STScI Logo

Space Telescope Imaging Spectrograph
CCD First Order TDS Corrections

Time Dependent Sensitivity Corrections for CCD First-Order Spectroscopic Modes

As of 2003-Nov-13 we have implemented in the OTFR pipeline the use of the TDSTAB reference file for the CCD detector, which now allows CALSTIS to correct the calibrated fluxes of CCD spectral observations for time dependent changes in STIS sensitivity (TDS corrections). Note that similar corrections for first-order MAMA modes were implemented in May of 2002. The decline in sensitivity ranges from zero (i.e. unmeasurable) at long wavelengths (with the G750L and G750M gratings) to as large as 2% per year at the shortest wavelengths of the G230LB grating. The new TDS correction will be applied when this new reference file is used with CALSTIS 2.13b or higher. Corresponding changes to SYNPHOT throughput files will be made in the near future.

Users who wish to apply these corrections to STIS CCD spectroscopic data should either re-request their data from the HST archive or obtain the latest version of the tdstab reference file for the CCD detector, then update the TDSTAB keyword in the headers of their data files, and then recalibrate using the version of x1d found in CALSTIS 2.13b or later.

Note that these TDS corrections only take into account changes in the throughput of STIS's optical components. Sensitivity losses due to the increase in CCD charge transfer inefficiency (CTI) over time must be corrected separately. That correction requires CALSTIS 2.15b or later, and use of a CCDTAB reference file delivered on or after 2003-Sep-08. For further details see the the October 2003 STAN article on Calstis 2.15b.