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Chapter 11:
Techniques for Mounting Larger Nonmetallic Mirrors
In this chapter, we continue our considerations of mountings for mirrors by discussing techniques that can be used to mount nonmetallic mirrors in the size range from about 0.89 m (35 in.) in diameter to 8.4 m (331 in.). Weight minimization is increasingly important as mirror size increases. With the exception of space-borne applications, the mirrors considered here are too flexible for 3-point, rim, or hub mounting during use and so must be supported at many points. Axial supports, generally applied to the back of the mirror, radial supports, generally applied to the rim of the mirror, and "defining supports"€ (locating and orienting the mirror) pose major design issues. Some mirrors have these forces applied within the interior of the substrate at the neutral surface where gravitational moments acting on the localized volumes are balanced. Most of the large mirrors mentioned here are intended for astronomical applications as scientists begin to reap the benefits of new design, manufacturing, and control technologies that break the size limitations previously imposed on ground-based systems by residual manufacturing errors, gravitational effects, and atmospheric turbulence. Mountings for selected examples of historically important telescope mirrors as well as operational and developmental telescope mirrors are considered. Mountings for mirrors used or tested with their axes in fixed orientations, horizontal or vertical, also are discussed. Some challenges of mounting large (~8-m) aperture, thin mirrors on active mechanisms that maintain required optical performance under the command of control systems with optical surface figure or image quality sensors are pointed out. An example of such an "adaptive" mirror (for the Gemini telescopes) is discussed. Finally, some unique features of the mountings for large mirrors in two highly successful space borne telescopes (Hubble and Chandra) are reviewed.
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