From fa080de7afc95aa1c19a6e6fc0e0708ced2eadc4 Mon Sep 17 00:00:00 2001 From: Joseph Hunkeler Date: Wed, 8 Jul 2015 20:46:52 -0400 Subject: Initial commit --- noao/imred/ccdred/doc/flatfields.hlp | 177 +++++++++++++++++++++++++++++++++++ 1 file changed, 177 insertions(+) create mode 100644 noao/imred/ccdred/doc/flatfields.hlp (limited to 'noao/imred/ccdred/doc/flatfields.hlp') diff --git a/noao/imred/ccdred/doc/flatfields.hlp b/noao/imred/ccdred/doc/flatfields.hlp new file mode 100644 index 00000000..94766960 --- /dev/null +++ b/noao/imred/ccdred/doc/flatfields.hlp @@ -0,0 +1,177 @@ +.help flatfields Jun87 noao.imred.ccdred + +.ih +NAME +flatfields -- Discussion of CCD flat field calibrations +.ih +DESCRIPTION +This topic describes the different types of CCD flat fields and +the tasks available in the \fBccdred\fR and spectroscopy packages for +creating them. Flat field calibration is the most important operation +performed on CCD data. This operation calibrates the relative response +of the detector at each pixel. In some cases this is as simple as +taking a special type of observation called a flat field. However, in +many cases this calibration observation must be corrected for +iillumination, scanning, wavelength, and aperture effects. + +The discussion is in three sections; direct imaging, scan mode, +and spectroscopy. Though there are many similarities between these +modes of operation there are important differences in how corrections +are applied to the basic flat field observations. The application of +the flat field calibrations to the observations using \fBccdproc\fR is +the same in all cases, however. +.sh +1. Direct Imaging +The starting point for determining the flat field calibration is an +observation of something which should have uniform response at all +points on the detector. In addition the color of the light falling at +each pixel should be the same as that in an observation so the same +filter must be used when determining the flat field (the issue of the +matching the color of the objects observed at the appropriate pixels is +ignored here). The best calibration observation is of a blank sky. If +an accurate blank sky observation can be obtained then this is all that +is needed for a flat field calibration. This type of flat field might +be called a \fIsky flat\fR, though this term is more often used for a +type of flat field described below. There are two difficulties with +this type of calibration; finding a really blank sky and getting a +sufficiently accurate measurement without using all the observing +time. + +It is usually not possible to get a blank sky observation accurate +enough to calibrate the individual pixels without introducing +undesirable noise. What is generally done is to use a lamp to either +uniformly illuminate a part of the dome or directly illuminate the +field of view. The first type of observation is called a \fIdome +flat\fR and the second is called a \fIprojection flat\fR. We shall call +both of these types of observations \fBlamp flat fields\fR. If the +iillumination is truely uniform then these types of observations are +sufficient for flat field calibration. To get a very accurate flat +field many observations are made and then combined (see +\fBflatcombine\fR). + +Unfortunately, it is sometimes the case that the lamp flat fields +do not illuminate the telescope/detector in the same way as the actual +observations. Calibrating with these flat fields will introduce a +residual large scale iillumination pattern, though it will correctly +calibrate the relative pixel responses locally. There are two ways to +correct for this effect. The first is to correct the flat field +observation. The second is to apply the uncorrected flat field to the +observations and then apply an \fIiillumination\fR correction as a +separate operation. The first is more efficient since it consists of a +single correction applied to each observation but in some cases the +approximate correction is desired immediately, the observation needed +to make the correction has not been taken yet, or the residual +iillumination error is not discovered until later. + +For the two methods there are two types of correction. One is to +use a blank sky observation to correct for the residual iillumination +pattern. This is different than using the sky observation directly as +a flat field calibration in that only the large scale pattern is +needed. Determining the large scale iillumination does not require high +signal-to-noise at each pixel and faint objects in the image can be +either eliminated or ignored. The second method is to remove the large +scale shape from the lamp flat field. This is not as good as using a +blank sky observation but, if there is no such observation and the +iillumination pattern is essentially only in the lamp flat field, this +may be sufficient. + +From the above two paragraphs one sees there are four options. +There is a task in the \fBccdred\fR package for each of these options. +To correct a lamp flat field observation by a blank sky observation, +called a \fIsky flat\fR, the task is \fBmkskyflat\fR. To correct the +flat field for its own large scale gradients, called an \fIiillumination +flat\fR, the task is \fBmkillumflat\fR. To create a secondary +correction to be applied to data processed with the