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).

Figure 7.1: Dark Current Signal vs Time.

The dark current calibration files are created from a stack of dark up-the-ramp datasets which are taken on a regular basis throughout each observing cycle. For each sample sequence, the dark current calibration file is created by calculating the robust (outlier rejected) mean signal for each pixel in each read. Calculated uncertainties in the dark current calibration signals (in the error arrays of these files) are propagated into the error arrays of the calibrated science observations at the time of the dark current subtraction by calwf3. Figure 7.2 illustrates the large-scale dark current structure, showing the measured signal rate in a high signal-to-noise dark current calibration ramp. In general, the upper left quadrant of the detector has the highest dark current, while the upper right has the lowest.

Figure 7.2: Dark Current Image.

7.4.2 Banding

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.

Figure 7.3: Examples of banded images.

Top Left: 64-pixel-high band in a SPARS50 full-frame science image. Top Right: 128-pixel-high band in a SPARS10 256 × 256 subarray dark calibration image. Bottom panels: 3-sigma clipped robust mean along the x-axis of the two images in the top panels. Note the central banded regions and the two higher spikes from the rows that bound them.


Observation Planning

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.