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.
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WFC3 Data Handbook
- • Acknowledgments
- Preface
- Chapter 1: WFC3 Instruments
- Chapter 2: WFC3 Data Structure
- Chapter 3: WFC3 Data Calibration
- Chapter 4: WFC3 Images: Distortion Correction and AstroDrizzle
- Chapter 5: WFC3-UVIS Sources of Error
- Chapter 6: WFC3 UVIS Charge Transfer Efficiency - CTE
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Chapter 7: WFC3 IR Sources of Error
- • 7.1 WFC3 IR Error Source Overview
- • 7.2 Gain
- • 7.3 WFC3 IR Bias Correction
- • 7.4 WFC3 Dark Current and Banding
- • 7.5 Blobs
- • 7.6 Detector Nonlinearity Issues
- • 7.7 Count Rate Non-Linearity
- • 7.8 IR Flat Fields
- • 7.9 Pixel Defects and Bad Imaging Regions
- • 7.10 Time Variable Background Contamination
- • 7.11 IR Photometry Errors
- • 7.12 References
- Chapter 8: Persistence in WFC3 IR
- Chapter 9: WFC3 Data Analysis
- Chapter 10: WFC3 Spatial Scan Data