2.2 ACS/WFC Observations in Cycle 33 and Beyond

Shared-risk Observations

As a result of the NASA Hubble Operational Paradigm Change Review (OPCR), the Wide Field Channel of ACS is offered as shared-risk in Cycle 33 and may receive minimal calibration and support. It is expected that Cycle 32 (including Bridge Programs) and prior ACS/WFC programs will be allowed to be observed into Cycle 33.  The periodic annealing of the ACS/WFC CCDs is anticipated to continue indefinitely, to maintain the health of the detector.

WFC support in future cycles is contingent on the outcome of the 2025 Senior Review, with the possibility that WFC observations will be terminated in Cycle 34 and beyond. Given that WFC support may change rapidly, users are encouraged to consult the OPCR website, the Late Breaking News section of the Cycle 33 Call for Proposals, and the ACS website for the latest status. Users may also contact the HST Help Desk for specific questions.

Observers interested in using ACS/WFC are encouraged to explore whether their scientific goals can be accomplished with other HST modes, especially WFC3/UVIS, which remains fully supported in Cycle 33.  A comparison of ACS/WFC and WFC3/UVIS is provided in Section 2.4. More details about WFC3/UVIS can be found in the WFC3 Instrument Handbook.

Reduced-Gyro Mode

In June 2024, NASA announced that HST will transition to operating in "reduced-gyro mode" (RGM). Key impacts of this switch include:

• Less flexible scheduling of observations due the a reduced field of regard on a given day. Scheduling is further limited by additional observing constraints (e.g., limited orient ranges), so users are encouraged to only impose special requirements on their observations that are absolutely required. Coordinated parallels can be especially impacted. Previously efficient observing strategies that interchange the ACS and WFC3 fields of view at six-month intervals with a 180 degree roll are are no longer practical. 
• Limited-to-no ability to observe fast moving targets (>5 arcseconds per second), corresponding to anything within roughly the orbit of Mars.
• Slight increases in target acquisition times, leading to slight reductions in science exposures.

Observers should refer to the RGM Tips and the RGM Primer for complete details. Despite the reduced efficiency in RGM mode, HST will be able to provide the same quality of science data for the majority of observing programs.