6.1 Overview

WFC3/UVIS was installed on board HST in May of 2009.  As of the summer of 2024, the instrument has completed its fifteenth year of operation in the harsh radiation environment of low-Earth orbit.

There are three main consequences of radiation damage:  (1) an increase in the number of hot/warm pixels; (2) an increase in the overall dark-current level; and (3) an increase in the number of charge traps, which cause CTE (charge-transfer-efficiency) losses and sink pixels. Some of the hot pixels can be annealed away by warming the detector to ~20° C during the monthly anneal procedures (e.g. Figure 6 in WFC3 ISR 2016-08), but there is a gradual build-up of permanent hot pixels over time. There is no indication that annealing reduces CTE losses.

This chapter describes the various aspects of CTE losses in the WFC3/UVIS detector. The first section provides a general overview and compares CTE losses in WFC3/UVIS to those in ACS/WFC. The next section discusses the nature of WFC3/UVIS losses in more detail. Section 6.4 describes the pixel-based model in terms of how it was constructed and how the pixel-based-reconstruction procedure can be run. Section 6.5 gives a brief overview of the empirical formula-based corrections available for point sources. The final section discusses how best to deal with CTE losses in WFC3/UVIS images in terms of prevention, mitigation, and correction.

The CTE losses described and characterized in this chapter are in the parallel transfer of charge, aka y-CTE.  WFC3/UVIS does experience some charge-transfer losses in the serial direction (x-CTE), but these losses are much smaller than the parallel-transfer losses.  WFC3 ISR 2024-07 provides a discussion of the current impact of serial CTE on the WFC3/UVIS detector.  In brief, while y-CTE affects both the astrometry and photometry of bright sources at the 5% level and can have a pathological effect on faint sources, the impact of x-CTE is almost entirely astrometric and is only 1% of a pixel for bright stars and 2.5% for fainter stars.  The ISR provides more information and describes the stand-alone pixel-based corrections for those few projects that may require them.