Robotic fabrication of large f/1.4 parabolas

Abstract

The technology for rapid fabrication of large optical components utilizing robots has recently been perfected at Itek. This paper discusses the steps necessary to fabricate large aperture, f/1.4 parabolas. Raw glass blanks are initially machined to 5-µm rms surfaces. Aspherizing leaves a subsurface damage (SSD) layer of up to 50µm as measured through replication techniques. Robotic loose abrasive grind iterations figure the surface and remove the SSD layer. A 30 machine hour cycle using a 9µm loose abrasive and subaperture tools gives way to several similar uniform removal (DC) microgrind iterations with a 3µm loose abrasive. A surface error map is computed after each cycle using interferometric and profilometric data. The DC removal phase ends when the SSD layer has been minimized. Through an iterative approach, the high areas of the glass are selectively hit. This figuring process yields a 0.1-µm rms surface. Polishing begins in a fashion similar to grinding. Several 50-h cycles remove the microground SSD layerwhile maintaining the surface figure. Next, the part is figured to final specification of 0.02-µm rms. The work has been perfected as Itek has produced this mirror in a total of 46 fabrication days.