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+.RP
+.TL
+User's Guide to the CCDRED Package
+.AU
+Francisco Valdes
+.AI
+IRAF Group - Central Computer Services
+.K2
+P.O. Box 26732, Tucson, Arizona 85726
+June 1987
+Revised February 1988
+.AB
+The IRAF CCD reduction package, \fBccdred\fR, provides tools
+for the easy and efficient reduction of CCD images.  The standard
+reduction operations are replacement of bad pixels, subtraction of an
+overscan or prescan bias, subtraction of a zero level image,
+subtraction of a dark count image, division by a flat field calibration
+image, division by an illumination correction, subtraction of a fringe
+image, and trimming unwanted lines or columns.  Another common
+operation provided by the package is scaling and combining images with
+a number of algorithms for rejecting cosmic rays.  Data in the image
+header is used to make the reductions largely automated and
+self-documenting though the package may still be used in the absence of
+this data.  Also a translation mechanism is used to relate image header
+parameters to those used by the package to allow data from a variety of
+observatories and instruments to be processed.  This guide provides a brief
+description of the IRAF CCD reduction package and examples of reducing
+simple CCD data.
+.AE
+.NH
+Introduction
+.LP
+     This guide provides a brief description of the IRAF CCD reduction
+package \fBccdred\fR and examples of reducing simple CCD data.  It is a
+generic guide in that it is not tied to any particular type of data.
+There may be more specific guides (or "cookbooks") for your data.
+Detailed descriptions of the tasks and features of the package are
+provided in the help documentation for the package.
+
+     The purpose of the CCDRED package is to provide tools for the easy
+and efficient reduction of CCD images.  The standard reduction
+operations are replacement of bad columns and lines by interpolation
+from neighboring columns and lines, subtraction of a bias level
+determined from overscan or prescan columns or lines, subtraction of a
+zero level using a zero length exposure calibration image, subtraction
+of a dark count calibration image appropriately scaled to the dark time
+exposure, division by a scaled flat field calibration image, division
+by an illumination image (derived from a blank sky image), subtraction
+of a scaled fringe image (also derived from a blank sky image), and
+trimming the image of unwanted lines or columns such as the overscan
+strip.  Any set of operations may be done simultaneously over a list of
+images in a highly efficient manner.  The reduction operations are
+recorded in the image header and may also be logged on the terminal and
+in a log file.
+
+     The package also provides tools for combining multiple exposures
+of object and calibration images to improve the statistical accuracy of
+the observations and to remove transient bad pixels.  The combining
+operation scales images of different exposure times, adjusts for
+variable sky background, statistically weights the images by their
+signal-to-noise, and provides a number of useful algorithms for
+detecting and rejecting transient bad pixels.
+
+     Other tasks are provided for listing reduction information about
+the images, deriving secondary calibration images (such as sky
+corrected flat fields or illumination correction images), and easily
+setting the package parameters for different instruments.
+
+     There are several important features provided by the package to
+make the reduction of CCD images convenient; particularly to minimize
+record keeping.  One of these is the ability to recognize the different
+types of CCD images.  This ability allows the user to select a certain
+class of images to be processed or listed and allows the processing
+tasks to identify calibration images and process them differently from
+object images.  The standard CCD image types are \fIobject\fR,
+\fIzero\fR level, \fIdark\fR count, and \fIflat\fR field.  For more on
+the image types see \fBccdtypes\fR.
+
+     The tasks can also identify the different filters (or other subset
+parameter) which require different flat field images.  This means you don't
+have to separate the images by filter and process each set separately.
+This feature is discussed further in \fBsubsets\fR.
+
+     The tasks keep track of the reduction steps completed on each
+image and ignore images which have been processed.  This feature,
+along with recognizing the image types and subsets, makes it possible to
+specify all the images to a task with a wildcard template, such as
+"*.imh", rather than indicating each image by name.  You will find this
+extremely important with large sets of observations.
