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. The performance of the sensor as a function of parameters such as star magnitude, seeing, and integration time is discussed. The phasing accuracy has always been below 15 nm root mean square wavefront error under normal conditions of operation and the limiting star magnitude achieved on-sky with this sensor is 15.7 in the red, which would be sufficient to phase segmented telescopes in closed-loop during observations.

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

Future extremely large telescopes will adopt segmented primary mirrors with several hundreds of segments. Cophasing of the segments together is essential to reach high wavefront quality. The phasing sensor must be able to maintain very high phasing accuracy during the observations, while being able to phase segments dephased by several micrometers. The Zernike phase contrast sensor has been demonstrated on-sky at the Very Large Telescope. We present the multiwavelength scheme that has been implemented to extend the capture range from ±λ/2 on the wavefront to many micrometers, demonstrating that it is successful at phasing mirrors with piston errors up to ±4.0  μm on the wavefront. We discuss the results at different levels and conclude with a phasing strategy for a future extremely large telescope.

On-sky multiwavelength phasing of segmented telescopes with the Zernike phase contrast sensor.

Segmented aperture telescopes require an alignment procedure with successive steps from coarse alignment to monitoring process in order to provide very high optical quality images for stringent science operations such as exoplanet imaging. The final step, referred to as fine phasing, calls for a high sensitivity wavefront sensing and control system in a diffraction-limited regime to achieve segment alignment with nanometric accuracy. In this context, Zernike wavefront sensors represent promising options for such a calibration. A concept called the Zernike unit for segment phasing (ZEUS) was previously developed for ground-based applications to operate under seeing-limited images. Such a concept is, however, not suitable for fine cophasing with diffraction-limited images. We revisit ZELDA, a Zernike sensor that was developed for the measurement of residual aberrations in exoplanet direct imagers, to measure segment piston, tip, and tilt in the diffraction-limited regime. We introduce a novel analysis scheme of the sensor signal that relies on piston, tip, and tilt estimators for each segment, and provide probabilistic insights to predict the success of a closed-loop correction as a function of the initial wavefront error. The sensor unambiguously and simultaneously retrieves segment piston and tip-tilt misalignment. Our scheme allows for correction of these errors in closed-loop operation down to nearly zero residuals in a few iterations. This sensor also shows low sensitivity to misalignment of its parts and high ability for operation with a relatively bright natural guide star. Our cophasing sensor relies on existing mask technologies that make the concept already available for segmented apertures in future space missions.

/pdf/fine-cophasing-of-segmented-aperture-telescopes-with-zelda-a-4q3c33ryh9.pdf

Fine cophasing of segmented aperture telescopes with ZELDA, a Zernike wavefront sensor in the diffraction-limited regime

Future Extremely Large Telescopes will adopt segmented primary mirrors with several hundreds of segments. Cophasing of the segments together is essential to reach high wavefront quality. The phasing sensor must be able to maintain very high phasing accuracy during the observations, while being able to phase segments dephased by several micrometers. The Zernike phase contrast sensor has been demonstrated on-sky at the Very Large Telescope. We present the multi-wavelength scheme that has been implemented to extend the capture range from \pmlambda/2 on the wavefront to many micrometers, demonstrating that it is successful at phasing mirrors with piston errors up to \pm4.0 micron on the wavefront. We discuss the results at different levels and conclude with a phasing strategy for a future Extremely Large Telescope.

On-sky multi-wavelength phasing of segmented telescopes with the Zernike phase contrast sensor

All future Extremely Large Telescopes (ELTs) will be segmented. However, as their size grows, they become increasingly sensitive to external disturbances, such as gravity, wind and temperature gradients and to internal vibration sources. Maintaining their optical quality will rely more and more on active control means. This thesis studies active optics of segmented primary mirrors, which aims at stabilizing the shape and ensuring the continuity of the surface formed by the segments in the face of external disturbances.The modelling and the control strategy for active optics of segmented mirrors are examined. The model has a moderate size due to the separation of the quasi-static behavior of the mirror (primary response) from the dynamic response (secondary, or residual response). The control strategy considers explicitly the primary response of the telescope through a singular value controller. The control-structure interaction is addressed with the general robustness theory of multivariable feedback systems, where the secondary response is considered as uncertainty.Scaling laws allowing the extrapolation of the results obtained with existing 10m telescopes to future ELTs and even future larger telescopes are addressed and the most relevant parameters are highlighted. The study is illustrated with a set of examples of increasing sizes, up to 200 segments. This numerical study confirms that scaling laws, originally developed with simple analytical models, can be used in confidence in the preliminary design of large segmented telescopes.

Extremely large segmented mirrors: dynamics, control and scale effects

In the framework of the Active Phasing Experiment (APE), four different phasing techniques are tested. The
ZErnike Unit for Segment phasing sensor (ZEUS) is integrated on the APE bench. APE has been tested in
the laboratory before it will be installed on one of the Nasmyth platform of a Very Large Telescope (VLT)
Unit Telescope to perform on sky tests. The ZEUS phasing sensor concept has its origins in the Mach-Zehnder
interferometer equipped with a spatial filter in its focal plane. In this paper, the ZEUS phasing sensor is
described together with its theoretical background and deployment within the APE experiment. The algorithms
and its elements used to reconstruct the wavefront are described. Finally, the preliminary results obtained in
the laboratory are presented.

/pdf/preliminary-results-obtained-with-the-zeus-phasing-sensor-2osuklslze.pdf

Preliminary results obtained with the ZEUS phasing sensor within the APE experiment

The purpose of the Active Phasing Experiment, designed under the lead of ESO, is to study new phasing technologies
and to validate wavefront control concepts for Extremely Large Telescopes. The Active Phasing Experiment is currently
tested in the laboratory at the ESO headquarters and will be tested on sky at a Nasmyth focus of a VLT unit telescope at
the end of 2008. The test bench contains four different phasing sensors which are tested in parallel to compare them
under the same conditions. They have been developed by Istituto Nazionale di Astrofisica in Florenze, Instituto
Astrofisica Canarias in Tenerife, Laboratoire d'Astrophysique de Marseille and ESO. It includes also an Active
Segmented Mirror which simulates the segmentation of a primary mirror. A non-contact optical metrology has been
developed by Fogale Nanotech to control it. The VLT focus and the VLT atmospheric conditions are simulated in the
laboratory with a turbulence generator producing a seeing between 0.45 and 0.85 arcsec. Once installed on a VLT unit
telescope, the control system of the Active Phasing Experiment will be able to control the phasing of the ASM, but also
the guiding and the active optics of the VLT. This proceeding gives a brief summary of the opto-mechanical aspects of
the Active Phasing experiment, describes its control system and gives an analysis of the preliminary results obtained in
the laboratory.

Active Phasing Experiment: preliminary results and prospects

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Preliminary results obtained with the ZEUS phasing sensor within the APE experiment

Active Phasing Experiment: preliminary results and prospects