Further testing, by covering a flight spare CCD with a 2mm pinhole grid in an opaque metallic mask and illuminating it with a flat field source, showed that even when a pinhole was centered over a pixel only about 70% of the light was detected in that pixel.
For practical purposes, the effect can be modeled as equivalent to about 40 mas RMS gaussian jitter in the WFC, and 18 mas in the PC (as compared with the typical real pointing jitter of ~3 mas delivered by the excellent HST pointing control system). Alternatively, at least in the V band, it can be modeled by convolving a simulated image by the following kernel, which gives the pixel response function averaged within pixels:
One clue is the wavelength dependence of the observed sharpness: the results from the 2mm pinhole grid test get worse at longer wavelengths. This may reflect the greater penetration into the silicon of low energy photons, which facilitates the diffusion of photoelectrons across the pixel boundaries defined by the frontside gate structure.
There is also evidence for sub-pixel QE variations at the 10% level. There is an implied dependence on pixel phase for stellar photometry. This has been seen at about the 1% level in on-orbit data. The work of Jorden, Deltorn, and Oates (Greenwich Observatory Newsletter 9/93) has yielded quite similar results, and suggests that sub-pixel response must be taken into account when seeking to understand the behavior of all CCD detectors forming undersampled images.