  Intrapixel nonuniformity is known to exist in CCD and CMOS image sensors,
though the effects in backside illuminated (BSI) CCDs are too small to be a
concern for most astronomical observations. However, projects like the Large
Synoptic Survey Telescope require precise knowledge of the detector
characteristics, and intrapixel effects may need more attention. By scanning
CCD and CMOS cameras with a small light spot (unresolved by the optics), we
find in the images that the spot's flux, centroid displacement, and ellipticity
vary periodically on the pixel scale in most cases. The amplitude of variation
depends on not only the detector but also how well the spot is sampled by the
pixels. With a spot radius of 2 pixels (encircling 80% energy) as measured, the
flux and the ellipticity extracted from the BSI CCD camera vary by 0.2-0.3%
(rms) and 0.005 (rms), respectively, while the deviation of the centroid
position (rms ~ 0.01 pixel) is not correlated with the pixels. The effects are
more pronounced for the BSI CMOS camera and even worse for the frontside
illuminated CMOS camera. The results suggest that a closer examination of the
intrapixel effects is needed for precision astronomy.
