The potential well of the pixel-defining electrodes does not extend all the way to the surface of the CCD. Photo-electrons generated near the surface may then diffuse to neighbor pixels, causing a degradation of the image sharpness. The W14-2 CCD has been ``ultra thinned'' to reduce this problem, but measurements of the Modulation Transfer Function made by [Andersen, M. I. 96] shows the problem still is significant.
The measurement displayed in figure 4 is made at 632 nm. The degradation is expected to be worse at shorter wavelengths and vice versa due to the color-dependent transparency of the CCD. From this, an effective pixel size in excess of 2 physical pixels is inferred.
Figure 4:
MTF of the W14-2 CCD measured at a wavelength of 632nm.
Compared to the ideal sinc-shaped MTF of a pixel, the measured MTF
represents a severe loss of resolution.