf\fR, \fIsigma\fR, +\fIthreshold\fR, \fIdatamin\fR, and \fIdatamax\fR parameters +.IP[2] +calculates the convolution kernel whose mathematical function when +convolved with the input image is to +compute the amplitude of the best-fitting Gaussian of +full-width half-maximum \fIfwhmpsf\fR at each point in the input image +.IP[3] +convolves the input image with the convolution kernel after +eliminating bad data with the \fIdatamin\fR and \fIdatamax\fR parameters, +and writes the results to a temporary convolved image +.IP[4] +searches for local maxima in the convolved image whose amplitudes are greater +than the detection threshold, and greater than the amplitudes of any neighbors +within a region the size of the convolution kernel +.IP[5] +computes approximate centers, magnitudes, and shape +statistics for these local maxima +.IP[6] +eliminates local maxima whose centers are outside the image, and whose +sharpness and roundness statistics are outside the limits set by the user +.IP[7] +writes the centers, approximate magnitudes, sharpness and roundness +statistics, and id number for the remaining local maxima, to the +output star list +.IP[8] +deletes the convolved image + +.NH 4 +The Daofind Algorithm Parameters + +.PP +The critical \fBdaofind\fR algorithm parameters are \fIfwhmpsf\fR, +\fIdatamin\fR, \fIdatamax\fR, \fIsigma\fR, and \fIthreshold\fR. +These parameters are verified at startup time by \fBdaofind\fR. +.PP +The \fIfwhmpsf\fR parameter should be close +to the true full-width at half-maximum of the psf in order +to optimize the detection algorithm for stellar objects. If \fIfwhmpsf\fR +is too far from the true value, stars may be omitted from the star list +and/or non-stellar objects added to it. +.PP +The \fIdatamin\fR and \fIdatamax\fR parameters are used to flag and remove +bad data from the convolved image. +If \fIdatamin\fR and \fIdatamax\fR are +too far from the true value stars may be omitted from the star list +and/or non-stellar objects added to it. +.PP +The \fIsigma\fR parameter should be close to the true standard deviation of +the sky background in an uncrowded region of the frame. This parameter +in combination with \fIthreshold\fR +determines the detection threshold in counts for faint objects. If it is +incorrect either too few or too many objects will be detected. +.PP +The \fIthreshold\fR parameter should normally be set to some small +number between 3.0 and 5.0. If threshold is too big only +the brightest stars will be detected. If threshold is too small too +many noise spikes will be detected. + +.NH 4 +Running Daofind Non-Interactively + +.PP +The following example shows how to run \fBdaofind\fR in non-interactive mode. + +.YS +da> daofind test default + +FWHM of features in scale units (2.5) (CR or value): + New FWHM of features: 2.5 scale units 2.5 pixels +Standard deviation of background in counts (10.) (CR or value): + New standard deviation of background: 10. counts +Detection threshold in sigma (4.) (CR or value): + New detection threshold: 4. sigma 40. counts +Minimum good data value (50.) (CR or value): + New minimum good data value: 50. counts +Maximum good data value (24500.) (CR or value): + New maximum good data value: 24500. counts + +Image: test.imh fwhmpsf: 2.5 ratio: 1. theta: 0. nsigma: 1.5 + + 40.97 4.02 -1.663 0.612 0.017 1 + 23.06 7.03 -1.214 0.636 -0.019 2 + 18.02 7.96 -1.318 0.622 0.010 3 + 25.99 22.01 -2.167 0.658 0.001 4 + 35.98 22.00 -2.499 0.572 -0.039 5 + 8.02 22.97 -2.239 0.550 0.068 6 + 30.97 25.01 -1.934 0.711 -0.044 7 + 28.96 33.92 -1.087 0.418 0.132 8 + 35.98 42.03 -2.332 0.639 0.108 9 + +threshold: 40. relerr: 1.140 0.2 <= sharp <= 1. -1. <= round <= 1.\fR +.YE + +If this is the first time \fBdaofind\fR has been run the results will appear +in the file "test.coo.1". +.PP +The detected objects can be marked on the image display using the +\fBtvmark\fR task as shown below. + +.YS +da> display test 1 fi+ +da> tvmark 1 test.coo.1 col=204 +.YE + +In this example the detected stars will be marked on the displayed image +as red dots. If too many faints stars have been missed the user +can rerun \fBdaofind\fR with a lower value of the \fIthreshold\fR +parameter. + +.NH 4 +Running Daofind Interactively + +.PP +\fBDaofind\fR may also be run in interactive mode. +Most users will only exercise this option for small images which do not +require long cpu/elapsed times to perform the convolution. +.PP +The following example shows how to run \fBdaofind\fR interactively. + +.YS +da> display test 1 fi+ + +da> daofind test default inter+ +.YE + +.IP ... +Type the \fBv\fR keystroke command to verify the critical algorithm +parameters. +.LP + +.YS +FWHM of features in scale units (2.5) (CR or value): + New FWHM of features: 2.5 scale units 2.5 pixels +Standard deviation of background in counts (10.) (CR or value): + New standard deviation of background: 10. counts +Detection threshold in sigma (4.) (CR or value): + New detection threshold: 4. sigma 40. counts +Minimum good data value (50.) (CR or value): + New minimum good data value: 50. counts +Maximum good data value (24500.) (CR or value): + New maximum good data value: 24500. counts +.YE + +.IP ... +Type the \fBspacebar\fR keystroke command to detect the objects and write them +out to the star list file. +.LP + +.YS +Image: test.imh fwhmpsf: 2.5 ratio: 1. theta: 0. nsigma: 1.5 + + 40.97 4.02 -1.663 0.612 0.017 1 + 23.06 7.03 -1.214 0.636 -0.019 2 + 18.02 7.96 -1.318 0.622 0.010 3 + 25.99 22.01 -2.167 0.658 0.001 4 + 35.98 22.00 -2.499 0.572 -0.039 5 + 8.02 22.97 -2.239 0.550 0.068 6 + 30.97 25.01 -1.934 0.711 -0.044 7 + 28.96 33.92 -1.087 0.418 0.132 8 + 35.98 42.03 -2.332 0.639 0.108 9 + +threshold: 40. relerr: 1.140 0.2 <= sharp <= 1. -1. <= round <= 1. + +Output file: test.coo.1 +.YE + +.IP ... +Change \fIthreshold\fR to 3.0 with the colon command \fB:threshold 3.0\fR. +.IP ... +Type the \fBspacebar\fR keystroke command to detect the objects and write +them out to a new star list file. +.LP + +.YS +Image: test.imh fwhmpsf: 2.5 ratio: 1. theta: 0. nsigma: 1.5 + + 40.97 4.02 -1.975 0.577 0.017 1 + 23.06 7.03 -1.526 0.604 -0.019 2 + 18.02 7.96 -1.631 0.587 0.010 3 + 25.99 22.01 -2.480 0.626 0.001 4 + 35.98 22.00 -2.811 0.537 -0.039 5 + 8.02 22.97 -2.551 0.515 0.068 6 + 30.97 25.01 -2.246 0.681 -0.044 7 + 21.27 25.94 -0.146 0.804 -0.558 8 + 28.96 33.92 -1.400 0.379 0.132 9 + 35.98 42.03 -2.645 0.606 0.108 10 + +threshold: 30. relerr: 1.140 0.2 <= sharp <= 1. -1. <= round <= 1. + +Output file: test.coo.2 +.YE + +.IP ... +Change \fIthreshold\fR to 5.0 with the colon command \fB:threshold 5.0. +.IP ... +Type the \fBspacebar\fR keystroke command to detect the objects and write them +out to a new coordinate file. +.LP + +.YS +Image: test.imh fwhmpsf: 2.5 ratio: 1. theta: 0. nsigma: 1.5 + + 40.97 4.02 -1.420 0.577 0.017 1 + 23.06 7.03 -0.972 0.604 -0.019 2 + 18.02 7.96 -1.076 0.587 0.010 3 + 25.99 22.01 -1.925 0.626 0.001 4 + 35.98 22.00 -2.257 0.537 -0.039 5 + 8.02 22.97 -1.997 0.515 0.068 6 + 30.97 25.01 -1.692 0.681 -0.044 7 + 28.96 33.92 -0.845 0.379 0.132 8 + 35.98 42.03 -2.090 0.606 0.108 9 + +threshold: 50. relerr: 1.140 0.2 <= sharp <= 1. -1. <= round <= 1. + +Output file: test.coo.3 +.YE + +.IP ... +Type the \fBq\fR keystroke, first in the image display window then the +text window to quit the task. +.LP + +If this is the first run of \fBdaofind\fR, +the three star list files for the \fIthreshold\fR values of +4.0, 3.0, and 5.0 will be written to "test.coo.1", "test.coo.2", and +"test.coo.3" respectively. +.PP +The \fBdaofind\fR results for different thresholds can be evaluated by +marking the detected objects on the image display using the \fBtvmark\fR +task and different +colors for each threshold. In the following example objects detected +at threshold=3.0 are marked in red, at threshold=4.0 in green, at threshold= +5.0 in blue. + +.YS +da> display test 1 fi+ +da> tvmark 1 test.coo.2 col=204 point=3 +da> tvmark 1 test.coo.1 col=205 point=3 +da> tvmark 1 test.coo.3 col=206 point=3 +.YE + +Note that the identical stars were detected at thresholds 4.0 and 5.0 but +the faint star at 21,26 was only detected at threshold=3.0. +.PP +In this example the user decides that threshold = 4.0 is the +"best" threshold, sets the \fIthreshold\fR parameter appropriately as shown +below, and deletes the the star lists for threshold = 3.0 and threshold = 5.0. + +.YS +da> findpars.threshold = 4.0 +da> delete test.coo.2,test.coo.3 +.YE + +.NH 4 +The Daofind Output + +.PP +The quantities xcenter, ycenter, mag, sharpness, roundness, and id +are recorded for each detected object. Each is described briefly below. +.IP [1] +\fIXcenter\fR and \fIycenter\fR are the coordinates of +the detected object in fractional pixel units. They are computed by +fitting one-dimensional Gaussian functions of full-width at half-maximum +\fIfwhmpsf\fR to the x and y marginal pixel distributions centered on +the star. The computed coordinates can be overlaid on the +displayed image with the \fBtvmark\fR command. +.IP [2] +The estimated magnitude is measured relative to the detection threshold +and is defined as + +.YS +mag = -2.5 * log10 (density / (relerr * threshold * sigma)) +.YE + +where density is the peak density of the object in the convolved image, +relerr an internally +computed factor measuring the amount by which the standard error in one pixel +in the input image must be multiplied to obtain the standard error +in one pixel in the convolved image, and threshold and sigma +are the values of the corresponding \fIthreshold\fR and \fIsigma\fR +parameters. For stellar +objects the computed magnitude is directly proportional to the true +magnitude of the star. Stars with a peak density exactly equal to +the detection threshold will have a magnitude of 0.0. The remaining +stars will have negative magnitudes. +.IP [3] +The sharpness statistic is the ratio of the amplitude of the best fitting +delta function at the position of a detected object to the amplitude of +the best fitting gaussian at the same position as shown below. + +.YS +sharpness = (data - <data>) / density +.YE + +The amplitude of the best fitting gaussian is simply the density +of the detected object in the convolved image. +The amplitude of the best fitting delta function is defined +as corresponding original image data value minus the average of all +the neighboring +pixels in the image <data>. Typical values of sharpness are of ~0.6 for +approximately gaussian stars and \fInsigma\fR = 1.5. +Hot pixels will have sharpness values >> 1 and cold pixels will have +sharpness values +of ~0, hence reasonable limits for the \fIsharphi\fR and \fIsharplo\fR +parameters are 1.0 and 0.2 respectively. +Increasing the size of convolution box defined by the +\fInsigma\fR parameter from its default value of 1.5 to a larger value +(smaller values should be avoided !) while keeping the \fIfwhmpsf\fR the +same, will increase the average value of the sharpness statistic because +more pixels further from the center of the star are included in the +computation of <data>. If \fInsigma\fR is changed +the \fBfindpars\fR parameters \fIsharphi\fR and \fIsharplo\fR +will also need to be changed. +.IP [4] +The roundness statistic is computed by fitting a one-dimensional +gaussian function of full-width at half-maximum \fIfwhmpsf\fR to the +x and y marginal pixel distributions. + +.YS +roundness = 2.0 * (hx - hy) / (hx + hy) +.YE + +hx and hy are the heights of the best fitting one-dimensional +gaussians in x and y. A totally +round object will have a roundness of ~ 0.0. If the object is very +elongated in x roundness will be a large negative number; a large positive +number if it is elongated in y. The roundness statistic is +effective at filtering out bad columns and rows of data. It is not +effective at filtering out objects elongated at intermediate angles. +.IP [5] +Id is a sequence number which identifies the star. + +.NH 4 +Examining the Daofind Output + +.PP +The easiest way to check that \fBdaofind\fR is performing correctly is to mark +the detected stars on the image display with \fBtvmark\fR. +.PP +If the marked image suggests that \fBdaofind\fR is detecting too few or +too many stars the first items to check are the the values of the +\fIsigma\fR and \fIthreshold\fR parameters since these parameters determine the +detection threshold. Sigma should be +the standard deviation of the sky pixels in an uncrowded region of the image. +Threshold should normally be some number between 3.0 and 5.0. If sigma +and threshold are reasonable the user should compare the observed value of +sigma with the predicted value derived from the median background level and the +effective gain and readout noise values. If there is a significant +mismatch in these numbers the user should check the reduction history of the +image. The number of spurious detections goes up dramatically for +thresholds less than ~3.0 * sigma. A plot of number +of detections versus threshold will show a change in slope at some point below +this threshold. Users who wish to detect faint objects while keeping +spurious detections at a manageable minimum should +set the detection threshold to a value just above the threshold at +which this change in slope occurs. +.PP +Users should also check the values of the +parameters \fIsharplo\fR, \fIsharphi\fR, +\fIroundlo\fR, and \fIroundhi\fR parameters to ensure that +detected objects are not being unfairly filtered out. In particular the values +of \fIsharplo\fR and \fIsharphi\fR should be changed if the \fInsigma\fR +parameter is changed. +.PP +Finally the user should check +the \fIfwhmpsf\fR, \fInsigma\fR, \fIdatamin\fR and \fIdatamax\fR parameters +for correctness since these parameters control the computation of the +convolution kernel and the density enhancement image. +.PP +Histograms of the various columns in the \fBdaofind\fR output can be +plotted using the \fBpdump\fR and \fBphistogram\fR tasks. The following +example shows how to plot a histogram of the magnitudes. + +.YS +da> pdump test.coo.1 mag yes | phistogram STDIN binwidth=.1 +.YE + +The various columns can also be plotted against each other. +The following example shows how to plot magnitude error versus magnitude. + +.YS +da> pdump test.coo.1 mag,merr yes | graph point+ +.YE + +.PP +By setting the \fBdaofind\fR \fIstarmap\fR and \fIskymap\fR parameters the +user can save and examine the density enhancement image and the corresponding +background density image. The sum of these two images should yield a +close representation of the original image except for regions of +bad data and edge pixels. Due to the nature of the convolution kernel +the starmap image will have a mean value of +~0.0 in the sky regions, an rms \(~= relerr * sigma in the sky regions, +and positive peaks of intensity surrounded by negative valleys at the positions +of bright stars. The skymap image will have a mean value \(~= sky in +the sky regions, an rms \(~= sqrt (sigma ** 2 / N + K * (relerr * sigma) ** 2), +(N is the number of pixels in the gaussian kernel and K is the average power +in the gaussian kernel), and dips in intensity surrounded by +bright rings at the position of the stars. + +.NH 3 +Rgcursor and Rimcursor + +.PP +The LISTS package tasks \fBrimcursor\fR and \fBrgcursor\fR can be used to +generate coordinate lists interactively. For example a coordinate +list can be created using the image display and the image display cursor +as shown below. + +.YS +da> display test 1 fi+ + +da> rimcursor > test.coo +.YE + +.IP ... +Move cursor to stars of interest and tap the space bar. +.IP ... +Type <EOF> to terminate the list. + +.PP +A coordinate list can also be created using a contour plot and the graphics +cursor as shown below. + +.YS +da> contour test + +da> rgcursor > test.coo +.YE + +.IP ... +Move the cursor to the stars of interest and tap the space bar. +.IP ... +Type <EOF> to terminate the list. + +.PP +In both cases the text file "test.coo" contains the x and y coordinates of +the marked stars in image pixel units. The output of \fBrimcursor\fR or +\fBrgcursor\fR can +be read directly by the DAOPHOT \fBphot\fR task. + +.NH 3 +User Program + +.PP +Any user program which produces a text file with the stellar coordinates +listed one per line with x and y in columns 1 and 2, can be used to produce +DAOPHOT coordinate files which can be read by the \fBphot\fR task. + +.NH 3 +Modifying an Existing Coordinate List + +.PP +The LISTS package routine \fBlintran\fR +can be used to perform simple coordinate transformations on +coordinate lists including shifts, magnifications, and rotations. + +.NH 2 +Initializing the Photometry with Phot + +.PP +The \fBphot\fR task computes initial centers, +sky values, and initial magnitudes for all the objects in the input +star list. The centers and magnitudes are used as starting +values for the non-linear +least-squares psf computation and fitting routines in the \fBpsf\fR, +\fBpeak\fR, \fBnstar\fR, and \fBallstar\fR tasks, and to estimate +signal-to-noise +values in the \fBgroup\fR task. The individual sky values +computed by \fBphot\fR are used directly by the \fBpsf\fR task to compute +the psf model, by the +\fBpeak\fR, \fBnstar\fR, +and \fBallstar\fR tasks to compute the group sky values, and by the +\fBgroup\fR task to estimate signal-to-noise ratios. + +.NH 3 +The Phot Algorithm + +.PP +By default the \fBphot\fR task performs the following functions: +.IP [1] +reads in the \fBphot\fR task parameters including the input image name, +the input coordinate file name, the output photometry file name, +the \fBdatapars\fR, \fBcenterpars\fR, \fBfitskypars\fR, and \fBphotpars\fR +algorithm parameters, and determines whether the task mode of operation +is interactive or non-interactive +.IP [2] +reads in the initial coordinates of a star from the coordinate list and/or +the image cursor, and computes new coordinates for the star using the centering +algorithm defined by the \fIcalgorithm\fR parameter (if \fIcalgorithm\fR +is not "none") using data in a box whose size is defined by the \fIcbox\fR +parameter +.IP [3] +computes the sky value for the star using the default algorithm +specified by the \fIsalgorithm\fR parameter and the data in an annulus of +pixels defined by the \fIannulus\fR and \fIdannulus\fR parameters +.IP [4] +computes the instrumental magnitude and magnitude error for each star +inside the aperture radii +specified by the \fIapertures\fR parameter using fractional pixel techniques, +the computed sky value, the standard deviation of the sky pixels, and the +gain of the CCD +.IP [6] +sets the instrumental magnitude scale for the image using the +\fBphotpars\fR \fIzmag\fR parameter and the exposure time specified by +the \fBdatapars\fR \fIexposure\fR or \fIitime\fR parameters +.IP [7] +sets the magnitude(s) to INDEF for stars which are saturated or contain +bad data, +for which the aperture is partially off the image, for which a sky +value could not be computed, or for which the +signal is fainter than the background +.IP [8] +writes the results to the output photometry file + +.NH 3 +The Phot Algorithm Parameters + +.PP +The critical \fBphot\fR algorithm parameters are \fIcalgorithm\fR, +\fIsalgorithm\fR, +\fIannulus\fR, \fIdannulus\fR, \fIapertures\fR, \fIdatamin\fR and +\fIdatamax\fR. These parameters are verified by \fBphot\fR +at startup time. +.PP +The \fIcalgorithm\fR parameter tells \fBphot\fR how to compute +centers for the objects in the coordinate list. Calgorithm should be "none" +if the coordinate +list was computed by \fBdaofind\fR or the coordinates are known to +be precise; otherwise calgorithm should one of "centroid", "gauss", or +"ofilter". "centroid" is quick and sufficiently accurate in most cases; +"gauss" and "ofilter" take longer but are more accurate. If calgorithm is +not "none", +\fBphot\fR will ask the user to verify the centering box size +\fIcbox\fR. \fIcbox\fR should be set to 5 or ~2 * \fIfwhmpsf\fR pixels wide +whichever is greater. +.PP +The \fIsalgorithm\fR parameter tells \fBphot\fR how to compute the sky +values. If the +fluctuations in the sky background are due primarily to crowding the +default choice +"mode" should be used. If the fluctuations in the sky background +are due to nebulosity or large galaxies +and the sky statistics are confused, "median", "centroid" or +"crosscor" might be the best choice. In cases where the +background is very low and the sky histogram is sparse or +undersampled "mean" might be the best choice. +.PP +The \fIannulus\fR and \fIdannulus\fR parameters tell \fBphot\fR the +position of the sky annulus with respect to the star. The sky region +must be far enough away from the star to avoid contamination from +the star itself, but close enough +to be representative of the intensity distribution under the star. Values of +~ 4.0 * \fIfwhmpsf\fR for both parameters are good starting values. +.PP +The \fIapertures\fR parameter tells \fBphot\fR the radius of the +photometry aperture. The photometry through this aperture sets the +instrumental magnitude scale for all the subsequent DAOPHOT +reductions. \fIApertures\fR should be ~ 1.0 * \fIfwhmpsf\fR. +.PP +The \fIdatamin\fR and \fIdatamax\fR parameters are used to detect +bad data in the photometry and sky apertures. Bad data is removed from +the sky pixel list before sky fitting takes place so it is important +that datamax and datamin, but particularly datamin, be correct. +Stars which have bad data in the photometry apertures will have their +magnitudes set to INDEF and be flagged with an error. + +.NH 3 +Running Phot Non-interactively + +.PP +The following example shows how to run \fBphot\fR in non-interactive +mode using the results of \fBdaofind\fR as input. + +.YS +da> phot test default default + +Centering algorithm (none) (CR or value): + New centering algorithm: none +Sky fitting algorithm (mode) (CR or value): + Sky fitting algorithm: mode +Inner radius of sky annulus in scale units (10.) (CR or value): + New inner radius of sky annulus: 10. scale units 10. pixels +Width of the sky annulus in scale units (10.) (CR or value): + New width of the sky annulus: 10. scale units 10. pixels +File/list of aperture radii in scale units (3.0) (CR or value): 3.0 + Aperture radius 1: 3. scale units 3. pixels +Minimum good data value (50.) (CR or value): + New minimum good data value: 50. counts +Maximum good data value (24500.) (CR or value): + New maximum good data value: 24500. counts + +test 40.97 4.02 100.7955 17.218 ok +test 23.06 7.03 100.3257 17.650 ok +test 18.02 7.96 99.40262 17.484 ok +test 25.99 22.01 101.3196 16.800 ok +test 35.98 22.00 101.1601 16.373 ok +test 8.02 22.97 98.89139 16.603 ok +test 30.97 25.01 101.2904 17.051 ok +test 28.96 33.92 100.6189 17.782 ok +test 35.98 42.03 101.043 16.594 ok + +.YE + +\fBPhot\fR looks for an input star list called "test.coo.?", +creates a file called "test.mag.?", and verifies +the critical parameters. By default the verbose switch is set to "yes", +so a short summary of the results for each star is printed on the +terminal as it is computed. +.PP +\fBPhot\fR may also be run non-interactively from a coordinate list created +with the image cursor list task \fBrimcursor\fR as shown below. Note +that centering has been turned on, and the verify switch has been turned off. + +.YS +da> display test 1 fi+ + +da> rimcursor > cursor.coo + +da> page cursor.coo + +41.02 4.033 101 \040 +22.918 6.969 101 \040 +18.123 7.849 101 \040 +25.951 21.939 101 \040 +35.736 21.744 101 \040 + 7.947 23.016 101 \040 +30.843 24.777 101 \040 +28.984 33.779 101 \040 +36.127 41.705 101 \040 + +da> phot test cursor.coo default calg=centroid verify- + +test 40.92 4.04 100.8871 17.222 ok +test 23.17 6.97 100.6163 17.666 ok +test 18.04 7.92 99.55305 17.487 ok +test 25.96 21.97 101.4161 16.801 ok +test 35.94 21.98 101.2101 16.373 ok +test 8.05 23.00 98.74371 16.601 ok +test 30.94 25.02 101.3224 17.052 ok +test 28.91 33.85 100.6207 17.786 ok +test 35.96 42.08 100.9039 16.591 ok\fR +.YE + +The "centroid" algorithm +computes a new center by doing an intensity-weighted sum of the +x and y marginals, whereas the \fBdaofind\fR algorithm +fits a 1D gaussian to the marginal pixel distributions in x and y. +The following example shows the results for the almost equivalent +\fBphot\fR centering algorithm "gauss". + +.YS +da> phot test cursor.coo default calg=gauss verify- + +test 41.00 4.03 100.8698 17.219 ok +test 23.11 7.03 100.3567 17.653 ok +test 18.02 7.96 99.40262 17.484 ok +test 25.98 21.97 101.4021 16.801 ok +test 35.96 21.99 101.2101 16.373 ok +test 8.02 22.98 98.77907 16.601 ok +test 30.97 25.02 101.2904 17.051 ok +test 28.93 33.94 100.6726 17.783 ok +test 35.97 42.02 100.976 16.593 ok\fR +.YE + +The positions produced by the "gauss" algorithm are closer to +the positions computed by the \fBdaofind\fR task, +than those computed by the "centroid" algorithm. +However as the positions computed by \fBphot\fR are used as initial +positions by the DAOPHOT tasks, +it is usually not necessary to go to the more expensive "gauss" algorithm. + +.NH 3 +Running Phot Interactively + +.PP +\fBPhot\fR can also be configured to run interactively using the +image display and image +cursor for coordinate input. In this mode the user loads the image +into the display and runs +\fBphot\fR interactively by turning the interactive switch on as +shown below. +When the program is ready to accept input the cursor will begin +blinking in the display window. The following series of steps will +do photometry on stars selected with the image cursor. + +.YS +da> display test 1 fi+ + +da> phot test "" default interactive+ calgorithm=centroid +.YE + +.IP ... +Execute the \fBv\fR keystroke command to verify the critical parameters. +.LP + +.YS +Centering algorithm (centroid) (CR or value): + New centering algorithm: centroid +Centering box width in scale units (5.) (CR or value): + New centering box width: 5. scale units 5. pixels +Sky fitting algorithm (mode) (CR or value): + Sky fitting algorithm: mode +Inner radius of sky annulus in scale units (10.) (CR or value): + New inner radius of sky annulus: 10. scale units 10. pixels +Width of the sky annulus in scale units (10.) (CR or value): + New width of the sky annulus: 10. scale units 10. pixels +File/list of aperture radii in scale units (3.) (CR or value): + Aperture radius 1: 3. scale units 3. pixels +Standard deviation of background in counts (10.) (CR or value): + New standard deviation of background: 10. counts +Minimum good data value (50.) (CR or value): + New minimum good data value: 50. counts +Maximum good data value (24500.) (CR or value): + New maximum good data value: 24500. counts +.YE + +.IP ... +Move the cursor to the stars of interest and tap the \fBspacebar\fR to do +the photometry. +.LP + +.YS +test 40.92 4.04 100.8871 17.222 ok +test 23.17 6.97 100.6163 17.666 ok +test 18.04 7.92 99.55305 17.487 ok +test 25.96 21.97 101.4161 16.801 ok +test 35.94 21.98 101.2101 16.373 ok +test 8.05 23.00 98.74371 16.601 ok +test 30.94 25.02 101.3224 17.052 ok +test 28.91 33.85 100.6207 17.786 ok +test 35.96 42.08 100.9039 16.591 ok +.YE + +.IP ... +Type \fBq\fR to quit image, and \fBq\fR again to exit task. +.LP + +The coordinate file name has been set to "" so that initial +positions for the stars to be measured will be read from the image +cursor, and the centering algorithm has been temporarily changed +on the command line +from "none" to "centroid" so that new centers will be computed. +The user simply points the cursor to the stars to be measured and taps the +space bar to measure the star. This option is often useful for +picking up stars missed by \fBdaofind\fR in a previous iteration, +or in cases where the user only wishes to measure a small group of stars. + +.NH 3 +The Phot Output + +.PP +\fBPhot\fR produces a large output file containing many parameters, +intermediate and final results. The principle +quantities of interest are: 1) the position +of the star xcenter and ycenter, 2) the sky value, +its standard deviation, and the number of pixels used to compute +it, msky, stdev, and nsky 3) the total +counts inside the aperture and the effective area of the aperture, +sum and area 4) the magnitude and magnitude error in the +aperture, mag and merr, +and 5) the exposure time, airmass, filter, and time of observation, itime, +xairmass, ifilter, and otime. +.IP [1] +\fIXcenter\fR and \fIycenter\fR are the computed coordinates for +the detected objects in fractional pixels. They can be overlaid on the +displayed image with the \fBtvmark\fR command. These numbers should be compared +with the initial coordinates xinit and yinit, to which they will be equal +if the centering algorithm was "none", or to which they should be close +if the centering algorithm is "centroid", "gauss", or "ofilter" assuming +that the original x and y positions were reasonable. +.IP [2] +\fIMsky\fR, \fIstdev\fR and \fInsky\fR are the estimated sky value in counts, +its standard deviation in counts, and the number of pixels used to compute it. +Users should, check that the position of the sky annulus is reasonable +and, check that the msky, stdev, and nsky values are reasonable +for a few isolated stars before proceeding. +.IP [3] +\fISum\fR and \fIarea\fR are the total counts (star + sky) in the +photometry aperture and area is area of the aperture in pixels squared +and should be roughly +equal to PI * r ** 2 where r is the radius of the photometry aperture +in pixels. +.IP [4] +\fIMag\fR and \fImerr\fR are the magnitude and magnitude error respectively +computed as follows. + +.YS + mag = zmag - 2.5 * log10(sum - area * msky) + 2.5 * log10(itime) + +merr = 1.0857 * sqrt((sum - area * msky) / gain + area * stdev ** 2 + + area ** 2 * stdev ** 2 / nsky) / (sum - area * msky) +.YE + +Users should check that the exposure time \fIitime\fR is correct since it +is used to determine the instrumental magnitude scale. The correct value +of gain is also required in order to get a correct estimate of the +magnitude error. Stdev is the observed standard deviation +of the sky pixels not the predicted value. +.IP [5] +The remaining quantities itime, \fIxairmass\fR, \fIifilter\fR, and \fIotime\fR +should be checked for correctness, e.g., were they read correctly from the image +header. + +.NH 3 +Examining the Results of Phot + +.PP +The user can check the results of \fBphot\fR in several +ways. The following command will mark all +stars in the \fBphot\fR output file on the display in red. + +.YS +da> display test 1 +da> pdump test.mag.1 xcenter,ycenter yes | tvmark 1 STDIN col=204 +.YE + +The following command will mark all the stars whose magnitudes are INDEF on +the screen in green. + +.YS +da> pdump test.mag.1 xcenter,ycenter "mag == INDEF" | tvmark \\ + 1 STDIN col=205 +.YE + +The following command will plot magnitude error versus magnitude for all +the stars in the photometry file. + +.YS +da> pdump test.mag.1 mag,merr yes | graph STDIN point+ +.YE + +The following command will plot a histogram of the magnitude distribution. + +.YS +da> pdump test.mag.1 mag,merr yes | phist STDIN plot_type=box +.YE + +The photometry file can be examined interactively with the \fBpexamine\fR +task as shown below. + +.YS +da> pexamine test.mag.1 "" test +.YE +.IP ... +A vector plot of magnitude error versus magnitude appears on the screen. +.IP ... +To examine individual stars in the vector plot move the graphics cursor to a +star and type \fBo\fR to get a record listing for the star, +followed by \fBr\fR, \fBc\fR, or \fBs\fR to see a radial profile plot, +contour plot, or surface plot respectively, of the star. +.IP ... +To activate the image cursor type \fBi\fR, move the cursor to a star +and type \fBo\fR to get a record listing +for the star, followed by \fBr\fR, \fBc\fR or \fBs\fR to draw the desired plot. +To reactivate the graphics cursor type \fBg\fR. +.IP ... +To plot magnitude error versus x coordinate for all the stars in the file, type +\fB:xcolumn xcenter\fR and \fB:ycolumn merr\fR followed by \fBp\fR to +redraw the plot. +.IP ... +To plot a histogram of the magnitudes of the objects type \fBh\fR. +.IP ... +Type \fBq\fR to quit. + +.NH 2 +Creating a Psf Star List with Pstselect + +.PP +The psf model fitting routines require a list of bright isolated stars +well distributed over the image to use as psf model templates. +The \fBpstselect\fR +task is used to select suitable candidate stars from the photometry file +for input to the psf modeling task \fBpsf\fR. + +.NH 3 +The Pstselect Algorithm + +.PP +By default the \fBpstselect\fR task performs the following functions: +.IP[1] +reads the task parameters including the input image name, input +photometry file, and output psf star list, reads the \fBdatapars\fR +and \fBdaopars\fR algorithm parameters, and determines whether the +task will be run interactively or non-interactively +.IP [2] +reads the dimensions of the input image from the input image header, and +the ids, x and y coordinates, magnitudes, and sky values of up to +\fImaxnstar\fR stars from the input photometry file +.IP [2] +assigns a large negative number to the magnitudes of all stars whose +measured magnitudes are INDEF in the input photometry file +.IP [3] +sorts the stars in order of increasing magnitude so that +the saturated and brightest stars are at the beginning of the list +.IP [4] +selects the +brightest \fImaxnpsf\fR stars (where maxnpsf is a number chosen by the user) +which are, not saturated, more than \fIfitrad\fR pixels away from the edge +of the input image, have no bad data within \fIfitrad\fR pixels, +and have no brighter neighbor stars within +(\fIpsfrad\fR + \fIfitrad\fR + 2) pixels +.IP [5] +writes the ids, x and y coordinates, magnitudes, and sky values +of the selected stars as read from the input photometry list +to the output psf star list + +.NH 3 +The Pstselect Algorithm Parameters + +.PP +The critical \fBpstselect\fR algorithm parameters are \fIpsfrad\fR, +\fIfitrad\fR, \fIdatamin\fR, and \fIdatamax\fR. +.PP +\fIPsfrad\fR and \fIfitrad\fR are used by \fBpstselect\fR to eliminate +potential psf stars which have bright neighbors. +For the test image these parameters are currently set +to 4 * \fIfwhmpsf\fR + 1 and 1 * \fIfwhmpsf\fR or 11 and 3 pixels respectively. +However as \fBpstselect\fR is the first task to actually use the values +of these parameters, the user should +check them here one more time before running \fBpstselect\fR. +\fIFitrad\fR should be ~ 1 * \fIfwhmpsf\fR +for optimal psf model computation and fitting so the user leaves it +at its current value of 3.0. \fIPsfrad\fR should be set to the radius at which +the profile of the brightest stars of interest disappear into the +noise. Normally 4 * \fIfwhmpsf\fR + 1 pixels is a good starting value for +this quantity. +If \fIpsfrad\fR is too small the fitted stars will not subtract completely +from the input image, if it is too big DAOPHOT +will consume cpu time doing unnecessary data extractions +and subtractions. +One way to check the value of the \fIpsfrad\fR parameter is to +use the \fBdaoedit\fR task to examine +radial profiles of isolated stars in the input image as shown below. + +.YS +da> display test 1 fi+ + +da> daoedit test +.YE + +.IP ... +Move cursor to star at 36,42 and press the \fBr\fR key. +.IP ... +Examine the resulting radial profile and note that the stellar profile +disappears into the noise at ~4 pixels. +.IP ... +Move the cursor to the star at 8,23 and press the \fBr\fR key. +.IP ... +Examine the radial profile and note that this stellar profile +also disappears into the noise at ~4 pixels. +.IP ... +Set \fIpsfrad\fR to 5.0 pixels by typing the command \fB:psfrad 5.0\fR. +.IP ... +Type \fBq\fR to quit the \fBdaoedit\fR task. +.YE + +.PP +The \fBpexamine\fR task and the input photometry file can also be +used to examine the radial profiles of isolated stars in the +photometry file. + +.YS +da> display test 1 fi+ + +da> pexamine test.mag.1 "" test +.YE + +.IP ... +A plot of magnitude error versus magnitude appears on the screen. +.IP ... +Type \fBi\fR to activate the image cursor. +.IP ... +Move the cursor to the star at 36,42 and type \fBr\fR, adjust the +outer radius of the plot with the command \fB:router\fR if necessary, +e.g., \fB:router 10\fR. +.IP ... +Examine the radial profile and note that it disappears into the noise at a +radius of ~4 pixels. +.IP ... +Move the cursor to the star at 8,23 and type \fBr\fR. +.IP ... +Examine the radial profile and note that that it +also disappears into the noise at a radius of ~4 pixels. +.IP ... +Type \fBq\fR to quit the pexamine task. +.LP + +The new value of \fIpsfrad\fR can be stored by editing the \fBdaopars\fR +parameter set with \fBepar\fR in the usual manner or on the command +line as shown below. + +.YS +da> daopars.psfrad = 5.0 +.YE + +.PP +Why is the value of 5.0 pixels for \fBpsfrad\fR so different from the +original estimate of 11.0 ? +There are two reasons. Firstly the stars in artificial image +test are quite faint, +with the brightest peaking at ~400 counts above background. Their stellar +profiles disappear into the noise quite quickly. Secondly the artificial +stars are gaussian in shape with a sigma \(~= 1.0 pixels. +Unlike real stars they have almost all their light +in the core and none in the wings. For realistic optical images +11.0 pixels rather than 5.0 would be a more reasonable choice +for \fIpsfrad\fR than 5.0. +.PP +The \fIdatamin\fR and \fIdatamax\fR parameters are used to reject +psf stars with bad data within \fIfitrad\fR pixels. +If \fIdatamin\fR and \fIdatamax\fR are set correctly before +the \fBphot\fR task is run, these parameters are redundant as stars +with bad data inside the photometry aperture will have INDEF magnitudes. +.PP +At this point the user should check that the current value of the +\fImaxnstar\fR parameter is larger than the total number of stars +in the photometry file written by the \fBphot\fR task. If \fImaxnstar\fR +is too small, \fBpstselect\fR cannot read the entire input photometry +file into memory and potential psf stars may be missed. + +.NH 3 +How Many Psf Stars Should Be Selected ? + +.PP +How many stars should the user select to create the psf model ? +An absolute minimum +set by the mathematics is 1 star for a constant psf model, +3 stars for a linearly variable psf model, and +6 stars for a quadratically variable psf model. A more reasonable minimum +suggested by Stetson (1992) is 3 stars +per degree of freedom or, +3 stars for a constant psf model, 9 stars for a linearly variable psf model, +and 18 stars for a quadratically variable psf model. If a variable psf model +is required, it is vitally important that the psf star list +sample the region of interest in the input image completely and +reasonably uniformly. +As the contribution of each psf star to the psf model is weighted by +its signal-to-noise, the psf stars may cover a range in magnitude without +compromising the resulting psf model. + +.NH 3 +Running Pstselect Non-interactively + +.PP +The following example shows how to run the \fBpstselect\fR task +in non-interactive mode. + +.YS +da> pstselect image default default 3 + +Psf radius in scale units (5.): + New psf radius: 5. scale units 5. pixels +Fitting radius in scale units (3.): + New fitting radius: 3. scale units 3. pixels +Minimum good data value (50.) (CR or value): + New minimum good data value: 50. counts +Maximum good data value (24500.) (CR or value): + New maximum good data value: 24500. counts + +Star 5 has been added to the PSF star list + X: 35.98 Y: 22.00 Mag: 16.372 Dmin: 82.96088 Dmax: 535.1335 +Star 9 has been added to the PSF star list + X: 35.98 Y: 42.03 Mag: 16.594 Dmin: 80.25255 Dmax: 489.9732 +Star 6 has been added to the PSF star list + X: 8.02 Y: 22.97 Mag: 16.603 Dmin: 71.00896 Dmax: 436.3393 + +Total of 3 PSF stars selected\fR +.YE + +By default \fBpstselect \fR looks for an input photometry file called +"test.mag.?" and writes an output psf star list called +"test.pst.?". + +.NH 3 +Running Pstselect Interactively + +.PP +\fBPstselect\fR may also be run interactively. In this mode of operation +the stars selected +by \fBpstselect\fR are examined by the user and accepted or rejected on +the basis of the appearance of their mesh, contour or radial profile plots +until a total of \fImaxnpsf\fR psf stars is reached. +Stars from the input photometry file which do not meet the +\fBpstselect\fR task selection criteria, +can be added to the psf star list by the user with the image cursor +until a total of \fImaxnpsf\fR psf stars have been selected. +.PP +The following example shows how to run \fBpstselect\fR in interactive +mode. + +.YS +da> pstselect image default default 3 inter+ plottype=radial + +Psf radius in scale units (5.): + New psf radius: 5. scale units 5. pixels +Fitting radius in scale units (3.): + New fitting radius: 3. scale units 3. pixels +Minimum good data value (50.) (CR or value): + New minimum good data value: 50. counts +Maximum good data value (24500.) (CR or value): +.YE + +.IP ... +The image cursor appears on the screen ready to accept user input. +.IP ... +Type \fBn\fR to display the first potential psf star found by +\fBpstselect\fR, \fBa\fR to select the star, or \fBd\fR to delete it. +.IP ... +Repeat the previous step for 2 more stars. +.IP ... +Type \fBl\fR to list the selected psf stars. +.IP ... +Type \fBq\fR to quit the task. + +.PP +By default \fBpstselect \fR looks for an input photometry file called +"test.mag.?" and writes an output psf star list called +"test.pst.?" as before. + +.NH 3 +The Pstselect Output + +.PP +The output psf star list consists of the ids, x and y coordinates, +magnitudes, and sky values of the selected psf stars copied from the +input photometry file without change. + +.NH 3 +Examining and/or Editing the Results of Pstselect + +.PP +The \fBpdump\fR and \fBtvmark\fR commands can be used to mark +and label the selected psf stars on the image display as shown in the +following example. + +.YS +da> display test 1 fi+ +da> pdump test.pst.1 xcenter,ycenter,id yes | tvmark 1 STDIN \\ + col=204 label+\fR +.YE + +Bad stars can be removed from the psf star list using the displayed and +labeled image and +the text editor, the \fBpselect\fR task, or +the \fBpexamine\fR task. +.PP +The following command shows how to create a new psf star list +without the psf star whose id is "5" using the \fBpselect\fR task. + +.YS +da> pselect test.pst.1 test.pst.2 "id != 5"\fR +.YE + +.PP +The same operation can be accomplished using the \fBpexamine\fR task +as shown below. + +.YS +da> pexamine test.pst.1 test.pst.2 test +.YE + +.IP ... +A message appears on the screen to the affect that +pexamine cannot plot x versus y since the default y +column merr is not in the input file. +.IP ... +The user types \fBh\fR to plot a histogram of the magnitudes +and notes that there are three stars in the histogram. +.IP ... +The user decides that the star with id number 5 marked on the +display should be deleted because it is too crowded. +.IP ... +The user types the \fBi\fR key to bring up the image cursor, moves +it to star number 5, types the \fBd\fR key to delete the star, and +the \fBp\fR key to replot the data. +.IP ... +The user types the \fBf\fR key to make the deletions permanent and the \fBe\fR +key to exit the task, and update the output catalog. +.LP + +Finally the user marks the new list on the display image using a +different marking color. + +.YS +da> pdump test.pst.2 xcenter,ycenter,id yes | tvmark 1 STDIN \\ + col=205 label+ +.YE +.LP + +.NH 2 +Computing the Psf Model with Psf + +.PP +The \fBpsf\fR task computes the psf model used by the +\fBpeak\fR, \fBnstar\fR, and \fBallstar\fR tasks to do psf fitting +photometry, by the \fBgroup\fR task to estimate magnitudes for stars +whose initial magnitudes are INDEF, and by the \fBaddstar\fR and +\fBsubstar\fR tasks to add stars to and subtract stars from an image. + +.NH 3 +The Psf Algorithm + +.PP +By default the \fBpsf\fR task performs the following functions: +.IP [1] +reads the \fBpsf\fR task parameters including, the input image name, +the input photometry file name, the input psf star list name, +the output psf image name, +the output psf star list name, the output psf star group file name, and +the \fBdatapars\fR and \fBdaopars\fR algorithm parameters +.IP [2] +reads the ids, x and y coordinates, magnitudes, and sky values +of the first \fImaxnstar\fR stars in the input photometry file +.IP [3] +reads the ids, x and y coordinates, magnitudes, and sky values +of the candidate psf stars from the input psf star list and/or +the image cursor, rejecting stars +which are not in the input photometry file, are within \fIfitrad\fR pixels +of the edge of the image, are saturated (if +the parameter \fIsaturated\fR is "no"), +or have bad data within \fIfitrad\fR pixels +.IP [4] +computes the analytic component of the psf model +specified by the parameter \fIfunction\fR using, data within \fIfitrad\fR +pixels of each psf star, weights proportional to the signal-to-noise +ratio in each psf star, and non-linear least-squares fitting techniques +.IP [5] +computes the residuals of each psf star from the best fit analytic function +within a radius of \fIpsfrad\fR + 2 pixels +.IP [6] +scales the residuals for each psf star to match the intensity of the first +psf star, subsamples the scaled residuals in x and y by a factor of 2, +weights the residuals by the signal-to-noise ratio of the psf star, +and combines the scaled, subsampled, and weighted residuals +to create 0, 1, 3, or 6, depending on the \fIvarorder\fR parameter, +psf model look-up tables +.IP [7] +repeats steps [5] and [6] \fInclean\fR times, down-weighting the contributions +to the psf model look-up table(s) of pixels with particularly large +residuals each time through the loop +.IP [8] +estimates magnitudes for the saturated psf stars (if any exist and +if the parameter \fIsaturated\fR is "yes"), +by fitting the current psf model to the wings of the saturated stars +using the \fBpeak\fR task fitting algorithm, +.IP [9] +computes the residuals of each saturated psf star (if any exist and they +were successfully fit) from the best fit +analytic function within a radius of \fIpsfrad\fR + 2 pixels, weights +the residuals by a factor of 0.5, and adds +the contribution of the scaled, subsampled, and weighted residuals +to the psf model look-up table(s) +.IP [10] +writes the computed analytic function parameters and +look-up tables to the output psf image +.IP [11] +identifies all stars within (psfrad + 2 * fitrad + 1) pixels of +a psf star as psf star neighbors, and stars within (2 * fitrad) pixels of +the psf star neighbors as friends of the neighbors +.IP [12] +writes the ids, x and y coordinates, magnitudes, and sky values of +the final list of psf stars to the output psf star list, and the group +and star ids, +x and y coordinates, magnitudes, and sky values of the psf stars, +psf star neighbors, and friends of the psf star neighbors +to the output psf star group file + +.NH 3 +Choosing the Appropriate Analytic Function + +.PP +DAOPHOT offers several choices for the functional form of the analytic +component of the psf model (see Appendix 8.2 for details). +To achieve the best fits and to minimize interpolation errors in +the psf model look-up tables, +users should choose the analytic function that most closely +approximates the stellar psf. The options are: +.IP [1] +\fBgauss\fR (2 parameters), +a 2D elliptical gaussian function aligned along the x and y axes of the image. +Gauss is generally the best choice for well-sampled, fwhmpsf >= 2.5 pixels, +ground-based images because the interpolation errors are small +and evaluation is efficient as the function is separable in x and y. +.IP [2] +\fBmoffat25\fR and \fBmoffat15\fR (3 parameters), +elliptical Moffat functions of beta 2.5 and 1.5 respectively which can +be aligned along an arbitrary position angle. The Moffat functions +are good choices for under-sampled ground-based data. +.IP [3] +\fBlorentz\fR (3 parameters), +an elliptical Lorentz function which can be aligned along an arbitrary position +angle. The Lorenz function is a good choice for old ST data since it has +extended wings. +.IP [4] +\fBpenny1\fR (4 parameters), +a