Unusual Horizontal Charge Overflow from Saturated CCD Pixels in the ACS WFC: Discovery and Remediation
Abstract
When CCD pixels become saturated during an integration, excess charge may begin to spill over into neighboring pixels. This effect is commonly referred to as `blooming', and the architecture of the CCD detector makes it such that this blooming is expected to occur only in the same column as the saturated pixel (vertically). However, for the ACS/WFC CCD, we recently noticed a previously-undocumented effect whereby a relatively small fraction (< 10%) of excess charge from saturated pixels spills over into neighboring pixels in the same row (horizontally). In this report, we present the characteristics of this horizontal spilling of charge, or `x-bloom', and a resulting pipeline to automatically identify and flag affected pixels. We demonstrate that proper accounting for the charge displaced by x-bloom, in addition to that lost to traditional vertical bloom, enables high accuracy photometry of arbitrarily saturated sources. This result can significantly increase the science value of archival WFC data, since a large fraction of archival images contain saturated sources which are subject to x-bloom.
- Publication:
-
Instrument Science Report ACS 2020-7
- Pub Date:
- October 2020
- Bibcode:
- 2020acs..rept....7C
- Keywords:
-
- HST;
- Hubble Space Telescope;
- Space Telescope Science Institute;
- STScI;
- Advanced Camera for Surveys;
- ACS