2.1 Impacts of Reduced Gyro Mode on Planning Observations

During Cycle 32, HST began operating in reduced gyro mode (RGM) following months of repeated observatory safings due to the performance of Gyro 3. Thus far, the scientific performance of STIS has not changed; however, observers should be aware that their science programs may require more orbital resources than when HST was operating in 3 gyro mode. A detailed discussion of the impacts of RGM is available in  The HST RGM Primer.

Observers should consult Sections 2.6 and 2.7 for an overview of new limits and existing policies for splitting observations of the same target into separate visits, as this is needed for an accurate accounting of overheads (Section 9.2) when determining how many orbits are required. All scheduling, timing, and orientation requirements (both relative and absolute) can reduce the schedulability of an observation, and they must be identified and justified in the "Special Requirements" section of the Phase I.

Coronagraphic observations (Section 12.11) are expected to be particularly impacted because strategies for maximizing achievable contrast (as discussed by Debes et al. 2019) involve tight ORIENT constraints for broad azimuthal coverage and linked visits for minimizing PSF variations. The July 2024 STAN suggested two mitigations for improved schedulability in RGM:

1. Increase the azimuthal coverage in a single orbit visit by choosing two different aperture positions that are orthogonal to one another, e.g., BAR5 + WEDGEA1.0 or BAR5 + BAR10 with the available-but-unsupported POS-TARGs to the lower left and lower right corners (Debes et al 2019).

2. Reduce the number of science target visits linked to a single PSF reference.  E.g., instead of requiring several observations of a science target at different roll angles to execute in back-to-back orbits together with one PSF star observation, pair each science target visit with its own PSF reference visit.