The ACS/WFC Focus-Diverse ePSF Webtool

The effects of telescope breathing cause the focus level of HST to change as a function of time during each orbit. This results in modest, but non-negligible, changes to the point-spread functions (PSFs) of stars. For optimal PSF fitting photometry, the library PSFs must be adjusted to account for the changing focus levels. Anderson and Bedin (2017) used a phylogram-based technique to show that it is possible to accurately model focus-related variations in the PSF and thus determine the focus level for a given exposure with measurements of just a handful of bright, isolated stars in the individual WFC3/UVIS images. Bellini et. al (2018) extended this analysis to ACS/WFC, with similar conclusions. Using the results of the Bellini et al. (2018) analysis and subsequent extension to the 11 most popular ACS/WFC filters, we have developed a webtool and Python API to deliver focus-diverse effective PSFs (ePSFs) to the community. The webtool is available for public access at acspsf.stsci.edu.