two component model consisting of an elliptical gaussian core +which can be aligned along an arbitrary position angle and +lorentzian wings aligned along the x and y axes of the image. +The Penny1 function is a good choice for a purely analytic psf model. +.IP [5] +\fBpenny2\fR (5 parameters), +a two component model consisting of an elliptical gaussian core +aligned along an arbitrary position angle and lorentzian wings aligned +along an arbitrary position angle which may be different from that of the +core. The Penny2 function is a good choice for a purely analytic psf model. +.IP [6] +\fBauto\fR (2, 3, 4 or 5 parameters), +try each of the 6 analytic psf functions in turn and select the one which +yields the smallest scatter in the fit. Users should use +this option with caution +since the greater number of free parameters in some models may +artificially produce a fit with less scatter without significantly +improving the resulting photometry. +.IP [7] +\fBlist\fR (2, 3, 4 or 5 parameters), +check only those functions in a user specified list, e.g. +"gauss,moffat25,lorentz" and select the one that gives the smallest +scatter. +.PP +Users uncertain of which analytic function to choose should leave +\fIfunction\fR set to "gauss" +and only if the results prove unsatisfactory experiment with one of the +more complicated analytic functions. + +.NH 3 +The Analytic Psf Model + +.PP +A purely analytic psf model may be computed +by setting the \fBdaopars\fR parameter \fIvarorder\fR = -1. +Analytic psf models are +constant, i.e. they have the same shape everywhere in the +image. +In the majority +of cases this is NOT the best modeling option, +as a better representation of the true psf is almost always obtained by +computing an empirical psf model composed of an +analytic function plus one look-up table. +.PP +An analytic psf model may be required to model severely undersampled +data because interpolation errors can produce large uncertainties +in the computed look-up tables and the resulting fits. +.PP +Fields which are so crowded that +isolated psf stars are non-existent, may also require psf modeling and +psf star neighbor subtraction with an analytic psf model, +before a more accurate higher order model free of ghosts produced +by the psf star neighbors can be computed. +This step is particularly important if the field is very crowded AND the +psf is known to be variable. + +.NH 3 +The Empirical Constant Psf Model + +.PP +Most users with typical ground-based optical +data choose to compute an empirical constant psf +model composed of an analytic component and a single look-up table, +by setting the \fBdaopars\fR parameter \fIvarorder\fR = 0. +This type of model is constant, i.e. the psf model has +the same shape everywhere in the image. +.PP +Because of interpolation errors, severely undersampled data may be better +fit with a purely analytic psf model as described in the previous section. +.PP +Fields which are so crowded that +isolated psf stars are non-existent may require psf modeling and +psf neighbor star subtraction with an analytic psf model, +before an accurate look-up table free of ghosts caused by the bright +psf star neighbors can be computed. + +.NH 3 +The Empirical Variable Psf Model + +.PP +Psf models which vary linearly or quadratically +with position in the image can be computed by setting +the \fIvarorder\fR parameter to 1 or 2 respectively. +In the first case a total of 3 look-up tables will be computed; +in the second case 6 look-up tables will be computed. +Users should always begin their analysis with \fIvarorder\fR = -1 or +0 if their data is from a telescope/instrument combination that +is unfamiliar to them. Only if the patterns of the residuals around stars +fit and subtracted with a constant psf model show systematic variations +with position +in the image, should the user proceed to experiment with the variable +psf models. +.PP +In very crowded regions it may be necessary to compute a good +variable psf model iteratively, starting with \fIvarorder\fR = -1 +and proceeding to \fIvarorder\fR = 2 by, computing the psf model, +fitting the psf model to the psf stars and their neighbors, subtracting +the psf star neighbors but not the psf stars from the original image, +increasing \fIvarorder\fR by 1, and recomputing the +psf model using the subtracted image, until all the psf stars and their +neighbors subtract out cleanly. + +.NH 3 +Rejecting Bad Data from the Psf Model + +.PP +The \fBpsf\fR task uses the \fBdatapars\fR parameters \fIdatamin\fR +and \fIdatamax\fR to flag bad data. +If the \fBdaopars\fR parameter \fIsaturated\fR is "no", a prospective +psf star +will be rejected outright if it has high or low bad data inside the fitting +radius; if \fIsaturated\fR is "yes" a star with low bad data will +be rejected outright but one with high bad data will be flagged +as saturated and accepted. Except in rare cases (see below) users should leave +\fIsaturated\fR set to "no". Stars with bad data outside the fitting radius +but inside the psf radius are flagged, and the user warned, but are still +accepted as psf stars. +.PP +All data within one fitting radius of the unsaturated psf stars is weighted by +the signal-to-noise ratio of the psf star and used to compute the +analytic component of the psf model. +Pixels which deviate strongly from the current best fit +analytic function are down-weighted during the course of the fit. +.PP +After the analytic function is fit, the residuals of the psf star data +from the best fit analytic function are computed, scaled to the magnitude +of the first psf star, weighted by the signal-to-noise in the psf star, +subsampled for a factor +of 2 in x and y, and added into the look-up table(s). +If there are too few psf stars with +good data to compute a particular element of the look-up table(s), +\fBpsf\fR will quit with an error. If the \fBdaopars\fR parameter +\fInclean\fR > 0, deviant pixels contributing to the psf model look-up tables +are down-weighted and the look-up table(s) are recomputed \fInclean\fR +times. +.PP +For images where all the bright candidate psf stars are saturated and all the +remaining +candidate psf stars are faint, it may be necessary to use the faint stars to +compute the analytic component of the psf model and bright saturated stars +to compute the look-up tables(s). +In this circumstance the user must set the parameter \fIsaturated\fR +to "yes" and include several saturated stars in the psf star list. +After the analytic function and an initial set of look-up tables(s) +is computed without using the saturated psf stars, the \fBpeak\fR task +fitting algorithm is used to compute accurate magnitudes for the +saturated psf stars by fitting the wings of the saturated stars to +the current psf model. New look-up table(s) are computed +which include the contributions weighted by 0.5 of the saturated psf stars. + +.NH 3 +The Model Psf Psfrad and Fitrad + +.PP +The \fBdaopars\fR parameter \fIpsfrad\fR defines the region over which +the psf model is defined. This radius should equal the radius at which the +radial profile of the brightest star of interest disappears into the noise, +e.g. \(~= 5 pixels for the test image as determined +in the section describing the +\fBpstselect\fR task. The fitting radius defines the region of data around +each psf star used to compute the analytic component of the psf model +and should be the larger of the numbers 3 and 1 * \fIfwhmpsf\fR pixels. + +.NH 3 +Modeling the Psf Interactively Without a Psf Star List + +.PP +The psf can be modeled interactively without an initial list of candidate +psf stars by displaying the image and selecting candidate psf stars +with the image +cursor. Good candidate psf stars must be in the input photometry file, +have no neighbors within \fIfitrad\fR +pixels, and be free of cosmetic blemishes. +.PP +The following example shows how to model the psf interactively without +using an initial psf star list. + +.YS +da> display test 1 fi+ + +da> psf test default "" default default default + +Analytic psf function(s) (gauss): + Analytic psf function(s): gauss +Order of variable psf (0): + Order of variable psf: 0 +Psf radius in scale units (5.): + New psf radius: 5. scale units 5. pixels +Fitting radius in scale units (3.): + New fitting radius: 3. scale units 3. pixels +Minimum good data value (50.) (CR or value): + New minimum good data value: 50. counts +Maximum good data value (24500.) (CR or value): + New maximum good data value: 24500. counts + +Warning: Graphics overlay not available for display device. + +Computing PSF for image: test +9 stars read from test.mag.1 +.YE + +.IP ... +A message appears on the screen telling the user how +many stars have been read from the photometry file +(users should make sure that this is the entire star list) +and the image cursor begins blinking. +.IP ... +The user types the \fBa\fR keystroke at pixel 36,42 +followed by another \fBa\fR keystroke after the default +plot appears, +to add star 9 psf star list. Star 6 at pixel 8,23 is added +to the psf star list in the identical manner. +.LP + +.YS +Star 9 has been added to the PSF star list + X: 35.98 Y: 42.03 Mag: 16.594 Dmin: 80.25255 Dmax: 489.9732 +Star 6 has been added to the PSF star list + X: 8.02 Y: 22.97 Mag: 16.603 Dmin: 71.00896 Dmax: 436.3393 +.YE + +.IP ... +The user types the \fBl\fR keystroke command to list the selected psf stars. +.LP + +.YS +Current PSF star list + Star: 9 X: 35.98 Y: 42.03 Mag: 16.59 Sky: 101.0 + Star: 6 X: 8.02 Y: 22.97 Mag: 16.60 Sky: 98.9 +.YE +.LP + +.IP ... +The user types the \fBf\fR keystroke command to compute the psf model. +.LP + +.YS +Fitting function gauss norm scatter: 0.03422394 + +Analytic PSF fit + Function: gauss X: 25. Y: 25. Height: 523.8066 Psfmag: 16.594 + Par1: 1.082032 Par2: 1.162063 + +Computed 1 lookup table(s) +.YE + +.IP ... +The user reviews the model fit with the \fBr\fR keystroke command and +decides to keep both psf stars. +.LP + +.YS +PSF star 9 saved by user +PSF star 6 saved by user +.YE + +.IP ... +The user types the \fBf\fR keystroke command to remodel the psf. +.LP + +.YS +Fitting function gauss norm scatter: 0.03422394 + +Analytic PSF fit + Function: gauss X: 25. Y: 25. Height: 523.8066 Psfmag: 16.594 + Par1: 1.082032 Par2: 1.162063 + +Computed 1 lookup table(s) +.YE + +.IP ... +The user types the \fBw\fR keystroke command to save the psf model +followed by the \fBq\fR keystroke command, executed twice, to quit the task. +.LP + +.YS +Writing PSF image test.psf.1.imh +Writing output PSF star list test.pst.1 +Writing output PSF star group file test.psg.1 +.YE + +.PP +At this point the user has created an initial psf model in the image +test.psf.1, a list of the psf stars in test.pst.1, and a list of the +psf stars and their neighbors in the file +test.psg.1 respectively. +.PP +Users may occasionally see "Star not found" messages when +selecting psf stars with the image cursor. This may mean: 1) that the star +is truly not in the input photometry file (this can be checked +with the \fBtvmark\fR task), 2) that the image cursor +is more than \fImatchrad\fR pixels from the position of the star +in the input photometry file (either position the image cursor more +carefully by hand or increase the value of the \fImatchrad\fR parameter), or, +3) that the input photometry file contains +more than \fImaxnstar\fR stars (increase the value of the parameter +\fImaxnstar\fR so that it is greater than the number of stars in +the photometry file). + +.NH 3 +Fitting the Psf Model Interactively Using an Initial Psf Star List + +.PP +The \fBpsf\fR task can also be run interactively using an initial list of +psf stars chosen by the user with the \fBpstselect\fR task. +If the \fBpsf\fR task parameter \fIshowpsf\fR is "yes" (the default), +the psf stars are read from the psf star list one at a time, +a mesh, contour, or radial profile plot is displayed in the graphics window, +and the user can accept or delete the star with the \fBa\fR or \fBd\fR +keystroke commands. If +\fIshowplots\fR is "no", the psf star list is read without intervention +by the user. In both cases new stars can be added to the end of the psf +star list with the image cursor in the usual manner. +.PP +A sample run is shown below. + +.YS +da> display test 1 fi+ + +da> pdump test.pst.1 xcenter,ycenter,id yes | tvmark 1 STDIN \\ + col=205 label+ +.YE + +.IP ... +The user marks and labels the initial list of psf stars on the image display. +.LP + +.YS +da> psf test default test.pst.1 default default default + +Analytic psf function(s) (gauss): + Analytic psf function(s): gauss +Order of variable psf (0): + Order of variable psf: 0 +Psf radius in scale units (5.): + New psf radius: 5. scale units 5. pixels +Fitting radius in scale units (3.): + New fitting radius: 3. scale units 3. pixels +Minimum good data value (50.) (CR or value): + New minimum good data value: 50. counts +Maximum good data value (24500.) (CR or value): + New maximum good data value: 24500. counts + +Warning: Graphics overlay not available for display device. + +Computing PSF for image: test +9 stars read from test.mag.1 +.YE + +.IP ... +The user rejects or accepts the stars in the .pst file by typing the +\fBd\fR or \fBa\fR keystroke commands respectively after the +default plot appears. +.LP + +.YS +Star 5 rejected by user +Star 9 has been added to the PSF star list + X: 35.98 Y: 42.03 Mag: 16.594 Dmin: 80.25255 Dmax: 489.9732 +Star 6 has been added to the PSF star list + X: 8.02 Y: 22.97 Mag: 16.603 Dmin: 71.00896 Dmax: 436.3393 + +2 PSF stars read from test.pst.1 +.YE + +.IP ... +The user types the \fBl\fR keystroke command to view the psf star +list one more time. +.LP + +.YS +Current PSF star list + Star: 9 X: 35.98 Y: 42.03 Mag: 16.59 Sky: 101.0 + Star: 6 X: 8.02 Y: 22.97 Mag: 16.60 Sky: 98.9 +.YE + +.IP ... +The user computes the psf model with the \fBf\fR keystroke command. +.LP + +.YS +Fitting function gauss norm scatter: 0.03422394 + +Analytic PSF fit + Function: gauss X: 25. Y: 25. Height: 523.8066 Psfmag: 16.594 + Par1: 1.082032 Par2: 1.162063 + +Computed 1 lookup table(s) +.YE + +.IP ... +The user saves the psf model with the \fBw\fR keystroke command. +.LP + +.YS +Writing PSF image test.psf.1.imh +Writing output PSF star list test.pst.2 +Writing output PSF star group file test.psg.1 +.YE + +.IP ... +The user types the \fBq\fR keystroke command to quit the task. +.LP + +The user notes that the output psf star list is given a version number +of 2 in this example, since version 1 was written by the \fBpstselect\fR task. + +.NH 3 +Fitting the Psf Model Interactively Without an Image Display + +.PP +Users without access to an image display, may still run \fBpsf\fR +interactively by redirecting +the image cursor commands to the terminal as shown below. + +.YS +da> set stdimcur = text + +da> psf test default test.pst.1 default default default + +Analytic psf function(s) (gauss): + Analytic psf function(s): gauss +Order of variable psf (0): + Order of variable psf: 0 +Psf radius in scale units (5.): + New psf radius: 5. scale units 5. pixels +Fitting radius in scale units (3.): + New fitting radius: 3. scale units 3. pixels +Minimum good data value (50.) (CR or value): + New minimum good data value: 50. counts +Maximum good data value (24500.) (CR or value): + New maximum good data value: 24500. counts + +Warning: Graphics overlay not available for display device. + +Computing PSF for image: test +9 stars read from test.mag.1 +.YE + +.IP ... +The user rejects or accepts the stars in the .pst file by typing +the \fBd\fR or \fBa\fR keystroke commands respectively at the prompt. +.LP + +.YS +Star 5 rejected by user +Star 9 has been added to the PSF star list + X: 35.98 Y: 42.03 Mag: 16.594 Dmin: 80.25255 Dmax: 489.9732 +Star 6 has been added to the PSF star list + X: 8.02 Y: 22.97 Mag: 16.603 Dmin: 71.00896 Dmax: 436.3393 + +2 PSF stars read from test.pst.1 +.YE + +.IP ... +The user types the \fBl\fR keystroke command at the prompt to +view the psf star list one more time. +.LP + +.YS +Current PSF star list + Star: 9 X: 35.98 Y: 42.03 Mag: 16.59 Sky: 101.0 + Star: 6 X: 8.02 Y: 22.97 Mag: 16.60 Sky: 98.9 +.YE + +.IP ... +The user computes the psf model by typing the \fBf\fR keystroke +command at the prompt. +.LP + +.YS +Fitting function gauss norm scatter: 0.03422394 + +Analytic PSF fit + Function: gauss X: 25. Y: 25. Height: 523.8066 Psfmag: 16.594 + Par1: 1.082032 Par2: 1.162063 + +Computed 1 lookup table(s) +.YE + +.IP ... +The user saves the psf model by typing the \fBw\fR keystroke command +at the prompt. +.LP + +.YS +Writing PSF image test.psf.1.imh +Writing PSF output star list test.pst.2 +Writing PSF output star group file test.psg.1 +.YE + +.IP ... +The user types the \fBq\fR keystroke command at the prompt to quit the task. +.LP + +Additional stars can be added to the psf star list by commands of +the form \fB":a id#"\fR or \fB"100.2 305.6 1 a"\fR typed in at the +terminal prompt. +The user should remember to reset the image cursor to the logical +image cursor with the command \fB"reset stdimcur = stdimage"\fR +after running the \fBpsf\fR task in "no image display" mode. + +.NH 3 +Fitting the Psf Model Non-interactively + +.PP +Finally the psf model can be fit non-interactively by setting the +\fIinteractive\fR parameter to "no", and using the list of psf stars produced by +the \fBpstselect\fR task as input. This is the preferred method for computing +the psf model when the number of psf stars is large (e.g. the psf +model to be computed is variable). + +.YS +da> psf test default test.pst.1 default default default inter- +.YE + +.NH 3 +The Output of Psf + +.PP +\fBPsf\fR writes an output psf star list test.pst.