+
+     A fundamental aspect of the package is that the processing
+modifies the images.  In other words, the reduction operations are
+performed directly on the image.  This "feature" further simplifies
+record keeping, frees the user from having to form unique output image
+names, and minimizes the amount of disk space required.  There
+are two safety features in this process.  First, the modifications do
+not take effect until the operation is completed on the image.  This
+allows you to abort the task without messing up the image data and
+protects data if the computer crashes.  The second feature is that
+there is a package parameter which may be set to make a backup of the
+input data with a particular prefix such as "orig" or "imdir$".  This
+backup feature may be used when there is sufficient disk space, when learning
+to use the package, or just to be cautious.
+
+     In a similar effort to efficiently manage disk space, when combining
+images into a master object or calibration image there is an option to
+delete the input images upon completion of the combining operation.
+Generally this is desirable when there are many calibration exposures,
+such as zero level or flat field images, which are not used after they
+are combined into a final calibration image.
+
+     The following sections guide you through the basic use of the
+\fBccdred\fR package.  Only the important parameters which you might
+want to change are described.  It is assumed that the support personnel
+have created the necessary instrument files (see \fBinstruments\fR)
+which will set the default parameters for the data you will be
+reducing.  If this is not the case you may need to delve more deeply
+into the details of the tasks.  Information about all the parameters
+and how the various tasks operate are given in the help documentation
+for the tasks and in additional special help topics.  Some useful help
+documentation is indicated in the discussion and also in the
+\fBReferences\fR section.
+.NH
+Getting Started
+.LP
+     The first step is to load \fBccdred\fR.  This is done by loading
+the \fBnoao\fR package, followed by the image reduction package
+\fBimred\fR, and finally the \fBccdred\fR package.  Loading a
+package consists of typing its name.  Note that some of these packages may be
+loaded automatically when you logon to IRAF.
+
+     When you load the \fBccdred\fR package the menu of tasks or commands
+is listed.  This appears as follows:
+
+.nf
+.KS
+.ft L
+    cl> ccdred
+      badpiximage       ccdtest           mkfringecor       setinstrument
+      ccdgroups         combine           mkillumcor        zerocombine
+      ccdhedit          cosmicrays        mkillumflat       
+      ccdlist           darkcombine       mkskycor          
+      ccdproc           flatcombine       mkskyflat         
+.ft R
+.KE
+.fi
+
+A summary of the tasks and additional help topics is obtained by typing:
+
+.ft L
+    cl> help
+.ft R
+
+This list and how to get additional help on specific topics is described
+in the \fBReferences\fR section at the end of this guide.
+
+     The first command to use is \fBsetinstrument\fR, which sets the package
+appropriately for the CCD images to be reduced.  The support personnel
+should tell you the instrument identification, but if not a list
+of known instruments may be listed by using '?' for the instrument name.
+
+.nf
+.ft L
+    cl> setinstrument
+    Instrument ID (type ? for a list) \fI<enter instrument id or ?>
+        <Set ccdred package parameters using eparam>
+        <Set ccdproc task parameters using eparam>
+.ft R
+.fi
+
+This task sets the default parameters and then allows you to modify the
+package parameters and the processing parameters using the parameter
+editor \fBeparam\fR.  If you are not familiar with \fBeparam\fR see the
+help or CL introduction documentation.  For most terminals you move up
+and down through the parameters with the terminal arrow keys, you
+change the parameters by simply typing the desired value, and you exit
+with control Z or control D.  Note that you can change parameters for
+any task at any time with \fBeparam\fR and you do not have to run
+\fBsetinstrument\fR again, even if you logout, until you need to reduce
+data from a different instrument.
+
+     The \fBccdred\fR package parameters control general I/O functions of
+the tasks in the package.  The parameters you might wish to change are
+the output pixel type and the verbose option.  Except when the input
+images are short integers, the noise is significantly greater than one
+digital unit, and disk space is critical, it is probably better to
+allow the processing to convert the images to real pixel datatype.  The
+verbose parameter simply prints the information written to the log file
+on the terminal.  This can be useful when little else is being done and
+you are just beginning.  However, when doing background processing and
+other IRAF reduction tasks it is enough to simply look at the end of
+the logfile with the task \fBtail\fR to see the current state of the
+processing.
+
+     The \fBccdproc\fR parameters control the CCD processing.  There are
+many parameters but they all may be conveniently set at this point.
+Many of the parameters have default values set appropriately for the
+instrument you specified.  The images to be processed can be specified
+later.  What needs to be set are the processing operations that you
+want done and the parameters required for each operation.  The
+processing operations are selected by entering yes or no for each one.
