Routine monitoring of the spectroscopic sensitivity of COS has revealed a change in the sensitivity of some COS gratings over the course of Cycle 17. This primarily affects the FUV gratings and the bare aluminum NUV gratings (G225M and G285M). As of mid-summer (when the time-dependent sensitivity files were delivered, see below) the decline in sensitivity from the initial on-orbit determinations was about 7% for the G160M gratings, about 5% for G130M, and ranged from 3% at the shortest wavelengths of G140L to about 8% at the longest wavelengths covered by G140L. For the G225M the change in sensitivity by mid-summer was about 2% from the initial on-orbit flux calibration, and for G285M it was about 8%. The early results of the spectroscopic sensitivity monitoring are described in COS ISR 2010-15, Osten et al., released on August 2. These data were used to establish time-dependent sensitivity (TDS) reference files which were delivered on July 14, 2010 to the calibration database system for use with COS/NUV and COS/FUV. These files are now routinely used in on-the-fly-reprocessing of COS data obtained through the archive. These files should correct fluxes to an accuracy of +/-2%. The FUV absolute flux calibration should be accurate to +/- 5% (Massa et al. 2010a), while the NUV absolute flux calibration should be accurate to +/-2% (Massa et al. 2010b).
The sensitivities used by the COS Exposure Time Calculator (ETC v18.2 and later) take into account the time-dependent sensitivities discussed above by projecting the sensitivities midway into Cycle 18.
Monitoring of the spectroscopic sensitivity of STIS continues for the CCD and MAMA detectors using external spectrophotometric white dwarf standard stars. New time-dependent sensitivity (TDS) reference files that incorporated the results of post-repair throughput measurements were delivered for the CCD on Septemer 10, 2009,, however, new TDS files for the FUV and NUV MAMA detectors were only delivered on July 12, 2010. Differences from the pre-SM4 throughput extrapolation previously used are substantial, so users who require the best flux calibration should re-retrieve their data from the HST archive.