Latest AAS Posters (January 2013)
|Mitigation of CTE Losses in ACS/WFC: Overview of Methods
||Abstract: The charge transfer efficiency (CTE) of the ACS/WFC CCD detectors is declining with time due to the cumulative effects of radiation damage. The methods that have recently become available to observers to mitigate CTE losses are reviewed. These include post-observation corrections, adjusting the observing strategy to minimize CTE losses, and the use of a short post-flash to increase the background.
|Mitigation of CTE Losses in ACS/WFC: Observed Sky Backgrounds
||Abstract: Over time, exposure to the harsh radiation environment of space has diminished the charge transfer efficiencies (CTE) of the ACS CCDs4. While post-processing techniques1 can combat this loss of charge, it is also possible to ameliorate CTE losses in advance: by observing with a natural background high enough (~20 e-) to “pave over” charge traps. HST observers can use the estimates provided here to anticipate the natural background that should be present in their exposures, and can then determine whether they want to supplement that background to improve CTE by lengthening their exposures or through using a post-flash.
|Mitigation of CTE Losses in ACS/WFC: Post-Flash Capabilities
||Abstract: The charge transfer efficiency (CTE) correction that is currently being applied in the Advanced Camera for Surveys Wide Field Channel (ACS/WFC) pipeline effectively corrects lost charge for most cases. However, when the background level is low a large percent of the original signal is lost during read out and cannot be recovered by the CTE correction algorithm.To address these cases, the ACS team is investigating the post-flash capabilities of ACS. Here we present results from our initial analysis of the repeatability, count rate, and gradient across the CCDs.
|Mitigation of CTE Losses in ACS/WFC: Optimal Background Parameters From Simulated Images
||Abstract: The Advanced Camera for Surveys (ACS) team has been exploring ways to further mitigate the effects of charge transfer inefficiency (CTI) on the wide field channel (WFC), in particular at low background levels where losses are so large that the current methods cannot recover the original signal. Using post flash increases the background levels and mitigates losses, although this also increases the effective background noise. We simulated images to represent typical astronomical scenes with various levels of post flash and explored the relationship between signal preserved and noise added. From this we present guidelines for guest observers on how best to optimize signal to noise ratios in their observations.