69-in. Mirror for Perkins Observatory

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    DR. HABLAN T. STETSON, director of the Perkins Observatory, Ohio Wesleyan University, Delaware, Ohio, informs us that the large 69-inch mirror for the new telescope was installed at the Observatory on Dec. 14. The disk of glass, weighing 3000 lb., was cast as an experiment in American glass making in the optical shops of the U.S. Bureau of Standards, in May 1927. The exceptional annealing of the glass, which required eight months of controlled temperature, facilitated greatly the polishing and figuring of the mirror's surface, which was executed in the factory of J. W. Fecker, in Pittsburgh. Exhaustive tests carried on at Pittsburgh by Dr. Stetson, Dr. J. S. Plaskett, of the Dominion Astrophysical Observatory, and I. C. Gardner, chief of the Division of Optical Instruments, of the U.S. Bureau of Standards, showed the surface to be of the highest order of excellence. Quantitative measurements revealed departures from the mean focus of twenty-five feet to be not greater than 0.01 in. for any zone. The completion of the Perkins telescope brings into the central States the third largest telescope in existence, the two other instruments of greater aperture being the 72.in. reflector of Victoria, B.C., and the 100-in. reflector at Mt. Wilson, California. These great telescopes in North America have all led the way to important additions to astronomical knowledge, and it may be confidently anticipated that the new instrument, under Dr. Stetson's direction, will carry on the tradition.

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    69-in. Mirror for Perkins Observatory. Nature 129, 17 (1932) doi:10.1038/129017c0

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