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Title:
A Method for Weak Lensing Observations
Authors:
Kaiser, Nick; Squires, Gordon; Broadhurst, Tom
Publication:
Astrophysical Journal v.449, p.460 (ApJ Homepage)
Publication Date:
08/1995
Origin:
APJ; KNUDSEN
Astronomy Keywords:
COSMOLOGY: OBSERVATIONS, COSMOLOGY: DARK MATTER, GALAXIES: FORMATION, COSMOLOGY: GRAVITATIONAL LENSING, COSMOLOGY: LARGE-SCALE STRUCTURE OF UNIVERSE
DOI:
10.1086/176071
Bibliographic Code:
1995ApJ...449..460K

Abstract

We develop and test a method for measuring the gravitational lensing-induced distortion of faint background galaxies. We first describe how we locate the galaxies and measure two-component "polarization" or ellipticity statistic ealpha whose expectation value should be proportional to the gravitational shear gammaalpha. We then show that an anisotropic instrumental point-spread function (PSF) perturbs the polarization by deltaealpha = PsalphabetaPbeta, where palpha is a measure of the PSF anisotropy and Psalphabeta is the "linearized smear polarizability tensor." By estimating Psalphabeta for each object we can determine palpha from the foreground stars and apply a correction PsalphabetaPbeta to the galaxies. We test this procedure using deep high-resolution images from the Hubble Space Telescope (HST) which are smeared with an anisotropic PSF and then have noise added to simulate ground-based observations. We find that the procedure works very well. A similar analysis yields a linear shear polarizability tensor Pgammaalphabeta which describes the response to a gravitational shear. This calibrates the polarization-shear relation, but only for galaxies which are well resolved. To empirically calibrate the effect of seeing on the smaller galaxies we artificially stretch HST images to simulate lensing and then degrade them as before. These experiments provide a rigorous and exacting test of the method under realistic conditions. They show that it is possible to remove the effect of instrumental PSF anisotropy and that the method provides an efficient and quantitative measurement of the gravitational shear.

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