HARMONI at ELT: designing a laser guide star wavefront sensors for the ELT

TL;DR

This paper details the design and development of laser guide star wavefront sensors for the HARMONI instrument on the ELT, focusing on optical, mechanical, and verification strategies for high-precision adaptive optics correction.

Contribution

It introduces a novel optical and mechanical design for LGSS using freeform lenses and discusses the rotator system for stable pupil alignment in ELT adaptive optics.

Findings

Final optical and mechanical design of LGSS presented

Innovative use of freeform lenses to reduce optical components

Strategy for AO system verification without access to deformable mirror

Abstract

HARMONI is the first light visible and near-IR integral field spectrograph for the ELT covering a large spectral range from 450nm to 2450nm with resolving powers from 3500 to 18000 and spatial sampling from 60mas to 4mas. It can operate in two Adaptive Optics modes-SCAO and LTAO-or with no AO. The project is preparing for Final Design Reviews. The laser Tomographic AO (LTAO) system provides AO correction with very high sky-coverage thanks to two systems: the Laser Guide Star Sensors (LGSS) and the Natural Guide Star Sensors (NGSS). LGSS is dedicated to the analysis of the wavefront coming from 6 laser guide stars created by the ELT. It is made of 6 independent wavefront sensor (WFS) modules mounted on a rotator of 600mm diameter to stabilise the pupil onto the microlens array in front of the detector. The optical design accepts elongated spots of up to 16 arcsec with no truncation using a CMOS detector from SONY. We will present the final optical and mechanical design of the LGSS based on freeform lenses to minimize the numbers of optical components and to accommodate for the diversity of sodium layer configurations. We will focus on rotator design, illustrating how we will move 1 tons with 90" accuracy in restrictive environment. Finally, we will present the strategy to verify the system in HARMONI context. The main challenge for the verification being how to test an AO system without access to the deformable mirror, part of the ELT.