CALWF3 v1.8

CALWF3 v1.8 was released for internal use at STScI on October 26, 2009, but was never installed in OPUS or released to the public, because it has already been superseded by v1.8.1. There was one update to calwf3 in this version.

Relevant PRs for v1.8:

PR 63788 FLATCORR routines modified to use mean gain for calibration

The UVIS and IR FLATCORR routines, which divide a science image by the flatfield reference image and apply the gain calibration, were both upgraded to solve an issue regarding the gain calibration. The gain calibration is performed by multiplying each image quadrant by the gain value applicable to that quadrant, using the quadrant-dependent gain values stored in the detector parameters reference table (CCDTAB). But because the intrinsic differences in gain between quadrants are also included in the different signal levels recorded in the flatfield images, the science images were essentially over-corrected for the gain variations, resulting in DC offsets between quadrants in calibrated images. The gain calibration routines in the FLATCORR steps were therefore updated to compute the mean gain of all 4 detector quadrants, from the values listed in the CCDTAB, and apply the mean gain to the entire science image, thus eliminating the double correction problem. Note that the mean gain is only used when processing direct images and not for grism images, because grism images are not divided by a flatfield during calwf3 processing and therefore don't suffer from the double correction.

Created 11/05/09 Modifed 12/10/09