Calibration of geometric distortion in the ACS detectors

The off-axis location of the Advanced Camera for Surveys causes 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. We present our best current calibration of the geometric distortion of each of the detectors.