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
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 see the HST
MultiDrizzle Handbook - Section 5.5.7
) or to allow the dark subtraction to stand.
Because the mean dark current in the WFC3 CCDs is so low 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.
Using an old superdark reference file can produce a poor dark correction: either
leaving too many hot pixels uncorrected and unflagged, or creating many negative “holes” caused by the correction of hot pixels which were not actually hot in the science data (i.e., if the detectors were warmed to anneal hot pixels in the interim).
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 of 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.
Dead pixels, specifically dead columns, are identified through visual inspection
from both individual, and stacks of, internal frames. Bad pixel locations are propagated into the bad pixel mask (header keyword BPIXTAB and the file name *_bpx.fits) which is applied by calwf3
in the standard data reduction pipeline.
summarizes the number of hot and dead pixels in each chip. The hot pixel range is the number of hot pixels observed between a single anneal procedure conducted during Cy17, i.e., immediately after an anneal and just preceding the next anneal. Typically, ~1000 new hot pixels appear every day.
We have chosen a limit of 54 e−
/hr (0.015 e−
/s/pix) 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
, where the red vertical lines indicate the dates of the anneal procedures, 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), and the brown vertical line is the switch between SMOV darks (1800-s) and Cycle 17 darks (900-s). Typically, about 90% of the hot pixels are 'fixed' during an anneal procedure, though the fraction varies slightly from procedure to procedure.
The WFC3 CCDs detectors will degrade over time due to exposure to the space
environment. This damage manifests itself in two ways: as an increase in the number of individual hot pixels as well as in an overall higher dark current. Based on a fit to the Cycle 17 900-s dark frames, the median dark current (excluding hot pixels) is increasing by >0.5 e−
/hr/pix/year. The number of permanent hot pixels, i.e., pixels that the anneals are unable to fix, is growing by 0.05-0.1% per month.
The UVIS channel flat-field reference files currently in use by the WFC3
calibration task CALWF3
were created at the Goddard Space Flight Center under thermal vacuum conditions using an external illumination source. These flat fields take into account the pixel-to-pixel variations in QE (P-flats). However, because the overall illumination pattern of the ground-based flats did not precisely match the illumination attained on-orbit from the OTA, there are low-frequency variations in sensitivity over the detector field of view present in the ground-based flats. These variations can be removed using L-flat corrections, which have been derived from in-flight dithered observations of the globular cluster Omega Centauri.