New and Improved Saturated Pixel Flagging for the ACS/WFC

Accurate characterization of the saturation level of the ACS/WFC CCD is crucial for proper flagging of affected pixels, which users and calibration routines require knowledge of. In this work, we present a new analysis of the saturation level that offers significant improvements and advantages over previously used methods. Unlike previous work, we measure the onset of saturation directly by identifying the precise charge level at which the brightest pixel of point sources begins to spill charge into neighboring pixels. This results in a sharp decrease in the fraction of charge contained in the central pixel, coupled with a sharp increase in the fraction of charge contained in neighboring pixels. Through this analysis, we find that the saturation level has a strong spatial dependence over the detector area and exhibits significant (+/- ~6% about the mean) variations, in agreement with previous work. Despite this qualitative agreement, we find that the saturation level currently used in the CALACS calibration pipeline to flag affected pixels is much too high, causing it to routinely miss many clearly saturated pixels. When using our new saturation map to perform the flagging, we find visually superior results and as many as ~15% more pixels being flagged as saturated in any given frame. We announce plans to implement our new saturation map into CALACS, and discuss extensions of this work.