Co-phasing segmented mirrors: theory, laboratory experiments and measurements on sky

Future Extremely Large Telescopes (ELTs) will be segmented. To
achieve the desired image quality, in particular for high contrast
imaging, the segments of the primary mirror must be phased to an
accuracy which corresponds to a small fraction of the observing
wavelength. Information on the misalignments of the segments of the
primary mirror will be provided with a phasing camera. The Zernike
phase contrast method is a novel technique to phase the primary
mirrors of segmented telescopes. The present thesis aims at
studying the phasing of segmented mirrors using the Zernike phase
contrast method. The theoretical background of this sensor and the
algorithm used to retrieve the piston, tip, and tilt information
are described. The Zernike Phase Contrast Sensor has been tested in
the laboratory and on-sky on a Unit Telescope of the ESO Very Large
Telescope with a segmented mirror conjugated to its primary mirror
to emulate a segmented telescope. The performance of the sensor as
a function of various parameters such as star magnitude, seeing,
and integration time is discussed. The phasing accuracy obtained on
sky has always been below 15 nm root mean square wavefront error
under normal conditions of operation. Equivalent limiting star
magnitudes of 18 and 15.7 achieved in the laboratory and on-sky,
respectively, with this sensor would be sufficient to phase
segmented telescopes in closed-loop during observations.