lamp flat field +image the tasks are \fBmkskycor\fR and \fBmkillumcor\fR which are, +respectively, based on a blank sky observation and the lamp flat field +iillumination pattern. + +With this introduction turn to the individual documentation for these +four tasks for further details. +.sh +2. Scan Mode +There are two types of scan modes supported by the \fBccdred\fR +package; \fIshortscan\fR and \fIlongscan\fR (see \fBccdproc\fR for +further details). They both affect the manner in which flat field +calibrations are handled. The shortscan mode produces images which are +the same as direct images except that the light recorded at each pixel +was collected by a number of different pixels. This improves the flat +field calibration. If the flat field images, of the same types +described in the direct imaging section, are observed in the same way +as all other observations, i.e. in scan mode, then there is no +difference from direct imaging (except in the quality of the flat +fields). There is a statistical advantage to observing the lamp or sky +flat field without scanning and then numerically averaging to simulate +the result of the scanning. This improves the accuracy of +the flat fields and might possibly allow direct blank sky observations +to be used for flat fields. The numerical scanning is done in +\fBccdproc\fR by setting the appropriate scanning parameters. + +In longscan mode the CCD detector is read out in such a way that +each output image pixel is the sum of the light falling on all pixels +along the direction of the scan. This reduces the flat field calibration +to one dimension, one response value for each point across the scan. +The one dimensional calibration is obtained from a longscan observation +by averaging all the readout lines. +This is done automatically in \fBccdproc\fR by setting the appropriate +parameters. In this case very good flat fields can be obtained from +one or more blank sky observations or an unscanned lamp observation. Other +corrections are not generally used. +.sh +3. Spectroscopy +Spectroscopic flat fields differ from direct imaging in that the +spectrum of the sky or lamp and transmission variations with wavelength +are part of the observation. Application of such images will introduce +the inverse of the spectrum and transmission into the observation. It +also distorts the observed counts making signal-to-noise estimates +invalid. This, and the low signal in the dispersed light, makes it +difficult to use blank sky observations directly as flat fields. As +with direct imaging, sky observation may be used to correct for +iillumination errors if necessary. At sufficiently high dispersion the +continuous lamp spectrum may be flat enough that the spectral signature +of the lamp is not a problem. Alternatively, flux calibrating the +spectra will also remove the flat field spectral signature. The +spectroscopic flat fields also have to be corrected for regions outside +of the slit or apertures to avoid bad response effects when applying +the flat field calibration to the observations. + +The basic scheme for removing the spectral signature is to average +all the lines or columns across the dispersion and within the aperture +to form an estimate of the spectrum. In addition to the averaging, a +smooth curve is fit to the lamp spectrum to remove noise. This smooth +shape is then divided back into each line or column to eliminate the +shape of the spectrum without changing the shape of the spectrum +in the spatial direction or the small scale response variations. +Regions outside of the apertures are replaced by unity. +This method requires that the dispersion be aligned fairly close to +either the CCD lines or columns. + +This scheme is used in both longslit and multiaperture spectra. +The latter includes echelle, slitlets, aperture masks, and fiber feeds. +For narrow apertures which do not have wider slits for the lamp +exposures there may be problems with flexure and defining a good +composite spectrum. The algorithm for longslit spectra is simpler and +is available in the task \fBresponse\fR in the \fBlongslit\fR package. +For multiaperture data there are problems of defining where the spectra +lie and avoiding regions off of the aperture where there is no signal. +The task which does this is \fBapnormalize\fR in the \fBapextract\fR +package. Note that the lamp observations must first be processed +explicitly for bias and dark count corrections. + +Longslit spectra may also suffer the same types of iillumination +problems found in direct imaging. However, in this case the iillumination +pattern is determined from sky observations (or the flat field itself) +by finding the large scale pattern across the dispersion and at a number +of wavelengths while avoiding the effects of night sky spectrum. The +task which makes this type of correction in the \fBlongslit\fR package +is \fBiillumination\fR. This produces an iillumination correction. +To make sky flats or the other types of corrections image arithmetic +is used. Note also that the sky observations must be explicitly +processed through the flat field stage before computing the iillumination. +.ih +SEE ALSO +.nf +ccdproc, guide, mkillumcor, mkillumflat, mkskycor, mkskyflat +apextract.apnormalize, longslit.response, longslit.iillumination +.fi +.endhelp -- cgit