# containing the ids, x and +y coordinates, magnitudes, and sky values, copied from the +input photometry file, +of the psf stars actually used to compute the final psf model. +Because of the ability +to add and subtract stars within \fBpsf\fR itself, this list may be different +from the input psf star list if any. A sample output psf star list is +shown below. + +.YS +#K IRAF = NOAO/IRAFV2.10EXPORT version %-23s +#K USER = davis name %-23s +#K HOST = tucana computer %-23s +#K DATE = 05-28-93 mm-dd-yr %-23s +#K TIME = 14:34:31 hh:mm:ss %-23s +#K PACKAGE = daophot name %-23s +#K TASK = psf name %-23s +#K IMAGE = test imagename %-23s +#K PHOTFILE = test.mag.1 filename %-23s +#K PSTFILE = test.pst.1 filename %-23s +#K PSFIMAGE = test.psf.1 imagename %-23s +#K GRPSFILE = test.psg.1 filename %-23s +#K OPSTFILE = test.pst.2 filename %-23s +#K SCALE = 1. units/pix %-23.7g +#K OTIME = 00:07:59.0 timeunit %-23s +#K IFILTER = V filter %-23s +#K XAIRMASS = 1.238106 number %-23.7g +#K PSFRAD = 5. scaleunit %-23.7g +#K FITRAD = 3. scaleunit %-23.7g +# +#N ID XCENTER YCENTER MAG MSKY \\ +#U ## pixels pixels magnitudes counts \\ +#F %-9d %-10.3f %-10.3f %-12.3f %-12.3f +# +9 35.980 42.029 16.594 101.043 +6 8.022 22.970 16.603 98.891 +.YE + +.PP +\fBPsf\fR also writes an output psf star group photometry file +test.psg.? containing the group ids, and the star ids, +x and y coordinates, magnitudes, and sky values +copied from the input photometry file, +for the psf stars and their +neighbors. A sample psf star group file is shown below. + +.YS +#K IRAF = NOAO/IRAFV2.10EXPORT version %-23s +#K USER = davis name %-23s +#K HOST = tucana computer %-23s +#K DATE = 05-26-93 mm-dd-yr %-23s +#K TIME = 16:10:48 hh:mm:ss %-23s +#K PACKAGE = daophot name %-23s +#K TASK = psf name %-23s +#K IMAGE = test imagename %-23s +#K PHOTFILE = test.mag.2 filename %-23s +#K PSTFILE = test.pst.2 filename %-23s +#K PSFIMAGE = test.psf.2 imagename %-23s +#K GRPSFILE = test.psg.2 filename %-23s +#K SCALE = 1. units/pix %-23.7g +#K OTIME = 00:07:59.0 timeunit %-23s +#K IFILTER = V filter %-23s +#K XAIRMASS = 1.238106 number %-23.7g +#K PSFRAD = 5. scaleunit %-23.7g +#K FITRAD = 3. scaleunit %-23.7g +# +#N ID GROUP XCENTER YCENTER MAG MSKY \\ +#U ## ## pixels pixels magnitudes counts \\ +#F %-9d %-6d %-10.3f %-10.3f %-12.3f %-14.3f +# +9 1 35.980 42.029 16.594 101.043 +8 1 28.958 33.924 17.781 100.619 +6 2 8.022 22.970 16.603 98.891 +.YE + +There are two stellar groups, one group per psf star, +in this file. The first psf star +has a single neighbor so there are two stars in the first group. +The first star in a group is always the psf star. The header parameters +record the input and output image and file names, the name of the +computed psf model, and the values +of the parameters +\fIpsfrad\fR and \fIfitrad\fR used to define the psf star groups. +.PP +The psf image contains, in the image header, the values of the +parameters that were used +to compute the psf model, the best fit values of the parameters of +the chosen analytic function, and a record of all the +psf stars used to compute the psf, and in the image pixels, the best fit +look-up table(s) of the residuals from the analytic function +subsampled by a factor of 2. +The psf image look-up table(s) can be plotted and examined just like any +other IRAF image. +.PP +A sample psf image header is shown below. + +.YS +test.psf.2[23,23][real]: PSF for image: test + No bad pixels, no histogram, min=unknown, max=unknown + Line storage mode, physdim [23,23], length of user area 1540 s.u. + Created Wed 16:10:47 26-May-93, Last modified Wed 16:10:47 26-May-93 + Pixel file 'tucana!/d0/iraf/davis/test.psf.2.pix' [ok] + IRAF = 'NOAO/IRAFV2.10EXPORT' + HOST = 'tucana ' + USER = 'davis ' + DATE = '05-26-93' + TIME = '16:10:48' + PACKAGE = 'daophot ' + TASK = 'psf ' + IMAGE = 'test ' + PHOTFILE= 'test.mag.2' + PSTFILE = 'test.pst.2' + PSFIMAGE= 'test.psf.2' + GRPSFILE= 'test.psg.2' + SCALE = 1. + PSFRAD = 5. + FITRAD = 3. + DATAMIN = 50. + DATAMAX = 24500. + NCLEAN = 0 + USESAT = F + FUNCTION= 'gauss ' + PSFX = 25. + PSFY = 25. + PSFHEIGH= 523.8066 + PSFMAG = 16.594 + NPARS = 2 + PAR1 = 1.082032 + PAR2 = 1.162063 + VARORDER= 0 + FEXPAND = F + NPSFSTAR= 2 + ID1 = 9 + X1 = 35.98 + Y1 = 42.029 + MAG1 = 16.594 + ID2 = 6 + X2 = 8.022 + Y2 = 22.97 + MAG2 = 16.603 +.YE + +This psf image header records that: the psf model +was computed with a gaussian +analytic function (function = gauss), the analytic +function has two parameters (npars=2) +whose values are 1.08 and 1.16 (par1 and par2 are the fwhm of the function +in x and y respectively in this case), the psf is constant but there is +1 look-up +table (varorder = 0), no saturated stars were used to compute the psf +(usesat=no), and no cleaning of bad pixels was done to compute the +lookup table (nclean=0). The number of psf stars and their positions +and magnitudes +are also listed. The psf model is defined over a radius of 5 +pixels (psfrad = 5.0), resulting in a square look-up table with dimensions +of 2 * (nint (2 * psfrad) + 1) + 1 pixels in x and y, and a fitting radius +of 3 (fitrad = 3.0) was used to compute the analytic portion of the psf +model. +.PP +The height of the best fit gaussian psfheigh is 523.81 counts. +The psf model has been assigned a magnitude of 16.594 which is the +magnitude of the first psf star in the input photometry file. +All fits to the psf model are scaled with respect to this +magnitude. Therefore a star which is twice as bright as the psf model will +have a fitted magnitude of ~15.841. +.PP +Psfx and psfy define the distance from the center of the input image to the +center of the edge pixels in x and y respectively, e.g psfx = +(ncols - 1.0) / 2.0 and psfy = (nlines - 1) / 2.0. These numbers are used to +evaluate the psf model only if the model is variable, \fIvarorder\fR > 0. +.PP +If the value of \fIvarorder\fR in this example had been 1 or 2 the +psf model image would have been 3-dimensional with 3 and 6 23 by 23 +pixel look-up +tables in planes 1-3 and 1-6 of the image respectively. +In both cases planes 1-3 would contain the 0th, 1st order in x, +and 1st order in y Taylor expansion coefficients around the analytic function. +In the latter case planes 4-6 would contain the +2nd order in x, 2nd order in xy, and 2nd order in y Taylor expansion +coefficients. If the value of \fIvarorder\fR +had been -1 no look-up tables would have been computed and the +psf model image would consist of an image header but no pixel file. + +.NH 3 +Checking the Psf Model + +.PP +To check the accuracy of the psf model, the user must fit each psf +star and its neighbors and friends +as a group using the \fBnstar\fR task, and the psf model and psf star group +photometry file +produced by \fBpsf\fR as shown below. In the following example +the user has chosen to set the rejections +file to "", so that all stars, even those too faint to be properly +fit, will be placed in the same output file. + +.YS +da> nstar test test.psg.1 default default "" + +Recenter the stars (yes): + Recenter the stars: yes +Refit the sky (no): + Refit the sky: no +Psf radius in scale units (5.): + New psf radius: 5. scale units 5. pixels +Fitting radius in scale units (3.): + New fitting radius: 3. scale units 3. pixels +Maximum group size in number of stars (60): + New maximum group size: 60 stars +Minimum good data value (50.) (CR or value): + New minimum good data value: 50. counts +Maximum good data value (24500.) (CR or value): + New maximum good data value: 24500. counts + +Group: 1 contains 2 stars + ID: 9 XCEN: 35.98 YCEN: 42.01 MAG: 16.60 + ID: 8 XCEN: 28.96 YCEN: 33.91 MAG: 17.73 +Group: 2 contains 1 stars + ID: 6 XCEN: 8.02 YCEN: 22.97 MAG: 16.63 +.YE + +The results of the fits will appear in test.nst.? as shown below. + +.YS +da> page test.nst.1 + +#K IRAF = NOAO/IRAFV2.10EXPORT version %-23s +#K USER = davis name %-23s +#K HOST = tucana computer %-23s +#K DATE = 05-28-93 mm-dd-yy %-23s +#K TIME = 14:46:13 hh:mm:ss %-23s +#K PACKAGE = daophot name %-23s +#K TASK = nstar name %-23s +#K IMAGE = test imagename %-23s +#K GRPFILE = test.psg.1 filename %-23s +#K PSFIMAGE = test.psf.1 imagename %-23s +#K NSTARFILE = test.nst.1 filename %-23s +#K REJFILE = "" filename %-23s +#K SCALE = 1. units/pix %-23.7g +#K DATAMIN = 50. counts %-23.7g +#K DATAMAX = 24500. counts %-23.7g +#K GAIN = 1. number %-23.7g +#K READNOISE = 0. electrons %-23.7g +#K OTIME = 00:07:59.0 timeunit %-23s +#K XAIRMASS = 1.238106 number %-23.7g +#K IFILTER = V filter %-23s +#K RECENTER = yes switch %-23b +#K FITSKY = no switch %-23b +#K PSFMAG = 16.594 magnitude %-23.7g +#K PSFRAD = 5. scaleunit %-23.7g +#K FITRAD = 3. scaleunit %-23.7g +#K MAXITER = 50 number %-23d +#K MAXGROUP = 60 number %-23d +#K FLATERROR = 0.75 percentage %-23.7g +#K PROFERROR = 5. percentage %-23.7g +#K CLIPEXP = 6 number %-23d +#K CLIPRANGE = 2.5 sigma %-23.7g +# +#N ID GROUP XCENTER YCENTER MAG MERR MSKY \\ +#U ## ## pixels pixels magnitudes magnitudes counts \\ +#F %-9d %-6d %-10.3f %-10.3f %-12.3f %-14.3f %-12.3f +# +#N NITER SHARPNESS CHI PIER PERROR \\ +#U ## ## ## ## perrors \\ +#F %-17d %-12.3f %-12.3f %-6d %-13s +# +9 1 35.982 42.006 16.601 0.019 101.043 \\ + 4 -0.019 0.512 0 No_error +8 1 28.962 33.912 17.730 0.074 100.619 \\ + 4 0.026 1.093 0 No_error +6 2 8.017 22.968 16.628 0.021 98.891 \\ + 3 0.022 0.558 0 No_error +.YE + +In this example the chi values computed by \fBnstar\fR for the two +psf stars are lower than expected, ~ 0.5 instead of ~ 1.0, meaning that the +observed errors are less than the predicted errors. +This occurs because there are only 2 psf stars, +and therefore the model psf and the fitted psf stars are not totally +independent. +For example, if only one psf star is used to compute +the psf model, the chi +computed by \fBnstar\fR for that star would be ~ 0.0 and for any others +such as its neighbors ~ 1.0. +.PP +After checking that the chi values look reasonable, the user subtracts the +fitted psf stars and their neighbors from the input image +with the \fBsubstar\fR task, and examines the residuals +of the fit around the psf stars as shown below. After \fBsubstar\fR is +run the subtracted +image is displayed and the psf stars are marked in green and the +psf neighbor stars are marked in red. + +.YS +da> substar test default "" default default + +Psf radius in scale units (5.): + New psf radius: 5. scale units 5. pixels +Minimum good data value (50.) (CR or value): + New minimum good data value: 50. counts +Maximum good data value (24500.) (CR or value): + New maximum good data value: 24500. counts + +SUBTRACTING - Star: 6 X = 8.02 Y = 22.97 Mag = 16.63 +SUBTRACTING - Star: 8 X = 28.96 Y = 33.91 Mag = 17.73 +SUBTRACTING - Star: 9 X = 35.98 Y = 42.01 Mag = 16.60 + +A total of 3 stars were subtracted out of a possible 3 + +da> display test.sub.1 1 fi+ +da> pdump test.nst.1 xc,yc,id yes | tvmark 1 STDIN col=204 label+ +da> pdump test.pst.1 xc,yc,id yes | tvmark 1 STDIN col=205 label+ +.YE + +.PP +The psf stars and their neighbors should subtract out cleanly +with no systematic patterns in the residuals as a function of distance +from the star (note this may not be the case if the psf is purely analytic +or so severely undersampled that the interpolation errors near the +center are very large), with magnitude, or with position in the image. +There should be no hidden +underlying neighbors revealed after the subtraction (these psf stars +should be rejected) or neighbors +that are not in the photometry file (this can be fixed up later). +The amplitudes of the fit residuals +should be consistent with the noise if sufficient stars are +used to determine the psf model. +.PP +The displayed and marked subtracted image and the output of \fBnstar\fR can +be examined in more detail with the \fBpexamine\fR task as shown below. + +.YS +da> pexamine test.nst.1 "" test.sub.1 +.YE +.IP ... +A plot of magnitude versus magnitude error for the psf stars and their +neighbors will appear. +.IP ... +Change the default plot to mag versus chi with the command \fB:ycolumn chi\fR +followed by the \fBp\fR keystroke command. +.IP ... +Activate the image cursor with the \fBi\fR keystroke command. +.IP ... +Move to psf star number 9 and type \fBr\fR to examine the radial profile +of the subtracted star, followed by \fBo\fR to get a listing of its position, +magnitude, magnitude error, sky value, etc. +.IP ... +Examine the radial profiles of the other subtracted psf stars and their +neighbors. +.IP ... +Type \fBq\fR to quit the task. + +.PP +For the test image "test", examination of the radial profiles of the +subtracted stars shows that the residuals are +consistent with the noise in the image and have no unusual features +leading to the conclusion that the current psf model is a good representation +of the true psf. + +.NH 3 +Removing Bad Stars from the Psf Model + +.PP +Bad psf stars detected after the psf star and neighbor subtraction, +for example those with +a cosmetic blemish, a close double, or an underlying neighbor star, +should be removed altogether from +the psf star list. This can be done by editing the psf star list written +by \fBpsf\fR with the text editor, with the \fBpselect\fR task and an +expression, e.g. "id != 5", specifying the star to be deleted, or +with the interactive \fBpexamine\fR task using the delete and update keys, +and rerunning \fBpsf\fR with the new psf star list. +.PP +This step is not required for the test image as both psf stars and their +neighbors subtracted cleanly from the image. + +.NH 3 +Adding New Stars to a Psf Star Group + +.PP +Occasionally stars that are too faint to be included +in initial star list produced by \fBdaofind\fR and measured with \fBphot\fR, are +nevertheless sufficiently bright and close to a psf star +to affect the computation of the psf model. +Ideally the psf stars should have no such companions and/or the look-up +table cleaning option in the \fBpsf\fR task should minimize the problem of +undetected neighbors. However in some cases it is necessary for +the user to intervene and add faint stars to the photometry list. +.PP +The easiest way to accomplish this +is to run \fBphot\fR interactively, selecting the +missing neighbor stars with the image cursor and appending the results +for the new neighbor stars to the photometry file produced by the first +run of \fBnstar\fR. + +.YS +da> phot test "" psf.mag inter+ calgorithm=centroid +.YE + +.IP ... +Point cursor to undetected psf star