An Empirical Pixel-Based Correction for Imperfect CTE. I. HST's Advanced Camera for Surveys

TL;DR

This paper presents an empirical pixel-based correction method for charge-transfer efficiency (CTE) losses in HST's ACS images, restoring flux, position, and shape by modeling and reversing charge traps effects.

Contribution

It introduces a novel empirical model to correct CTE losses in ACS images, applicable across different epochs and potentially adaptable to other instruments.

Findings

Restores flux, position, and shape in science images.

Effectively models charge traps affecting electrons.

Applicable to various background levels and star types.

Abstract

We use an empirical approach to characterize the effect of charge-transfer efficiency (CTE) losses in images taken with the Wide-Field Channel of the Advanced Camera for Surveys. The study is based on profiles of warm pixels in 168 dark exposures taken between September and October 2009. The dark exposures allow us to explore charge traps that affect electrons when the background is extremely low. We develop a model for the readout process that reproduces the observed trails out to 70 pixels. We then invert the model to convert the observed pixel values in an image into an estimate of the original pixel values. We find that when we apply the image-restoration process to science images with a variety of stars on a variety of background levels, it restores flux, position, and shape. This means that the observed trails contain essentially all of the flux lost to inefficient CTE. The Space Telescope Science Institute is currently evaluating this algorithm with the aim of optimizing it and eventually providing enhanced data products. The empirical procedure presented here should also work for other epochs (eg., pre-SM4), though the parameters may have to be recomputed for the time when ACS was operated at a higher temperature than the current -81 C. Finally, this empirical approach may also hold promise for other instruments, such as WFPC2, STIS, the ACS's HRC, and even WFC3/UVIS.