March 2025 STAN

In this STAN, the STIS Team presents Cycle 33 documentation updates and news to STIS users submitting Phase I proposals, as well as updates regarding the STScI Helpdesk, updates to the FUV-MAMA Time Dependent Sensitivity corrections, and improvements to the post-SM4 STIS NUV-MAMA dark correction.

Cycle 33 Documentation and Phase I Reminders

HST is now operating in reduced gyro mode(RGM), with the biggest impact being reduced scheduling opportunities for targets. Therefore, special requirements must be justified quantitatively in the Phase I proposal. Special requirements for STIS include ORIENT, scheduling or timing constraints, ToOs, CCD and MAMA observations in one visit, and HST visits that exceed 30 buffer dumps carrying TIME-TAG data. We remind users to keep special requirements to a minimum and to justify all requirements that are needed in the appropriate section of the proposal, and to quantitatively specify known special requirements in the APT file so that they can be automatically approved at the Phase II level. Any special requirements that are not disclosed at the Phase I level risk termination if they render the approved science unachievable. Proposals to use STIS FUV- or NUV-MAMA detectors must also demonstrate that the proposed observations do not violate the bright object restrictions. A comprehensive summary, including a table on count rate limits, for drafting a suitable observing plan under these conditions are detailed in Section 7.7.

Because targets with declination > 60 degrees are least impacted by RGM scheduling constraints, we encourage users where possible to favor targets above 60 degrees of declination for improved scheduling, particularly for CCD observations. Note that this is not a contributing factor in judging the merit of a proposal, but purely for making scheduling of approved programs easier. Since moving to RGM operations we are still working to optimize efficiency for constraints imposed by RGM operations, such as the reduced field of regard.

Special uses of STIS, including recommendations for users interested in coronagraphic observations, can be found in Chapter 12 of the IHB. We encourage PIs to utilize the STIS Coronagraphic Visualization Tool and the STIS Coronagraphic Observation Feasibility Tool available on the Data Analysis and Software Tools webpage to aid their proposal planning. For coronagraphic observations requiring multiple ORIENTs, PIs are reminded that HST has a limited roll angle at any given time. Observations with large ORIENT differences will be scheduled at different times. Observers can view the "Roll Ranges Report" in the Visit Planner tool in APT to see the available ORIENT ranges when their object is schedulable. If coronagraphy programs have many linked visits, causing tight scheduling constraints on these programs, users may consider planning for smaller groups of linked visits and more orbits up front.

STIS users preparing a program for Cycle 33 may refer to the Proposal Planning Toolbox, which includes deadlines, further documentation, and tools for submission. For additional inquiries, please contact the HST Help Desk. Information on STIS in preparation of Cycle 33 proposals has been released in the HST Call for Proposals and Primer, and the STIS Instrument Handbook has been updated to reflect the current status of the instrument. PIs are encouraged to review Chapter 10.1 in the STIS Instrument Handbook (IHB) for a summary and checklist for HST Phase I proposals. 

STScI Helpdesk Updates

We would like to inform users that the STScI Help Desk email address (help@stsci.edu) will be deactivated by the end of this year. Moving forward, any help desk inquiries that you may have are to be relayed through the existing ServiceNow web portals for the relevant mission/division:

- HST: https://stsci.service-now.com/hst or hsthelp.stsci.edu
-JWST: https://stsci.service-now.com/jwst or jwsthelp.stsci.edu
- Roman: https://stsci.service-now.com/roman or romanhelp.stsci.edu
- OPO: https://stsci.service-now.com/opo or opohelp.stsci.edu
- MAST: https://stsci.service-now.com/mast or masthelp.stsci.edu
- ITSD (internal): https://stsci.service-now.com/stars or stars.stsci.edu

STScI has used the ServiceNow platform for the help desk since early 2017, but we have kept the email address active as a courtesy to the user base. These portals offer announcements, knowledge base articles that address frequently asked questions, options to submit your questions directly to the appropriate experts, and access to your support history. A free MyST account is required to use these services. For any issues accessing ServiceNow, please contact helpsn@stsci.edu.