+The following items briefly describe each of the possible processing
+operations and the additional parameters required.
+
+.LP
+\fIfixpix\fR - Fix bad CCD lines and columns?
+.IP
+The bad pixels (cosmetic defects) in the detector are given in a file
+specified by the parameter \fIfixfile\fR.  This information is used
+to replace the pixels by interpolating from the neighboring pixels.
+A standard file for your instrument may be set by \fBsetinstrument\fR
+or if the word "image" is given then the file is defined in the instrument
+data file.  For more on the bad pixel file see \fBinstruments\fR.
+.LP
+\fIoverscan\fR - Apply overscan strip correction?
+.IP
+The overscan or prescan region is specified by the parameter
+\fIbiassec\fR.  This is given as an IRAF image section.  The overscan
+region is averaged along the readout axis, specified by the parameter
+\fIreadaxis\fR, to create a one dimensional bias vector.  This bias is
+fit by a function to remove cosmic rays and noise.  There are a number
+of parameters at the end of the parameter list which control the
+fitting.  The default overscan bias section and fitting parameters for
+your instrument should be set by \fBsetinstrument\fR.  If the word
+"image" is given the overscan bias section is defined in the image
+header or the instrument translation file.  If an overscan section is
+not set you can use \fBimplot\fR to determine the columns or rows for
+the bias region and define an overscan image section.  If you are
+unsure about image sections consult with someone or read the
+introductory IRAF documentation.
+.LP
+\fItrim\fR - Trim the image?
+.IP
+The image is trimmed to the image section given by the parameter
+\fItrimsec\fR.  A default trim section for your instrument should be
+set by \fBsetinstrument\fR, however, you may override this default if
+desired.  If the word "image" is given the data
+image section is given in the image header or the instrument
+translation file.  As with the overscan image section it is
+straightforward to specify, but if you are unsure consult someone.
+.LP
+\fIzerocor\fR - Apply zero level correction?
+.IP
+The zero level image to be subtracted is specified by the parameter
+\fIzero\fR.  If none is given then the calibration image will be sought
+in the list of images to be processed.
+.LP
+\fIdarkcor\fR - Apply dark count correction?
+.IP
+The dark count image to be subtracted is specified by the parameter
+\fIdark\fR.  If none is given then the calibration image will be sought
+in the list of images to be processed.
+.LP
+\fIflatcor\fR - Apply flat field correction?
+.IP
+The flat field images to be used are specified by the parameter
+\fIflat\fR.  There must be one flat field image for each filter
+or subset (see \fBsubsets\fR) to be processed.  If a flat field
+image is not given then the calibration image will be sought
+in the list of images to be processed.
+.LP
+\fIreadcor\fR - Convert zero level image to readout correction?
+.IP
+If a one dimensional zero level readout correction vector is to be subtracted
+instead of a two dimensional zero level image then, when this parameter is set,
+the zero level images will be averaged to one dimension.  The readout axis
+must be specified by the parameter \fIreadaxis\fR.  The default for your
+instrument is set by \fBsetinstrument\fR.
+.LP
+\fIscancor\fR - Convert flat field image to scan correction?
+.IP
+If the instrument is operated in a scan mode then a correction to the
+flat field may be required.  There are two types of scan modes, "shortscan"
+and "longscan".  In longscan mode flat field images will be averaged
+to one dimension and the readout axis must be specified.  Shortscan mode
+is a little more complicated.  The scan correction is used if the flat
+field images are not observed in scan mode.  The number of scan lines
+must be specified by the parameter \fInscan\fR.  If they are observed in
+scan mode, like the object observations, then the scan correction
+operations should \fInot\fR be specified.  For details of scan mode operations
+see \fBccdproc\fR.  The scan parameters
+should be set by \fBsetinstrument\fR.  If in doubt consult someone
+familiar with the instrument and mode of operation.
+.LP
+
+     This description of the parameters is longer than the actual operation of
+setting the parameters.  The only parameters likely to change during processing
+are the calibration image parameters.
