6.7 Sink Pixels

With the advent of post-flashing in 2012, a new type of image defect was identified, see WFC3 ISR 2014-04 for the discovery and WFC3 ISR 2014-19 for a more thorough analysis. Such 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. Sink pixels simply have 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 these electrons do not shuffle out of the pixel during the readout process, and are thus not recorded with that pixel. Investigations suggest that sink pixels are created by cosmic ray events ( WFC3 ISR 2014-19, WFC3 ISR 2014-22); thus it appears that most of the sink pixels may be a consequence of on-orbit radiation damage. 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 be recovered.

The impact of sink pixels on the background level depends on their locations in an image. For images with high backgrounds (~85 e¯) and for sink pixels near the readout register, the sink pixels have little effect on upstream pixels in the same column. However, for 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. A single sink pixel can affect both downstream pixels and several upstream pixels, depending on the background (see Figure 3 of WFC3 ISR 2014-19). So although sink pixels are rare (~0.05% of the detector), in low-background imaging they can corrupt as much as ~0.5% of the detector.

Since the behavior of the sink pixels is scene-dependent, the WFC3 team has decided to take a conservative approach to flag all pixels in a given image that are likely to be affected. The impact of a sink pixel on the scene depends on the local scene itself. If a source lands on a sink pixel (or even on the streak of a sink pixel), then the electrons in the source will limit the trail behind the sink in the same way that more background would. In order to calibrate the impact of background on each sink pixel,41,762 sink pixels were identified as producing streaks that affect ~6 x 10^5 pixels WFC3 ISR 2014-22). From this map a new reference is generated: a SNKCFILE that contains the modified-Julian date (MJD) of the appearance of the sink pixel on orbit, as well as information on whether the downstream adjacent pixels are affected by the sink pixel, and on how far upstream the sink pixel trails extend (see Figure 6.2).

The current calwf3 (version 3.3+) uses the SNKCFILE to populate the data quality array of a science image with 1024 for flagged pixels. 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 CTE-correction i.e. both flt and flc will have sink pixel flagged.

Figure 6.2: Schematic of the SNKCFILE sink-pixel reference file.

The figure is reproduced from WFC3 ISR 2016-01 and adapted from WFC3 ISR 2014-22. The figure shows the sink (blue), enhanced downstream (green), and low upstream (red) pixels. Empty pixels have zero and are not impacted by sink pixels. The sink pixel (blue) value refers to the MJD of the appearance of the sink pixel. The red values tell the pipeline when to flag each upstream pixel, based on the value in the sink pixel. If the sink pixel value is less than the listed number, the upstream pixel gets flagged. If the downstream pixel is -1 in the SNKCFILE, then it is always flagged as impacted.