neighbors and hit spacebar. +.IP ... +Type \fBq\fR to quit. +.LP + +.YS +da> prenumber psf.mag idoffset=5000 +.YE + +.IP ... +Renumber the stars in the new file starting with a number greater +than the number of stars in the original photometry file in order +to insure that all the stars have unique ids. +.LP + +.YS +da> pfmerge test.psg.1,psf.mag test.psf.psg +.YE + +.IP ... +Combine the psf star group photometry file produced by the \fBpsf\fR task +with the photometry file for the new psf star neighbors produced by +the \fBphot\fR task. +.LP + +Users should note that there is no input coordinate list to \fBphot\fR in +this case. Therefore the coordinate list is set to "", interactive mode +is on, and centering is turned on. +The remaining photometry parameters should be set exactly as they were in the +first run of \fBphot\fR. +.PP +This step is not required for the test image as all the significant +psf star neighbors were detected by the first run of the \fBdaofind\fR task. + +.NH 3 +Refitting the Psf Model With the New Psf Star Groups + +.PP +After the \fBpsf \fR and \fBphot\fR results have been merged, the +user must regroup the psf stars and their neighbors with +the \fBgroup\fR task, and refit the +new groups with the \fBnstar\fR task. + +.YS +da> group test test.psf.psg default test.grp critov=.2 +.YE + +.IP ... +Regroup the stars using a very small value for the critical overlap +parameter. +.LP + +.YS +da> nstar test test.grp default default "" +.YE + +.IP ... +Rerun nstar on the new psf star groups. +.LP + +.YS +da> substar test test.nst.2 "" default default +.YE + +.IP ... +Check that the new psf star groups subtract cleanly from the original +image. +.PP +This step is not required for the test image as all the significant +psf star neighbors were detected by the first run of the \fBdaofind\fR task. + + +.NH 3 +Computing the Final Psf Model + +.PP +Once the psf star and psf star neighbors subtract out cleanly +from the input image with the current psf model, +a final psf model should be computed using an image from which all +the psf star neighbors but not the psf stars have been subtracted. +To do this the user runs the \fBsubstar\fR task, setting the input photometry +file to the final output of \fBnstar\fR, and the exclude file to the final +psf star list written by the \fBpsf\fR task, and reruns \fBpsf\fR. +An example of this procedure is shown below. + +.YS +da> substar test test.nst.2 test.pst.2 default default +da> psf test.sub.3 test.grp test.pst.2 test.psf.2 test.pst.3 \\ + test.psg.3 inter- +.YE + +After this step the user should have a good psf model and can proceed to do +psf fitting photometry. +.PP +This step is not required for the test image as the single psf star +neighbor is sufficiently far from the psf star to have a negligible +effect on the computation of the psf model. + +.NH 3 +Visualizing the Psf Model with the Seepsf Task + +.PP +The psf analytic function parameters are stored in the psf image header +and the look-up table(s) in the psf image pixels. The look-up table(s) are +subsampled by a factor of 2 with respect to the image, and cannot +be used directly to visualize what the psf model looks like +at the scale of the image. The task \fBseepsf\fR can be used to +do this conversion as shown below. + +.YS +da> seepsf test.psf.3 psf3 +.YE + +The output image will contain a picture of what an ideal star of magnitude +equal to the magnitude of the psf (see the psfmag keyword in the psf +image header) should look like at the center of the image. +.PP +In the case of a variable psf the appearance of the psf model can be examined at +various +places in the image by specifying a position at which to compute the psf +model. + +.YS +da> seepsf test.psf.3 psf.13.8 xpsf=13 ypsf=8 +.YE + +.PP +The total power in a variable psf should be constant +as a function of position in the image even though its shape is +different. The variable psf fitting code +in DAOPHOT does perform flux conservation. Users can check this by using +the \fBimstatistics\fR task to check that there is no net power in the +look-up tables 2-3 or +2-6 if the psf order is 1 or 2. Similarly they can use \fBseepsf\fR to compute +the psf at various positions in the input image and \fBimstat\fR to check +that the net power in the psf is constant over the image +even though the shape of the +psf is variable. + +.NH 3 +Problems Computing the Psf Model + +.PP +Computing the psf model is the most crucial step in DAOPHOT. +The \fBdaofind\fR and \fBphot\fR +steps are usually straight-forward, and the principal fitting task +\fBallstar\fR runs entirely in batch once started. However computing +a good psf model requires user input. +.PP +This section suggests a few +things to check if the computed psf model is not doing a good +job of fitting and subtracting the psf or program stars. The user +should check: +.IP [1] +that the sky annulus chosen in the \fBphot\fR step is neither too +close or too far from the stars. If the sky annulus is too close +the computed skies will tend to be too high and the psf model +will have too small an amplitude, +producing false halos around the fitted and subtracted stars. If the sky annulus +is too far away the computed sky value will not represent the sky under the +psf star well, adding scatter to the photometry. +.IP [2] +the psf radius. +If the stars appear to be well fit in the cores but have residual halos with a +sharp inner edge around +them then the psf radius may be too small. The psf radius needs to +be big enough to give good subtractions for the brightest stars of interest. +.IP [3] +that the analytic component of the psf function is appropriate for +the data. +If the data is somewhat undersampled, fwhmpsf < 2.5 pixels, +one of the Moffat functions may give a better fit to the data than +the Gauss function. If that data is extremely undersampled an analytic +function may do better than one involving look-up tables. +.IP [4] +the psf stars. +One or more of the psf stars may not be stars, may be doubles, or contain +bad data. Although psf does try to detect and down-weight bad data it may +not be completely successful. Users need to examine the subtracted image +for objects with bad residuals and for stars with large fitted chi values +and eliminate them. +.IP [5] +for psf variability with position in the image. The true psf may be variable +and inadequately fit by a constant psf model. The user should examine the +residuals around the subtracted psf stars to see if there are patterns +with position in the image and +increase the order of the psf model if these are detected. +Large fitted chi values may also be an indication of a poor psf model. +.IP [6] +the distribution of the psf stars. +If the psf is variable the user must ensure that the psf stars adequately +cover the region of interest in the image. For example if there are no +psf stars in a certain portion of the image the psf may not +be well represented there. +.IP [7] +the data. If the psf is a higher order than quadratic \fBpsf\fR may +not be able to model it adequately. The user should check the image data +reduction history, and investigate any image combining, bad pixel and cosmic +ray removal +operations, etc., that may have fundamentally altered the data. +The data should also be checked for linearity. +.IP [8] +the noise model. If the chi values for the fitted psf stars are unusually +large or small, the effective readout noise and gain for the image +may not be correct. The user should check that these values are +being read from the image header correctly and that they are appropriate +for the data. + + +.NH 2 +Doing Psf Fitting Photometry with Peak, Nstar, or Allstar + +.PP +There are three psf fitting +photometry tasks in DAOPHOT. The \fBpeak\fR task fits the current psf +model to stars individually; the \fBnstar\fR task fits the psf model to +stars in fixed stellar groups simultaneously; the \fBallstar\fR task +groups and fits the psf model to stellar groups dynamically +and subtracts the fitted stars from the input image. +\fBAllstar\fR is the +task of choice for the majority of users, but all three options are +discussed in the following sections. + +.NH 3 +Fitting Single Stars with Peak + +.PP +\fBPeak\fR is the simplest psf fitting task. +\fBPeak\fR fits the psf model to the stars in the +input photometry list individually. Because \fBpeak\fR cannot fit stars in +groups as the \fBnstar\fR and \fBallstar\fR tasks do, and in +uncrowded frames aperture photometry is often simpler and just as accurate, +\fBpeak\fR has few unique functions. +However \fBpeak\fR can be useful in cases where the user wishes to: 1) improve +the signal to noise of faint stars by taking advantage of +\fBpeak's\fR optimal weighting scheme, 2) do astrometry of single +stars, 3) fit and remove single stars from the frame in order to +examine the underlying light distribution. + +.NH 4 +The Peak Algorithm + +.PP +By default the \fBpeak\fR task performs the following functions: +.IP [1] +reads the task parameters, including the name of +the input image, the input photometry file, the psf model, the +output photometry and rejections files, and the +\fBdatapars\fR and \fBdaopars\fR algorithm parameter sets +.IP [2] +reads the id, x and y coordinates, magnitude, and sky value +of a star from the input photometry file +.IP [3] +rejects the star if it has an undefined sky value, too +few good data pixels to obtain a fit, or is too faint +.IP [4] +extracts the image data within one fitting radius of each star and +performs an optimally weighted non-linear least-squares fit of the psf model +to the extracted data +.IP [5] +rejects the star if its signal-to-noise ratio is too low +or a unique solution cannot be found +.IP [6] +computes the best fit x, y, and magnitude for the star +.IP [7] +writes the id, new x and y coordinates, sky value, new magnitude, magnitude +error, number of iterations required to fit the star, chi statistic, +and sharpness +statistic for the fitted star to the +output photometry file, and the id, last computed x and y position, +and sky value of the rejected star, to +the rejections file +.IP [8] +repeats steps [2]-[7] for each star in the input photometry list + +.NH 4 +Running Peak + +.PP +A sample run of the \fBpeak\fR task is shown below. The user is prompted +for all the input and output file names and asked to verify the +critical parameters \fIrecenter\fR, \fIfitsky\fR, \fIpsfrad\fR, \fIfitrad\fR, +\fIdatamin\fR, and \fIdatamax\fR. + +.YS +da> peak test default default default default + +Recenter the stars (yes): + Recenter the stars: yes +Refit the sky (no): + Refit the sky: no +Psf radius in scale units (5.): + New psf radius: 5. scale units 5. pixels +Fitting radius in scale units (3.): + New fitting radius: 3. scale units 3. pixels +Minimum good data value (50.) (CR or value): + New minimum good data value: 50. counts +Maximum good data value (24500.) (CR or value): + New maximum good data value: 24500. counts + +Star: 1 X: 40.97 Y: 4.01 Mag: 17.22 Sky: 100.74 + FIT: Star: 1 X: 41.01 Y: 4.03 Mag: 17.22 Sky = 100.74 +Star: 2 X: 23.06 Y: 7.03 Mag: 17.65 Sky: 100.33 + FIT: Star: 2 X: 23.02 Y: 7.05 Mag: 17.75 Sky = 100.33 +Star: 3 X: 18.02 Y: 7.96 Mag: 17.48 Sky: 99.40 + FIT: Star: 3 X: 17.98 Y: 8.00 Mag: 17.57 Sky = 99.40 +Star: 4 X: 25.99 Y: 22.01 Mag: 16.80 Sky: 101.34 + FIT: Star: 4 X: 25.99 Y: 22.01 Mag: 16.81 Sky = 101.34 +Star: 5 X: 35.98 Y: 22.00 Mag: 16.37 Sky: 101.12 + FIT: Star: 5 X: 35.97 Y: 22.02 Mag: 16.39 Sky = 101.12 +Star: 6 X: 8.02 Y: 22.97 Mag: 16.60 Sky: 98.89 + FIT: Star: 6 X: 8.02 Y: 22.97 Mag: 16.63 Sky = 98.89 +Star: 7 X: 30.97 Y: 25.01 Mag: 17.05 Sky: 101.29 + FIT: Star: 7 X: 30.96 Y: 25.04 Mag: 17.05 Sky = 101.29 +Star: 8 X: 28.96 Y: 33.92 Mag: 17.78 Sky: 100.62 + FIT: Star: 8 X: 28.96 Y: 33.93 Mag: 17.74 Sky = 100.62 +Star: 9 X: 35.98 Y: 42.03 Mag: 16.59 Sky: 101.04 + FIT: Star: 9 X: 35.98 Y: 42.01 Mag: 16.60 Sky = 101.04\fR +.YE + +In this example the user chose to recenter the stars (almost always +the best choice), and not to refit the sky (usually the best choice). +Recentering should only be turned off if the initial centers in the input +photometry file are known to be very accurate, e.g. they +are derived from a better seeing +image or one that has gone through some image restoration program. +Users who elect to refit the sky, +should realize that they will almost certainly need to increase +the fitting radius to +obtain a reasonable fitted sky value. Increasing the fitting radius +however may also increase the scatter caused by neighboring stars. +.PP +The fitted stars can be subtracted from the input image with the \fBsubstar\fR +task as shown below. + +.YS +da> substar test test.pk.1 "" default default\fR +.YE + +.PP + +.NH 4 +The Peak Output + +.PP +Peak writes the quantities: id, xcenter, ycenter, mag, merr, msky, +niter, chi, sharp, pier, and perror to the output photometry file and the +rejections file. +.IP [1] +Id is the id number of the star as read from the input photometry file. +.IP [2] +Xcenter and ycenter are the best fit position of the star. +If the star was rejected xcenter and ycenter will be the computed +values of x and +y at the time it was rejected. If \fIrecenter\fR is "no", xcenter and ycenter +will be the position of the star in the input photometry file. +.IP [3] +Mag and merr are the best fit magnitude and magnitude error respectively. +The instrumental magnitude is computed relative to the +magnitude assigned to the +psf model. +Mag and merr are set to INDEF if the star cannot be fit to the psf model. +.IP [4] +Msky is the sky value in the input photometry file if fitsky = "no", +otherwise it is the +fitted sky value. If the star is not fit for some reason, +msky is the computed sky value +at the time the star was rejected. +.IP [5] +Niter is the number of iterations it took to fit the star. If this +number if equal to the \fBdaopars\fR parameter \fImaxiter\fR the user should be +suspicious of the computed positions, magnitudes, and sky values. +However as the convergence criteria are +conservative the star may still be reasonably well fit. +Niter is set to 0 if the star cannot be fit to the psf model. +.IP [6] +Chi and sharp are measures of the goodness of fit and the shape +of the star respectively. Chi should be ~ 1.0. If it is not then, +either the object is not a single star, the noise model including +one or more of the gain, readout noise, flat-fielding error, and +interpolation error parameters for the image are incorrect, the +psf model is a poor representation of the true psf, or the input +image does not conform to the requirements of the DAOPHOT package. +Sharp is a measure of the difference between the observed width +of the object and the width of the psf model. Stars should have a sharpness +value ~ 0.0, resolved objects a sharpness of > 0.0, and cosmic rays and similar +blemishes a sharpness of < 0. Chi and sharp are set to INDEF if the star +cannot be fit to the psf model. +.IP [7] +Pier and perror are an integer error code and error string respectively. +If no error was encountered during the fit pier is 0 and perror is +"No_error". Stars are rejected by the \fBpeak\fR task if 1) the sky value of +the star is INDEF 2) there are too few good data pixels to fit the star +3) the fitting matrix is singular meaning a unique solution could not +be found 4) the star is too faint, i.e. its signal / noise < 2.0. A fifth +condition, the solution did not converge by \fImaxiter\fR iterations, is not +used to reject the star, although users should be suspicious of a star +for which niter = \fImaxiter\fR. + + +.NH 3 +Fitting Stars with Group, Grpselect, Nstar and Substar + +.PP +Stars can be fit simultaneously in fixed groups using the +\fBnstar\fR task. +This psf fitting technique requires grouping the stars +into physically meaningful +associations with the \fBgroup\fR and/or the \fBgrpselect\fR tasks, +fitting the stars in each group simultaneously with the \fBnstar\fR task, +and subtracting +the fitted stars from the image with the \fBsubstar\fR task. +\fBNstar\fR