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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.