Consequences of CCD imperfections for cosmology determined by weak lensing surveys: From laboratory measurements to cosmological parameter bias

TL;DR

This study investigates how specific CCD sensor imperfections, like tree-rings and pixel-size variations, affect weak lensing measurements and cosmological parameter estimation, finding that these effects are negligible for LSST-like surveys.

Contribution

The paper develops methods to quantify and correct for CCD imperfections' impact on weak lensing, applying them to LSST prototype sensors to assess bias in cosmological parameters.

Findings

Tree-ring and pixel-size variations cause negligible bias in LSST cosmological parameters.

Methods to extract spurious shear from flat-field images are successfully developed.

Laboratory measurements of LSST prototype CCDs validate the minimal impact of sensor imperfections.

Abstract

Weak gravitational lensing causes subtle changes in the apparent shapes of galaxies due to the bending of light by the gravity of foreground masses. By measuring the shapes of large numbers of galaxies (millions in recent surveys, up to tens of billions in future surveys) we can infer the parameters that determine cosmology. Imperfections in the detectors used to record images of the sky can introduce changes in the apparent shape of galaxies, which in turn can bias the inferred cosmological parameters. In this paper we consider the effect of two widely discussed sensor imperfections: tree-rings, due to impurity gradients which cause transverse electric fields in the Charge-Coupled Devices (CCD), and pixel-size variation, due to periodic CCD fabrication errors. These imperfections can be observed when the detectors are subject to uniform illumination (flat field images). We develop methods to determine the spurious shear and convergence (due to the imperfections) from the flat-field images. We calculate how the spurious shear when added to the lensing shear will bias the determination of cosmological parameters. We apply our methods to candidate sensors of the Large Synoptic Survey Telescope (LSST) as a timely and important example, analyzing flat field images recorded with LSST prototype CCDs in the laboratory. We find that tree-rings and periodic pixel-size variation present in the LSST CCDs will introduce negligible bias to cosmological parameters determined from the lensing power spectrum, specifically w, {\Omega}m and {\sigma}8.