  Flat-field images with thick, fully-depleted CCDs exhibit response variations
near the edges of the chip and at other locations, such as the regoins
bordering mid-frame blooming stop implants. Two possible origins for these
repsonse variations have been suggested: either photometric response (quantum
efficiency) or effective pixel area is modified in these regions. In the latter
case source position and shape distortions would be expected in these regions,
with consequent impact on astrometric and weak lensing measurements. As an
experimental check to distinguish between the two effects and to gauge the
magnitude of distortion, we performed a measurment scanning an artificial star
image across the affected region of one device.
