Campus, Varian 355
Atmospheric turbulence is the dominant contribution to the single-exposure point spread function (PSF) for the LSST. Both the magnitude and spatial correlations of the PSF shape parameters must be very accurately known for cosmic shear measurements, and are expected to decrease with exposure time. But how quickly do they decrease? Is the nominal LSST exposure time (30 seconds) in the “long exposure time” regime in which many surveys operate?
These questions are investigated using sequential images of stars from the Differential Speckle Survey Instrument (DSSI) mounted on the Gemini South telescope less than 2 kilometers from the LSST site on the Cerro Pachón ridge in Chile. The DSSI time series of short-exposure images are in essence movies of the PSF. We stack the images to approximate PSF averaging in real data for different exposure times and use these to test the fidelity of simulations implemented in GalSim, a software package for simulating astronomical objects and PSFs. We extract PSF parameters — e.g., size and ellipticity — and compare the real and simulated behavior as a function of exposure time. These studies could help inform discussions of possibly variable exposure times for LSST visits — for example, to provide more uniform depth of visits.