Characterization and Mitigation of ACS/WFC Signal-Dependent Bias Shift

Astronomy and Astrophysics – Astronomy

Scientific paper

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Scientific paper

Subsequent to HST Servicing Mission 4 in 2009, the four read-out amplifiers of the Advanced Camera for Surveys Wide Field Channel (ACS/WFC) exhibit similar but distinct shifts in their pixel-to-pixel bias levels that are signal-dependent. For zero-second bias frames, the bias shift manifests as a smooth gradient across each of the ACS/WFC image quadrants. The peak-to-peak amplitude of the fluctuation for bias frames is large (14-20 e-) but highly repeatable, so the effect is cleanly removed with calibration super-biases.
The signal-dependent component of the bias
shift is linearly proportional to the pixel signal and decays nearly linearly over the course of several hundred serial transfers. Typically the amplitude of the signal-dependent shift is far below the pixel-to-pixel ACS/WFC read noise of 4e-. However, ACS/WFC fields containing many consecutive high-intensity pixels (e.g., Saturn) can develop excess bias-level distortions of 30e- or more, potentially compromising science with these images. The amplitude of the bias shift is electronically well determined, and thus near-perfectly removable from ACS/WFC images.
We present the formula for modeling and removing the effect, using a small number of parameters fit independently to bias frames and to images of Saturn recorded through each ACS/WFC amplifier. The ACS Instrument Team is currently considering the incorporation of this correction into the automated image reduction pipeline, or at least the distribution of this code to the community as a stand-alone in the STSDAS software suite.

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