7.6 CCD Gain Selection

Please check for updates on the ACS website.

WFC is offered as shared risk in Cycle 33 and may receive minimal calibration. See the ACS website, Call for Proposals, and OPCR webpage for the latest status.

As quantified in Table 4.1, the WFC CCDs have selectable gain values of 0.5, 1.0, 1.4, and 2.0 electrons per digital number. Various factors should influence the gain selected in Phase II for your science program: level of support and calibrations provided, influence of associated readout noise on data quality, dynamic range on the bright end, and data compressibility for WFC in limited applications.

Only GAIN=2.0 is fully supported for the WFC. Calibration support will not be provided for the GAIN=0.5, 1.0, and 1.4 settings; users proposing unsupported gain values should provide special justification and discussion of necessary calibrations. Please note that post-SM4 WFC images appear to have substantial (>10^-3 fractional) amplifier crosstalk when the ADC saturates.

GAIN=2.0 completely samples the full well depth of ~77,400 e^– (ACS ISR 2020-02). Furthermore, charge is conserved even beyond filling the full well depth; for point sources at GAIN=2.0 it is possible to obtain valid aperture photometry several magnitudes beyond saturation by summing over all pixels affected by the bleeding due to saturation (see Section 4.3.1). GAIN=2.0 provides critical sampling of the readout noise, supporting robust background sky-level determination even at low values.

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ACS Instrument Handbook
- • Acknowledgments
- • Change Log
- • Chapter 1: Introduction
- Chapter 2: Considerations and Changes After SM4
  - • 2.1 SM4 Repair of ACS
  - • 2.2 ACS/WFC Observations in Cycle 33 and Beyond
  - • 2.3 Comparison of WFC and UVIS
  - • 2.4 Comparison of HRC and UVIS
  - • 2.5 Dithering Considerations
  - • 2.6 CTE Considerations
  - • 2.7 SBC Considerations
- Chapter 3: ACS Capabilities, Design and Operations
  - • 3.1 ACS Location in the HST Focal Plane
  - • 3.2 Instrument Capabilities
  - • 3.3 Instrument Design
  - • 3.4 Basic Instrument Operations
  - • 3.5 ACS Quick Reference Guide
- Chapter 4: Detector Performance
  - • 4.1 Overview
  - • 4.2 The CCDs
  - • 4.3 CCD Operations and Limitations
  - • 4.4 The SBC MAMA
  - • 4.5 SBC Operations and Limitations
  - • 4.6 SBC Bright-Object Limits
- Chapter 5: Imaging
- Chapter 6: Polarimetry, Coronagraphy, Prism and Grism Spectroscopy
  - • 6.1 Polarimetry
  - • 6.2 Coronagraphy
  - • 6.3 Grism and Prism Spectroscopy
- Chapter 7: Observing Techniques
  - • 7.1 Designing an ACS Observing Proposal
  - • 7.2 SBC Bright Object Protection
  - • 7.3 Operating Modes
  - • 7.4 Patterns and Dithering
  - • 7.5 A Road Map for Optimizing Observations
  - • 7.6 CCD Gain Selection
  - • 7.7 ACS Apertures
  - • 7.8 Specifying Orientation on the Sky
  - • 7.9 Parallel Observations
  - • 7.10 Pointing Stability for Moving Targets
- Chapter 8: Overheads and Orbit-Time Determination
  - • 8.1 Overview
  - • 8.2 ACS Exposure Overheads
  - • 8.3 Orbit Use Determination Examples
- Chapter 9: Exposure-Time Calculations
  - • 9.1 Overview
  - • 9.2 Determining Count Rates from Sensitivities
  - • 9.3 Computing Exposure Times
  - • 9.4 Detector and Sky Backgrounds
  - • 9.5 Extinction Correction
  - • 9.6 Exposure-Time Examples
  - • 9.7 Tabular Sky Backgrounds
- Chapter 10: Imaging Reference Material
  - • 10.1 Introduction
  - • 10.2 Using the Information in this Chapter
  - 10.3 Throughputs and Correction Tables
    - • WFC F435W
    - • WFC F475W
    - • WFC F502N
    - • WFC F550M
    - • WFC F555W
    - • WFC F606W
    - • WFC F625W
    - • WFC F658N
    - • WFC F660N
    - • WFC F775W
    - • WFC F814W
    - • WFC F850LP
    - • WFC G800L
    - • WFC CLEAR
    - • HRC F220W
    - • HRC F250W
    - • HRC F330W
    - • HRC F344N
    - • HRC F435W
    - • HRC F475W
    - • HRC F502N
    - • HRC F550M
    - • HRC F555W
    - • HRC F606W
    - • HRC F625W
    - • HRC F658N
    - • HRC F660N
    - • HRC F775W
    - • HRC F814W
    - • HRC F850LP
    - • HRC F892N
    - • HRC G800L
    - • HRC PR200L
    - • HRC CLEAR
    - • SBC F115LP
    - • SBC F122M
    - • SBC F125LP
    - • SBC F140LP
    - • SBC F150LP
    - • SBC F165LP
    - • SBC PR110L
    - • SBC PR130L
  - • 10.4 Geometric Distortion in ACS
- • Glossary

7.6 CCD Gain Selection

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