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Evolution of Sink Pixels in ACS/WFC and Connection to Charge Transfer Efficiency

Alyssa M. Guzman, Jenna E. Ryon

Abstract

In our study spanning 2015-2021, we examined sink pixels (SPs) in the Advanced Camera for Surveys Wide Field Channel (ACS/WFC) using dark and SP reference files. SPs are pixels with values $\le$ $-10$ electrons below the local background of LED-flashed short (0.5 sec) darks, that collect and trap significant charge during readout. Analyzing seven years of short dark data, we assessed SP creation and persistence. In this time frame, 5,430 SPs were created in WFC1 and 5,649 SPs in WFC2, with creation rates of about 2.15 pixels/day and 2.23 pixels/day, respectively. These calculations allowed us to detect 44,068 SPs, not including SP trails, in the detector by the end of 2021, constituting approximately 0.25\% of the science frame. We found it is rare for SPs to return to a typical, non-negative pixel value. We observed more flagged SPs near the serial register than the chip gap. Skewed histograms for the $y$-position distribution, exhibiting a local peak in the distribution of SPs very near the chip gap described as the ``bounce-back" effect, were evident for both WFC1 and WFC2, while the $x$-position distribution remained uniform. Examining CTE-corrected images from 2015, 2018, and 2021 revealed consistent trends, with the gradient getting steeper over time due to CTE losses, which is also worse for pixels further from the serial register. We simulated the CTE-impacted readout of a short dark exposure with uniformly distributed SPs, to assess how CTE influences SP detectability. While the gradient effect was reproduced, the local peak near the chip gap was not. Filling in of SPs by CTE charge-release during readout appears to explain most of the gradient in the $y$-position density of SPs.

Evolution of Sink Pixels in ACS/WFC and Connection to Charge Transfer Efficiency

Abstract

In our study spanning 2015-2021, we examined sink pixels (SPs) in the Advanced Camera for Surveys Wide Field Channel (ACS/WFC) using dark and SP reference files. SPs are pixels with values electrons below the local background of LED-flashed short (0.5 sec) darks, that collect and trap significant charge during readout. Analyzing seven years of short dark data, we assessed SP creation and persistence. In this time frame, 5,430 SPs were created in WFC1 and 5,649 SPs in WFC2, with creation rates of about 2.15 pixels/day and 2.23 pixels/day, respectively. These calculations allowed us to detect 44,068 SPs, not including SP trails, in the detector by the end of 2021, constituting approximately 0.25\% of the science frame. We found it is rare for SPs to return to a typical, non-negative pixel value. We observed more flagged SPs near the serial register than the chip gap. Skewed histograms for the -position distribution, exhibiting a local peak in the distribution of SPs very near the chip gap described as the ``bounce-back" effect, were evident for both WFC1 and WFC2, while the -position distribution remained uniform. Examining CTE-corrected images from 2015, 2018, and 2021 revealed consistent trends, with the gradient getting steeper over time due to CTE losses, which is also worse for pixels further from the serial register. We simulated the CTE-impacted readout of a short dark exposure with uniformly distributed SPs, to assess how CTE influences SP detectability. While the gradient effect was reproduced, the local peak near the chip gap was not. Filling in of SPs by CTE charge-release during readout appears to explain most of the gradient in the -position density of SPs.
Paper Structure (11 sections, 10 figures)

This paper contains 11 sections, 10 figures.

Table of Contents

  1. Introduction
  2. Data and Analysis
  3. Evolution of Sink Pixels
  4. Initial Results
  5. Sink Pixels and CTE Effects
  6. FLT Short Darks
  7. SP Reference Files
  8. FLC Short Darks
  9. Simulation of Short Darks and Further Results
  10. Conclusion
  11. Acknowledgements

Figures (10)

  • Figure 1: A 50x50 pixels region of 2021-12-07 snk.fits showing SPs and their trails for WFC1 (left) and WFC2 (right). SPs cause trails in the opposite direction of readout because they trap charge as the data is read out.
  • Figure 2: Four example pixels from WFC1 that were flagged as SPs, along with their creation dates and their locations at the bottom right of each subplot. The red line represents the pre-break mean value of this pixel, before becoming a SP, while the blue line is the post-break mean value. The purple dashed line marks the flagged date of the SP.
  • Figure 3: Accumulation plot of SPs for WFC1 (left) and WFC2 (right). The number of new SPs appearing in each anneal since January 2015 is plotted in blue, while the fitted line is red. There are approximately 60.43 pixels per month that are becoming SPs in WFC1 and approximately 62.63 pixels per month becoming SPs in WFC2.
  • Figure 4: Four examples out of 11 healed SPs from WFC1 that became regular pixels in the time frame of the data set, along with their flagged dates and their locations at the top left of each subplot. The red line represents the pre-break mean value of this SP, before becoming a regular pixel, while the blue line is the post-break mean value. The purple dashed line marks the flagged date of the healed pixel.
  • Figure 5: Locations of pixels flagged as SPs in WFC1 (top) and WFC2 (bottom) science frame from the flash-subtracted short darks (left column), using 2021-12-07 as the reference anneal. The two histograms are the distribution of SPs in the $y$ (middle column) and $x$ (right column) direction. The $y$-pixel range goes from 0-2047 and the $x$-pixel range is 0-4095.
  • ...and 5 more figures