Effects of Post-Observation Data Processing on HST/ACS Wide Field Channel Polarimetry

Polarimetric observations with Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) use three polarization filters rotated at 60º angles relative to each other, and the calculation of the Stokes parameters from ACS polarimetric data is sensitive to changes in the relative count rates in the three polarization filters. After nearly 18 years in Low Earth Orbit, the charge transfer efficiency (CTE) has degraded significantly from its initial value, and thus ACS observations are now routinely corrected using a pixel-based CTE correction. The efficacy of the CTE correction has been examined for most ACS applications; however, no examination of the effect of the CTE correction on the calculation of Stokes parameters has been carried out. In this study, we quantify the impact of the ACS Wide Field Channel (WFC) CTE correction algorithm on the characterization of the electric field vector using simulated ACS/WFC observations of point sources in a grid that samples a range of CTE-impacting parameters. The results from this work will inform future GO programs about best strategies for polarimetric observations, and will also instruct users about any potential limitations in archival polarimetry data.