Calibrating the wavefront sensor for the 6.5-m MMT with a phase-shifting interferometer

In an adaptive optical system, it is essential that the wavefront sensor be accurately calibrated and aligned to the wavefront corrector. For the case of the Shack Hartmann sensor, there are at least three quantities which must be measured: a. the response of the subapertures to local wavefront tilt, b. the location of the zero point for each subaperture from a plane wave input, and c. the spatial relationship between the wavefront sensor subapertures and the wavefront corrector actuators. This paper present a method which verifies the calibration of a Shack Hartmann wavefront sensor by simultaneously measuring static wavefront with a phase shifting interferometer. These measurements were made using an apparatus we constructed in the laboratory to build and test the adaptive optical system for the 6.5 m MMT.