We have updated the IRAF/STSDAS task "daydark", which has been available in the stsdas.hst_calib.stis package to create CCD dark reference files with a "daily" update of the hot pixels. The main improvement over the previous version is the following.
STIS Side-2 CCD darks (taken after July 2, 2001) will now be corrected for the dependence on CCD temperature. The Side-2 CCD electronics lack an active temperature controller for the STIS CCD, so that the dark current fluctuates along with the CCD temperature. The new version of "daydark" normalizes the CCD dark current to a value appropriate for a reference CCD housing temperature of 18 degrees C, using a scaling of 7% increase in dark current per degree of housing temperature (see STIS Instrument Science Report 2001-03). The task does this scaling automatically for CCD data retrieved from the HST archive after January 7, 2002. For data observed and retrieved between July 2, 2001, and January 7, 2002, the user has two options: (a) retrieve the data again from the archive (using OTFR) so that "daydark" will do the scaling automatically, or (b) enter a list of CCD housing temperatures into the new input parameter "houstemp". The housing temperature appropriate for a given CCD exposure (taken after July 2, 2002) can be retrieved using a table available through the STIS web site at http://www.stsci.edu/hst/stis/performance/background. While option (a) above is the easiest option and yields sufficiently high quality for the great majority of STIS observations, the CCD housing temperature table in option (b) currently has the finest sampling in time. Hence, option (b) could, in certain cases, yield slightly better dark scaling than option (a) (the latter uses the value of science header keyword OCCDHTAV, which is currently evaluated from a more sparsely sampled table). Ultimately, the science header keyword OCCDHTAV will be populated from the finely sampled housing temperature table from option (b). A future Spectrographs Newsletter will report on that.
We have made the new script available immediately, through the STIS web site at http://www.stsci.edu/hst/stis/software/analyzing, and it will be part of the upcoming STSDAS release.