The Effectiveness of Post-flashing ACS/WFC Observations of Unresolved Sources
January 04, 2020N. Miles, N. A. Grogin, and A. Bellini
Throughout the almost 18 year lifetime of the Advanced Camera for Surveys Wide Field Channel (ACS/WFC), the charge transfer efficiency (CTE) of its two CCDs has been steadily declining as a result of radiation damage. One way to minimize the effects of an imperfect CTE is by increasing the background level. This is done by either increasing the exposure time or by using the ACS/WFC LED to post-flash the observation. By increasing the background level, many of the charge traps responsible for the imperfect CTE become filled prior to readout. As of late 2019, the ACS Team generally cautions against the use of post-flash because of the additional noise imparted by the post-flash correction. Here we present the results of an analysis into the effectiveness of post-flashing observations of unresolved sources as a means to mitigate the effects of imperfect CTE. We generate a series of images containing artificial stars of varying magnitudes and different amounts of post-flash background. We use the CTE forward modeler available in ACSTOOLS to simulate the effects of readout. Finally, for each source we compare the recovered signal to its known value to examine CTE losses as a function of both S/N and background level when post-flashing an observation of unresolved sources.