Adapting the pyramid wavefront sensor for pupil fragmentation of the ELT class telescopes

TL;DR

This paper proposes a modified pyramid wavefront sensor scheme with spatial filtering to effectively detect and correct pupil fragmentation and petal modes in ELT telescopes, enhancing their adaptive optics performance.

Contribution

It introduces a double path wavefront sensor scheme with spatial filtering to measure and correct both turbulence and petal modes caused by pupil fragmentation in ELT telescopes.

Findings

The spatial structure of petal modes affects pyramid wavefront sensor sensitivity.

A double path wavefront sensor scheme can measure turbulence and petal modes simultaneously.

The proposed scheme restores the full spatial resolution of ELT adaptive optics.

Abstract

The next generation of Extremely Large Telescope (24 to 39m diameter) will suffer from the so-called "pupil fragmentation" problem. Due to their pupil shape complexity (segmentation, large spiders ...), some differential pistons may appear between some isolated part of the full pupil during the observations. Although classical AO system will be able to correct for turbulence effects, they will be blind to this specific telescope induced perturbations. Hence, such differential piston, a.k.a petal modes, will prevent to reach the diffraction limit of the telescope and ultimately will represent the main limitation of AO-assisted observation with an ELT. In this work we analyse the spatial structure of these petal modes and how it affects the ability of a Pyramid Wavefront sensor to sense them. Then we propose a variation around the classical Pyramid concept for increasing the WFS sensitivity to this particular modes. Nevertheless, We show that one single WFS can not accurately and simultaneously measure turbulence and petal modes. We propose a double path wavefront sensor scheme to solve this problem. We show that such a scheme,associated to a spatial filtering of residual turbulence in the second WFS path dedicated to petal mode sensing, allows to fully measure and correct for both turbulence and fragmentation effects and will eventually restore the full capability and spatial resolution of the future ELT.