Ultra-high precision NiP mirror fabrication using ion beam figuring for space applications

Nickel phosphorous (NiP) is a broadly used optical material for wide-field visible to mid-infrared instrumentation in space applications. The process chain usually involves applying an electroless metal deposition onto a mirror substrate, which is then machined by single-point diamond turning (SPDT). However, the SPDT process leaves low- and mid-spatial frequency errors, which degrades the optical performance. In this work, we demonstrate the use of ion beam figuring (IBF) to correct the low-spatial frequency errors. IBF is a noncontact technique used in the final step of a mirror fabrication, which can precisely correct the surface form errors via a deterministic, stable, and fully computer-controlled process. We report on an IBF process which improves the surface quality of the NiP-coated flat and spherical mirrors. For the flat mirror, the root mean square (RMS) height error over a clear aperture (CA) area of 25 × 15 mm² has been improved from 16.3 nm to 3.4 nm after the IBF process. Similarly, for a spherical mirror, the surface irregularity has been reduced from 13.8 nm to 4.4 nm RMS. These irregularities were eventually limited by the diamond turning marks, which could not be corrected or attenuated with the IBF. For further improvement of the surface quality of the NiP mirror, one flat mirror was polished on a lapping tool (chemical and mechanical polishing) and processed through IBF. The surface quality of the NiP mirror achieved a 1.9 nm RMS surface irregularity over a CA of 20 × 10 mm².