http://arxiv.org/abs/1409.6273
Current and future imaging surveys will measure cosmic shear with statistical precision that demands a deeper understanding of potential systematic biases in galaxy shape measurements than has been achieved to date. We use analytic and computational techniques to study the impact on shape measurements of two atmospheric chromatic effects for ground-based surveys such as the Dark Energy Survey (DES) and the Large Synoptic Survey Telescope (LSST): (i) atmospheric differential chromatic refraction (DCR) and (ii) wavelength dependence of seeing. We investigate the effects of using the point spread function (PSF) measured with stars to determine the shape of a galaxy that has a different spectral energy distribution (SED) than the stars. For (i), we extend a study by Plazas & Bernstein based on analytic calculations that show that DCR leads to significant biases in galaxy shape measurements for future surveys, if not corrected. For (ii), we find that the wavelength dependence of seeing leads to significant biases for galaxy shape measurements – even for current ground-based surveys. For both effects, we investigate correction techniques based on multi-filter photometry. Using simulated galaxy images, we find a form of chromatic “model bias” that arises when fitting a galaxy image with a model that has been convolved with a stellar, instead of galactic, point spread function. We find that PSF-level corrections can reduce biases to levels that meet the requirements for the LSST survey. We conclude that achieving the ultimate precision for weak lensing from current and future ground-based imaging surveys requires a detailed understanding of the wavelength dependence of the PSF in the atmosphere, and from other sources such as optics and sensors. The source code for this analysis is available at https://github.com/DarkEnergyScienceCollaboration/chroma.
J. Meyers and P. Burchat
Tue, 23 Sep 14
56/60
Comments: 20 pages, 16 figures, submitted to ApJ
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