Stressed-lap polishing of large aspheric mirrors

The stressed-lap polishing technique has been developed to figure large, highly aspheric mirrors for optical and infrared telescopes. The Steward Observatory Mirror Laboratory plans to cast and polish lightweight honeycomb sandwich mirrors up to 8.4-m diameter, with focal ratios as fast as 1.14. These mirrors must be figured to an accuracy corresponding to better than 0.1 arcsec FWHM image quality. Three primary mirrors have been cast at 1.8-m diameter and three at 3.5-m diameter, and two of these are being polished with stressed laps: a 1.8-m f/1.0 ellipsoid for the Vatican Advanced Technology Telescope, and a 3.5-m f/1.5 paraboloid for a telescope at the Air Force Phillips Laboratory. Both mirrors are far more aspheric than any existing astronomical telescope mirror, with peak-to-valley departures of 445μm and 253μm from the best-fitting sphere. Through active control of the shape of the polishing tool, the stressed lap combines the natural smoothing and corrective effect of a large, stiff, rapidly moving tool with the ability to maintain an accurate fit to a severely aspheric surface.