17258

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 comma with the ability to explore apparent coma comma astigmatism comma 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.