Tech Briefs

Industrial-grade, lightweight mirrors used in military and aeronautics have tight specifications brought on by demanding performance parameters. For example, a mirror that is used in an orbiting telescope would have to be extremely lightweight, stiff, and be configured to operate in extreme temperatures. These parameters traditionally work against each other. A material that is stiff is typically heavy, and a mirror that is lightweight and machinable may greatly distort when exposed to extreme heat or cold. Furthermore, materials that fit some of these parameters may not be easily machined to create a mirror — an art that requires high-precision tooling.

Stiffness can be added to a mirror through material selection and mirror design. Adding a back plate to the mirror can easily double the stiffness; however, current designs tailored to minimize weight do not provide a suitable surface on which to mount a back plate. To minimize weight, all nonessential material must be removed. Traditional designs include an eggcrate design with a series of ribs and a pattern of pockets in the walls of the ribs that may be square, triangular, or any shape that can be produced by machining and/or casting. Past attempts at mounting back plates have resulted in mounting distortions that render a mirror effectively useless for the intended projects. Distortions caused by errors and unevenness of mirror mounting surfaces can translate approximately 98% to the front mirror contour surface.

An ultra-lightweight metal mirror was developed with internal structural features that tailor the stiffness of the mirror for unique capability to resist distortion. The unique configuration of the mirror design is made possible by additive manufacturing, which allows a complex internal structure to be built from a Direct Laser Metal (DLM) Sintering process. Internal structure, as well as optical surfaces and mounting surfaces, is made much thinner than standard practice with up to 100:1 height-of-structure-to-thickness ratio.

Direct Metal Laser Sintering (DMLS) allows fabrication of extremely thin wall and complex structures that may be essentially impossible to produce by metal removal processes. Complex aspheric contours may be easily produced as simple spherical optical surfaces. The DMLS process is a layer-by-layer manufacturing process. A single layer of metal may be laser welded upon a previous layer, resulting in time-dependent temperature profiles within a component being fabricated.

The internal support structure of the metal mirror optimizes stiffness and minimizes areal density consistent with suitable strength and low cost. The walls comprise a thickness of approximately 0.01 to 0.025". The thickness of the walls does not affect the configuration of the internal support structure. The thickness of the walls may affect the weight of the mirror, and additive manufacturing may result in a lower wall thickness.

The pockets are configured to provide maximum support to a front mirror surface as well as minimum weight. Removing material from the internal portion of the pockets allows for reduction in weight while not compromising the stiffness and support to the mirror surface. Reduction in weight is a critical concern for any object intended for use in outer space. The holes provide a reduction in weight while not compromising strength and support of the internal support structure. They remove excess material from columns that may form between the pockets.

Producing holes that remove weight from the internal support structure is achieved with additive manufacturing because the internal support structure is built up layer-by-layer with the holes in the internal support structure determined by choosing where material may or may not be added to each layer. The result is a lighter-weight mirror substrate with negligible difference in mirror structural stiffness. In addition to a significant reduction in weight of the mirror substrate by the absence of unnecessary material, there is an advantage to the production of holes in designing the internal support structure to allow a system of highly efficient bracing of back and front surfaces of the mirror. The system of efficient bracing of the back and front surfaces of the mirror allows tailoring the stiffness of those surfaces relative to one another, providing the capability to make a rear mounting surface on the back side of the mirror stiffer than the front side of the mirror. This unique ability limits the mounting distortions.

The flanges add stiffness to the rear mounting surface, and offer stiffness both to the internal support structure through strengthening the transition points of the pockets, and the rear mounting surface by providing more contact area for the rear-mounted surface. Stiffening mirror mounting surfaces is possible through additive manufacturing, and may aid in limiting distortion of a mirror optical surface. The internal support structure tailors stiffness and flexibility relative to the front mirror contour so that distortions are reduced, or dampened, by internal flexing of the internal support structure.

This work was done by John Casstevens of Dallas Optical Systems 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-33372-1

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