The alignment test system for the AXAF-I high-resolution mirror assembly (HRMA) performs a double-pass Hartmann test, in which a beam of light emitted from the focus of the HRMA is reflected back through the HRMA by an autocollimating flat (ACF). Measurements of the displacement of the beam on its return to the focal plane can be used to determine the misalignment of the HRMA, and also some information about errors in the shapes of the HRMA mirrors. Beam displacements caused by imperfections in the ACF can be distinguished from those caused by the HRMA by taking measurements with the ACF in different positions. This paper shows how to use such measurements to calibrate the system to compensate for errors caused by the ACF. Simulated results based on measured ACF surface data are presented.