Chromatic CCD effects on weak lensing measurements for LSST

TL;DR

This paper investigates how wavelength-dependent effects of CCD sensors can bias weak lensing measurements in LSST, highlighting the importance of correcting for sensor-induced chromaticity to ensure accurate cosmic shear analysis.

Contribution

The study develops toy models and compares them with simulations to quantify CCD chromatic effects on weak lensing, emphasizing their potential impact on LSST cosmic shear results.

Findings

Sensor contributions to PSF chromaticity are smaller than atmospheric effects.

CCD chromaticity can significantly bias shear measurements if uncorrected.

Biases are more pronounced in red filters and off-axis objects.

Abstract

Wavelength-dependent point spread functions (PSFs) violate an implicit assumption in current galaxy shape measurement algorithms that deconvolve the PSF measured from stars (which have stellar spectral energy distributions (SEDs)) from images of galaxies (which have galactic SEDs). Since the absorption length of silicon depends on wavelength, CCDs are a potential source of PSF chromaticity. Here we develop two toy models to estimate the sensitivity of the cosmic shear survey from the Large Synoptic Survey Telescope to chromatic effects in CCDs. We then compare these toy models to simulated estimates of PSF chromaticity derived from the LSST photon simulator PhoSim. We find that even though sensor contributions to PSF chromaticity are subdominant to atmospheric contributions, they can still significantly bias cosmic shear results if left uncorrected, particularly in the redder filter bands and for objects that are off-axis in the field of view.