  We characterize the astrometric distortion at the edges of thick,
fully-depleted CCDs in the lab using a bench-top simulation of LSST observing.
By illuminating an array of forty thousand pinholes (30mu m diameter) at the
object plane of a f/1.2 optical reimager, thousands of PSFs can be imaged over
a 4Kx4K pixel CCD. Each high purity silicon pixel, 10mu m square by 100mu m
deep, can then be individually characterized through a series of sub-pixel
dithers in the X/Y plane. The unique character [response, position, shape] of
each pixel as a function of flux, wavelength, back side bias, etc. can be
investigated. We measure the magnitude and onset of astrometric error at the
edges of the detector as a test of the experimental setup, using a LSST
prototype CCD. We show that this astrometric error at the edge is sourced from
non-uniformities in the electric field lines that define pixel boundaries. This
edge distortion must be corrected in order to optimize the science output of
weak gravitational lensing and large scale structure measurements for the LSST.