is the task of choice when the user wishes to explicitly +control the grouping process or fit stars in a small number of +widely separated groups efficiently. +\fBNstar\fR is most commonly used to fit the psf model to the psf stars and +their neighbors. + +.NH 4 +The Group and Nstar Algorithms + +.PP +By default the \fBgroup\fR task performs the following steps: + +.IP [1] +reads the task parameters, including the name of the input image, the input +photometry file, the psf model, the output photometry +file, and the \fBdatapars\fR and \fBdaopars\fR +algorithm parameter sets +.IP [2] +reads the ids, x and y coordinates, magnitudes, and sky values +of up to \fImaxnstar\fR stars in the input photometry file, computes an +approximate magnitude for the stars with INDEF magnitudes, and sorts +the stars in increasing order of y +.IP [3] +finds all the stars which are within \fIpsfrad\fR + \fIfitrad\fR + 1 pixels +of a given star, evaluates the psf of the brighter star at a distance +of \fIfitrad\fR pixels from +the fainter, and if this value is larger than \fIcritovlap\fR +times the expected +error per pixel, or the two stars are within \fIfitrad\fR + 1 pixels of +each other, adds the star to the group +.IP [4] +writes the group and star ids, x and y coordinates, magnitudes and sky values +for all the groups, to the output group photometry file. + +.PP +By default the \fBnstar\fR task performs the following steps: + +.IP [1] +reads the task parameters including the name of +the input image, the input group file, the psf image, the +output group photometry and rejections files and the \fBdatapars\fR and +\fBdaopars\fR algorithm parameter sets +.IP [2] +reads the group and star ids, x and y coordinates, magnitudes, and +sky values for all the stars in a group from the input group photometry +file +.IP [3] +extracts the data within psfrad + fitrad pixels +around the group and +performs a weighted least-squares fit of the psf model to the extracted +data +.IP [4] +rejects stars which have an undefined sky value, which are too faint (more +than 12.5 magnitudes fainter than the psf), which are +too noisy (faintest star in the group less than a 1.0, 1.5, or 2.0 +sigma detection after 5, 10, and 15 iterations or convergence respectively), +for which there are too few good +pixels to compute a fit, for which a unique solution cannot +be found, which have merged with another star (fainter star < 0.37 * +fwhmpsf from a brighter star in the group), which are both too +noisy and too +close to a brighter star (a star is between .37 and 1.0 fwhm of +a brighter star and is a 1.0, 1.5, or 2.0 sigma detection before +iterations 5, 10, and 15 respectively), or which are in a group +too large (> than the value of the \fImaxgroup\fR parameter) to be reduced. +.IP [5] +estimates new x and y coordinates and magnitudes for each star in +the group +.IP [6] +iterates until all the stars in the group satisfy the convergence criteria +backing up the iteration counter by 1 each time a star is rejected from +the group to allow the remaining stars time to settle into a new fit +.IP [7] +writes the star and group ids, new x and y coordinates, sky values, +new magnitudes and magnitude errors, chi and sharpness statistics +for the fitted stars to the +output group photometry and rejections files +.IP [8] +repeats steps [2]-[7] for each group in the input group photometry file. + +.NH 4 +Running Group, Grpselect, and Nstar + +.PP +Before \fBnstar\fR can be run the stars must be grouped with the +\fBgroup\fR task as shown below. + +.YS +da> group test default default default + +Psf radius in scale units (5.): + New psf radius: 5. scale units 5. pixels +Fitting radius in scale units (3.): + New fitting radius: 3. scale units 3. pixels +Critical overlap in stdevs per pixel (1.): .2 + New critical overlap: 0.2 stdevs per pixel +Minimum good data value (50.) (CR or value): + New minimum good data value: 50. counts +Maximum good data value (24500.) (CR or value): + New maximum good data value: 24500. counts + +Size of Number of +group groups + +1 4 +2 1 +3 1 + +Total of 9 stars in 6 groups\fR +.YE + +The critical overlap parameter \fIcritovlap\fR determines the degree +to which crowding and/or random photometric errors are expected/allowed +to influence the photometry. If the default value of 1 is required to group +all the stars into associations of <= the current value of +\fImaxgroup\fR stars, then unavoidable random photometric errors and +crowding errors will affect the photometry about equally. +If a critical +overlap much greater than 1 is required, then crowding errors will +dominate the random photometric errors. If a critical overlap +much less than 1 does the +job then +unavoidable random photometric errors will dominate, and crowding errors +are relatively insignificant. +In +the previous example the user chose to set \fIcritovlap\fR to a value +much smaller +than 1 to test whether random photometric rather than crowding errors will +dominate the photometry. +.PP +If the first run of \fBgroup\fR separates all the stars into groups of less than +60 all is well and the user can proceed to the \fBnstar\fR task. Otherwise +the \fBgrpselect\fR task must be used to select out the larger groups +and subdivide them as shown in the following example. + +.YS +da> grpselect test.grp.1 small.grp 1 60 +.YE + +.IP ... +First separate out the small groups. +.LP + +.YS +da> grpselect test.grp.1 big.grp.1 61 10000 +.YE + +.IP ... +Next separate out the large groups. +.LP + +.YS +da> group test big.grp.1 default big.grp.2 critovlap=1.0 +.YE + +.IP ... +Rerun the group task on the large group file with a bigger value +of critovlap. +.LP + +.YS +da> pconcat small.grp,big.grp.2 all.grp +.YE + +.IP ... +Finally concatenate all the new group files together. +.LP +This step is not required for the test image since there are only a +few stars and the field is not very crowded. + +.PP +Run \fBnstar\fR on the grouped photometry file and \fBsubstar\fR on the +fitted photometry file. + +.YS +da> nstar test default default default default + +Recenter the stars (yes): + Recenter the stars: yes +Refit the sky (no): + Refit the sky: no +Psf radius in scale units (5.): + New psf radius: 5. scale units 5. pixels +Fitting radius in scale units (3.): + New fitting radius: 3. scale units 3. pixels +Maximum group size in number of stars (60): + New maximum group size: 60 stars +Minimum good data value (50.) (CR or value): + New minimum good data value: 50. counts +Maximum good data value (24500.) (CR or value): + New maximum good data value: 24500. counts + +Group: 1 contains 1 stars + ID: 1 XCEN: 41.01 YCEN: 4.03 MAG: 17.22 +Group: 2 contains 2 stars + ID: 2 XCEN: 23.04 YCEN: 7.07 MAG: 17.75 + ID: 3 XCEN: 17.99 YCEN: 8.00 MAG: 17.57 +Group: 3 contains 3 stars + ID: 5 XCEN: 35.98 YCEN: 22.01 MAG: 16.39 + ID: 7 XCEN: 30.99 YCEN: 25.04 MAG: 17.04 + ID: 4 XCEN: 26.00 YCEN: 22.01 MAG: 16.80 +Group: 4 contains 1 stars + ID: 6 XCEN: 8.02 YCEN: 22.97 MAG: 16.63 +Group: 5 contains 1 stars + ID: 8 XCEN: 28.96 YCEN: 33.93 MAG: 17.74 +Group: 6 contains 1 stars + ID: 9 XCEN: 35.98 YCEN: 42.01 MAG: 16.60 + +da> substar test default "" default default +.YE + +The parameter \fImaxgroup\fR specifies the +maximum number of stars that \fBnstar\fR will +fit simultaneously. The default value of 60 +is a conservative number based +on the observed numerical behavior of the matrix inversion routines. +.PP +For most crowded field photometry applications it is simpler and easier +to use the automated \fBallstar\fR task. + +.NH 4 +The Nstar Output + +.PP +By default \fBnstar\fR writes the quantities: id, group, xcenter, ycenter, +mag, merr, msky, niter, chi, sharp, pier, and perror to the output +photometry and rejections files. +.IP [1] +Id and group are the star and group id numbers in the input group +photometry file. +.IP [2] +Xcenter and ycenter are the best fit coordinates of the star. +If the star was rejected xcenter and ycenter will be the best fit +values of x and y at the time it was rejected. If \fIrecenter\fR is "no" +xcenter and ycenter will be the position of the star in the input +group photometry file. +.IP [3] +Mag and merr are the best fit magnitude and magnitude error respectively. +The instrumental magnitude is computed relative to the magnitude +of the psf model. +Mag and merr are set to INDEF if the star cannot be fit to the +psf model. +.IP [4] +Msky is always the individual sky for the star in the input photometry +file regardless of whether fitsky is "no" or "yes". In the former +case the actual value of the sky used in \fBnstar\fR is +the mean of all the sky values for all the stars in the group. In the +latter case it is a fitted parameter. +.IP [5] +Niter is the number of iterations it took to fit the star. If \fBniter\fR +is equal to the parameter \fImaxiter\fR the user should be +suspicious of the result. However since the convergence criteria are +quite tightly constrained the result may still be reasonable. +Niter is set to 0 if the star cannot be fit to the psf model. +.IP [6] +Chi and sharp are measures of the goodness of fit and the star's shape +respectively. Chi should be ~ 1.0. If it is not then +either the object is not a single star, +the noise model including the ccd gain and readout noise, the flat +fielding error and the interpolation error parameters assumed for the image are +not correct, the +psf model is a poor representation of the true psf, or the input +image does not conform to the requirements of the DAOPHOT package. +Sharp is a measure of the difference between the observed width +of the object and the width of the psf model. Stars should have sharpness +values \(~= 0.0, resolved objects sharpness values > 0.0, and cosmic rays +and similar blemishes sharpnesses values < 0.0. Chi and sharp are set to INDEF +if the star cannot be fit to the psf model. +.IP [7] +Pier and perror are an integer error code and error string respectively. +If no error was encountered during the fit, pier is 0 and perror is +"No_error". + +.NH 3 +Fitting Stars With Allstar + +.PP +\fBAllstar\fR groups, fits +and subtracts the fitted stars from the input image without intervention +by the user. +Because the grouping process is dynamic and the best +fit stars are fit and subtracted first, fewer weak stars +and noise spikes migrate to the position of stronger stars in \fBallstar\fR +than is the case with \fBnstar\fR. \fBAllstar\fR replaces the functionality +of the tasks \fBgroup\fR, \fBgrpselect\fR, \fBnstar\fR, and \fBallstar\fR. +\fBAllstar\fR is the task of choice for doing crowded +field photometry with DAOPHOT. + +.NH 4 +The Allstar Algorithm + +.PP +By default the \fBallstar\fR task performs the following steps: +.IP [1] +reads the task parameters including the name of the input image, the input +photometry file, the psf +model, the output photometry and rejections files, the output +subtracted image, and the \fBdatapars\fR and \fBdaopars\fR algorithm +parameter sets +sets +.IP [2] +reads the ids, x and y coordinates, magnitudes, and sky values for +the first \fImaxnstars\fR stars in the input photometry file, rejecting +at the start stars which have undefined sky values or which +are too close to another star +.IP [3] +reads the original image into a working array and initializes +two scratch arrays containing 1) the noise model and 2) the residuals from +the current best fit for all the stars +.IP [4] +at the beginning of each iteration: +1) groups the stars into physical associations that contain fewer +than \fImaxgroup\fR +stars, regrouping as necessary until all the groups are less than +\fImaxgroup\fR or until the group is too dense to reduce, 2) +subtracts the current best fit for all the stars that are still unfit from +the working copy of the image and stores the results in the residuals +array 3) initializes the weight array for all the unfitted stars +.IP [5] +during each iteration: 1) extracts the data within fitrad pixels +around each star in each group from the residuals image, 2) +performs a weighted non-linear least-squares fit of the psf model to +the extracted +data, ignoring bad pixels and down-weighting pixels that deviate too +far from the psf model, and 3) computes new x and y coordinates and +magnitudes for each star in each group +.IP [6] +after the fourth iteration 1) writes the id, new x and y coordinates, +sky value, new magnitude and magnitude error, number of iterations +required to fit the star, and the chi and sharpness statistic of stars +which meet the convergence criteria, to +the output photometry file, 2) subtracts the fitted star permanently from +the working copy of the image, 3) updates the +noise model in the weight array, 4) and eliminates the star from +the active star list +.IP [7] +after the fourth iteration rejects stars which: 1) are too faint +(more than 12.5 magnitudes fainter +than the psf model), 2) have too low a signal-to- +noise ratio (1.0, 1.5 and 2.0 sigma detection after 5, 10, and 15 iterations +respectively), 3) have too few good +pixels to compute a fit, 4) do not permit a unique solution, +5) have merged with another star (star is +< 0.37 * fwhmpsf from a brighter star), 6) are both too noisy +and too close to a neighbor star (star is between 0.37 and 1.0 * fwhmpsf from +a brighter star in the group and is a 1.0, 1.5, or 2.0 sigma +detection before iterations 5, 10, and 15 respectively), +or 7) are part of a group too dense to be reduced. +.IP [8] +writes out the final version of the work array into the output subtracted +image + +.NH 4 +Running Allstar + +.PP +\fBAllstar\fR is run as shown below. + +.YS +da> allstar test default default default default default + +Recenter the stars (yes): + Recenter the stars: yes +Refit the sky (no): + Refit the sky: no +Psf radius in scale units (5.): + New psf radius: 5. scale units 5. pixels +Fitting radius in scale units (3.): + New fitting radius: 3. scale units 3. pixels +Maximum group size in number of stars (60): + New maximum group size: 60 stars +Minimum good data value (50.) (CR or value): + New minimum good data value: 50. counts +Maximum good data value (24500.) (CR or value): + New maximum good data value: 24500. counts + +NITER = 1 +NITER = 2 +NITER = 3 +NITER = 4 +FITTING: ID: 1 XCEN: 41.01 YCEN: 4.03 MAG: 17.22 +FITTING: ID: 4 XCEN: 26.00 YCEN: 22.01 MAG: 16.80 +FITTING: ID: 7 XCEN: 30.99 YCEN: 25.04 MAG: 17.04 +FITTING: ID: 6 XCEN: 8.02 YCEN: 22.97 MAG: 16.63 +FITTING: ID: 8 XCEN: 28.96 YCEN: 33.93 MAG: 17.74 +FITTING: ID: 9 XCEN: 35.98 YCEN: 42.01 MAG: 16.60 +NITER = 5 +FITTING: ID: 2 XCEN: 23.04 YCEN: 7.05 MAG: 17.75 +FITTING: ID: 3 XCEN: 18.00 YCEN: 7.99 MAG: 17.56 +FITTING: ID: 5 XCEN: 35.98 YCEN: 22.01 MAG: 16.39 +.YE + +.PP +Users can easily run \fBallstar\fR as a background job as shown below. + +.YS +da> allstar test default default default default default verify- \\ + >& allstar.out & +.YE + +.NH 4 +The Allstar Output + +.PP +\fBAllstar\fR writes the following +quantities: id, xcenter, ycenter, mag, merr, msky, niter, chi, sharp, +pier, and perror to the output photometry and +rejections files. +.IP [1] +Id is the id number of the star as read from the input photometry file. +.IP [2] +Xcenter and ycenter are the best fit position of the star. +If the star was rejected xcenter and ycenter will be the computed +values of x and +y at the time it was rejected. If \fIrecenter\fR is "no", xcenter and ycenter +will be the position of the star in the input photometry file. +.IP [3] +Mag and merr are the best fit magnitude and magnitude error respectively. +The instrumental magnitude is computed relative to the magnitude of the +psf model. +Mag and merr are set to INDEF if the star cannot be fit to the psf model. +.IP [4] +Msky is the sky value in the input photometry file if \fIfitsky\fR = "no", +otherwise it is the +recomputed sky value. If \fIfitsky\fR is "yes" the sky is recomputed +every third iteration after the current best fit for the star is +subtracted from the image data. The new sky value is set to the +average of 40% of the sky pixels, centered on the median sky