Improved UVIS Aperture Corrections derived from Focus Diverse PSF Maps

Abstract: In crowded fields, small-aperture photometry can reduce contamination errors from neighboring sources compared to larger aperture photometry. However, the UVIS encircled energy (EE) varies with detector position and focus variations on orbital timescales for aperture radii less than 10 pixels ($\sim$0.4 arcseconds). Using a set of focus-diverse empirical PSFs by Anderson (2018), we compute 2D spatial maps of the aperture correction between 5-10 pixels and find a maximum change of $\sim$0.01 mag over all focus levels for a given detector position. The upper-left and lower-right corners of the UVIS detector are more focus-sensitive than the rest of the field of view, where the mean correction is systematically $\sim$0.01 mag higher in Amp A for bluer filters (F275W, F336W, F438W) and $\sim$0.01 mag higher in Amp D for redder filters (F606W, F814W) at all focus levels. We test the new aperture correction maps in globular clusters, and we find reduced scatter, better agreement between the two CCDs, and a small shift in the absolute photometry when compared to a single (constant) aperture correction per image. These improvements are specific to photometry with apertures $<$ 10 pixels in radius; results from larger apertures are not affected. Using published EE tables can introduce systematic uncertainties in absolute photometry due to its tendency to vary with detector position and focus level, with larger errors for smaller apertures. Users requiring photometric accuracy better than $\sim$1% for small apertures can use isolated stars in the individual FLT/FLC frames (or PSF cutouts at a similar detector position and focus level) to compute encircled energy corrections and accurately account for the amount of flux at radii larger than their photometric apertures.