Results of Ion Figuring W.M. Keck Telescope Primary Mirror Segments

Since mid-1990 several W.M. Keck 10 meter Telescope primary mirror segments have been final figured using the ion beam figuring process. This optical fabrication technique uses a neutral ion beam to remove residual surface figure errors after the polishing process has been completed. The initial surface figure errors for the primary mirror segments have ranged from 0.090 to 0.350 µm rms; after a single ion figuring process correction cycle the errors have been reduced to 0.025 to 0.090 µm rms. The process and approach used, along with recent processing results will be discussed.