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The ambitious goals of precision cosmology with wide-field optical surveyssuch as the Dark Energy Survey (DES) and the Large Synoptic Survey Telescope(LSST) demand, as their foundation, precision CCD astronomy. This in turnrequires an understanding of previously uncharacterized sources of systematicerror in CCD sensors, many of which manifest themselves as static effectivevariations in pixel area. Such variation renders a critical assumption behindthe traditional procedure of flat fielding--that a sensor's pixels comprise auniform grid--invalid. In this work, we present a method to infer a curl-freemodel of a sensor's underlying pixel grid from flat field images, incorporatingthe superposition of all electrostatic sensor effects--both known andunknown--present in flat field data. We use these pixel grid models to estimatethe overall impact of sensor systematics on photometry, astrometry, and PSFshape measurements in a representative sensor from the Dark Energy Camera(DECam) and a prototype LSST sensor. Applying the method to DECam data recoversknown significant sensor effects for which corrections are currently beingdeveloped within DES. For an LSST prototype CCD with pixel-responsenon-uniformity (PRNU) of 0.4%, we find the impact of "improper" flat-fieldingon these observables is negligible in nominal .7" seeing conditions. Theseerrors scale linearly with the PRNU, so for future LSST production sensors,which may have larger PRNU, our method provides a way to assess whetherpixel-level calibration beyond flat fielding will be required.

Is Flat Fielding Safe for Precision CCD Astronomy?