WFC3 Data Handbook V. 4.0

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Chapter 6:
Charge Transfer Efficiency (CTE)

6.1 CTE Losses And Background

6.2 The Nature Of CTE Losses

6.3 The Pixel-Based Model

6.4 Empirical Corrections

6.5 Dealing With CTE Losses In WFC3/UVIS Images

6.6 References

WFC3/UVIS was installed in May of 2009. By the middle of Cycle 21 it will have been in the harsh radiation environment of space for 5 years. There are two main consequences of radiation damage: an increase in the number of hot/warm pixels and an increase in the number of charge traps, which lead to CTE (charge-transfer efficiency) losses. Some of the hot pixels can be annealed away, 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 with a small addition of background. Section 6.3 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 6.4 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.