Superdark reference files are generated every four days, with typically between 10 - 18 dark images in each superdark. The individual darks are recalibrated with the latest superbias file and most recent calwf3
software version, stacked to remove cosmic rays, converted from DN to electrons, and normalized to 1 sec. Any pixels with values > 0.015e-/sec are considered hot; their values are left unchanged in the science extension and flagged with a value of 16 in the DQ extension which is propagated into the final *_flt.fits DQ extensions. In this way, observers can decide whether to ignore hot pixels (for instructions on how to control which bit masks are used during drizzling, please consult the HST AstroDrizzle
Handbook) or to allow the dark subtraction to stand. Because the mean dark current in the WFC3 CCDs is so low (~5e-/hr/pix in mid-2013) it is very difficult to achieve a useful signal-to-noise for pixels that have normal levels of dark current. Subtracting these uncertain values from science images during calwf3
processing would introduce noise into the calibrated images; therefore all good (non-hot) pixels in the SCI extensions of superdark reference images are set to the median value of the good pixels in the chip. Users can verify whether the darkfile most appropriate to their observations has been installed for pipeline use in several different ways:
Two types of bad pixels are routinely monitored using on-orbit WFC3 data: hot pixels and dead pixels. Hot pixels, i.e., those pixels with a higher than normal dark current, are identified in dark frames using a threshold 0.015 e-/sec (54 e-/hr). The cutoff was chosen based on the tail of the dark histogram (see Figure 5.1
) as well as visual examination of the dark frames. The number of hot pixels increases over time due to on-orbit radiation damage; periodic anneal procedures, where the UVIS detector is warmed to ~20C, successfully fix about 90% of the hot pixels. Hot pixel locations and levels are provided in the UVIS superdark reference files which are subtracted from science data though dithering can mitigate their effect as well.
We have chosen a limit of 0.015 e-/s/pix (54 e-/hr) as a threshold above which we consider a pixel to be "hot", based on the tail of the histogram as well as a visual examination of 900-s dark frames taken during Cycle 17. Figure 5.1
shows a histogram of CR-free pixels from 900-s darks taken at three different times after the April 2010 anneal procedure: immediately following the procedure (red line), about 10 days later (green line) and about 18 days later (blue line). The increase in hot pixels due to on-orbit radiation damage is apparent; the anneal procedures have been found to fix 80-90% of the hot pixels which accumulate over time. The hot pixel cutoff is shown with a vertical line at 54 e-/hr; at this threshold, the growth rate for WFC3 hot pixels is ~1000 pix/day.
While the number of hot pixels increases over time due to the continual damage the detectors sustain in the harsh on-orbit radiation environment, periodic anneal procedures, which warm the CCD chips, are able to reduce the number of hot pixels. Figure 5.2
shows the number of hot pixels as a function of time since the installation of WFC3 on HST. The monthly anneal intervals are represented by the grey regions while the orange vertical lines represent the Science Instrument Command and Data Handling Unit (SIC&DH) failures, when WFC3 was safed (prior to Oct 2009, WFC3 safings warmed the chips slightly).
The WFC3 CCDs detectors will degrade over time due to exposure to the space environment. This damage manifests itself in the darks as an increase in the number of individual hot pixels as well as in an overall higher dark current (CTE losses, another manifestation of damage, is discussed in Chapter 7
). Based on a fit the unpost-flashed dark frames taken since launch, the median dark current (excluding hot pixels) is increasing by ~1.4 e-/hr/pix/year and is currently at ~5 e-/hr/pix (Figure 5.2
). The number of permanent hot pixels, i.e., pixels that the anneals are unable to fix, is growing by about 35/chip/day, or 0.05-0.1% per month.