Title: 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.