Characterization of Residual Charge Images in LSST Camera e2v CCDs

TL;DR

This paper investigates residual charge images in LSST Camera CCDs, revealing their dependence on clocking schemes and proposing methods to minimize these effects to improve image quality for the Rubin Observatory LSST.

Contribution

The study characterizes residual charge effects in LSST Camera CCDs and identifies their dependence on clocking parameters, proposing mitigation strategies for systematic image artifacts.

Findings

Residual charge strongly depends on parallel clocking voltages and timing.

Distinct causes identified for residual charge in bright spot and trail regions.

Proposed clocking adjustments may reduce residual charge effects.

Abstract

LSST Camera CCDs produced by the manufacturer e2v exhibit strong and novel residual charge images when exposed to bright sources. These manifest in images following bright exposures both in the same pixel areas as the bright source, and in the pixels trailing between the source and the serial register. Both of these pose systematic challenges to the Rubin Observatory Legacy Survey of Space and Time instrument signature removal. The latter trail region is especially impactful as it affects a much larger pixel area in a less well defined position. In our study of this effect at UC Davis, we imaged bright spots to characterize these residual charge effects. We find a strong dependence of the residual charge on the parallel clocking scheme, including the relative levels of the clocking voltages, and the timing of gate phase transition during the parallel transfer. Our study points to independent causes of residual charge in the bright spot region and trail region. We propose potential causes in both regions and suggest methodologies for minimizing residual charge. We consider the trade-offs to these methods including decreasing the camera's full well and dynamic range at the high end. Some of these results and suggestions have been reviewed by the camera commissioning team and may result in changes made to the clocking voltage scheme on the LSST Camera.