+
+     When processing many images using the same calibration files a modest
+performance improvement can be achieved by keeping (caching) the
+calibration images in memory to avoid disk accesses.  This option
+is available by specifying the amount of memory available for image
+caching with the parameter \fImax_cache\fR.  If the value is zero then
+the images are accessed from disk as needed while if there is
+sufficient memory the calibration images may be kept in memory during
+the task execution.
+.NH
+Processing Your Data
+.LP
+     The processing path depends on the type of data, the type of
+instrument, types of calibration images, and the observing
+sequence.  In this section we describe two types of operations common
+in reducing most data; combining calibration images and performing the
+standard calibration and correction operations.  Some additional special
+operations are described in the following section.
+
+     However, the first thing you might want to try before any
+processing is to get a listing of the CCD images showing the CCD image
+types, subsets, and processing flags.  The task for this is
+\fBccdlist\fR.  It has three types of of output; a short one line per
+image format, a longer format which shows the state of the processing,
+and a format which prints the image names only (used to create files
+containing lists of images of a particular CCD image type).  To get a
+quick listing type:
+
+.nf
+.ft L
+    cl> ccdlist *.imh
+    ccd001.imh[544,512][short][unknown][V]:FOCUS L98-193
+    ccd007.imh[544,512][short][object][V]:N2968 V 600s
+    ccd015.imh[544,512][short][object][B]:N3098 B 500s
+    ccd024.imh[544,512][short][object][R]:N4036 R 600s
+    ccd045.imh[544,512][short][flat][V]:dflat 5s
+    ccd066.imh[544,512][short][flat][B]:dflat 5s
+    ccd103.imh[544,512][short][flat][R]:dflat 5s
+    ccd104.imh[544,512][short][zero][]:bias
+    ccd105.imh[544,512][short][dark][]:dark 3600s
+.ft R
+.fi
+
+     The example shows only a sample of the images.  The short format
+listing tells you the name of the image, its size and pixel type, the
+CCD image type as seen by the package, the subset identifier (in this
+case the filter), and the title.  If the data had been processed then
+there would also be processing flags.  If the CCD image types do not
+seem right then there may be a problem with the instrument
+specification.
+
+     Many of the tasks in the \fBccdred\fR package have the parameter
+\fIccdtype\fR which selects a particular type of image.  To list
+only the object images from the previous example:
+
+.nf
+.ft L
+    cl> ccdlist *.imh ccdtype=object
+    ccd007.imh[544,512][short][object][V]:N2968 V 600s
+    ccd015.imh[544,512][short][object][B]:N3098 B 500s
+    ccd024.imh[544,512][short][object][R]:N4036 R 600s
+.ft R
+.fi
+
+If no CCD image type is specified (by using the null string "")
+then all image types are selected.  This may be
+necessary if your instrument data does not contain image type identifications.
+.NH 2
+Combining Calibration Images
+.LP
+     If you do not need to combine calibration images because you only
+have one image of each type, you can skip this section.  Calibration
+images, particularly zero level and flat field images, are combined in
+order to minimize the effects of noise and reject bad pixels in the
+calibrations.  The basic tool for combining images is the task
+\fBcombine\fR.  There are simple variants of this task whose default
+parameters are set appropriately for each type of calibration image.
+These are the ones you will use for calibration images leaving
+\fBcombine\fR for combining object images.  Zero level images are
+combined with \fBzerocombine\fR, dark count images with
+\fBdarkcombine\fR, and flat field images with \fBflatcombine\fR.
+
+     For example, to combine flat field images the command is:
+
+.nf
+.ft L
+    cl> flatcombine *.imh
+    Jun  1 14:26 combine: maxreject
+            Images      N    Exp   Mode  Scale Offset Weight
+        ccd045.imh      1    5.0  INDEF  1.000     0.  0.048
+        ccd046.imh      1    5.0  INDEF  1.000     0.  0.048
+        	\fI<... list of files ...>\fL
+        ccd065.imh      1    5.0  INDEF  1.000     0.  0.048
+        ----------- ------ ------
+         FlatV.imh     21    5.0
+.ft R
+.fi
+
+This output is printed when verbose mode is set.  The same information
+is recorded in the log file.  In this case the flat fields are combined
+by rejecting the maximum value at each point in the image (the
+"maxreject" algorithm).  The images are scaled by the exposure times,
+which are all the same in this example.  The mode is not evaluated for
+exposure scaling and the relative weights are the same because the
+exposure times are the same.  The example only shows part of the
+output; \fBflatcombine\fR automatically groups the flat field images by
+filter to produce the calibration images "FlatV", "FlatB", and
+"FlatR".
