March 2022 STAN
This STAN includes updates to the available STIS observing modes for Cycle 30 Phase I and details of the increased temperature of STIS following its latest recovery.
STIS Mode Changes for Cycle 30
Observers should be aware of two new changes to available STIS modes of operation beginning in Cycle 30.
Observers interested in the most accurate absolute flux calibration (< 2-3%) of first order spectra taken on either the FUV-MAMA or NUV-MAMA now have greater control over the placement of spectra on the detector. Normally, GO observations are shifted to slightly different locations on the detector on a monthly basis to minimize uneven charge depletion of the detectors (see IHB Section 7.6.2). However, the majority of calibration programs that monitor the absolute and relative sensitivity of the UV spectral modes disable this monthly offsetting (by use of the MSMOFF mode) to maximize the repeatability of the measurements. This previously restricted mode is now available-but-unsupported to allow observers to place their first order MAMA spectra at these best-calibrated regions of the detector. The use of MSMOFF to disable monthly offsetting requires scientific justification at the Phase I level.
The NUV PRISM optical element is no longer supported but can still be used as an available-but-unsupported mode. This change is due to the ever growing uncertainty in the PRISM's absolute flux calibration. The time dependent sensitivity of the PRISM is known to deviate from that of the other STIS NUV modes, but its sensitivity is not routinely monitored since no science observations have used the PRISM in the post-SM4 era.
Observers planning to use the PRISM who also require absolute flux calibration must budget orbits to observe their own standard star calibrators at the Phase I level. Additionally, users must demonstrate both (1) that their observations pass NUV-MAMA bright object safety limits assuming little to no change in the sensitivity since the last measurement, and (2) that observations are technically feasible assuming more significant degradation in sensitivity. See STIS ISR 2017-06 and references therein for the most recent assessment of the PRISM's sensitivity and IHB Section 4.4 for more details of the NUV PRISM.
Questions about either STIS mode change can be directed to the HST Help Desk.
STIS Running Hot Post-Recovery
The ambient temperature of STIS has been elevated since all four science instruments on HST were recovered from the “safing” event last fall. STIS went into safe mode following an anomaly detected in October 23, 2021 and was recovered on December 7, 2021. As part of the mitigation strategy to avoid future safings, the retired instrument NICMOS was turned on in a limited capacity for monitoring. It is likely that powering up NICMOS has caused the increase in temperature for HST.
For the STIS CCD, the temperature is tracked using telemetry from the CCD housing. The housing temperature serves as a proxy measurement for the CCD detector temperature. Since the switch to the Side-2 electronics, STIS lost the capability to directly measure the detector temperature (see STIS ISR 2001-03). The figure below is a plot from the CCD Housing Temperature Monitor, and in the most recent (right-most) cluster of data the CCD Housing Temperature is elevated by ~2-3 °C relative to the historical average.
The impact that this has on science done with the CCD is likely extremely small but could affect observations of the faintest targets working at the lowest signal-to-noise. The current temperature correction performed by the calibration pipeline scales the dark level by a flat percentage-per-°C factor down to a reference temperature of 18 °C. A 2-3°C increase of STIS means that a larger scaling factor is required to achieve the reference temperature. The current temperature correction only approximates the temperature dependence of the dark rate (discussed in STIS ISR 2018-05), and so a larger scaling factor may introduce more noise in science data after dark correction. However, historical investigation has shown that the current temperature correction still behaves relatively well, within a few percent of more complex correction algorithms (as in STIS ISR 2018-05), even in the current regime of 22-23°C. This, in conjunction with some preliminary investigation of the correction quality in the most recent reference files, suggests that the impact to user data is extremely small. Regardless, we encourage users to keep this in mind if their data would be impacted by even small changes to the dark rate.
The MAMA detectors are less sensitive to temperature change relative to the CCD. This is in part because the STIS CCD requires special consideration as it is not capable of fine thermal control on Side-2. The dark rate monitors for both MAMA detectors show no signs of elevation since the return to science operations, indicating that the temperature correction for both detectors is performing nominally in the heightened thermal environment.
If you see any rise or difference in the behavior of the dark rate in your data, don’t hesitate to reach out to the HST Help Desk .