Extreme Adaptive Optics in the mid-IR: The METIS AO system

TL;DR

This paper discusses the challenges and design considerations for implementing high-performance adaptive optics systems at mid-infrared wavelengths on extremely large telescopes, focusing on the METIS instrument for the E-ELT.

Contribution

It provides an overview of atmospheric effects on mid-IR AO performance and proposes a first-order system concept for METIS on the E-ELT.

Findings

High Strehl ratios (>95%) achieved on smaller telescopes with low-order AO.

Performance drops significantly on larger telescopes without advanced AO systems.

Special considerations are needed to reach >90% Strehl ratios at mid-IR wavelengths on ELTs.

Abstract

Adaptive Optics at mid-IR wavelengths has long been seen as either not necessary or easy. The impact of atmospheric turbulence on the performance of 8-10 meter class telescopes in the mid-IR is relatively small compared to other performance issues like sky background and telescope emission. Using a relatively low order AO system, Strehl Ratios of larger than 95% have been reported on 6-8 meter class telescopes. Going to 30-42 meter class telescopes changes this picture dramatically. High Strehl Ratios require what is currently considered a high-order AO system. Furthermore, even with a moderate AO system, first order simulations show that the performance of such a system drops significantly when not taking into account refractivity effects and atmospheric composition variations. Reaching Strehl Ratios of over 90% at L, M and N band will require special considerations and will impact the system design and control scheme of AO systems for mid-IR on ELTs. In this paper we present an overview of the effects that impact the performance of an AO system at mid-IR wavelengths on an ELT and simulations on the performance and we will present a first order system concept of such an AO system for METIS, the mid-IR instrument for the E-ELT.