Proceedings PaperSpace Telescope Mirror Substrate
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NASA's Space Telescope calls for a concave-convex 98"-diameter lightweight mirror substrate. The material chosen was Corning's Code 7971 ULE glass, which was fabricated into a 10"-thick lightweight core faced with 1"-thick front and back plates. ULE glass is manufactured by a very unusual process, and has some equally unusual properties. The manufacturing method is known as flame hydrolysis, in which silicon and titanium tetrachloride vapors are reacted in a flame to produce tiny molten glass particles. These particles are collected on rotating tables to form clear glass discs 60" in diameter and 5-6" thick, from which the substrate must be fabricated. Perhaps the most unusual property of ULE material is its weldability. Discs are sawed into thin slabs that are welded together in an open room to form inner and outer rings and a square-celled monolithic honeycomb core. Other discs are heated to 1600°C and allowed to flow out to the needed diameter to produce the face plates. The entire assembly is sealed together at 1600°C and sagged to curvature to produce the rough blank, which is then annealed. Another unusual property is the linear correlation of the coefficient of thermal expansion (C.T.E.) and ultrasonic velocity. This permits non-destructive evaluation and documentation of the actual material used for the component parts, which assures that the final blank will meet the demanding requirements for homogeneity of C.T.E. as well as near-zero C.T.E.