The sharpness of the CCD PSF is somewhat degraded by charge diffusion into adjacent pixels. The effect is usually described in terms of the pixel response function (PRF), which gives the distribution of flux from within the pixel into adjacent pixels. Charge diffusion results in ~0.5 magnitude loss in the WFC limiting magnitude at short wavelengths (the worst case). At longer wavelengths and at all wavelengths for the HRC the reduction in the limiting magnitude is ~0.2 magnitudes or less. Due to variations in the CCD thickness, charge diffusion is not constant over the field of view. At different wavelengths, the CCD pixel response functions can be represented by the following kernels (for the center of the field):
Figure 5.8:
More details on ACS CCD charge diffusion are given in ACS ISR 2003-06
.
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