Final surface error correction of an off-axis aspheric petal by ion figuring

The final surface figure error correction of a 1.3 m ULETM frit-bonded, ultra- lightweight, off-axis primary mirror petal was successfully completed using the ion figuring process. The petal was a concave aspheric optical element. Ion figuring is an optical fabrication method that provides highly controlled error correction of previously polished surfaces using a directed, inert and neutralized ion beam to physically sputter material from the optic surface. The surface figure error of the petal following conventional polishing was 5.02 (lambda) p-v, 0.62 (lambda) rms, and was improved to 0.17 (lambda) p-v, 0.015 (lambda) rms in four process (test-ion figure) iterations ((lambda) equals 632.8 nm). A multi-iteration process sequence was selected to address the various surface figure error volume and spatial frequency components and involved applying three different beam removal functions. The benefits of ion figuring a complex shaped optic using multiple figuring-testing iterations were clearly demonstrated.