iraf.net

NUMBER:	474
MODULE:	mscred.ccdproc
SYSTEM:	-V4.1: December 5, 2000
DATE:	Thu Dec  7 14:22:10 MST 2000
FROM:	valdes

BUG:	When a flat field is processed in a certain way as noted below,
	applying it to object data with CCDPROC will cause each flat field
	extension to be normalized differently.  This will be evident in
	the log file where the scale factor for the flat field extensions
	will be listed with different values rather than the same value.
	The flat fielded object image will then appear with different gain
	levels of the same relative magnitude as the flat field in each
	extension.  Note that small residual gain differences are normal
	due to dome flats not exactly matching the sky and these are
	removed by using sky flats.

	The incorrect behavior occurs when the flat field is processed
	after combining to a master flat field followed by making a
	new version of the flat field, such as when making a pupil ghost
	corrected flat field.  The error occurs because a time stamp for
	the CCDMEAN scaling keyword, the keyword CCDMEANT, is added by
	CCDPROC and checked against the time the image was last modified.
	After the first processing all the CCDMEAN keywords will correctly have
	the same value.  However, any modified version of the flat field
	will then invalidate the time stamp causing the next execution of
	CCDPROC that applies the flat field to compute new CCDMEAN values
	separately for each extension.  If the CCDMEANT keyword is absent
	then CCDPROC will not recompute the normalization value.  Thus the
	workaround is to delete the CCDMEANT keyword from all extensions
	either before making a new version of the flat field or in the flat
	field version to be applied by CCDPROC.

	In the most common case where individual flat fields are processed
	with scaling followed by combining to a master flat using the task
	FLATCOMBINE the bug does not occur because the scaling and
	combining operation eliminates the CCDMEANT keyword.  Also when the
	flat field is not modified after it has been processed by CCDPROC
	the error will not occur.  So almost all users will not have
	encountered this problem.

STATUS:	The MSCRED release of December 7, 2000 fixes this problem by
	having CCDPROC eliminate the CCDMEANT keyword when the flat
	field is first processed.