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+.help ccdgeometry Sep87 noao.imred.ccdred
+.ih
+NAME
+ccdgeometry - Discussion of CCD geometry and header parameters
+.ih
+DESCRIPTION
+The \fBccdred\fR package maintains and updates certain geometry
+information about the images.  This geometry is described by four image
+header parameters which may be present.  These are defined below by the
+parameter names used in the package.  Note that these names may be
+different in the image header using the image header translation
+feature of the package.
+
+.ls DATASEC
+The section of the image containing the CCD data.  If absent the
+entire image is assumed to be data.  Only the pixels within the
+data section are modified during processing.  Therefore, there may be
+additional calibration or observation information in the image.
+If after processing, the data section is the entire image it is
+not recorded in the image header.
+.le
+.ls CCDSEC
+The section of the CCD to corresponding to the data section.  This
+refers to the physical format, columns and lines, of the detector.  This is
+the coordinate system used during processing to relate calibration
+data to the image data; i.e. image data pixels are calibrated by
+calibration pixels at the same CCD coordinates regardless of image pixel
+coordinates.  This allows recording only parts of the CCD during data
+taking and calibrating with calibration frames covering some or all of
+the CCD.  The CCD section is maintained during trimming operations.
+Note that changing the format of the images by image operators outside
+of the \fBccdred\fR package will invalidate this coordinate system.
+The size of the CCD section must agree with that of the data section.
+If a CCD section is absent then it defaults to the data section such
+that the first pixel of the data section has CCD coordinate (1,1).
+.le
+.ls BIASSEC
+The section of the image containing prescan or overscan bias information.
+It consists of a strip perpendicular to the readout axis.  There may be
+both a prescan and overscan but the package currently only uses one.
+This parameter may be overridden during processing by the parameter
+\fIccdproc.biassec\fR.  Only the part of the bias section along the
+readout is used and the length of the bias region is determined by
+the trim section.  If one wants to limit the region of the bias
+strip used in the fit then the \fIsample\fR parameter should be used.
+.le
+.ls TRIMSEC
+The section of the image extracted during processing when the trim
+operation is selected (\fIccdproc.trim\fR).  If absent when the trim
+operation is selected it defaults to the data section; i.e. the processed
+image consists only of the data section.  This parameter may be overridden
+during processing by the parameter \fIccdproc.trimsec\fR.  After trimming
+this parameter, if present, is removed from the image header.  The
+CCD section, data section, and bias section parameters are also modified
+by trimming.
+.le
+
+The geometry is as follows.  When a CCD image is recorded it consists
+of a data section corresponding to part or all of the CCD detector.
+Regions outside of the data section may contain additional information
+which are not affected except by trimming.  Most commonly this consists
+of prescan and overscan bias data.  When recording only part of the
+full CCD detector the package maintains information about that part and
+correctly applies calibrations for that part of the detector.  Also any
+trimming operation updates the CCD coordinate information.  If the
+images include the data section, bias section, trim section, and ccd
+section the processing may be performed entirely automatically.
+
+The sections are specified using the notation [c1:c2,l1:l2] where c1
+and c2 are the first and last columns and l1 and l2 are the first and
+last lines.  Currently c1 and l1 must be less than c2 and l2
+respectively and no subsampling is allowed.  This may be added later.
+.endhelp