Active optics in astronomy - Freeform mirror for the MESSIER telescope proposal

TL;DR

This paper proposes a novel active optics method using stress polishing to create freeform mirrors for a wide-field space telescope, combining elasticity and optical design optimization.

Contribution

It introduces a new stress polishing technique for freeform mirror fabrication in space telescopes, integrating elasticity and optical optimization for high-quality imaging.

Findings

Successful design of a freeform mirror using stress polishing

Integration of elasticity and optical optimization methods

Feasibility demonstrated through simulations and analysis

Abstract

Active optics techniques in astronomy provide high imaging quality. This paper is dedicated to highly deformable active optics that can generate non-axisymmetric aspheric surfaces-or freeform surfaces-by use of a minimum number of actuators. The aspheric mirror is obtained from a single uniform load t h a t acts over the surface of a closed-form substrate whilst under axia l reaction to its elliptical perimeter ring during spherical polishing. MESSIER space proposal is a wide-field low-central-obstruction folded-two-mirror-anastigmat or here called briefly three-mirror-anastigmat (TMA) telescope. The optical design is a folded reflective Schmidt. Basic telescope features are 36cm aperture, f/2.5, with 1.6 o 2.6 o field of view and a curved field detector allowing null distortion aberration for drift-scan observations. The freeform mirror is generated by spherical stress polishing that provides super-polished freeform surfaces after elastic relaxation. Preliminary analysis required use of the optics theory of 3rd-order aberrations and elasticity theory of thin elliptical plates. Final cross-optimizations were carried out with Zemax raytracing code and Nastran FEA elasticity code in order to determine the complete geometry of a glass ceramic Zerodur deformable substrate.