Surface error correction of a Keck 10-m telescope primary mirror segment by ion figuring

ABSTRACTThe ion figuring process has been successfully used to correct the residual surface figure error on a 1 . 8m Zerodur off—axis segment of the Keck telescope primary mirror. The segment is one of 36 hexagonal mirrorsegments comprising the full aperture of the lOm primary mirror. Ion figuring is an optical fabrication methodwhich provides highly deterministic error correction of previously polished optical surfaces using a directed, inertand neutralized Argon ion beam to physically sputter material from the surface. Figure error correction is accom-pushed by varying the velocity of the constant—output ion source as it scans across the optic surface . The surfacefigure error was reduced from 0.726 im rms to 0.090 jim rms in two test—figure iterations. The demonstrationprovided information and requirements for future processing of Zerodur and other glass—ceramic materials, andclearly showed the applicability of ion beam figuring to the final correction of large, complex optics.1. INTRODUCTIONA successful demonstration of segmented mirror technology was achieved in November 1990 when the firstnine of 36 hexagonal segments forming the W.M. Keck lOm diameter telescope were assembled and used to makea "first light" image of the stars. The Zerodur hexagonal segments are all off—axis sections of the hyperbolic lOmdiameter, 35m radius—of—curvature primary mirror (PM) .