Position-dependent CCD charge diffusion: Results from HST/ACS

摘要

Charge diffusion can vary significantly over the area of a thinned CCD due to local differences in the device thickness. This effect can cause variations in the widths of point spread functions over the field of view. Analyses of on-orbit Hubble Space Telescope Advanced Camera for Surveys (ACS) Wide Field Camera (WFC) star cluster images show a direct relationship between stellar PSF widths and CCD thickness measurements. PSF model fitting was used to separate variations caused by aberrations from charge diffusion effects. The results show that in the ACS/WFC, which undersamples the PSF by a factor of two at 0.5 μm, the core width of a star's image is determined mostly by charge diffusion and varies by 30% - 40% over the field, with larger differences at shorter wavelengths. This effect should be considered for any instrument that uses thinned CCDs and does not fully sample the optical PSF.