` Background photo courtesy of NOAO. Taken with LBNL p-channel CCD with extended red sensitivity. Pixel Area Variation in CCDs and Implications for Precision Photometry Roger Smith & Gustavo Rahmer (Caltech) E2V Sample test. SPIE Astronomical Telescopes and Instrumentation, Marseille, , 6pm. Poster HOW FLAT FIELDING CAN HURT YOU What causes granularity in flats? If we average lots of bright flat fields, the shot noise drops below the intrinsic pixel to pixel variation. This variation has been widely assumed to be due to sensitivity. It is usually removed by dividing by a flat field in which the lower spatial frequencies have been suppressed, since low spatial frequencies are presumed to be due to illumination effects. but is this correct ? Sensitivity variation Conventional wisdom When pixel boundaries are relocated, incident flux is allocated differently between pixels but total signal is conserved. There is no error from aperture photometry. Profile fitting may be affected by the shift in centers. actual ideal Pixel boundaries are defined by the peak of the barrier potentials. These are defined by implants between columns and by electrode voltages between lines. Cumulative errors (scale, alignment) in the lithography are very small, but any implant or electrode can be offset slightly. An electrode or mask can be etched to a different width or can have local edge roughness, and of course there can be impurities or crystal defects which distort electric fields. So, although the average pitch is well controlled there can be some randomness in boundary positions. Pixel Response Non-Uniformity is typically