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+.help overview Apr98 noao.imred.crutil
+
+.ce
+\fBThe Cosmic Ray Package: CRUTIL\fR
+
+The cosmic ray package provides tools for identifying and removing cosmic
+rays in images.  The tasks are:
+
+.nf
+ cosmicrays - Remove cosmic rays using flux ratio algorithm
+  craverage - Detect CRs against average and avoid objects
+  crcombine - Combine multiple exposures to eliminate cosmic rays
+     credit - Interactively edit cosmic rays using an image display
+      crfix - Fix cosmic rays in images using cosmic ray masks
+     crgrow - Grow cosmic rays in cosmic ray masks
+   crmedian - Detect and replace cosmic rays with median filter
+   crnebula - Detect and replace cosmic rays in nebular data
+.fi
+
+The best way to remove cosmic rays is using multiple exposures of the same
+field.  When this is done the task \fBcrcombine\fR is used to combine the
+exposures into a final single image with cosmic rays removed.  The images
+are scaled (if necessary) to a common data level either by multiplicative
+scaling, an additive background offset, or some combination of both.
+Cosmic rays are then found as pixels which differ by some statistical
+amount away for the average or median of the data.
+
+A median is the simplest way to remove cosmic rays.  This is an option
+with \fBcrcombine\fR.  But this does not make optimal use of the data.
+An average of the pixels remaining after some rejection operation is better.
+If the noise characteristics of the data can be described by a gain and
+read noise then cosmic rays can be optimally rejected using the
+"crreject" algorithm.  This works on two or more images.  There are
+a number of other rejection algorithms which can be used as described in
+the task help.
+
+The rest of the tasks in the package are used when only a single exposure
+is available.  These include interactive editing with \fBcredit\fR.  The
+replacement algorithms in this task may also be used non-interactively if
+you have a list of pixel coordinates as input.  Other tasks automatically
+identifying pixels which are significantly higher than surrounding pixels.
+
+The simplest of these tasks is \fBcrmedian\fR.  This replaces
+cosmic rays with a median value and produces a cosmic ray
+mask which is a simple type of integer image where good pixels have a value
+of zero and bad pixels have a non-zero value.  The tasks \fBcrgrow\fR and
+\fBcrfix\fR are provided to use this type of cosmic ray mask.  The former
+will flag additional pixels within some radius of the flagged pixels in the
+mask.  The latter is the basic tool for replacing the identified pixels in
+the data by neighboring data.  It uses linear interpolation along lines or
+columns.  The median task is simple but it often will flag the cores of
+stars or other small but real features.
+
+The task \fBcraverage\fR is similar to \fBcrmedian\fR in that it compares
+the pixel values against a smoothed version.  Instead of a median it uses
+an average with the central pixel excluded.  It is more sophisticated
+in that it also compares the average against a larger median to see if
+the region corresponds to an object.  Thus it can detect objects and
+the task could be used as a simple object detection task in its own right.
+Because the hardest part of cosmic ray detection from a single image is
+avoiding truncation of the cores of stars this task does not allow cosmic
+rays to be detected where it thinks there is an object.  This task is
+also more versatile in allow separate mask values and works on a list
+of images.
+
+Somewhat more sophisticated algorithms are available in the tasks
+\fBcosmicrays\fR and \fBcrnebula\fR.  These attempt to determine if a
+deviant pixel is the core of a star or part of a linear nebular feature
+respectively.
+
+The best use of these tasks is to experiment and iterate.  In particular,
+one may want to iterate a task several times and use both \fBcosmicrays\fR
+and \fBcraverage\fR.
+
+Good hunting!
+.endhelp