Revisiting x-CTE in WFC3/UVIS

TL;DR

This paper revisits the serial charge-transfer efficiency (x-CTE) in WFC3/UVIS, quantifies its impact on images, and develops a correction model, highlighting its smaller effect compared to parallel CTE but potential importance for high-precision astrometry.

Contribution

The study provides a detailed characterization of serial CTE in WFC3/UVIS after extended space exposure and introduces a pixel-based correction model for x-CTE effects.

Findings

Serial CTE has a smaller impact than parallel CTE on images.

Serial CTE trails are sharper initially but extend farther and wrap around rows.

A pixel-based correction model for serial CTE has been developed and released.

Abstract

Most discussion of charge-transfer-efficiency (CTE) losses involves the parallel transfer of charge in the y-direction down the chip columns (y-CTE). Serial charge-transfer efficiency (x-CTE) refers to the horizontal transer of charge. Serial CTE losses were first assessed in WFC3/UVIS in 2014 (WFC3/ISR 2014-02), where it was found that bright stars were shifted by roughly 0.0015 pixel away from the readout amplifier. Now that the WFC3/UVIS CCD has spent more than three times as long in space, imperfect serial CTE should have three times the impact on the images, so we revisit the effect and characterize serial CTE in a similar manner to our models for parallel-CTE by using a combination of warm pixels, cosmic rays, saturated stars, and overscan pixels. Overall, serial CTE has a much smaller impact on images than does parallel CTE. As of 2023, parallel CTE can have a roughly 5 percent impact on bright sources and a greater than 50 percent impact on faint sources, and serial CTE can have a roughly 1% impact on bright sources and a roughly 3 percent impact on fainter sources. The first few pixels in the serial CTE trails are much sharper than the parallel trails, but there is a very faint component to the serial trails that extends much farther than the parallel trails, even wrapping around to the next row. We develop a pixel-based model for the trapping and release of charge in the serial register and release a stand-alone beta-version of this pixel-based serial-CTE correction. Most images are not significantly impacted by the x-CTE effect, however HST users that require high-precision astrometry could benefit from this correction. At the very least, curious users can quantify its potential impact on their science.