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+%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}