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.