Phase contrast techniques for wavefront sensing and calibration in adaptive optics

The wavefront sensor is the most critical component of an adaptive optics (AO) system. Most astronomical systems use one of a small number of alternatives, notably the Shack-Hartmann or the curvature sensor; these are sensitive to the first and second derivative of the wavefront phase, respectively. In this paper, we explore a novel adaptation of the phase-contrast technique developed for microscopy by Zernike to measure phase directly, and show that it is potentially useful in astronomical adaptive optics, both for closed-loop wavefront sensing and for off-line calibration of the system PSF. The phase-contrast WFS should enjoy an advantage in lower read noise, as well as a natural match to the piston-type deformable mirror actuators commonly in use with most current Shack-Hartmann systems, and favorable error propagation during wavefront reconstruction. It appears that it might be possible to implement versions with the reasonably broad spectral bandwidth desired for astronomical applications, and to integrate them with relatively minor modifications into existing AO system architectures.