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.
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