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Space Telescope Imaging Spectrograph
Incorrect Dark Subtraction

Incorrect Dark Subtraction for Early 2001 CCD Data

Due to a failure of the STIS Side-1 electronics in May 2001, some thermal control of the STIS CCD was lost, resulting in temperature variations that drive variations in the dark rate. These variations appear in all CCD data obtained during Side-2 operations (staring in July 2001). Since January 2002, the CALSTIS pipeline has scaled the dark rate in Side-2 data by the CCD temperature prior to dark subtraction. This correction utilizes the OCCDHTAV header keyword, which provides the CCD housing temperature. For the correct reduction of Side-1 CCD data, this keyword should be set to a negative number or absent. Unfortunately, we have discovered that for Side-1 data taken in 2001 (January -May), this header keyword has been populated incorrectly by the pipeline, resulting in an incorrect dark subtraction for these data. Users who retrieved STIS CCD data in 2002 from observations during the first 5 months of 2001 should re-retrieve their data with On-The-Fly-Reprocessing (OTFR), in order to obtain the correct dark subtraction. Alternatively, they can set OCCDHTAV in the science extensions of their FITS files to -1, and re-run CALSTIS on their local machine.