<title>Progress in the stressed-lap polishing of a 1.8-m f/1 mirror</title>

Abstract

We are in the process of polishing a 1.8-rn f/i ellipsoid with an actively stressed lap. As a preliminary exercise, we have polished the mirror as a sphere using a rigid subdiameter lap. The overall surface error was 25 nm rms, and the surface met a specification corresponding to i/8-arcsec image quality. A stressed lap 600 mm in diameter was designed and built to polish the mirror as an f/i ellipsoid. It consists of an aluminum disk which changes shape continuously under the influence of 12 moment-generating actuators. These actuators are programmed to produce the shape changes necessary to make the lap fit the mirror surface as it moves across that surface and rotates. In this paper we describe the principles and design of the lap, test results, and progress to date in polishing the 1.8-rn mirror.