Irregularity in Ceiling of Analog-to-Digital Conversion for Post-SM4 ACS/WFC Images
Abstract
We document a recently noticed phenomenon in which post-SM4 ACS/WFC raw images appear to have a lower pixel value ceiling than expected. During readout of an exposure, if a given pixel would have a value in excess of 2^16 - 1 = 65,535 DN after converting to DN from electrons given the gain setting, the A-to-D converter should simply record a value of 65,535 for that pixel. This is the case for pre-SM4 images, but for post-SM4 images, there appears to be a bug which causes the ceiling value to be slightly lower, and somewhat variable, typically in the range of 64,500 +/- 1,000 DN. The cause of this issue is presently unknown, but it affects only a small number (~100) of archival images and is not expected to affect any future ACS observing programs. The ACS team has now rectified the data quality flagging to catch such A-to-D saturated pixels, which should be treated as unusable.
- Publication:
-
Instrument Science Report ACS 2020-3
- Pub Date:
- March 2020
- Bibcode:
- 2020acs..rept....3C
- Keywords:
-
- HST;
- Hubble Space Telescope;
- Space Telescope Science Institute;
- STScI;
- Advanced Camera for Surveys;
- ACS