New COS/FUV GSAGTAB Delivered
Due to continuous charge extraction, the COS FUV detector experiences a decline in signal over time that we call gain sag. To avoid sagging, the high voltage level of the detector is raised, and the location of the spectra are eventually moved to a new life time position (a pristine area of the detector). The gain sag table (GSAGTAB) tracks the locations of sagged pixels and the date that they become sagged.
Two new COS FUV gain sag table reference files were delivered for use in the COS calibration pipeline. 23e1646sl_gsag.fits is used with COS blue modes and 23e16470l_gsag.fits is used with all other COS FUV observing modes. Since the last GSAGTAB delivery, October of 2015, new gain sag holes have appeared around the lifetime position two (LP2) and lifetime position three (LP3) regions. Currently, the LP4 region at the current high voltage has no sagged pixels. However, the past GSAGTAB for segment FUVA did not account for hot spot activity and flagged non-sagged pixels in LP4 data. Users that have data taken after October 2015, particularly FUVA data at LP4, should re-retrieve their data from the HST Archive to ensure that the sagged areas are handled properly by the pipeline.