5.3 Dark Current and Hot Pixels

5.3.1 Dark Current

Superdark reference files are generated on a daily basis, with typically between 10 - 18 dark images (acquired over 4-5 days) in each superdark. The individual darks are recalibrated with the latest superbias file and most recent calwf3 software version, stacked to remove cosmic rays and converted from DN to e-/sec. Any pixels with values > 54 e-/hr 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 science image *_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 DrizzlePac Handbook) or to allow the dark subtraction to stand. Because the mean dark current in the WFC3 CCDs is so low (~8e-/hr/pixel in late-2017) it is very difficult to achieve, with only 10-18 dark images, 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 an anneal-cycle-averaged value (i.e. each good pixel in the superdark reference image is set to the average value of that pixel over all individual darks in an anneal cycle, of order 100, see WFC3 ISR 2016-08) in the chip. Users can verify whether the darkfile most appropriate to their observations has been installed for pipeline use in several different ways:

  • re-retrieving the images from the MAST, which automatically updates the headers and recalibrates the data
  • by checking the CRDS ( Calibration Reference Data System).
  • using Starview to obtain a list of best reference files

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).

5.3.2 Hot Pixels

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 0.015 e-/sec (54 e-/hr). The cutoff was chosen based on the tail of the dark histogram in early on-orbit data (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 a small percentage 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 of 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) that is applied by calwf3 in the standard data reduction pipeline. Currently there are ~8000 dead pixels in chip 1 (amps A and B) and ~16000 in chip 2 (amps C and D).

Figure 5.1: Histograms of three dark images (hot pixel threshold of 0.015 e-/sec marked with vertical dotted line).

We have chosen a limit of 0.015 e-/s/pixel (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 a fraction 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 pixel/day (see WFC3 ISR 2016-08 for more information on darks and 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 alternating grey and white regions while the red vertical lines represent the Science Instrument Command and Data Handling Unit (SIC & DH) lockups, when (prior to Oct 2009, WFC3 SIC&DH lockups warmed the chips i.e. essentially another anneal).

Since Nov 8, 2012 (green vertical line), the UVIS darks have been post-flashed, i.e., a background of 12 e-/pixel is applied to each image. Post-flashing preserves faint signal in pixels far from the amps that otherwise would have been undetectable due to CTE losses. The result is a jump in the number of *detected* hot pixels. In addition, the suppression of CTE trails by the post-flash also causes a commensurate reduction in the overall measured dark current. CTE trails are also removed by the pixel-based algorithms. Dark current and hot pixel plots are updated, as resources allow, on the WFC3 Performance Monitoring webpage.

The WFC3 CCD detectors 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 6). Based on a fit to non post-flashed dark frames taken since launch, the median dark current (excluding hot pixels) is increasing by ~0.5 e-/hr/pixel/year and is currently at ~8 e-/hr/pixel ( 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.

Figure 5.2: Percentage of UVIS hot pixels (>0.015 e-/s) versus time

At top: Percent of UVIS hot pixels (>0.015 e-/s) versus time for Chip 1 (top left) and Chip 2 (right). At bottom: median UVIS dark current as a function of time. The green vertical line (Nov 8, 2012) marks the start of post-flash for UVIS dark frames (see text). Monthly anneal intervals are marked by the grey and white regions.The red vertical lines mark SIC & DH failures. (The published plots were updated on March 20th 2017.)