Pixel-based correction for Charge Transfer Inefficiency in the Hubble Space Telescope Advanced Camera for Surveys

TL;DR

This paper presents a physically motivated pixel-based correction method for Charge Transfer Inefficiency in HST ACS images, significantly reducing CTI trails and improving data quality for scientific analysis.

Contribution

It introduces a new correction scheme operating on raw data, effectively mitigating CTI effects across all flux levels in HST ACS images.

Findings

Reduced CTI trails by a factor of ~30

Achieved 97% correction efficiency

Enabled improved galaxy photometry and astrometry

Abstract

Charge Transfer Inefficiency (CTI) due to radiation damage above the Earth's atmosphere creates spurious trailing in Hubble Space Telescope (HST) images. Radiation damage also creates unrelated warm pixels - but these happen to be perfect for measuring CTI. We model CTI in the Advanced Camera for Surveys (ACS)/Wide Field Channel (WFC) and construct a physically motivated correction scheme. This operates on raw data, rather than secondary science products, by returning individual electrons to pixels from which they were unintentionally dragged during readout. We apply our correction to images from the HST COSMOS survey, successfully reducing the CTI trails by a factor of ~30 everywhere in the CCD and at all flux levels. We quantify changes in galaxy photometry, astrometry and shape. The remarkable 97% level of correction is more than sufficient to enable a (forthcoming) reanalysis of downstream science products, and the collection of larger surveys.