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Twenty-One Years of the Hubble Space Telescope’s Advanced Camera for Surveys: Calibration Update

Presentation #230.09 in the session Instrumentation for Space Missions.

Published onJul 01, 2023
Twenty-One Years of the Hubble Space Telescope’s Advanced Camera for Surveys: Calibration Update

The Advanced Camera for Surveys (ACS) has been a workhorse Hubble Space Telescope (HST) imager for over twenty-one years, subsequent to its Servicing Mission 3B installation in 2002. The ACS Solar Blind Channel (SBC) continues to offer unique access to high-resolution, high-sensitivity imaging in the far-UV. The once defunct ACS Wide Field Channel (WFC) has now been operating far longer (>14yrs) since its Servicing Mission 4 (SM4) repair than it had originally operated prior to its 2007 electronics failure. Despite the accumulating radiation damage to the WFC CCDs during their long stay in low Earth orbit, ACS remains heavily exploited by the HST community as both a prime and a parallel detector. Ongoing detector-calibration efforts by the STScI ACS Team consistently deliver the highest level of performance to the ACS users. We present results from the latest studies of detector performance for both WFC and SBC, including long-term monitoring of WFC and SBC dark current, WFC readout noise, and WFC CCD pixel stability. We also advertise updated ACS documentation and software tools for the user community. Highlights presented here include: 1) a re-analysis of the absolute gain values among the four WFC CCD readout amplifiers at the default (GAIN=2) setting, using a new algorithm that has revealed systematic gain-estimation biases due to degraded CCD charge-transfer efficiency (CTE) and due to increased CCD pixel charge-diffusion at high intensity levels; 2) a revised modeling of the empirical WFC aperture-photometric losses due to CTE, incorporating nonlinearity in flux-dependence that has been observed for bright point-sources; 3) a recent bug-fix to the sub-pixel phase sampling of the oft-used WFC four-point compact dither pattern, as well as a recently designed suite of four-point compact dither patterns optimized for joint sub-pixel phase sampling of both the WFC detector and the WFC3 detectors (both for WFC3/UVIS and for WFC3/IR) when observing in parallel; and 4) an advertisement of the ACS operating mode newly offered for HST Cycle 31: high-precision optical/near-IR imaging spectropolarimetry at high spatial resolution (~0.1 arcsec) by crossing the WFC grism and polarizer filters. More extensive details of this new ACS spectropolarimetry capability will be presented in a companion poster at this conference (AAS 242).

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