In 2012, the National Research Council called for a new generation of astronomical telescopes to enable discovery of habitable planets, facilitate advances in solar physics, and enable the study of faint structures around bright objects by developing high-contrast imaging and spectroscopic technologies to provide unprecedented sensitivity, field of view, and spectroscopy of faint objects. Large-aperture, lightweight mirrors for UV, Optical and IR (UVOIR) telescopes answer this need. NASA requires low-cost, ultra-stable, large-aperture, normal incidence mirrors with low mass-to-collecting-area ratios. Potential UV/optical missions require 4- to 8- or 16- meter monolithic or segmented primary mirrors with active/passive alignment and control of normal-incidence imaging systems with <10 nm RMS surface figures.

In all cases, the most important metric for an advanced optical system (after performance) is affordability or areal cost (cost per m2 of collecting aperture). Current normal incidence space mirrors cost $4 to $6 million per m2 of optical surface area. Trex's Phase I research effort seeks a cost reduction for precision optical components by 5 to 50 times, to less than $1 million to $100,000 per m2.

The labor, schedule, risk, and cost drivers for the large-aperture glass (ULE), metallic (beryllium), and ceramic (zero-dur and silicon carbide) mirrors of modern times are machining, lightweighting, and polishing. Concerning the latter, one of the hidden costs of large-diameter mirror polishing is the iterative metrology process of polish, measure, polish, measure, until requirements are achieved. These costs amplify as the diameter of the mirror increases, and the required surface figure error and surface finish of the mirror decrease. For instance, a single 1.5-meter-diameter lightweight ULE primary mirror for the EUV telescope of a high-altitude balloon experiment costs as much as $10 million.

Current silicon carbide telescope technology is limited by the obtainable surface figure accuracy of reaction-bonded silicon carbide (RB-SiC), thermal stability, and CTE (coefficient of thermal expansion). Trex CVC SiC surpasses RB-SiC in all relevant material properties, including achievable surface figure accuracy.

Trex, in collaboration with Advanced Bonding Technology (ABT), has developed a hybrid joining process whereby CVC SiC components are bonded together to make large optical assemblies or complex structures with the same thermal performance as the base SiC material — a type of additive manufacturing. No additives, adhesives, or bonding agents are used to affect bonding that would influence CTE or elastic modulus of the joint region. This technology will enable cost-effective production of multimeter mirror assemblies by substantially polishing simple CVC SiC plate structures, then bonding the plates together using various support structures. This same approach can be used to build telescope structures (e.g., Surrier truss or optical benches).

ABT developed a hybrid joining process whereby two or more CVD SiC articles (not to be confused with Trex CVC SiC) can be bonded together without the use of bonding agents or additives to produce a bond with the same thermal performance as the base SiC material. Component SiC parts are pre-machined to the desired dimensions and fixtured in a manner so as to yield the desired final structure. Normal high-temperature furnaces are used to facilitate the bond. Early feasibility demonstrations for a CVC SiC solid-state bonding process clearly showed the capability to make large complex mirrors and structures from small, simply shaped, and easily manufactured parts (i.e. additive manufacturing). The CVC SiC bonding technology will be competitive with glass and beryllium mirrors where the requirement for ultra-stability exists.

Low-cost, lightweight, dimensionally stable SiC mirrors have use in complex telescopes for astronomy, imaging, and remote sensing applications, including optical instruments/telescopes that enable imaging, surveillance, and reconnaissance missions for police and paramilitary units, firefighters, power and pipeline monitoring, search and rescue, atmospheric and ocean monitoring, imagery and mapping for resource management, and disaster relief and communications. The dual-use nature of complex telescopes will bring affordability to national defense missions as well.

This work was done by Lauren Bolton, Bill Goodman, and Fred Styer of Trex Enterprises Corp. for Marshall Space Flight Center. NASA is seeking partners to further develop this technology through joint cooperative research and development. For more information about this technology and to explore opportunities, please contact Clark Darty at This email address is being protected from spambots. You need JavaScript enabled to view it.. MFS-33377-1

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This article first appeared in the November, 2017 issue of Tech Briefs Magazine.

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