16518

This program will be used to cross-calibrate the HST focus in two WFC3-UVIS filters using interleaved exposures in F410M and F606W. It will supplement the existing CAL-OTA Focus and Optical Monitor program comma which executes every 2 months in F410M comma and will allow us to tie the observations to the WFC3 PSF database and phylogram-type plots (Anderson and Bedin 2017 comma MNRAS 470 comma p. 948) comma which include far more observations in F606W than in F410M. It will supplement the results obtained from interleaved exposures in F410M and F606W in program 15585.