|Space Telescope Science Institute|
|WFC3 Data Handbook v. 3.0|
7.3.1 Dark Current SubtractionThe 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 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 subtracts it read-by-read from the science observation. Details on the variability of the dark current behavior are also presented in WFC3 ISR 2012-11.Figure 7.1: 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 7.2 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.7.3.2 BandingBanding occurs when an IR observation (or observations) in a smaller subarray immediately precedes on 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, extends 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. The banded region is book ended on top and bottom by single row of pixels with even brighter levels. Figure 7.4 shows 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.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 imageFigure 7.4: Vertical brightness profiles.Plotted are 3-sigma clipped robust mean brightness profiles (of the two images in Figure 7.3) along the y-axis. Note the central banded region and the two higher spikes from the rows that bound itBanding can be prevented by avoiding mixed aperture sizes within the same orbit. If this is unavoidable, observations should be sequenced from largest to smallest aperture sizes.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. For further assistance, contact the STScI Helpdesk (firstname.lastname@example.org).