WFC3/UVIS: External CTE Monitoring 2009-2024

Abstract

This report examines Charge Transfer Efficiency (CTE) flux losses in the Wide Field Camera 3 UVIS detector aboard the Hubble Space Telescope. Spanning approximately 14 years of observations from October 2009 to February 2024, the study analyzes CTE flux loss trends across various total background levels and source fluxes. In addition to analyzing the present state of CTE flux losses, we provide updated coefficients for the empirical model for point source photometry corrections in both non-CTE-corrected (FLT) and CTE-corrected (FLC) data. Between 2009 and 2023, the rate of CTE flux loss for a 500-2000 e$^-$ source, farthest from the readout, in FLT data with a 1-3 e$^-$/pix background, is measured to be $\sim$0.05 $\Delta$mag/2051 pix/year. The recommended minimum total background level to mitigate CTE losses remains at 20-25 e$^-$/pix. At that level, we find that 500-2000 e$^-$ sources, farthest from the readout, in 2024 FLT data can suffer $\sim$23$\%$ flux loss/2051 pix. The FLC data provide some relief, but we measure a $\sim$12$\%$ flux loss/2051 pix in 2024. There continues to be a slight over-correction in some FLC results that contain backgrounds above 40 e$^-$/pix. In 2024, 8000-20000 e$^-$ sources farthest from the readout in a 40, 60, or 90 e$^-$/pix background are over-corrected by $\sim$1, 2, and 3$\%$, respectively.