The list of Investigators does not match the STScI records for this program. First submission by Isabel Rivera. -1518846928 ----- Attempting Submission 1 (Thu Jul 31 14:30:59 GMT 2025) ----- New PI and Co-Is added to this program. HST Focus and Optical Monitor Utilize WFC3/UVIS and ACS/WFC in parallel to observe stellar cluster members with multiple exposures over an orbit. Phase retrieval performed on the PSF in each image will be used to measure focus, with the ability to explore apparent coma, astigmatism, and third order spherical changes in WFC3. The goals are to 1) monitor the optical telescope assembly focal length for the purposes of maintaining focus within science tolerances and 2) determine focus offsets between the imagers and identify any science instrument-specific focus behavior and dependencies. In Cycle 30, we converted the standard CAL/OTA monitor to a CAL/WFC3 monitor. Historically, this bi-monthly program uses the WFC3/UVIS filter F410M to take shallow subarray exposures of the sparse open cluster NGC188. Full frame ACS/WFC exposures in filter F502N are made in parallel. Phase retrieval (PR) is performed on the stellar images to measure the focus of each exposure in equivalent secondary mirror despace. These focus measurements are used to track the evolution of the focus over months and years and to inform whether a focus adjustment is required. Starting from this cycle, Cycle 33, we are utilizing an alternative strategy using four full-frame WFC3/UVIS exposures per visit with exposure times seven times longer than the original subarray exposures. This approach provides complete detector coverage across both UVIS chips and enables many of the fainter stars in the field to be viable for phase retrieval analysis. One orbit every ~2 months with WFC3/UVIS and ACS/WFC in parallel for con-focality check. Since deployment, the HST OTA has shrunk by ~150 microns, resulting in over 20 compensating Secondary Mirror (SM) adjustments away from the Primary Mirror to maintain good focus to the science instruments. This program provides the observations to determine when a secondary mirror adjustment is required. 0 false false RESTRICTED STELLAR CLUSTER OPEN CLUSTER NO magnitude not relevant J2000 The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. /Users/irivera/Downloads/OP-cache/17965-v01.xml WFC3/UVIS with ACS in parallel for confocality check. 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