Toward Precision LSST Weak-Lensing Measurement. I. Impacts of Atmospheric Turbulence and Optical Aberration

Abstract

ABSTRACTThe weak-lensing science of the Large Synoptic Survey Telescope (LSST) project drives the need to carefully model and separate the instrumental artifacts from the intrinsic shear signal caused by gravitational lensing. The dominant source of the systematics for all ground-based telescopes is the spatial correlation of the point-spread function (PSF) modulated by both atmospheric turbulence and optical aberrations in the telescope and camera system. In this article, we present a full field-of-view simulation of the LSST images by modeling both the atmosphere and the system optics with the most current data for the telescope and camera specifications and the environment. To simulate the effects of atmospheric turbulence, we generated six-layer Kolmogorov/von Kármán phase screens with the parameters estimated from the on-site measurements. LSST will continuously sample the wavefront, correcting the optics alignment and focus. For the optics, we combined the ray-tracing tool ZEMAX and our simulated focal-plane data to introduce realistic residual aberrations and focal-plane height variations. Although this expected focal-plane flatness deviation for LSST is small compared with that of other existing cameras, the fast focal ratio of the LSST optics cause this focal-plane flatness variation and the resulting PSF discontinuities across the CCD boundaries to be significant challenges in our removal of the PSF-induced systematics. We resolve this complication by performing principal component analysis (PCA) CCD by CCD and by interpolating the basis functions derived from the analysis using conventional polynomials. We demonstrate that this PSF correction scheme reduces the residual PSF ellipticity correlation below 10-7 over the cosmologically interesting (dark-matter-dominated) scale 10′–3°. From a null test using the Hubble Space Telescope (HST) Ultra Deep Field (UDF) galaxy images without input shear, we verify that the amplitude of the galaxy ellipticity correlation function, after the PSF correction, is consistent with the shot noise set by the finite number of objects. We conclude that the current optical design and specification for the accuracy in the focal-plane assembly are sufficient to enable the control of the PSF systematics required for weak-lensing science with LSST.