Development of ultra-lightweight and thermally-stable cordierite ceramic mirrors

Cordierite ceramics offer several interesting advantages over traditional glass materials for ultra-lightweight and thermally-stable space optics. The authors have conducted a research and development activity to establish a technology for manufacturing lightweight cordierite mirrors for next-generation space telescopes. In this activity, a 0.7-m spherical cordierite mirror was manufactured as a mock-up. The mirror substrate was formed to a lightweight rib structure using a near net shape forming process. The rib pattern was designed assuming a hexagonalshaped mirror segment, and the original outer shape was circular for an easy polishing process before outline machining. The weight was 21.3 kg with the original circular shape, but will be reduced to 12.3 kg (at areal density of 38.5 kg/m²) by processing it into the hexagonal outline. The surface figure accuracy was 154 nm RMS (root mean square). As the outer shape will then be processed to become hexagonal, there was no need to further improve the accuracy. The prototype of a 0.3-m aspherical cordierite mirror was also manufactured with more difficulty. This mirror was parabolic in shape, assuming the primary mirror of a Cassegrain telescope, and its substrate was further reduced in weight. Thus, a weight of 1.91 kg was achieved (at areal density of 24.6 kg/m²), and the surface figure accuracy reached 35 nm RMS. As a result of these prototyping efforts, cordierite mirrors are expected to be applied in space optics that require extremely high observation performance, combined with large-size, ultra-lightweight, and high-precision features.