On-orbit alignment and imaging performance of the HST advanced camera for surveys

The Advanced Camera for Surveys (ACS), installed in the Hubble Space Telescope in March 2002, has significantly extended HST's imaging capabilities. We describe the on-orbit optical alignment procedures and results, detailing the excellent image quality performance achieved. Comparison is made with the instrument specifications, ground test results and published performance expectations. The residual aberration content over the field of each channel is described and compared with the optical model, and various other performance measures, including sharpness and encircled energy are treated. The effects of the telescope focus oscillations due to thermal variations ("breathing") and image positional stability are also discussed.