Cycle 27 STIS ETC Updates
ETC 27.1 was updated to use an evaluation date for calculations from mid-cycle 26 (MJD 58604; 2019-May-01) to mid-cycle 27 (MJD 58940; 2020-Apr-01). Though the underlying time-dependent sensitivity (TDS) trends remain unchanged, we modeled a small linear decrease in system throughput over this period of time. The greatest throughput changes occurred at the blue end of the FUV gratings, on the order of -1%, with typical changes much less.
Likewise, we extrapolated the STIS/CCD dark rate from [0.021, 0.025, 0.030] to [0.021, 0.026, 0.031] counts/s/pixel at the [top, middle, bottom] of the detector to reflect the new evaluation date. These trends were derived from temperature-corrected darks, and the "low" value at the top of the detector was adjusted upward by 1σ of the temperature correction distribution to give a conservative value for the detector's lowest dark rate region (near the read-out amplifier and E1 position).
The NUV and FUV dark rates and CCD read noise parameters remain unchanged.