Calibration biases in measurements of weak lensing

¹ Zentrum für Astronomie der Universität Heidelberg, ITA, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
e-mail: mbartelmann@ita.uni-heidelberg.de
² Zentrum für Astronomie der Universität Heidelberg, ARI, Mönchhofstr. 12–14, 69120 Heidelberg, Germany
³ Center for Cosmology and Astro-Particle Physics, The Ohio State University, 191 W. Woodruff Ave., Columbus, Ohio 43210, USA
⁴ Department of Physics, The Ohio State University, 191 W. Woodruff Ave., Columbus, Ohio 43210, USA

Received: 30 March 2011
Accepted: 13 October 2012

Abstract

As shown recently, the common KSB method for measuring weak gravitational shear creates a non-linear relation between the measured and the true ellipticity of objects. We investigate here what effect such a non-linear calibration relation may have on cosmological parameter estimates from weak lensing if a simpler, linear calibration relation is assumed. We show that the non-linear relation introduces a bias in the shear-correlation amplitude and thus a bias in the cosmological parameters Ω_m0 and σ₈. Its direction and magnitude depends on whether the point-spread function is narrow or wide compared to the galaxy images from which the shear is estimated. Substantial over- or underestimates of the cosmological parameters are equally possible, depending also on the variant of the KSB method. Our results show that for trustable cosmological-parameter estimates from measurements of weak lensing, one must verify that the method employed is free from ellipticity-dependent biases or monitor that the calibration relation inferred from simulations is applicable to the survey at hand.