+.NH 2
+Calibrations and Corrections
+.LP
+     Processing the CCD data is easy and largely automated.
+First, set the task parameters with the following command:
+
+.ft L
+    cl> eparam ccdproc
+.ft R
+
+You may have already set the parameters when you ran
+\fBsetinstrument\fR, though the calibration image parameters
+\fIzero\fR, \fIdark\fR, and \fIflat\fR may still need to be set or
+changed.  Once this is done simply give the command
+
+.nf
+.ft L
+    cl> ccdproc *.imh
+    ccd003: Jun  1 15:13 Overscan section is [520:540,*] with mean=485.0
+    ccd003: Jun  1 15:14 Trim data section is [3:510,3:510]
+    ccd003: Jun  1 15:14 Overscan section is [520:540,*] with mean=485.0
+    FlatV:  Jun  1 15:14 Trim data section is [3:510,3:510]
+    FlatV:  Jun  1 15:15 Overscan section is [520:540,*] with mean=486.4
+    ccd003: Jun  1 15:15 Flat field image is FlatV.imh with scale=138.2
+    ccd004: Jun  1 15:16 Trim data section is [3:510,3:510]
+    ccd004: Jun  1 15:16 Overscan section is [520:540,*] with mean=485.2
+    ccd004: Jun  1 15:16 Flat field image is FlatV.imh with scale=138.2
+                \fI<... more ...>\fL
+    ccd013: Jun  1 15:22 Trim data section is [3:510,3:510]
+    ccd013: Jun  1 15:23 Overscan section is [520:540,*] with mean=482.4
+    FlatB:  Jun  1 15:23 Trim data section is [3:510,3:510]
+    FlatB:  Jun  1 15:23 Overscan section is [520:540,*] with mean=486.4
+    ccd013: Jun  1 15:24 Flat field image is FlatB.imh with scale=132.3
+                \fI<... more ...>\fL
+.ft R
+.fi
+
+     The output shown is with verbose mode set.  It is the same as
+recorded in the log file.  It illustrates the principle of automatic
+calibration image processing.  The first object image, "ccd003", was
+being processed when the flat field image was required.  Since the
+image was taken with the V filter the appropriate flat field was
+determined to be "FlatV".  Since it had not been processed, the
+processing of "ccd003" was interrupted to process "FlatV".  The
+processed calibration image may have been cached if there was enough
+memory.  Once "FlatV" was processed (note that the flat field was not
+flattened because the task knows this image is a flat field) the
+processing of "ccd003" was completed.  The next image, "ccd004", is
+also a V filter image so the already processed, and possibly cached,
+flat field "FlatV" is used again.  The first B band image is "ccd013"
+and, as before, the B filter flat field calibration image is processed
+automatically.  The same automatic calibration processing and image
+caching occurs when using zero level and dark count calibration
+images.
+
+     Commonly the processing is done with the verbose mode turned off
+and the task run as a background job.  This is done with the commands
+
+.nf
+.ft L
+    cl> ccdred.verbose=no
+    cl> ccdproc *.imh &
+.ft R
+.fi
+
+The already processed images in the input list are recognized as having been
+processed and are not affected.  To check the status of the processing we
+can look at the end of the log file with:
+
+.ft L
+    cl> tail logfile
+.ft R
+
+After processing we can repeat the \fBccdlist\fR command to find:
+
+.nf
+.ft L
+    cl> ccdlist *.imh ccdtype=object
+    ccd007.imh[508,508][real][object][V][OTF]:N2968 V 600s
+    ccd015.imh[508,508][real][object][B][OTF]:N3098 B 500s
+    ccd024.imh[544,512][short][object][R][OTF]:N4036 R 600s
+.ft R
+.fi
+
+The processing flags indicate the images have been overscan corrected,
+trimmed, and flat fielded.
