Spurious shear induced by the tree rings of the LSST CCDs

TL;DR

This paper analyzes how tree rings in LSST CCD sensors induce spurious shear signals that could bias cosmic shear measurements, finding the effect to be negligible at the survey scale.

Contribution

It quantifies the spurious shear caused by tree rings in LSST sensors and assesses its impact on cosmic shear measurements.

Findings

Spurious shear correlation function is about 10^{-13} for LSST-scale areas.

The induced shear bias is negligible for cosmic shear analysis.

Additional detailed analysis is forthcoming.

Abstract

We present an analysis of the impact of the tree rings seen in the candidate sensors of the Large Synoptic Survey Telescope (LSST) on galaxy-shape measurements. The tree rings are a consequence of transverse electric fields caused by circularly symmetric impurity gradients in the silicon of the sensors. They effectively modify the pixel area and shift the photogenerated charge around, displacing the observed photon positions. The displacement distribution generates distortions that cause spurious shears correlated with the tree-rings patterns, potentially biasing cosmic shear measurements. In this paper we quantify the amplitude of the spurious shear caused by the tree rings on the LSST candidate sensors, and calculate its 2-point correlation function. We find that 2-point correlation function of the spurious shear on an area equivalent to the LSST field of view is order of about $10^{-13}$, providing a negligible contribution to the 2-point correlation of the cosmic shear signal. Additional work is underway, and the final results and analysis will be published elsewhere (Okura et al. (2015), in prep.)