The ACS point spread function has been studied in ground test measurements, and by using on-orbit data and models generated by the Tiny TIM software (http://www.stsci.edu/software/tinytim/tinytim.html
) developed by J. Krist and R. Hook. As with other HST instruments, the ACS point spread function is affected by both optical aberrations and geometric distortions. Point sources imaged with WFC and HRC experience blurring due to charge diffusion into adjacent pixels because of CCD subpixel variations, which reduces the limiting magnitudes that can be reached by WFC/HRC. The SBC PSF and the long-wavelength HRC PSF are also affected by a halo produced by the detectors themselves.
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