6.7 Sink Pixels

With the advent of post-flashing in 2012, a new type of image defect was identified (WFC3 ISR 2014-04 for the discovery, WFC3 ISR 2014-19 for additional analysis). The sink defects are caused by pixels that contain a modest number of charge traps (typically 20–100 e¯). When read out, these sink pixels do not correctly report the number of electrons that were generated in them by photons and thus exhibit lower counts than adjacent “normal” pixels (see Figure 4 of WFC3 ISR 2014-22). This phenomenon is distinct from normal pixel-to-pixel sensitivity variations, in that photons that interact with sink pixels do generate electrons, but some of those electrons do not shuffle out of the pixel during the readout process, and are thus not recorded with that pixel. Investigations suggest that most of the sink pixels may be a consequence of on-orbit radiation damage (WFC3 ISR 2014-19, WFC3 ISR 2014-22). Only a very small population of sink pixels were found in data taken before launch (WFC3 ISR 2014-19WFC3 ISR 2014-22). At present, no sink pixel has been found to heal or recover.

The impact of sink pixels depends both on their locations in an image and the image background. For images with high backgrounds (~85 e¯/pixel) and for sink pixels near the readout register, the sinks have little effect on upstream or downstream pixels in the same column. However, for images with lower backgrounds or for pixels far from the readout register, the interplay between CTE losses and sink pixels can extend the sink pixel profile to more than 10 pixels (see Figure 3 of WFC3 ISR 2014-19). So although sink pixels are rare (~0.25% of the detector), in low-background imaging they can corrupt as much as ~0.5% of the detector. The behavior of the sink pixels is also scene-dependent e.g. if a source lands on a sink pixel (or the streak of a sink pixel), then the electrons in the source will limit the trail behind the sink in the same way that a higher background would. Thus the WFC3 team has adopted a conservative approach to flag all pixels that are likely to be affected in a given image; observers can choose whether to use or ignore the sink pixel flags (DQF value of 1024, see Table 3.2) in the science image DQF files.

The calibration pipeline calwf3 (version 3.3+) uses the sink pixel reference file (SNKCFILE) to populate the data quality array of a science image with 1024 for all flagged sink pixels. The sinks are identified in the SNKCFILE with the modified-Julian date (MJD) of the appearance of the sink pixel; any upstream/downstream adjacent pixels affected by the sink pixel are flagged as well (see Figure 6.4 and Section 6 in WFC3 ISR 2014-22). More details on the flagging process are given in Section 3.2.7, as well as in WFC3 ISR 2014-22. Note that the sink pixels flagging is performed regardless of whether the CTE-correction is performed, i.e. both flt and flc will have sink pixels flagged in the DQ extensions.

Figure 6.4: Schematic of the SNKCFILE sink-pixel reference file.

A subsection of the SNKCFILE with sinks (blue), enhanced downstream (green), and low upstream (red) pixels marked. Empty pixels in the reference file have a value of zero i.e. those pixels are unaffected by sink pixels and their trails. The sink pixel value (blue) in the SNKCFILE encodes the date (in MJD) that the sink pixel appeared; science data acquired before that date will not have that sink or its up/downstream pixels flagged. If the downstream pixel is less than zero in the SNKCFILE, then it is always flagged as impacted. The red values allow the calwf3 pipeline to determine whether to flag an upstream pixel, based on the science data value at the location of the sink pixel. If the sink pixel value is less than the listed number, the upstream pixel gets flagged (see Section 6 in WFC3 ISR 2014-22 for more details). Note that the flag value propagated into the science image DQF is '1024' for any pixels impacted by sinks.  The figure is reproduced from WFC3 ISR 2016-01, adapted from Figure 13 in WFC3 ISR 2014-22