value, +which are inside the sky +annulus defined by the parameters \fIsannulus\fR and +\fIwsannulus\fR. The sky value is not recomputed +if there are fewer than 100 sky pixels in the specified sky annulus +even if \fIfitsky\fR is "yes". +If the star is not fit for some reason, +msky is the sky value at the time the star was rejected. +.IP [5] +Niter is the number of iterations it took to fit the star. If this +number is equal to \fImaxiter\fR the user should be +suspicious of the result. However as the convergence criteria are +conservative the star may still be reasonably well fit. +Niter is set to 0 if the star cannot be fit to the psf model. +.IP [6] +Chi and sharp are measures of the goodness of fit and the shape +respectively. Chi should be ~ 1.0. If it is not, then +either the object is not a single star, the noise model including +one or more of the gain, readout noise, flat-fielding error, and +interpolation error parameters for the image are incorrect, the +psf model is a poor representation of the true psf, or the input +image does not conform to the requirements of the DAOPHOT package. +Sharpness is a measure of the difference between the observed width +of the object and the width of the psf model. Stars should have a sharpness +value ~ 0.0, resolved objects a sharpness of > 0.0, and cosmic rays and similar +blemishes a sharpness of < 0. Chi and sharp are set to INDEF if the star +cannot be fit for some reason. +.IP [7] +Pier and perror are an integer error code and error string respectively. +If no error was encountered during the fit, pier is 0 and perror is +"No_error". + +.NH 2 +Examining the Output Photometry Files + +.PP +The identical tools can be used to examine the output of the \fBpeak\fR, +\fBnstar\fR, and \fBallstar\fR tasks. Some examples using the output of +\fBallstar\fR are shown below. +.PP +The following command produces a plot of magnitude error versus magnitude. + +.YS +da> pdump test.als.1 mag,merr yes | graph point+ +.YE + +The following command produces a plot of chi versus magnitude. + +.YS +da> pdump test.als.1 mag,chi yes | graph STDIN point+ +.YE + +The following command produces a plot of chi versus sharpness. + +.YS +da> pdump test.als.1 sharp,chi yes | graph STDIN point+ +.YE + +The output photometry file can also be examined interactively +with the \fBpexamine\fR task and the displayed subtracted image. +Note that the fitted stars are marked in green and the rejected stars +are marked in red on the display. + +.YS +da> display test.sub.1 1 fi+ + +da> pdump test.als.1 xcenter,ycenter yes | tvmark 1 STDIN col=205 + +da> pdump test.arj.1 xcenter,ycenter yes | tvmark 1 STDIN col=204 + +da> pexamine test.als.1 "" test.sub.1 +.YE + +.IP ... +A plot of magnitude error versus magnitude appears on the screen. +.IP ... +The user moves to a discrepant point in the graph and types o to get a +listing of the results for the star, r to get a radial profile plot +around the subtracted star, and concludes on the basis of the plots +that the bad chi value is due to the star being a close double. +.IP ... +The user types i to switch to image cursor mode, +moves to several other stars with poor subtractions and types +s to see a surface plot of the residuals. +.IP ... +The user types q to quit. + +.NH 2 +Problems with the Photometry + +.PP +Bad chi values in, and poor +subtractions of, \fBpeak\fR, \fBnstar\fR or \fBallstar\fR +photometry can usually be traced to: 1) a psf model which was +poorly determined in the first place, e.g. poor choice of +parameters, bad choice of +psf stars or too few stars used for determining a good variable psf model, +2) data reduction problems e.g. the mean sky value was subtracted from the +image, the image statistics have been altered, or cosmic ray removal +clipped the tops of the stars, to give a few of many examples, +or 3) the properties of the image, e.g. non-linearity, a psf which has +very high order variability or very undersampled data, make computation +of a good psf model difficult or impossible. +.PP +Bad chi values can also be caused by incorrect +values of gain and readout noise or by a data reduction operation +which has significantly affected the image statistics. +.PP +Poor sky fitting can also cause scatter +in the photometry. Users should carefully check the position of the +sky annulus used in \fBphot\fR if they are seeing poor subtractions. +If the images have a rapidly varying +background due, due for example to nebulosity, it might be useful to +check out the alternate sky fitting routines, median or centroid, in the +\fBphot\fR task. The refit sky option in \fBpeak\fR and \fBnstar\fR +should be exercised with caution +since a larger fitting radius is often required +to get a reasonable sky fit, than is required to get good positions +and magnitudes, and this in turn can cause more scatter +in the photometry due to the influence of neighbors. On the other +hand the refit sky option in \fBallstar\fR can often significantly +reduce scatter in very crowded regions since it can use data closer to +or even underneath (!) the star to improve the sky estimate. +Users who use this option must remember to set the inner +radius of the \fBallstar\fR sky annulus to avoid the inner stellar +core region where there is a lot of noise in the subtraction. +.PP +After running \fBsubstar\fR on a file produced by the \fBpeak\fR +task, users will sometimes see large holes in the data at the +position of some subtracted +stars. This is usually caused by fainter stars (which are fit individually) +migrating to the position of a brighter nearby star and then being +subtracted out twice by \fBsubstar\fR. Keeping the fitting radius small +will help minimize this problem, but if it is frequent and the frame +is somewhat crowded, the user should run \fBnstar\fR or \fBallstar\fR +instead of \fBpeak\fR. +.PP +A similar problem can be caused by users running \fBdaofind\fR +with a very low threshold and detecting a lot of noise spikes, which +then migrate to the positions of brighter stars +and cause scatter and holes in the subtracted \fBpeak\fR photometry, +or attach themselves to noise spikes in the stellar profiles and cause +scatter and holes in the subtracted \fBnstar\fR photometry. +Similar problems can affect \fBallstar\fR photometry but to a much +lesser degree since the +stars are grouped dynamically and subtracted from the input data as +they are fit. For all three photometry tasks spurious detections +can consume a lot of excess computer time because the stellar groups +become much larger. + +.NH 2 +Detecting Stars Missed By Daofind + +.PP +In very crowded fields many new stars, missed by the first run of \fBdaofind\fR, +will be detected after +the first run of \fBpeak\fR+\fBsubstar\fR, \fBnstar\fR+\fBsubstar\fR, +or \fBallstar\fR. If there +are many "missed" stars +\fBdaofind\fR should be run on the subtracted image after increasing the +\fIthreshold\fR parameter to avoid detecting the residuals +of previously subtracted stars. If there +are only a few such stars they can be "detected" by creating a coordinate +file using the subtracted image and \fBtvmark\fR in interactive mode. +Examples of both techniques are shown below. + +.YS +da> daofind test.sub.1 newstars.coo threshold=5.0 +.YE + +or + +.YS +da> display test.sub.1 1 fi+ + +da> pdump test.als.1 xcen,ycen yes | tvmark 1 STDIN col=204 + +da> tvmark 1 newstars.coo inter+ +.YE + +.IP ... +Move cursor to missing stars and tap the \fBa\fR key to append them to the +output coordinate file. + +.NH 2 +Initializing the Missing Star Photometry with Phot + +.PP +The next step is to get initial photometry for the "missing" +stars. The simplest way is +to run \fBphot\fR on the original image using the coordinate list created +by \fBdaofind\fR or \fBtvmark\fR, and the same algorithm parameters as +were used +in the first run of \fBphot\fR. It is also possible to use \fBphot\fR directly +in interactive mode to create a photometry file of missed stars. Both +options are shown below. + +.YS +da> phot test newstars.coo newstars.mag +.YE + +or + +.YS +da> display test.sub.1 1 fi+ + +da> pdump test.als.1 xcen,ycen yes | tvmark 1 STDIN col=204 + +da> phot test "" newstars.mag centroid=calgorithm inter+ +.YE + +.IP ... +Point the cursor to the missing stars and tap \fBspacebar\fR. +.LP + +Note that if the stars are or were marked with the cursor, the user must +turn centroiding on in order to center them correctly. + +.NH 2 +Merging Photometry Files with Pfmerge + +.PP +The photometry file containing the aperture photometry for the new stars +can be combined with the best psf fitting photometry already computed +by the \fBnstar\fR or \fBallstar\fR tasks +for the original star list, using the task \fBpfmerge\fR as shown below. +The \fBprenumber\fR task ensures that the new stars all have unique ids. + +.YS +da> pfmerge test.als.1,newstars.mag newstars.als.1 +da> prenumber newstars.als.1\fR +.YE + + +.NH 2 +Refitting the Stars with Allstar + +.PP +After the photometry files have been merged a final run of \fBallstar\fR or +\fBgroup\fR+\fBnstar\fR+\fBsubstar\fR on the combined file in order +to compute accurate magnitudes for the new stars should be made +as shown below. + +.YS +da> allstar test newstars.als.1 default default default default +.YE + +.NH 2 +Examining the Subtracted Image + +.PP +The user should search the subtracted image for any remaining unfit +stars and perform another iteration of \fBdaofind\fR, \fBphot\fR, +\fBpfmerge\fR and \fBallstar\fR to computed fitted magnitudes for +the new objects. + +.NH 2 +Computing an Aperture Correction + +.PP +The aperture correction is the number which must be added to the +fitted instrumental magnitudes computed by the \fBpeak\fR, +\fBnstar\fR, or \fBallstar\fR tasks to produce the total instrumental +magnitude. +.PP +In order to compute aperture corrections for an image with a constant psf model +the user must: +.IP [1] +identify several bright isolated stars in the input image or subtract +all the neighbors from around several bright stars such as the +psf stars using the current psf model and the \fBsubstar\fR task +.IP [2] +using a minimum aperture radius equal to the one used in \fBphot\fR to compute +initial aperture photometry for all the crowded field stars, +and a maximum aperture radius equal to the one through which +the instrumental magnitudes of the standard stars were or will be measured, +use the \fBphot\fR task to +do multi-aperture photometry of the stars identified in [1] through at +least five apertures +.IP [3] +run the \fBmkapfile\fR task in the PHOTCAL package on the aperture +photometry file produced in 2, to determine the aperture +correction for the image as shown below +.LP + +.YS +da> phot test "" test.apmags calg=centroid aperture="3,3.5,4.0,4.5 5.0" +.YE + +.IP ... +Do multi-aperture photometry of the selected stars. +.LP + +.YS +da> mkapfile test.apmags 5 test.apcors +.YE + +.IP ... +Compute the aperture correction between apertures 1 and 5. + +.PP +To compute compute aperture corrections for an image with a variable psf model +the user must: +.IP [1] +identify several bright isolated stars in the input image or subtract +all the neighbors from around several bright stars such as the +psf stars using the current psf model and the \fBsubstar\fR task +.IP [2] +using a photometry aperture equal to the one through which +the magnitudes of the standard stars were or will be measured, +use the \fBphot\fR task to +do aperture photometry of the stars identified in [1] +.IP [3] +extract the fitted magnitudes for these stars from existing \fBnstar\fR +or \fBallstar\fR photometry or recompute them using the +\fBnstar\fR or \fBallstar\fR tasks and the current psf model +.IP [4] +set the aperture correction to the mean difference between the fitted +magnitudes computed in [3] and the aperture photometry magnitudes +computed through the large aperture in [2] + +.NH +References + +.LP +.nf +Stetson, P. B. 1987 Pub .A.S.P., \fB99\fR, 191 +Stetson, P. B., Davis, L.E. and Crabtree, D.B. 1989, in + \fICCDs in Astronomy\fR, G.H. Jacoby, San Francisco: Astronomical + Society of the Pacific, 289 +Stetson, P. B. 19 Pub .A.S.P., \fB102\fR, 932 +Stetson, P.B, 1992, \fIUser's Manual for DAOPHOT II\fR +Stetson, P. B. 1992 in \fIAstronomical Data Analysis Software and Systems I\fR, + D.M. Worall, C. Biemesderfer, and J. Barnes, San Francisco: Astronomical + Society of the Pacific, 297 +.fi + +.NH +Appendices + +.NH 2 +The Instrumental Magnitude Scale + +.PP +The instrumental magnitude scale is set by the magnitude assigned +to the psf model, the quantity \fIpsfmag\fR stored in the psf image header. +Psfmag is the magnitude of the first psf star in the input photometry +file, usually but not always the file written by the \fBphot\fR task. +If magnitudes were measured through more than one aperture +in \fBphot\fR, the magnitude used will be the +magnitude through the smallest aperture. + +.NH 2 +The Analytic Psf Models + +.PP +The functional forms of the currently supported analytic psf models +are listed below. +The quantity A is a normalization factor. The Pn are +the parameters which are fit during the psf modeling process. + +.nf + z = x ** 2 / p1 ** 2 + y ** 2 / p2 ** 2 + gauss = A * exp (-0.5 * z) + + z = x ** 2 / p1 ** 2 + y ** 2 / p2 ** 2 + x * y * p3 + moffat15 = A / (1 + z) ** 1.5 + moffat25 = A / (1 + z) ** 2.5 + + z = x ** 2 / p1 ** 2 + y ** 2 / p2 ** 2 + x * y * p3 + lorentz = A / (1.0 + z) + + z = x ** 2 / p1 ** 2 + y ** 2 / p2 ** 2 + e = x ** 2 / p1 ** 2 + y ** 2 / p2 ** 2 + x * y * p4 + penny1 = A * ((1 - p3) / (1.0 + z) + p3 * exp (-0.693*e)) + + z = x ** 2 / p1 ** 2 + y ** 2 / p2 ** 2 + p5 * x * y + e = x ** 2 / p1 ** 2 + y ** 2 / p2 ** 2 + x * y * p4 + penny2 = A * ((1 - p3) / (1.0 + z) + p3 * exp (-0.693*e)) +.fi + +.NH 2 +The Error Model + +.PP +The predicted errors in the the DAOPHOT photometry are computed per +pixel as shown below, where terms 1, 2, 3, and 4 represent the readout +noise, the poisson noise, the flat-fielding error, and the +interpolation error respectively. The quantities readnoise, epadu, +I, M, p1, and p2 are the effective readout noise in electrons, the +effective gain in +electrons per ADU, the pixel intensity in ADU, the PSF model +intensity in ADU, the FWHM in x in pixels, and the FWHM in y in pixels. + +.nf + error = sqrt (term1 + term2 + term3 + term4) (ADU) + term1 = (readnoise / epadu) ** 2 + term2 = I / epadu + term3 = (.01 * flaterr * I) ** 2 + term4 = (.01 * proferr * M / p1 / p2) ** 2 +.fi + + +.NH 2 +The Radial Weighting Function + +.PP +The radial weighting function employed by all the psf fitting tasks +is shown below, where dx and dy are the distance of the pixel +in question from the centroid of the star being fit. + +.nf + wtr = 5.0 / (5.0 + rsq / (1.0 - rsq)) + rsq = (dx ** 2 + dy ** 2) / fitrad ** 2 +.fi + +.NH 2 +Total Weights + +.PP +The total weight assigned each pixel in the fit is the +following. + +.nf + wtp = wtr / error ** 2 +.fi + +.NH 2 +Bad Data Detection + +.PP +Pixels less than the good data minimum \fIdatamax\fR or greater than +the good data maximum \fIdatamax\fR are rejected immediately from the +fit. +.PP +After a few iterations and if clipexp > 0, a clipping scheme to +reject bad data is enabled. The weights of the pixels are +recomputed as follows. Pixels having a residual of cliprange sigma +will have their weight reduced by half. + +.nf + wt = wtp / (1.0 + (residual / error / chiold / + cliprange) ** clipexp) +.fi + +.NH 2 +Stellar Mergers + +.PP +In order for two stars to merge during the course of the psf fitting +process either their separation must be < 0.37 * FWHM of the psf model, +or their separation must be > 0.37 * FWHM but < 1.0 * FWHM of the +psf model and the signal-to-noise ratio of the fainter is less than 1.0, 1.5, +or 2.0 after iterations 4, 9, and 14 respectively. + +.NH 2 +Faint Stars + +.PP +Stars are considered to be too faint if they are more than 12.5 +magnitudes fainter than the psf, or if after a certain number of iterations, +they have a signal-to-noise ratio less than 2.0. |