The Use of High Resolution Phase-Measuring Interferometry in the Figuring of Large Symmetric and Asymmetric Aspheres

Abstract

In-situ testing of large optics greatly increases the speed and accuracy of the figuring process. Rapid test data turnaround and precise contour map displays allow an optician using traditional polishing techniqes to progress rapidly to final figure. At the Optical Sciences Center we have developed a high resolution phase-measuring interferometer that we used during the final figuring stages of a 20-in. diameter, 97-in. focal length off-axis parabola and a 72-in. diameter, f/2.7 symmetric lightweight parabola. Both mirrors were tested and figured in-situ, the off-axis mirror on the table of the swing-arm generator and the 72-in. mirror in a test tower in which the mirror was supported on a rotary table equipped with a polishing arm. High resolution phase maps of each surface were obtained with resolutions of approximately 9mm/pixel and 2mm/pixel on the 72- and 20-in. mirrors, respectively. The phase contour maps were displayed on a video monitor and photographed with a 35mm camera on Tri-X film. The negatives were mounted directly as slides into circular slide mounts and displayed with a slide projector directly onto the slurry coated mirror. The optician then worked the high zones and patches using traditional polishing methods. The circular slide mounts allowed for easy alignment of the contour maps with the fiducialized mirrors. Test turnaround time from one polishing run to the next was about 2 hours allowing for rapid convergence to the final figure.