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CALWF3 v2.0

CALWF3 v2.0 Release Notes

CALWF3 v2.0 was released for internal use at STScI on March 8, 2010 and was installed in the OPUS pipeline on March 11. There were several important changes and bug fixes in this version. GO's with IR images, in particular, should submit archive retrieval requests to have their data sets recalibrated.

Relevant PRs for v2.0

PR 63711 IR processing fixed to compute sampzero consistently

The SAMPZERO header keyword value contains the effective exposure time of the "zeroth" read of an IR exposure (the time elapsed between the detector reset and the zeroth read). The header value is only accurate for full-frame readouts and is therefore recomputed by CALWF3 when processing a sub-array exposure. This value is used in the ZSIGCORR and UNITCORR processing steps when performing calculations involving signal in the zeroth read. The value for sub-array modes is now calculated correctly and consistently, regardless of the combination of processing steps being performed in CALWF3.

PR 64259 FLATCORR updated to use mean gain for grism exposures

The UVIS and IR FLATCORR step in CALWF3 applies gain conversion as well as flat fielding. Flat field images naturally contain signal offsets between amplifier quadrants due to slightly different gains and therefore application of the flat field removes the relative differences in gain between images quadrants. The actual gain conversion is then done by multiplying the entire image by the mean gain of all quadrants. Because grism images do not get a real flat field image applied in calwf3 (flat fielding is done during spectral extraction) amplifier-dependent gain values were applied to each image quadrant. This scheme has been changed to be consistent with direct image processing, where the mean gain is applied to the entire image and the quadrant-dependent offsets are included in new "unity" flat field reference images that are applied to grism exposures. This removes the residual offsets (1-3%) between image quadrants that had been occuring in the calibrated products for grism exposures.

PR 64260 WF3REJ updated to not require input spt files

When the WF3REJ routine in CALWF3 is used to combine sets of images (e.g. CR-SPLIT's) it attempts to build a combined "spt" file to go along with the combined "crj" science product. If the "spt" files associated with each input science image are not available, it now only issues a warning and continues processing without building the combined "spt" product.

PR 64261 CALWF3 no longer produces dummy drz files

Since MultiDrizzle was enabled for WFC3 processing in the OTFR pipeline on Feb. 4, 2010, there's no longer a need for CALWF3 to produce empty "drz" products as a place holder. All pipeline "drz" products are now real images created by MultiDrizzle.

PR 64262 CALWF3 IR ZSIGCORR enhanced

The IR ZSIGCORR calibration step, which is used to estimate the amount of signal contained in each pixel in the initial (or "zeroth") read of an IR exposure, is also used to detect and flag pixels that have such a high signal rate that they already go into saturation in either the zeroth or first read. In its previous form, ZSIGCORR was not always detecting and flagging such pixels, which then resulted in incorrect values for those pixels in the output "flt" image. The routine has been reworked to correct this problem, such that pixels that saturate in the zeroth read are now properly flagged and zeroed out in the "flt" product.

PR 64504 DRIZCORR turned on for grism exposures

When MultiDrizzle was enabled in the OTFR pipeline for WFC3 images on Feb. 4, 2010, it was applied to all external exposures except for those using the grisms, because distortion-corrected products are generally not needed for grism exposures. In certain situations, however, this can lead to inconsistencies in the World Coordinate System (WCS) information contained in the "flt" image product headers for the direct images that usually accompanying grism images, because the WCS information is updated by MultiDrizzle. MultiDrizzle processing is now applied to all external images, even though the grism "drz" products are usually not of use for analysis, so that the WCS information in the "flt" headers for pairs of direct and grism exposures will always be consistent.

PR 64630 IR CRCORR compuation of ERR values corrected

The CALWF3 IR CRCORR step performs up-the-ramp fitting to each pixel to determine the countrate that is stored in the output "flt" science (SCI) image extension. The uncertainty in the fit is also computed and stored in the "flt" error (ERR) image extension. The computation of the ERR values was not correctly including contributions from read noise and Poisson noise, resulting in ERR values that were severely underestimated (by factors of 5 or more). The algorithm has been corrected to now produce appropriate ERR values in the "flt" products. Anyone using the ERR values for subsequent processing or analysis should have their data reprocessed.

Created 03/12/2010 Modified 06/29/2010 MJD