Fresnel phasing of segmented mirror telescopes.

Shack-Hartmann (S-H) phasing of segmented telescopes is based upon a physical optics generalization of the geometrical optics Shack-Hartmann test, in which each S-H lenslet straddles an intersegment edge. For the extremely large segmented telescopes currently in the design stages, one is led naturally to very large pupil demagnifications for the S-H phasing cameras. This in turn implies rather small Fresnel numbers F for the lenslets; the nominal design for the Thirty Meter Telescope calls for F=0.

On-sky performance of the Zernike phase contrast sensor for the phasing of segmented telescopes.

The Zernike phase contrast method is a novel technique to phase the primary mirrors of segmented telescopes. It has been tested on-sky on a unit telescope of the Very Large Telescope with a segmented mirror conjugated to the primary mirror to emulate a segmented telescope. The theoretical background of this sensor and the algorithm used to retrieve the piston, tip, and tilt information are described.