Properties of Ellipticity Correlation with Atmospheric Structure from Gemini South

Cosmic shear holds great promise for a precision independent measurement of$\Omega\rm_m$, the mass density of the universe relative to the criticaldensity. The signal is expected to be weak, so a thorough understanding ofsystematic effects is crucial. An important systematic effect is theatmosphere: shear power introduced by the atmosphere is larger than theexpected signal. Algorithms exist to extract the cosmic shear from theatmospheric component, though a measure of their success applied to a range ofseeing conditions is lacking. To gain insight into atmospheric shear, Gemini South imaging in conjunctionwith ground condition and satellite wind data were obtained. We find that undergood seeing conditions Point-Spread-Function (PSF) correlations persist wellbeyond the separation typical of high-latitude stars. Under these conditions,ellipticity residuals based on a simple PSF interpolation can be reduced towithin a factor of a few of the shot-noise induced ellipticity floor. We alsofind that the ellipticity residuals are highly correlated with wind direction.Finally, we correct stellar shapes using a more sophisticated procedure andgenerate shear statistics from stars. Under all seeing conditions in our dataset the residual correlations lie everywhere below the target signal level. Forgood seeing we find that the systematic error attributable to atmosphericturbulence is comparable in magnitude to the statistical error (shape noise)over angular scales relevant to present lensing surveys.Comment: To appear in ApJ April 10, 2007, 659