  The off-axis location of the Advanced Camera for Surveys (ACS) is the chief
(but not sole) cause of strong geometric distortion in all detectors: the Wide
Field Camera (WFC), High Resolution Camera (HRC), and Solar Blind Camera (SBC).
Dithered observations of rich star cluster fields are used to calibrate the
distortion. We describe the observations obtained, the algorithms used to
perform the calibrations and the accuracy achieved.
