Stress polishing of thin shells for adaptive secondary mirrors

Abstract

Context. Adaptive secondary mirrors (ASM) are, or will be, key components on all modern telescopes, providing improved seeing conditions or diffraction limited images, thanks to the high-order atmospheric turbulence correction obtained by controlling the shape of a thin mirror. Their development is a key milestone towards future extremely large telescopes (ELT) where this technology is mandatory for successful observations. Aims. The key point of actual adaptive secondaries technology is the thin glass mirror that acts as a deformable membrane, often aspheric. On 6 m−8 m class telescopes, these are typically 1 m-class with a 2 mm thickness. The optical quality of this shell must be sufficiently good not to degrade the correction, meaning that high spatial frequency errors must be avoided. The innovative method presented here aims at generating aspherical shapes by elastic bending to reach high optical qualities. Methods. This method is called stress polishing and allows generating aspherical optics of a large amplitude with a simple spherical polishing with a full sized lap applied on a warped blank. The main advantage of this technique is the smooth optical quality obtained, free of high spatial frequency ripples as they are classically caused by subaperture toolmarks. After describing the manufacturing process we developed, our analytical calculations lead to a preliminary definition of the geometry of the blank, which allows a precise bending of the substrate. The finite element analysis (FEA) can be performed to refine this geometry by using an iterative method with a criterion based on the power spectral density of the displacement map of the optical surface. Results. Considering the specific case of the Very Large Telescope (VLT) deformable secondary mirror (DSM), extensive FEA were performed for the optimisation of the geometry. Results are showing that the warping will not introduce surface errors higher than 0.3 nm rms on the minimal spatial scale considered on the mirror. Simulations of the flattening operation of the shell also demonstrate that the actuators system is able to correct manufacturing surface errors coming from the warping of the blank with a residual error lower than 8 nm rms.

Ultra-smooth finishing of aspheric surfaces using CAST technology

“…This new generation of giant telescopes and instrumentations needs new developments in terms of technology, performance, and optical surface quality [25]. In the cosmology to exoplanets detection context, the scientific programs push instrument complexity to the edge because the Active Optics methods enable the production of complex optical surfaces of very high quality.…”

Thin glass shells for active optics for future space telescopes

“…Methods for manufacturing thin glass optics were developed, often in the framework of adaptive optics solutions [8], which improve the performance of present ground-based telescopes. The manufacturing processes may involve the thinning and grinding of high-quality thick blanks in combination with stress polishing [9], stressed lap figuring [10], polishing and figuring [11][12][13] technologies. The baseline process of thinning a high value blank is a delicate step so far successfully applied for the production of monolithic units or few segments.…”

Manufacturing of thin glass shells for future space telescopes

“…Methods for manufacturing thin glass optics were developed, often in the framework of adaptive optics solutions [5], which allow boosting the performance of present ground-based telescopes. The manufacturing processes used up to now involve the thinning and grinding of high-quality thick blanks in combination with stress polishing [6], stressed lap figuring [7], polishing and figuring [8,9] technologies. The baseline process of thinning a high value blank is a delicate step adequate for the production of single pieces, but less suited for the production of several units required for the assembly of large segmented adaptive or active systems.…”