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The weak-lensing science of the LSST project drives the need to carefullymodel and separate the instrumental artifacts from the intrinsic lensingsignal. The dominant source of the systematics for all ground based telescopesis the spatial correlation of the PSF modulated by both atmospheric turbulenceand optical aberrations. In this paper, we present a full FOV simulation of theLSST images by modeling both the atmosphere and the telescope optics with themost current data for the telescope specifications and the environment. Tosimulate the effects of atmospheric turbulence, we generated six-layer phasescreens with the parameters estimated from the on-site measurements. For theoptics, we combined the ray-tracing tool ZEMAX and our simulated focal planedata to introduce realistic aberrations and focal plane height fluctuations.Although this expected flatness deviation for LSST is small compared with thatof other existing cameras, the fast f-ratio of the LSST optics makes this focalplane flatness variation and the resulting PSF discontinuities across the CCDboundaries significant challenges in our removal of the systematics. We resolvethis complication by performing PCA CCD-by-CCD, and interpolating the basisfunctions using conventional polynomials. We demonstrate that this PSFcorrection scheme reduces the residual PSF ellipticity correlation below 10^-7over the cosmologically interesting scale. From a null test using HST/UDFgalaxy images without input shear, we verify that the amplitude of the galaxyellipticity correlation function, after the PSF correction, is consistent withthe shot noise set by the finite number of objects. Therefore, we conclude thatthe current optical design and specification for the accuracy in the focalplane assembly are sufficient to enable the control of the PSF systematicsrequired for weak-lensing science with the LSST.

Towards Precision LSST Weak-Lensing Measurement - I: Impacts of Atmospheric Turbulence and Optical Aberration