  Reduction of images and science analysis from ground-based telescopes such as
the LSST requires detailed knowledge of the PSF of the image, which includes
components attributable to the instrument as well as components attributable to
the atmosphere. Because the atmospheric component is constantly changing, the
PSF is typically extracted from each image by measuring the size and shape of
star images across the CCD, then building a fitting function over the focal
plane which is used to model the PSF for analysis of extended sources such as
galaxies. Since the stars in each CCD field have a range of brightnesses,
accurate knowledge of the PSF for point sources of varying brightness is
essential. It has been found that in thick, fully-depleted CCDs, the
electrostatic repulsion of charge stored in the collecting wells gives rise to
a larger and slightly more elliptical PSF for brighter stars. This
"brighter-fatter" effect has been reported in some detail in the literature. In
this work, we report direct and indirect measurements of this effect in
prototype LSST sensors, and describe a detailed physics-based model of the
electrostatics and charge transport within the CCD.
