7.4 WFC3 Dark Current and Banding
7.4.1 Dark Current Subtraction
The dark current in the IR detector is the signal measured when no illumination source is present. In an ideal detector, this signal would grow linearly with integration time. In practice, the dark current behavior of the IR detector is dependent upon the timing pattern used to collect each observation and is not constant for the duration of a given
MULTIACCUM ramp. In certain situations, the measured dark current can even be negative. Figure 7.1 shows a plot of the mean measured dark current signal versus time for three different timing patterns. Note that the three curves are not superimposed one another, nor do they show a straight line for the entire duration of the ramps. Details are presented in WFC3 ISR 2009-21. For these reasons, there is a separate
MULTIACCUM dark current reference file for each sample sequence and mode (full-frame, subarray) combination. During pipeline processing, calwf3 subtracts the appropriate dark current ramp read-by-read from the science observation (see WFC3 ISR 2012-11).
The behavior of the IR darks has been analyzed in on-orbit data spanning Sep 2009 - June 2016. Although the darks possess a similar signal pattern across the detector, the median dark rates vary by as much as ~0.03 e¯/s (WFC3 ISR 2017-04). The distribution of these median values has a triangular shape with a mean and standard deviation of 0.049 ± 0.007 e¯/s). There is no apparent systematic long-term trend in the dark signal ( WFC3 ISR 2017-04).
Banding occurs when an IR observation (or observations) in a smaller readout format immediately precedes one in a larger subarray or a full-frame. Banded images exhibit a rectangular region containing pixels whose brightness levels are offset by typically +/- 3-5 DN from values in the rest of the image. The band is centered vertically in the larger (second) image, extending all the way across the image horizontally, and has a height equal to the height of the smaller (first) subarray observation. This is illustrated in Figure 7.3 (top panels). The banded region is outlined on top and bottom by single rows of pixels with even brighter levels. The bottom panels of Figure 7.3 show the respective vertical brightness profiles. For more information, please refer to WFC3 ISR 2011-04 and the last paragraph of Section 7.4.4 of the WFC3 Instrument Handbook.
Banding can be prevented by avoiding mixed aperture sizes within the same orbit. If a variety of aperture sizes are required, observations should be sequenced from largest to smallest aperture sizes.
Dealing with Banded Observations
Some early dark calibration files exhibited banding, so banding may have been imprinted during the calibration process. Observations retrieved from the archive are automatically reprocessed with the latest calibration files (which are now band-free), so simply re-retrieving one's observations from MAST may solve the problem.
If recalibration does not solve the problem, banded observations may still be scientifically viable. For brighter point sources, the effects of the low-level banding may not be significant and/or may be subtracted as sky. For fainter point sources or extended targets that straddle two or more bands, banded observations may still be scientifically salvageable if one can perform independent sky subtraction in each of the band regions.
WFC3 Data Handbook
- • Acknowledgments
- 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
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