Mitigation of the Brighter-Fatter Effect in the LSST Camera

TL;DR

This paper investigates the brighter-fatter effect in LSST CCDs, introduces a new correction method accounting for higher-order effects, and evaluates its performance to improve weak-lensing measurements.

Contribution

The authors develop a physically motivated correction technique for the brighter-fatter effect that includes higher-order terms, enhancing the accuracy of PSF modeling in LSSTCam.

Findings

The correction improves flat field measurements more than artificial star images.

Higher-order effects cause anisotropic evolution of the BFE, violating previous correction assumptions.

A new metric for sensor full-well capacity is proposed to optimize image quality.

Abstract

Thick, fully depleted charge-coupled devices (CCDs) are known to exhibit non-linear behavior at high signal levels due to the dynamic behavior of charges collecting in the potential wells of pixels, called the brighter-fatter effect (BFE). This particularly impacts bright calibration stars, which appear larger than their intrinsic shape, creating a flux-dependent point-spread function (PSF) that if left unmitigated, could make up a large fraction of the error budget in Stage IV weak-lensing (WL) surveys such as the Legacy Survey of Space and Time (LSST). In this paper, we analyze image measurements of flat fields and artificial stars taken at different illumination levels with the LSST Camera (LSSTCam) at SLAC National Accelerator Laboratory in order to quantify this effect in the LSST Camera before and after a previously introduced correction technique. We observe that the BFE evolves anisotropically as a function of flux due to higher-order BFEs, which violates the fundamental assumption of this correction method. We then introduce a new sampling method based on a physically motivated model to account these higher-order terms in the correction, and then we test the modified correction on both datasets. We find that the new method corrects the effect in flat fields better than it corrects the effect in artificial stars which we conclude is the result of a unmodeled curl component of the deflection field by the correction. We use these results to define a new metric for the full-well capacity of our sensors and advise image processing strategies to further limit the impact of the effect on LSST WL science pathways.