Design of pre-optics for laser guide star wavefront sensor for the ELT

TL;DR

This paper presents an optical design for a zoom system in laser guide star wavefront sensors tailored for the ELT-HARMONI instrument, enabling active refocusing with minimal wavefront error and low pupil distortion.

Contribution

It introduces a novel four-group lens zoom system with aspherical surfaces designed specifically for ELT wavefront sensing applications.

Findings

Residual wavefront error median 13.8-94.3 nm

Maximum wavefront error <206 nm

Exit pupil distortion 0.26-0.36%

Abstract

In the present paper, we consider the optical design of a zoom system for the active refocusing in laser guide star wavefront sensors. The system is designed according to the specifications coming from the Extremely Large Telescope (ELT)-HARMONI instrument, the first-light, integral field spectrograph for the European (E)-ELT. The system must provide a refocusing of the laser guide as a function of telescope pointing and large decentring of the incoming beam. The system considers four moving lens groups, each of them being a doublet with one aspherical surface. The advantages and shortcomings of such a solution in terms of the component displacements and complexity of the surfaces are described in detail. It is shown that the system can provide the median value of the residual wavefront error of 13.8-94.3 nm and the maximum value <206 nm, while the exit pupil distortion is 0.26-0.36% for each of the telescope pointing directions.