Tech Briefs

Production of precision optical mirrors by replication requires molds or mandrels of the complementary shape. For example, replicating a concave mirror requires a convex mandrel. Convex shapes are difficult to fabricate and test since they do not focus light. Convex mandrels are therefore costly when they are available. Their sizes are limited to 1-2 meters. Two-step or double replication is well known in the art. In the traditional method, a specific polymer resin system with fillers is used to replicate an existing concave mirror (designated as “mother”) to produce a convex intermediate designated as “daughter.” The same material is then used to replicate the daughter, creating a third-generation concave that is designated as “granddaughter.”

The problem with the traditional method is that polymer resins shrink upon curing (polymerization), so there is a difference in the shape of the daughter compared to the mother. The granddaughter similarly shrinks compared to the mother, therefore compounding the difference in shape compared to the mother. The end result is a double-replicated concave mirror whose shape departs significantly from the original mirror (mother).

An improved method of double optical replication was developed using carbon nanotube epoxy composite materials to reproduce a concave telescope mirror via a two-step process involving dissimilar materials. The process halts the progressive degradation in fidelity in a two-step replication process. The first replication step is done with a given combination of epoxy resin and fillers (formulation #1). The second step is made using the same epoxy, but with a different type of filler at a different mass ratio (formulation #2). The result is a concave mirror (granddaughter) whose shape more resembles that of the original unit (mother).

Potential benefit of this invention is that it enables low-cost fabrication of large-aperture telescope mirrors. By employing existing concave mirrors rather than the much more expensive convex shapes, the tooling cost is reduced by an order of magnitude. Furthermore, convex mandrels are limited in size, whereas monolithic concave mirrors have been made up to 8 meters. Another benefit is that it speeds production while reducing the chances of tooling failure. Glass mandrels are prone to damage by shear when used in replication, and failure of a convex mandrel can halt production. In the improved two-step process, multiple intermediate convex daughters can be made. Simultaneous fabrication of multiple units is thereby enabled, and the risks of single-point failure are reduced.

This work was done by Peter C. Chen of Lightweight Telescopes Inc. 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 Ronald C. Darty at This email address is being protected from spambots. You need JavaScript enabled to view it.. MFS-33345-1.

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