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

Abstract

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.

11 pages, 10 figures. Accepted for publication in A&A - sec. 13

References in corpus (2)

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Cited by in corpus (14)

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• Calibration of residual aberrations in exoplanet imagers with large numbers of degrees of freedom
• Large-scale fluctuations in the cosmic ionising background: the impact of beamed source emission
• High contrast at small separation -- II. Impact on the dark hole of a realistic optical set-up with two deformable mirrors
• Cascade adaptive optics with a second stage based on a Zernike wavefront sensor for exoplanet observations
• Redundant apodization for direct imaging of exoplanets I: Robustness to primary mirror segmentation-induced errors outside the segment diffraction limit
• MAORY/MORFEO and LIFT: can the low order wavefront sensors become phasing sensors?
• The Effect of Interstellar Absorption on Measurements of the Baryon Acoustic Peak in the Lyman-α Forest
• On-sky reconstruction of Keck Primary Mirror Piston Offsets using a Zernike Wavefront Sensor
• Estimating differential pistons for the Extremely Large Telescope using focal plane imaging and a residual network
• Mid-order wavefront control for exoplanet imaging: preliminary characterization of the segmented deformable mirror and Zernike wavefront sensor on HiCAT
• Concept validation of a high dynamic range point-diffraction interferometer for wavefront sensing in adaptive optics