RAP Figuring Slumped Mirrors to Remove Mid-Spatial Frequency Errors

Future X-ray telescopes require significant amounts of optical area. To accommodate this in a grazing incidence design, extremely thin mirrors are formed in concentric shell configurations. A slumping technique has been demonstrated with such thin, lightweight shells. However, the optical surface is found to contain a significant amount of mid-spatial frequency errors. Reactive Atom Plasma (RAP) is a figuring technique that does not impart mid-spatial frequencies to the optical substrate geometries and can additionally remove specific spectra from the figure error. RAP is a sub-aperture, atmospheric pressure, non-contact figuring technology that relies on a deterministic gas-phase etching of the optical surface with high material removal rates. Further, RAP has the ability to modulate tool footprint on the fly, allowing the removal of specific spatial frequencies from the error spectrum. RAP has already been demonstrated as a very credible approach for fabricating the lightweight wedges required for the assembly of such mirrors and is especially suitable for figuring extremely lightweight mirrors given the non-contact operation. In phase 1, we demonstrated the ability of the RAP process to impart minimal mid-spatial errors into the optical surface. A fully automated figuring platform with adjustable footprints is to be developed for phase 2.