Pixel area variation in CCDs and implications for precision photometry

Abstract

Images with smooth and moderately flat illumination are commonly used to calibrate pixel to pixel sensitivity variation without consideration that some structure on short spatial scales may be due to reallocation of area between pixels. Errors in the position of pixel boundaries have the effect of repartitioning charge between pixels but do not affect the total charge collected. Since the resulting errors tend to cancel when combining signal from adjacent pixels, this effect has gone largely unnoticed. However proposed wide field astronomical surveys, which strive to achieve high photometric precision with coarse spatial sampling, must take this into account. We present simple analysis techniques to identify how much flat field structure is due to systematic and random variations in pixel area, rather than sensitivity, as a function of row/column direction and spatial frequency. Analysis of data from CCDs made with radically different technologies and pixel sizes by different manufacturers suggests that pixel size variation in the column direction probably dominates QE variations on short spatial scales for all CCDs. Refinements to flat field calibration methods and tests to confirm their efficacy are proposed.

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