Cosmic shear requirements on the wavelength-dependence of telescope point spread functions

TL;DR

This paper examines how the wavelength dependence of telescope PSFs affects cosmic shear measurements, demonstrating that proper correction methods are essential for accurate dark energy parameter estimation in future surveys.

Contribution

It introduces and compares two correction methods for PSF wavelength dependence, showing that template-fitting effectively reduces biases below required thresholds.

Findings

Wavelength dependence of PSF causes biases in cosmic shear analysis.

Correcting PSF effects is crucial for next-generation telescopes.

Template-fitting method outperforms broad-band colour method in bias correction.

Abstract

Cosmic shear requires high precision measurement of galaxy shapes in the presence of the observational Point Spread Function (PSF) that smears out the image. The PSF must therefore be known for each galaxy to a high accuracy. However, for several reasons, the PSF is usually wavelength dependent, therefore the differences between the spectral energy distribution of the observed objects introduces further complexity. In this paper we investigate the effect of the wavelength-dependence of the PSF, focusing on instruments in which the PSF size is dominated by the diffraction-limit of the telescope and which use broad-band filters for shape measurement. We first calculate biases on cosmological parameter estimation from cosmic shear when the stellar PSF is used uncorrected. Using realistic galaxy and star spectral energy distributions and populations and a simple three-component circular PSF we find that the colour-dependence must be taken into account for the next generation of telescopes. We then consider two different methods for removing the effect (i) the use of stars of the same colour as the galaxies and (ii) estimation of the galaxy spectral energy distribution using multiple colours and using a telescope model for the PSF. We find that both of these methods correct the effect to levels below the tolerances required for per-cent level measurements of dark energy parameters. Comparison of the two methods favours the template-fitting method because its efficiency is less dependent on galaxy redshift than the broad-band colour method and takes full advantage of deeper photometry.