Rapid Stressed Mirror Polishing of Silicon Carbide Aspheric Mirrors

Aspheric mirror fabrication requires a series of sophisticated, time consuming manufacturing processes. Programs like Airborne Laser (ABL) which require relatively large aspheric mirrors which are aggressively lightweighted add further complexity to the manufacturing process, in this way, further extending the timescale required for mirror manufacturing. Stressed Mirror Polishing (SMP) has been demonstrated to be a novel optical manufacturing approach which can greatly reduce and simplify the production of large aspheric mirrors. SMP has been successfully applied to produce the 36+ hexagonal glass mirror segments (simple slabs) for the two Keck telescopes. Stressed mirror polishing allows the use of a large spherical grind/polish tool (10x - 50x larger grinding area than more traditional small tool polishing approaches) which greatly reduces the manufacturing time associated with aspheric optical fabrication. SSG's Tinsley Laboratories subsidiary has applied the SMP process to produce 18 of the two meter Keck mirror segments. Once the process was proven out Tinsley was producing these two meter aspheric optics at a rate of X per month. SMP has been proven to be an exceptionally fast process when it is applied to simple slab mirror geometries, however, attempts to apply the process to lightweighted glass mirror substrates has resulted in catastrophic failures. In these cases the non-uniform distribution of stresses into these rib-supported glass mirror substrates has resulted in substrate fractures. SSG proposes the application of the stressed mirror polishing process to aggressively lightweighted Reaction Bonded (RB) Silicon Carbide (SiC) mirror substrates. RB SiC is > 5x more fracture tough than ULE glass, making stressed mirror polishing of rib-supported SiC mirrors viable. SSG's near-net-shape RB SiC manufacturing process provides additional schedule benefits in that the lightweighted substrate can be produced with little to no machining, this process brings the substrate manufacturing process down from 12 - 16 months (based on frit bonded, lightweighted ULE as a comparison) to 2 - 3 months. We expect SMP of an aggressively lightweighted mirror substrate, suitable for a program like ABL, to result in significant surface quilting. The manufacturing process we proposed utilizes Tinsley's unique Computer Controlled Optical Surfacing (CCOS) technique to remove this print through as the final surface improvement step. Tinsley's CCOS process has been demonstrated to produce ultra-high quality surfaces (< 2 nm RMS aspheric surfaces produced for Hubble Space Telescope correction optics) while also being able to remove quilting from aggressively lightweighted optical surfaces (10 kg/m2 Beryllium AMSD mirror). The work proposed combines SSG's extensive experience with SiC flight optics and RB SiC manufacturing with Tinsley's expertise in stressed mirror polishing and computer controlled optical surfacing to create a novel, rapid, aspheri