Abstract: A system for measuring distance to an optical surface includes a range finder for transmitting a beam toward the optical surface, and a ring mirror disposed between the range finder and the optical surface. The ring mirror has (a) a circumferential reflecting surface for reflecting a portion of the transmitted beam back to the range finder, and (b) a central aperture for passing another portion of the transmitted beam toward the optical surface. The circumferential reflecting surface is effective in providing a zero reference point for the range finder. The central aperture is effective in passing the transmitted beam to the optical surface for sequentially addressing the optical surface at different locations on the optical surface. The transmitted beam may be steerable for (a) providing the zero reference point circumferentially about the reflecting surface, and (b) providing sequential addresses to locations on the optical surface.