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Active optics techniques in astronomy provide high imaging quality. Thispaper is dedicated to highly deformable active optics that can generatenon-axisymmetric aspheric surfaces-or freeform surfaces-by use of a minimumnumber of actuators. The aspheric mirror is obtained from a single uniform loadt h a t acts over the surface of a closed-form substrate whilst under axia lreaction to its elliptical perimeter ring during spherical polishing. MESSIERspace proposal is a wide-field low-central-obstructionfolded-two-mirror-anastigmat or here called briefly three-mirror-anastigmat(TMA) telescope. The optical design is a folded reflective Schmidt. Basictelescope features are 36cm aperture, f/2.5, with 1.6 o 2.6 o field of view anda curved field detector allowing null distortion aberration for drift-scanobservations. The freeform mirror is generated by spherical stress polishingthat provides super-polished freeform surfaces after elastic relaxation.Preliminary analysis required use of the optics theory of 3rd-order aberrationsand elasticity theory of thin elliptical plates. Final cross-optimizations werecarried out with Zemax raytracing code and Nastran FEA elasticity code in orderto determine the complete geometry of a glass ceramic Zerodur deformablesubstrate.

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