6.1 Overview

WFC3/UVIS was installed on board the HST in May of 2009. At the end of cycle 24, in the summer of 2017, the instrument has completed its eight year of operation in the harsh radiation environment of low-Earth orbit. There are three main consequences of radiation damage: an increase in the number of hot/warm pixels, an increase in dark current, and an increase in the number of charge traps, which lead to CTE (charge-transfer efficiency losses) and sink pixels. Some of the hot pixels can be annealed away, by warming the detector to ~20C 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, focusing on how the mini-channel makes it possible to minimize CTE losses. Section 6.4 describes the pixel-based model in more detail, 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 correction 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.