+
+     As you can see, processing images is very easy.  There is one source
+of minor confusion for beginning users and that is dealing with calibration
+images.  First, there is no reason that calibration images
+may not be processed explicitly with \fBccdproc\fR, just remember to set
+the \fIccdtype\fR to the calibration image type or to "".  When processing
+object images the calibration images to be used may be specified either
+with the task parameter for the particular calibration image or by
+including the calibration image in the list of input images.  Calibration
+images specified by parameter value take precedence and the task
+does not check its CCD image type.  Calibration images given in the
+input list must have a valid CCD image type.  In case too many
+calibration images are specified, say because the calibration images
+combined to make the master calibration images were not deleted and
+so are part of the image list "*.imh", only the first one will be used.
+Another point to know is that flat field, illumination, and fringe images
+are subset (filter) dependent and so a calibration image for each filter
+must be specified.
+.NH
+Special Processing Operations
+.LP
+     The special processing operations are mostly concerned with the
+flat field response correction.  There are also special processing
+operations available in \fBccdproc\fR for one dimensional readout
+corrections in the zero level and flat field calibrations.  These
+were described briefly above and in more detail in \fBccdproc\fR
+and are not discussed further in this guide.  The processing
+operations described in this section are for preparing flat fields
+for two dimensional spectroscopic data, for correcting flat fields
+for illuminations effects, for making a separate illumination correction,
+and for applying corrections for fringe effects.  For additional
+discussion about flat fields and illumination corrections see the
+help topic \fBflatfields\fR.
+.NH 2
+Spectroscopic Flat Fields
+.LP
+     For spectroscopic data the flat fields may have to be processed to
+remove the general shape of the lamp spectrum and to replace regions outside
+of the aperture where there is no flat field information with values that
+will not cause bad response effects when the flat field is applied to the
+data.  If the shape of the lamp spectrum is not important and if the
+longslit spectra have the regions outside of the slit either off the
+detector or trimmed then you may use the flat field without special
+processing.
+
+   First you must process the flat field images explicitly with
+
+.ft L
+    cl> ccdproc *.imh ccdtype=flat
+.ft R
+
+where "*.imh" may be replaced with any list containing the flat fields.
+If zero level and dark count corrections are required these calibration
+images must be available at this time.
+
+     Load the \fBtwodspec\fR package and then either the \fBlongslit\fR
+package, for longslit data, or the \fBapextract\fR package, for
+multiaperture data such as echelles, multifiber, or aperture mask
+spectra.  The task for removing the longslit quartz spectrum is
+\fBresponse\fR.  There is also a task for removing illumination
+effects, including the slit profile, from longslit spectra called
+\fBillumination\fR.  For more about processing longslit spectra see the
+help for these tasks and the paper \fIReduction of Longslit Spectra
+with IRAF\fR.  The cookbook \fIReduction of Longslit Spectroscopic
+Data Using IRAF (KPNO ICCD and Cryogenic Camera Data)\fR also provides
+a very good discussion even if your data is from a different instrument.
+
+     For multiaperture data the task for removing the relative shapes of
+the spectra is called \fBapnormalize\fR.  Again, consult the help documentation
+for this task for further details.  Since you will probably also be
+using the package for extracting the spectra you may be interested
+in the document \fIThe IRAF APEXTRACT Package\fR.
+.NH 2
+Illumination Corrections
+.LP
+     The flat field calibration images may not have the same illumination
+pattern as the observations of the sky due to the way the lamp illuminates the
+optical system.  In this case when the flat field correction is applied
+to the data there will be gradients in the sky background.  To remove
+these gradients a blank sky calibration image is heavily smoothed
+to produce an illumination image.  The illumination image
+is then divided into the images during processing to correct for the
+illumination difference between the flat field and the objects.
+Like the flat fields, the illumination corrections images may be subset
+dependent so there should be an illumination image for each subset.
+
+The task which makes illumination correction images is \fBmkskycor\fR.
+Some examples are
+
+.nf
+.ft L
+    cl> mkskycor sky004 Illum004
+    cl> mkskycor sky*.imh ""
+.ft R
+.fi
+
+In the first example the sky image "sky004" is used to make the illumination
+correction image "Illum004".  In the second example the sky images are
+converted to illumination correction images by specifying no output image
+names.  Like \fBccdproc\fR if the input images have not been processed they
+are first processed automatically.
