Properties of Ellipticity Correlation with Atmospheric Structure from Gemini South

Cosmic shear holds great promise for a precision independent measurement of Ωm, the mass density of the universe relative to the critical density. The signal is expected to be weak, so a thorough understanding of systematic effects is crucial. An important systematic effect is the atmosphere: shear power introduced by the atmosphere is larger than the expected signal. Algorithms exist to extract the cosmic shear from the atmospheric component, though a measure of their success applied to a range of seeing conditions is lacking. To gain insight into atmospheric shear, Gemini South imaging in conjunction with ground condition and satellite wind data were obtained. We find that under good seeing conditions Point-Spread-Function (PSF) correlations persist well beyond the separation typical of high-latitude stars. Under these conditions, ellipticity residuals based on a simple PSF interpolation can be reduced to within a factor of a few of the shot-noise induced ellipticity floor. We also find that the ellipticity residuals are highly correlated with wind direction. Finally, we correct stellar shapes using a more sophisticated procedure and generate shear statistics from stars. Under all seeing conditions in our data set the residual correlations lie everywhere below the target signal level. For good seeing we find that the systematic error attributable to atmospheric turbulence is comparable Lawrence Livermore National Laboratory, Livermore, CA 94551 University of California, Davis, CA 95616 Institute for Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, Livermore, CA 94551 Stanford Linear Accelerator Center, Menlo Park, CA 94025 National Optical Astronomy Observatory, Tucson, AZ Gemini Observatory, Casilla 603, La Serena, Chile Current address: National Science Foundation, Arlington, VA 22230 SLAC-PUB-12297 astro-ph/0701157 January 2007 Submitted to Astrophys.J.