SAG+: a chemo-mechanical shape adaptive grinding process for fast finishing of large mirrors and lenses

Space observation is inevitable to know about the formation of the universe and galaxies, and for imaging far objects. Therefore, astronomers always strive to build better telescopes with high resolution and stability. Methods to fabricate low-weight large-size precision telescope mirrors are being continuously developed around the globe. Material selection and processing method are two important aspects in fabricating high performance mirrors. Zerodur^TM, a nano-crystalline glass ceramic, has found wide ranging application in earth-bound and space-borne astronomical telescopes due to its near-zero thermal expansion coefficient. To finish this kind of material, Shape Adaptive Grinding (SAG) with contact type elastic abrasive tools is a viable process that achieves excellent surface roughness (Ra < 1 nm) and form error (P-V ~ 20 nm). However, the relatively high hardness of Zerodur^TM causes material removal by mechanical means to be rather low. In this paper, a new SAG+ process is proposed where cerium oxide slurry is used together with diamond based SAG tools to leverage chemo-mechanical interaction and improve the material removal rate. Cerium oxide acts as a reducing agent that destabilizes silica crystals. This allows the Si-O bond to weaken and enhances material removal during polishing. Various grades of resin bonded diamond SAG tools were used in combination with cerium oxide slurry to process lapped Zerodur^TM samples on an industrial robot. SAG+ could achieve at least 50% reduction in the polishing time while retaining similar material removal depth and surface finish. The capabilities of SAG+ were also demonstrated in pre-polishing of silicon X-ray mirror segments, whereby the surface roughness reduced by 47-62% for different grades of polishing tools. These results on robotic polishing of large mirror substrates employing proposed SAG+ technology seems a promising advancement towards sustainable manufacturing, where energy footprint can be significantly reduced by shortening the overall polishing cycle time.