+
+To apply the illumination correction
+
+.nf
+.ft L
+    cl> ccdproc *.imh ccdtype=object illumcor+ illum=Illum004
+    cl> ccdproc *.imh ccdtype=object illumcor+ illum=sky*.imh
+.ft R
+.fi
+
+The illumination images could also be set using \fBeparam\fR or given
+on the command line.
+.NH 2
+Sky Flat Fields
+.LP
+    You will notice that when you process images with an illumination
+correction you are dividing each image by a flat field calibration and
+an illumination correction.  If the illumination corrections are not
+done as a later step but at the same time as the rest of the processing
+one will get the same calibration by multiplying the flat field by
+the illumination correction and using this product alone as the
+flat field.  Such an image is called a \fIsky flat\fR since it is
+a flat field which has been corrected to yield a flat sky when applied
+to the observations.  This approach has the advantage of one less
+calibration image and two less computations (scaling and dividing the
+illumination correction).  As an added short cut, rather than compute
+the illumination image with \fBmkskycor\fR and then multiplying, the
+task \fBmkskyflat\fR does all this in one step.  Thus, \fBmkskyflat\fR
+takes an input blank sky image, processes it if needed, determines the
+appropriate flat field (sky flats are also subset dependent) from the
+\fBccdproc\fR parameters or the input image list, and produces an
+output sky flat.  Further if no output image is specified the task
+converts the input blank sky calibration image into a sky flat.
+
+     Two examples in which a new image is created and in which the
+input images are converted to sky flats are
+
+.nf
+.ft L
+    cl> mkskyflat sky004 Skyflat
+    cl> mkskyflat sky*.imh ""
+.ft R
+.fi
+.NH 2
+Illumination Corrected Flat Fields
+.LP
+     A third method to account for illumination problems in the flat fields
+is to remove the large scale pattern from the flat field itself.  This is
+useful if there are no reasonable blank sky calibration images and the
+astronomical exposures are evenly illuminated but the flat fields are not.
+This is done by smoothing the flat field images instead of blank sky
+images.  As with using the sky images there are two methods, creating
+an illumination correction to be applied as a separate step or fixing
+the original flat field.  The smoothing algorithm is
+the same as that used in the other tasks.  The tasks to make these types
+of corrections are \fBmkillumcor\fR and \fBmkillumflat\fR.  The usage
+is pretty much the same as the other illumination correction tasks
+except that it is more reasonable to replace the original flat fields
+by the corrected flat fields when fixing the flat field.  Examples
+of an illumination correction and removing the illumination pattern
+from the flat field are
+
+.nf
+.ft L
+    cl> mkillumcor flat025 Illum025
+    cl> mkillumflat flat*.imh ""
+.ft R
+.fi
+
+As with the other tasks, the input images are processed if necessary.
+.NH 2
+Fringe Corrections
+.LP
+    Some CCD detectors suffer from fringing effects due to the night
+sky emission lines which are not removed by the other calibration
+and correction operations.  To correct for the fringing you need a
+really blank sky image.  There is not yet a task to remove objects from
+sky images because this is often done with an interactive image display
+tool (which will soon be added).  The blank sky image is heavily smoothed
+to determine the mean sky background and then this is subtracted from the
+original image.  The image should then be essentially zero except for the
+fringe pattern.  This fringe correction image is scaled to the same
+exposure time as the image to be corrected and then subtracted to remove
+the fringing.  Note that since the night sky lines are variable there
+may need to be an additional scaling applied.  Determining this scaling
+requires either an interactive display tool or a very clever task.
+Such tasks will also be added in the future.
+
+     The task to make a fringe correction image is \fBmkfringecor\fR.
+the sky background is determined in exactly the same way as the illumination
+pattern, in fact the same sky image may be used for both the sky
+illumination and for the fringe correction.  The task works consistently
+with the "mk" tasks in that the input images are processed first if needed
+and then the output correction image is produced with the specified name
+or replaces the input image if no output image is specified.
