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Large telescope primary mirrors cast in borosilicate glass hold an exciting potential for future astronomy. The Steward Observatory mirror laposaJpory has resurrected the casting technology and is heading the research effort for this type of primary ". The University of Texas large telescope group, after conferring with Steward Observatory and NOAO, has ventured into the structural analysis of some permissible structured mirror designs for the benefit of the national telescope effort as well is to consider this type of mirror as a candidate for use in an additional telescope at McDonald Observatory'. Durin;pe past three years, various designs have been proposed both by Texas researchers and the Arizona groupsJt . Because of the intricate rib structure inherent in the castings, finite element models are fairly time-consuming to construct, therefore the number of numerical models constructed tends to be few, and methods of simplifying the analysis can often help to get an overall picture of mirror flexure. In examining local flexure, however, because of the extremely small values, a fair amount of work using fine meshes and accuracy in model representation has proved valuable in identifying sane unique problems for large mirrors. Certain proposed patterns and methods of support exhibit an unexpected and troublesome amount of deflection near the outer edge under self-weight loading. In a computer environment originally designed for batch operation, we have constructed a crude computer-aided engineering system, wherein iterative application of finite element analyses is combined with a small amount of rib redesign at each iteration, and have managed to improve the flexure situation somewhat, but a superlative design that uses the entire surface to define the aperture has yet to be found. The flexure of the individual ribs near the mirror's edge suggests that a more direct loading of the outermost support points be achieved, placing more material in direct compression and having the least amount of material subject to bending. An important outcome is that serious consideration should generally be given to producing a casting oversized enough to obviate edge flexure problems, contrary to the traditional practice of considering the mechanical edge of the optic as the aperture definition.
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