March 2026 STAN

In this STAN, the STIS Team presents Cycle 34 documentation updates and news to STIS users submitting Phase I proposals.

Cycle 34 Documentation and Phase I Reminders

HST continues to operate in reduced gyro mode (RGM), with the biggest impact being reduced scheduling opportunities for targets. Therefore, all special requirements must be requested and quantitatively justified in the "Special Requirements" section of Phase I proposals. 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. Special requirements should be kept to a minimum, and it's necessary 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.

Reminder for Disruptive ToO proposers

Please carefully consider and specify the response time needed for your science case. A standard disruptive ToO designation is defined as a < 3-week turnaround, and turnaround times can be as short as 2 - 5 days. However, achieving the shortest turnaround times depends on the ability of the observer to provide prompt, accurate, and complete information of the target’s flux, especially for those observations using the MAMA detectors. Observations requiring a < 2-day turnaround are defined as ultra-disruptive, which place significant demands on the HST schedule. Thus, only one ultra-disruptive ToO program is allowable per cycle, and such programs are subject to additional restrictions as described in the Cycle 34 HST Observation Types documentation .

SNAP Programs

For users developing a SNAP proposal, pay attention to the requirements and limitations outlined in the call for proposals, particularly for STIS/MAMA SNAPS. All STIS/MAMA observations require SAA-free orbits, regardless of the length of the visit, limiting the number of opportunities available. Additionally, excessively complex STIS/MAMA SNAP targets or configurations may not be implemented in Phase II due to limited resources available for bright-object checking. In order to boost the completion rate of a SNAP program, the STIS team recommends the use of CCD modes when scientifically feasible.

Special Uses of STIS

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.

General Reminders

STIS users preparing a program for Cycle 34 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 34 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. 

New Gaia Exposure Time Calculator (ETC) Normalizations

Section 4 of the STIS ETC allows users to normalize an input spectral distribution by various fluxes and bandpasses. As of ETC version 34.1.1, users may now select Gaia bandpasses for normalization. It is important to note that Gaia photometry is in the Vega system, and users will need to select "Vega Magnitude" if they are using magnitudes reported in Gaia DR3. Users are reminded to regularly check the STScI ETC News and Known Issues or the ETC Release Notes for the latest information on the ETC.

Instrument Science Reports posted to arXiv

As a way to increase the visibility of HST Instrument Science Reports (ISRs) within the astronomical community, the STIS team has decided to cross-post recently published ISRs to the astrophysics arXiv . The goal is to ensure that as many users as possible see the latest updates to HST/STIS instrument calibration, software, and best practices. All ISRs published in 2024 and 2025 have now been posted to arXiv, and in the future, any new ISRs will be added to arXiv as well.