Ion figuring is an efficient and deterministic process for fabricating high quality optical surfaces. The simultaneous phase shifting interferometer is a helium-neon laser-based Twyman-Green system capable of data acquisition at shutter speeds of up to 1/10,000th of a second, which eliminates vibration effects. Ion figuring and simultaneous phase shifting interferometry were successfully combined to produce an aspheric, 18 inch diameter, f/2, ULE mirror of exceptional surface figure quality. Through iterative cycles of SPSI testing and ion figuring, the mirror surface was processed to a final figure quality of 0.007 (lambda) rms (95% of data <EQ 0.028 (lambda) p-v). The radius of curvature of the mirror was held to within 0.00008 inch of the design radius. Details of the testing/figuring cycles are discussed along with information regarding the mirror mounting scheme, interferometer calibration, and data analysis techniques.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.