Updates to WFC3/UVIS Encircled Energy Values in Select Filters

Abstract

We present updated encircled energy (EE) curves for a subset of UVIS filters. These are derived from a reanalysis of the drizzled Point Spread Function (PSF) data underlying the current EE calibration together with new measurements from deep observations of the PSF wings. Improved centroiding and analysis techniques applied to the drizzled PSFs produce more accurate EE values at small radii ($r \le 10$ pixels), bringing the results into closer agreement with a large archival PSF study (Huynh et al. 2025). At large radii, we leverage deep observations of the PSF wings in six filters and compare the fraction of light between 2" and 6" with predictions from an optical model of the PSF (Hartig 2009). For filters with pivot wavelengths $ \gtrsim{4000} \mathrm{\mathring{A}}$, the model agrees with the empirical data, but over-estimates the EE for UV filters by $\sim0.5$% at 2". The revised EE solutions affect the UVIS zeropoints, which are derived from aperture photometry in a 0.4" (10 pixel) radius and corrected to 6" using EE tables (Calamida et al. 2022). Overall, the impact is small; the EE at 0.4" is larger by $ \gtrsim{0.5}$ % (0.005 mag) for several filters (F218W, F225W, F275W, F775W, F814W, F845M), especially for UVIS2. In contrast, the EE value is generally smaller at 0.4" for long-pass filters (F200LP, F350LP, F850LP), with F850LP differing by $\sim2 $ % (0.02 mag). Updated EE tables will be delivered together with a revised set of UVIS inverse sensitivity tables and zeropoints later in 2026. In the interim, we provide EE tables for commonly used filters in Appendix A.