Hubble Space Telescope
WFPC2 Target Location on CCD Pixel

For unresolved targets, the position of the point spread function (PSF) centroid on the pixel will influence the noise contributed by read-out, dark current, and the sky background. The noise is smallest when the PSF lands on the center of a pixel. The noise is largest when the PSF lands on pixel corner, such that the light is spread over a maximal number of pixels. The choice of "pixel center" or "pixel corner" determines which values are taken from WFPC2 Handbook Table 6.4.