Silicon Carbide (SiC) mirrors hold many advantages over traditional optical materials and are increasingly common in optical systems. The wide range of optical applications necessitates different approaches to the manufacturing and finishing of SiC mirrors. Three key advancements have led to this differentiation: 1) manufacturing of CVD clad SiC mirrors in near cost and schedule parity with Zerodur, 2) super-polish of amorphous Silicon claddings, 3) low-roughness polishing results of bare reaction-bonded SiC aspheres. Three approaches which utilize these advancements will be discussed, each with its own strengths and weaknesses for specific applications. The relative schedules and performance of these approaches will also be compared, with Zerodur used as a reference.
While post-polish has previously been shown to greatly enhance the surface quality, surface roughness, and surface
figure of single-point diamond turned Aluminum mirrors, the field of bare Aluminum polishing continues to
advance. New results demonstrating improvement in mid-spatial frequency errors and methods for adapting to a
wider range of Aluminum materials constitute the next generation of polished Aluminum mirrors. These results
show new levels of surface finish, correlated with BRDF measurements. Complimentary enhancements have been
made by achieving new levels of precision in the placement of the optical axis relative to datum features, enabling
significant alignment time savings.
Recent advancement has been made in producing difficult aspheric mirrors in bare aluminum to
visible imaging quality. Polished bare aluminum mirrors offer significant producability and cost
advantages for defense and surveillance systems, and can satisfy the environmental and
performance needs of many systems. We describe the finish of both bare VQ Al mirrors in terms
of power spectral density levels that can now be routinely achieved. Parameters are provided to
guide the designer in specifying VQ mirrors, and in considering trades with other materials.
At the DSS conference in 2011, Tinsley introduced to the optics industry a unique bare Aluminum polishing
capability that produced truly visible quality optical surfaces. This process, which can produce an optical
surface with roughness as low as 15Å, achieves these results without the need for claddings or coatings.
This makes Tinsley's bare Aluminum mirrors ideal for telescopes which must function over a wide range of
temperatures and cannot tolerate the bi-metallic effects associated with conventional Aluminum mirror
The single point diamond turning (SPDT) manufacturing process has inherent advantages in regards to both
the mechanical position of the asphere surface relative to mounting datums, and manufacturing leadtime.
This manufacturing process, when followed by Tinsley's bare Aluminum polishing process, can very quickly
yield high-precision aspheric mirrors. The technologies outlined above can now be leveraged to quickly
produce a high-precision telescope assembly which is athermal, and can be "snapped" together.
It is now well understood that with US Department of Defense (DoD) budgets shrinking and the
Services and Agencies demanding new systems which can be fielded more quickly, cost and schedule
are being emphasized more and more. At the same time, the US has ever growing needs for advanced
capabilities to support evolving Intelligence, Surveillance and Reconnaissance objectives. In response
to this market demand for ever more cost-effective, faster to market, single-channel, athermal optical
systems, we have developed new metal polishing technologies which allow for short-lead, low-cost
metal substrates to replace more costly, longer-lead material options.
In parallel, the commercial marketplace is being driven continually to release better, faster and cheaper
electronics. Growth according to Moore's law, enabled by advancements in photolithography, has
produced denser memory, higher resolution displays and faster processors. While the quality of these
products continues to increase, their price is falling. This seeming paradox is driven by industry
advancements in manufacturing technology. The next steps on this curve can be realized via polishing
technology which allows low-cost metal substrates to replace costly Silicon based optics for use in
ultra-short wavelength systems.