Toric mirrors and active optics: degenerated configuration for spherical monomode deformable mirrors

Providing toric mirrors with Active Optics techniques will allow generating aspheric surfaces which optical quality avoid high spatial frequencies errors. In order to demonstrate the feasibility of this technique, a link has been established between analytical calculations, finite element modelling and experimental validation. A particular configuration of a flat mono-mode deformable mirror (MDM), called "degenerated configuration", has been analytically calculated, showing how to generate a third order astigmatism aberration (Astm 3) by active deformation. This mirror has been manufactured and tested. A finite element model has been produced in order to correlate simulations with experiments. The deformed optical surface is projected on a Zernike polynomial base, indicating that Astm 3 mode is, within a very high precision, the only aberration generated on the optical surface. Another spherical concave MDM has been modelled as a VLT-SPHERE toric mirror of diameter 133mm, to demonstrate the feasibility of toric surfaces from active optics deformation of a spherical shell. Projection on Zernike base shows that the simulated deformed surface is a combination of a sphere and a quasi pure Astm 3 mode, corresponding to a toroidal surface. Other terms generated, like Astm 5, could benefit of a fine adjustment from the geometry of the substrate.