+As examples,
+
+.nf
+.ft L
+    cl> mkfringecor sky004 Fringe
+    cl> mkfringecor sky*.imh ""
+.ft R
+.fi
+.NH
+Demonstration
+.LP
+     A simple demonstration task is available.  To run this demonstration
+load the \fBccdtest\fR package; this is a subpackage of the main
+\fBccdred\fR package.  Then simply type
+
+.ft L
+	cl> demo
+.ft R
+
+The demonstration will then create some artificial CCD data and reduce
+them giving descriptive comments as it goes along.  This demonstration uses
+the "playback" facility of the command language and is actually substituting
+it's own commands for terminal input.  Initially you must type carriage return
+or space after each comment ending with "...".  If you wish to have the
+demonstration run completely automatically at it's own speed then type 'g'
+a the "..." prompt.  Thereafter, it will simple pause long enough to give
+you a chance to read the comments.  When the demo is finished you will
+need to remove the files created.  However, feel free to examine the reduced
+images, the log file, etc.  \fINote that the demonstration changes the
+setup parameters so be sure to run \fBsetinstrument\fI again and check
+the setup parameters.\fR
+.NH
+Summary
+.LP
+     The \fBccdred\fR package is very easy to use.  First load the package;
+it is in the \fBimred\fR package which is in the \fBnoao\fR package.
+If this is your first time reducing data from a particular instrument
+or if you have changed instruments then run \fBsetinstrument\fR.
+Set the processing parameters for the operations you want performed.
+If you need to combine calibration images to form a master calibration
+image use one of the combine tasks.  Spectroscopic flat fields may
+need to be processed first in order to remove the lamp spectrum.
+Finally, just type
+
+.ft L
+    cl> ccdproc *.imh&
+.ft R
+.SH
+References
+.LP
+     A general guide to using IRAF is \fIA User's Introduction to the IRAF
+Command Language\fR.  This document may be found in the IRAF documentation
+sets and is available from the National Optical Astronomy Observatories,
+Central Computer Services (NOAO-CCS).
+
+     A more detailed description of the \fBccdred\fR package including
+a discussion of the design and some of the algorithms see \fIThe IRAF
+CCD Reduction Package -- CCDRED\fR" by F. Valdes.  This paper is available
+from NOAO-CCS and appears in the proceedings of the Santa Cruz Summer
+Workshop in Astronomy and Astrophysics, \fIInstrumentation for Ground-Based
+Optical Astronomy: Present and Future\fR, edited by Lloyd B. Robinson and
+published by Springer-Verlag.
+
+     The task descriptions and supplementary documentation are available
+in printed form in the IRAF documentation sets, a special set
+containing documentation for just the \fBccdred\fR package, and on-line
+through the help task by typing
+
+.ft L
+    cl> help \fItopic\fR
+.ft R
+
+where \fItopic\fR is one of the following.
+
+.nf
+.ft L
+  badpiximage - Create a bad pixel mask image from a bad pixel file
+    ccdgroups - Group CCD images into image lists
+     ccdhedit - CCD image header editor
+      ccdlist - List CCD processing information
+      ccdproc - Process CCD images
+      ccdtest - CCD test and demonstration package
+      combine - Combine CCD images
+   cosmicrays - Detect and replace cosmic rays
+  darkcombine - Combine and process dark count images
+  flatcombine - Combine and process flat field images
+  mkfringecor - Make fringe correction images from sky images
+   mkillumcor - Make flat field illumination correction images
+  mkillumflat - Make illumination corrected flat fields
+     mkskycor - Make sky illumination correction images
+    mkskyflat - Make sky corrected flat field images
+setinstrument - Set instrument parameters
+  zerocombine - Combine and process zero level images
+
+          ADDITIONAL HELP TOPICS
+
+       ccdred - CCD image reduction package
+     ccdtypes - Description of the CCD image types
+   flatfields - Discussion of CCD flat field calibrations
+        guide - Introductory guide to using the CCDRED package
+  instruments - Instrument specific data files
+      subsets - Description of CCD subsets
+.ft R
+.fi
+
+Printed copies of the on-line help documentation may be made with the
+command
+
+.ft L
+    cl> help \fItopic\fL | lprint
+.ft R
+
+     In addition to the package documentation for \fBccdred\fR,
+\fBlongslit\fR, and \fBapextract\fR there may be specific guides for
+certain instruments.  These specific guides, called "cookbooks", give
+specific examples and parameter values for the CCD data.