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.
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
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.
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.
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.