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

TL;DR

This paper presents a multi-wavelength Zernike phase contrast sensor technique for on-sky phasing of segmented telescope mirrors, achieving high accuracy over extended piston ranges suitable for future Extremely Large Telescopes.

Contribution

It introduces a multi-wavelength scheme that extends the phasing capture range of the Zernike sensor, enabling effective on-sky phasing of large segmented mirrors with micrometer piston errors.

Findings

Successfully phased mirrors with piston errors up to ±4.0 micrometers.

Extended the capture range from λ/2 to several micrometers.

Validated the approach with on-sky demonstrations at the VLT.

Abstract

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.