As part of the ETC 20.2 release, the included COS and STIS throughputs have been updated to provide our best estimates for Cycle 20.
Predictions for the COS FUV channel throughput have greater uncertainties than those for the other detector channels.
The sudden and steep decline in COS FUV sensitivity observed in late 2011 is thought to have been related to an increase in solar activity and the corresponding increase in atmospheric density at HST's orbital altitude. Fortunately, the short-term solar activity has decreased since December 2011, and the decline in COS FUV sensitivity has correspondingly become slower than had been assumed in ETC 20.1.1 which had been based on data taken through December 2011. The throughput data for the COS FUV detectors have now been updated in the ETC 20.2, with predicted throughputs for January 2013 10-20% higher than the December 2011 prediction included in ETC 20.1.1.
In addition, the team has updated the STIS throughputs. No extraordinary trends were found in the calibration observations, but the smooth changes of the STIS throughputs as a function of time are now reflected in the throughputs prediction for April 2013 (mid-cycle 20). The changes in throughput relative to ETC 20.1.1 for the STIS CCD, NUV, and FUV are less than 5%.
The COS/STIS team has also reviewed all dark current rates for both COS and STIS based on recent calibration data. Comparing the ETC 20.1.1 and 20.2 predictions for April 2013, the STIS NUV dark rate has decreased by 30%, the COS NUV dark rate has increased by 11%, and the FUV dark rates (for both spectroscopic and spectroscopic target acquisition modes) have increased by 130%. We would like to emphasize that, while the ETC can only reflect an average of the dark current rate, the STIS FUV dark current rate in particular exhibits tremendous variations with position on the detector, due to the infamous glow region, and with the time during which the HV has been on. These effects are described in detail in section 7.5.2 of the STIS instrument handbook, in particular in Figures 7.18 and 7.22. STIS FUV MAMA users whose observations are sensitive to dark current (e.g., faint targets) are strongly encouraged to read the corresponding documentation to assess the feasibility of their observations and better constrain the exposure time needed to achieve the required accuracy.
Last, the resolution element of the new C1055 and C1096 COS FUV spectroscopic modes has been updated to 30 pixels. Its value was previously 6 pixels (as for the other G130M modes) in ETC 20.1.1, which did not reflect reality. The resolving power for the C1055 and C1096 G130M modes actually decreases with increasing wavelength, from R ~3000 at 950 A down to R ~ 1000 at 1150 A, and is thus much lower than the resolving power of the other G130M cenwaves. The reason for this difference is that these new modes operate out of focus. On the other hand, similar to the dispersion of the other G130M modes, the new C1055 and C1096 G130M modes have a dispersion of ~ 0.01 A/pixel. Thus, one resolution element for these new modes corresponds to about 30 pixels at 950 A, and 115 pixels at 1150 A. The previous version of the ETC (20.1.1) used a value of 6 pixels for the resolution element of the C1055 and C1096 modes, which led to overestimate the time required to achieve a given S/N per resolution element. The corrected value of the resolution element for C1055 and C1096 in ETC 20.2 therefore provides more accurate estimates of the S/N and time to reach a given S/N per resolution element.