Spot scan probe of lateral field effects in a thick fully-depleted CCD

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.