Experiments have demonstrated the feasibility of making mirror structures that comprise a thin face plate of chemical-vapor-deposited (CVD) pyrolytic graphite (PG) integrated with a thicker backing plate of CVD PG open-cell foam. The backing plate serves as a stiff structural support for the face plate, on which a precise mirror optical surface can then be formed. Lightweight, highly precise mirrors for telescopes (including telescopes in outer space) and for aiming laser beams could be fabricated following this approach.
Considered on the basis of cost and performance, CVD PG may be the most effective available structural material for precise, lightweight mirrors. CVD PG is produced by thermal decomposition of natural gas, which is available in abundance at low cost. PG is a highly ordered (with respect to molecular structure), high-stiffness phase of carbon that has a very low mass density (2.1 g/cm3), a very low coefficient of thermal expansion (<10-6 K-1), and an in-molecular-plane thermal conductivity of 372 W/(m·K) (rivaling that of copper). In addition, PG can be polished to high optical quality (a root-mean-square surface roughness <10 Å). The hardness of PG can be tailored to alter the degree of polishability and the cost of polishing.
What has made the fabrication of unitary, all-PG mirror structures possible is the development of a CVD process in which a fully dense PG face plate is deposited directly on a CVD PG foam support. The great advantage afforded by this process is that the structure produced contains no dissimilar materials, so that thermal distortions associated with differential thermal expansion can be expected to be minimal. The process is as applicable to complex, curved mirrors as it is to flat ones. The areal mass densities of CVD PG mirror structures are expected to be <10 kg/m2, and may even range as low as 5 kg/m2.
In flexure, compression, and thermal-expansion tests, CVD PG foam has been found to exhibit the high stiffness and low thermal expansion required of a lightweight structural-support material for mirrors. The rigidity of CVD PG was further demonstrated during grinding and polishing of mirrors. While the fabrication processes have not yet been optimized, it has been established that lightweight, stiff CVD PG mirror structures can be formed and mirror surfaces can be polished on their face plates, all at relatively low cost.