Updating STIS FUV-MAMA Time Dependent Sensitivity Corrections

The STIS team has observed a changing sensitivity trend for the FUV-MAMA observing modes. We will be releasing an update to the 2019 FUV-MAMA TDSTAB, with a new breakpoint at January 24, 2020, which will affect the sensitivity corrections of all FUV-MAMA data taken after this date. Data taken on or near January 24, 2020, will see only small changes, and FUV-MAMA data taken on and after 2022 may see changes between 2-3%. An example plot comparing the current and new TDSTABs is shown below. These plots are the percent change in sensitivity of G140L over a given wavelength bin (1300-1350 Å) from instrument launch in 1997 to the most recent available data (January 2025). This new breakpoint is the first to be generated using a newly written TDS code, which improves our fitting routine and optimizes the date of a new breakpoint based on the fits.

This new to-be-delivered TDSTAB will be used to update the throughputs used in the HST ETC for ETC version 33.2. New NUV-MAMA and CCD TDSTABs are also being considered, but any changes will be minor and intended primarily to update the throughput projections in ETC v33.2. This new FUV-MAMA TDSTAB will be delivered in the next few weeks. If you’d like to be notified of this and any future deliveries, please subscribe to the mailing list: STIS_REFFILES_UPD@MAILLIST.STSCI.EDU. To check for this delivery, go to the HST CRDS webpage at https://hst-crds.stsci.edu/. Under the ‘Latest References’ section select ‘stis’ then in the dropdown select ‘tdstab.’ To find the latest deliveries, sort by ‘Activation Date'.

Improvements to the Post-SM4 STIS NUV-MAMA Dark Correction

The STIS team has delivered an updated NUV Dark Correction Table (TDCTAB), 92l21262o_tdc.fits, to the HST Calibration Reference Data System (CRDS) for use with all post-SM4 NUV data. This file encodes the scaling relation for STIS NUV DARKFILEs as a function of detector temperature and date. The updated file contains additional break points starting at MJD 60297.29 and 60478.63 (2023-12-19 and 2024-06-17, respectively). Data throughout the post-SM4 era were used to fit a linear trend of dark rate with temperature, which was applied before fitting the time trends. Dual exponential curves were fit to model the time-dependence following large-scale instrument temperature swings that occurred during safing events. These curves were biased to produce consistent and reasonable results during monitoring data gaps and to temper extrapolation.

The team simultaneously delivered four new NUV-MAMA DARKFILEs, the first set of new DARKFILEs since 2009, corresponding to USEAFTER dates in mid-year 2012, 2015, 2018, and 2021:

USEAFTER    DARKFILE
2012-06-27  92l22044o_drk.fits
2015-06-16  92l22047o_drk.fits
2018-06-25  92l22045o_drk.fits
2021-07-18  92l22042o_drk.fits

Each DARKFILE was created using 3 years of NUV dark observations, summing ~150 raw darks with 1300s exposures. The files have been scaled with functions from the updated TDCTAB which more closely align with the post-SM4 dark rate. The DARKFILEs have been compared via histograms to pre- and post-SM4 reference superdarks available in CRDS for correctness in science, error, and data quality extensions. For validation, they have been used to calibrate QSOs from a range of dates (2013 - 2022), which show improvements in removing the dark current in both background level and in artifacts caused by detector glow in some low SNR datasets.

These files were delivered in CRDS contexts hst_1214.pmap and hst_1215.pmap on 2025-02-21. Affected archival datasets were recalibrated on 2025-02-26. Users wishing to retrieve data taken after 2012-06-27 recalibrated with these updates are advised to redownload their products from MAST or apply the CRDS bestrefs script to RAW data products and recalibrate with CalSTIS.