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

The ion figuring process has been successfully used to correct the residual surface figure error on a 1.8 m ZerodurTM off-axis segment of the new Keck telescope primary mirror. The segment is one of 36 hexagonal mirror segments composing the full aperture of the 10 m primary mirror. Ion figuring is an optical fabrication method that provides highly deterministic error correction of previously polished optical surfaces using a directed, inert, and neutralized Argon ion beam to physically sputter material from the surface. Figure error correction is accomplished by varying the velocity of the constant-output ion source as it scans across the optic surface. The surface figure error was reduced from 0.726 micrometers rms to 0.090 micrometers rms in two test-figure iterations. The demonstration provided information and requirements for future processing of ZerodurTM and other glass-ceramic materials, and clearly showed the applicability of ion beam figuring to the final correction of large, complex optics.