Fabrication and testing of the f/1 1.8-m Vatican Advanced Technology Telescope primary mirror

Abstract

Fabricating very large (up to 8.4 m) and very fast (down to f/1) aspherics is one of the principal goals of the Steward Observatory Mirror Laboratory. As a first step toward this goal we have been fabricating the 1.8-m f/1 primary mirror for the Vatican Advanced Technology Telescope. Using a novel stressed-lap polishing tool we have been able to achieve a smooth, high quality figure on the mirror that departs from the best-fit sphere by >700 waves. The tool is 60 cm in diameter, faced with pitch, and is actively bent under computer control to fit the asphere as it is stroked across the surface in conventional fashion. Initially, the mirror was polished as a sphere to the final aspheric specifications with a rigid lap of the same size to establish techniques for figuring the mirror with a tool of this size. The asphere was then diamond generated into the surface on the large optical generator to ~4 μm rms. Stressed-lap polishing then commenced without any further grinding. Both speed and pressure variation of the lap were used to achieve rotational symmetry.