Complete sub-aperture pre-polishing & finishing solution to improve speed and determinism in asphere manufacture

There are four fundamental steps to precision glass aspheric manufacturing: 1) grinding - to generate the rough shape, 2) pre-polishing - to remove subsurface damage and smooth grinding residuals, 3) metrology - to quantify surface figure errors and 4) finishing - to reach final figure and roughness specification. The aspheric pre-polish step is currently the least deterministic process, as conventional sub-aperture tools (e.g. pitch, polyurethane pad) inherently have removal rate variation due to tool misfit, pad wear, or slurry variation over time. This limits the final figure accuracy achievable or at a minimum, leads to significant unpredictability in cycle time. Magnetorheological Finishing (MRF (R)) offers a very deterministic finishing process, but is limited in its ability to smooth certain spatial frequencies. In this paper, we present a complete polishing process (pre-polishing + finishing) that utilizes a novel combination of MRF and conventional small-tool pitch polishing. This combined approach leverages the strengths of both processes, providing a fast, deterministic, scalable aspheric finishing process to compliment existing grinding technology. Results for a 300 mm rectangular aperture, off-axis section will be presented showing final peak-to-valley quality of better than twentieth wave.