  One of the scientific goals of the Large Synoptic Survey Telescope (LSST) is
to measure the evolution of dark energy by measuring subtle distortions of
galaxy shapes due to weak gravitational lensing caused by the evolving dark
matter distribution. Understanding the point spread function (PSF) for LSST is
a crucial step to accurate measurements of weak gravitational lensing.
Atmospheric contributions dominate the LSST PSF. Simulations of Kolmogorov
turbulence models are commonly used to characterize and correct for these
atmospheric effects. In order to validate these simulations, we compare the
predicted atmospheric behavior to empirical data. The simulations are carried
out in GalSim, an open-source software package for simulating images of
astronomical objects and PSFs. Atmospheric simulations are run by generating
large phase screens at varying altitude and evolving them over long time
scales. We compare the turbulence strength and temporal behavior of atmospheres
generated from simulations to those from reconstructed telemetry data from the
Gemini Planet Imager (GPI). GPI captures a range of spatial frequencies by
sampling the atmosphere with 18-cm subapertures. The LSST weak lensing analysis
will measure correlations of galaxy ellipticity, requiring very accurate
knowledge of the magnitude and correlations of PSF shape parameters. Following
from the first analysis, we use simulations and sequential short exposure
observations from the Differential Speckle Survey Instrument (DSSI) to study
the behavior of PSF parameters - e.g., ellipticity and size - as a function of
exposure time. These studies could help inform discussions of possible variable
exposure times for LSST visits for example, to provide more uniform depth of
visits.
