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WFC3 Data Handbook 2.1 May 2011
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WFC3 Data Handbook > Chapter 6: WFC3-IR Error Sources > 6.3 WFC3 Dark Current and Banding

6.3
6.3.1
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 6.1 shows a plot of the mean measured dark current signal versus time for three different timing patterns. Note that the three curves do not overlie 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. During pipeline processing, calwf3 uses the appropriate dark current ramp and subtracts it, read-by-read, from the science observation.
Figure 6.1: Dark Current Signal vs. Time.
The dark current calibration files are created from many dark observations, 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 6.2 provides a general idea of the large-scale dark current structure. This figure shows 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 6.2: Dark Current Image.
Dark current image for a high signal to noise observation. Histogram equalization stretch
from 0 to 0.4 e/s
6.3.2
Banding has been observed in some IR subarray dark calibration images, and some subarray and full-frame external science images. Banded images exhibit a rectangular region containing pixels with brightness levels that are significantly different (typically +/- 3-5 DN) from values in the rest of the image. This region is vertically centered within the image and extends all the way across the image horizontally into the reference pixels. The banded region is “bookended” on top and bottom by single row of pixels with “discontinuous” brightness levels (see sample images and plots below). The vertical extent of the banded region is always exactly one-half the image height: 64 pixels high in a 128x128 subarray image, 128 pixel high in a 256x256 image, and so one. Figure 6.3 shows two example images, and Figure 6.4 their respective vertical brightness profiles.
Figure 6.3: Examples of banded images.
Left: 64-pixel-high band in a SPARS50 full-frame external science image. Right: 128-pixel-high band in a SPARS10 256x256 subarray dark calibration image.
Figure 6.4: Vertical brightness profile.
Plotted above are 3-sigma clipped robust mean brightness profile (of the two images in Figure 6.3) along the y-axis. Note the central banded region and the discontinuous rows that bound it.
Behavior
One of the most puzzling properties of the banding anomaly is the fact that under the right conditions, isolated images of a type that normally do not exhibit any trace of banding can in fact show strong banding. Of particular interest is the fact that in nearly every case, a subarray image whose size exactly matches the vertical height of the band was taken minutes to an hour prior. For example, several 64x64 subarrary images were taken just minutes prior to the banded full-frame science image illustrated above—which has a 64-pixel wide band.
Calibration of banding is another open issue. It is not fully understood how banding affects external science images, and if the anomaly can be calibrated out. Further complicating the issue is the fact that many (but not all) subarray dark calibration files exhibit strong banding. Table 6.1 shows the preliminary results for all the subarrays that have been seen with the banding effect. We will continue to monitor all the subarrays in cycle 18.
Table 6.1: Preliminary results of banding survey
Mitigation
Because the effects of banding on calibrated images and dark calibration files is not fully understood, the best course of action for observers is to run calwf3 on one’s data to manually recalibrate twice — once with DARKCORR set to “OMIT”, and once with DARKCORR set to “PERFORM” (see Section 3.7 for examples). This will allow an assessment to be made of what effect, if any, banding has on one’s observations. Contact the STScI help desk for additional assistance.
In summary, banding is still not that well understood and very much an open issue. The behavior of the banding anomaly is complex enough that additional analysis is required to fully determine the root cause.

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