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Critical to the deployment of large surveillance optics into the space environment is the generation of high quality optics.
Traditionally, aluminum, glass and beryllium have been used; however, silicon carbide becomes of increasing interest
and availability due to its high strength. With the hardness of silicon carbide being similar to diamond, traditional
polishing methods suffer from slow material removal rates, difficulty in achieving the desired figure and inherent risk of
causing catastrophic damage to the lightweight structure.
Rather than increasing structural capacity and mass of the substrate, our proprietary sub-aperture aspheric surface
forming technology offers higher material removal rates (comparable to that of Zerodur or Fused Silica), a deterministic
approach to achieving the desired figure while minimizing contact area and the resulting load on the optical structure.
The technology performed on computer-controlled machines with motion control software providing precise and quick
convergence of surface figure, as demonstrated by optically finishing lightweight silicon carbide aspheres. At the same
time, it also offers the advantage of ideal pitch finish of low surface micro-roughness and low mid-spatial frequency
error. This method provides a solution applicable to all common silicon carbide substrate materials, including substrates
with CVD silicon carbide cladding, offered by major silicon carbide material suppliers.
This paper discusses a demonstration mirror we polished using this novel technology. The mirror is a lightweight silicon
carbide substrate with CVD silicon carbide cladding. It is a convex hyperbolic secondary mirror with 104mm diameter
and approximately 20 microns aspheric departure from best-fit sphere. The mirror has been finished with surface
irregularity of better than 1/50 wave RMS @632.8 nm and surface micro-roughness of under 2 angstroms RMS.
The technology has the potential to be scaled up for manufacturing capabilities of large silicon carbide optics due to its
high material removal rate.
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