5.1.1 Gain

Photoelectrons that accumulate in a CCD pixel are read out as a voltage and converted to data numbers (DN), often called Analog-to-Digital Units (ADUs), by the analog-to-digital converter (ADC). The ADC output is a 16-bit number, capable of producing a maximum of 65,535 DN (2¹⁶- 1) for each pixel. A straightforward scheme in which one DN corresponds to one electron would make it impossible to measure signals larger than 65,535 electrons. The conversion gain, defined as the number of electrons per DN, provides a way of changing the dynamic range of electrons that can be measured. Although it is possible to operate the WFC3 CCD detector at gains of 1, 1.5, 2, and 4 e-/DN, only a gain of 1.5 e-/DN is supported. This gain permits sampling of the entire dynamic range of the detectors with negligible impact on the readout noise. The absolute gain values for the WFC3 CCDs, routinely measured every year, have been stable to within 1-2% since launch in May 2009 (WFC3 ISR 2013-02, WFC3 ISR 2015-05, WFC3 ISR 2016-13, and WFC3 ISR 2018-17). The most recent absolute gain values, as a function of chip, amp, and binning (1x1, 2x2, 3x3) are summarized in Table 5.1. The gains were measured in flat-field images via the standard mean-variance technique: the inverse slope of the mean signal level plotted versus the variance yields the gain.

Table 5.1: WFC3/UVIS absolute gain values (measurement errors are: ~0.01 e-/DN)

CCD Chip	Amp	Gain at 1 x 1 (e-/DN)	Gain at 2 x 2 (e-/DN)	Gain at 3 x 3 (e-/DN)
1	A	1.57	1.57	1.56
1	B	1.57	1.57	1.56
2	C	1.60	1.58	1.57
2	D	1.58	1.58	1.57

5.1.2 Read Noise

The average read noise level measured in the bias frame science area pixels at the default gain setting (1.5e-/DN) are summarized in Table 5.2. Scatter in the measurements based on data acquired between June 2009 and July 2015 is less than 1%, depending on the amplifier (WFC3 ISRs 2017-17, 2015-13, 2009-26).

Table 5.2: WFC3/UVIS readout noise, in electrons, and uncertainty for unbinned and binned modes.

Mode	Amplifier A			Amplifier B			Amplifier C			Amplifier D
Binning	1 × 1	2 × 2	3 × 3	1 × 1	2 × 2	3 × 3	1 × 1	2 × 2	3 × 3	1 × 1	2 × 2	3 × 3
Mean	2.97	3.11	3.22	3.03	3.15	3.26	2.95	2.99	3.09	3.06	3.29	3.38
Uncertainty	0.01	0.02	0.04	0.01	0.01	<0.01	0.00	<0.01	0.01	0.01	0.02	<0.01

An analysis of the statistical behavior of the WFC3 ADCs shows some tendency for the least significant bit to be slightly biased at the readout speed adopted by the WFC3 electronics (see WFC3-ISR 2005-27). This minor effect should not degrade the photometric and noise characteristics of the WFC3/UVIS images.

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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

5.1 Gain and Read Noise

5.1.1 Gain

5.1.2 Read Noise

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