6.5 Empirical Corrections

Empirical corrections provide an alternate way to correct for CTE losses of observations of point sources or estimate future CTE loss levels. Noeske et al. (WFC3 ISR 2012-09) studied star clusters NGC6791 and 47 Tuc and examined how the measured fluxes of stars varied when particular stars were placed close to and far from the readout register. They documented the losses experienced by point sources as a function of (1) the electron flux of the point source, (2) the detector Y position of the source, (3) the background level, and (4) the date of the observation.

These corrections are provided in the form of polynomials and can be used to estimate how many electrons were originally present in the aperture before the readout process. The conclusions in this ISR and in the more recent follow-up studies ( WFC3 ISR 2017-09, WFC3 ISR 2016-17, and WFC ISR 2021-06) mirror the conclusions from the pixel-based-model study: namely, that background (12-20 electrons) can have a significantly ameliorative effect on charge-transfer efficiency.

The most recent empirical corrections are described in WFC ISR 2021-06, which also provides an analysis of the long term behavior of the UVIS CTE using observations of star clusters.

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WFC3 Data Handbook
- • Acknowledgments
- • What's New in This Revision
- Preface
  - • Handbook Structure
  - • Typographic Conventions
- Chapter 1: WFC3 Instruments
  - • 1.1 Instrument Overview
  - • 1.2 The UVIS Channel
  - • 1.3 The IR Channel
- Chapter 2: WFC3 Data Structure
  - • 2.1 Types of WFC3 Files
  - • 2.2 WFC3 File Structure
  - • 2.3 Data Storage Requirements
  - • 2.4 Headers and Keywords
- Chapter 3: WFC3 Data Calibration
  - • 3.1 The calwf3 Data Processing Pipeline
  - • 3.2 UVIS Data Calibration Steps
  - • 3.3 IR Data Calibration Steps
  - • 3.4 Pipeline Tasks
  - • 3.5 Manual Recalibration of WFC3 Data
- Chapter 4: WFC3 Images: Distortion Correction and AstroDrizzle
- Chapter 5: WFC3-UVIS Sources of Error
  - • 5.1 Gain and Read Noise
  - • 5.2 Bias Subtraction
  - • 5.3 Dark Current and Hot Pixels
  - • 5.4 UVIS Flat Fields
  - • 5.5 Image Anomalies
  - • 5.6 Generic Detector and Camera Properties
  - • 5.7 UVIS Photometry Errors
  - • 5.8 References
- Chapter 6: WFC3 UVIS Charge Transfer Efficiency - CTE
  - • 6.1 Overview
  - • 6.2 CTE Losses And Background
  - • 6.3 The Nature Of CTE Losses
  - • 6.4 The Pixel-Based Model
  - • 6.5 Empirical Corrections
  - • 6.6 Dealing With CTE Losses in WFC3 UVIS Images
  - • 6.7 Sink Pixels
  - • 6.8 References
- 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
  - • 7.11 IR Photometry Errors
  - • 7.12 References
- Chapter 8: Persistence in WFC3 IR
- Chapter 9: WFC3 Data Analysis
  - • 9.1 Photometry
  - • 9.2 Astrometry
  - • 9.3 Spectroscopy
  - • 9.4 STSDAS, STSCI_PYTHON and Astropy
  - • 9.5 Specific Tools for the Analysis of WFC3
  - • 9.6 References
- Chapter 10: WFC3 Spatial Scan Data
  - • 10.1 Analysis of Scanned Data
  - • 10.2 IR Scanned Data
  - • 10.3 UVIS Scanned Data
  - • 10.4 References

6.5 Empirical Corrections

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