Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.7408a&link_type=abstract
American Astronomical Society, 181st AAS Meeting, #74.08; Bulletin of the American Astronomical Society, Vol. 24, p.1240
Astronomy and Astrophysics
Astronomy
Scientific paper
In April 1992 the University of Arizona Mirror Lab successfully cast the first U.S. telescope mirror blank that exceeds in size the Hale telescope mirror cast in 1935. The new blank goes well beyond the simple ribbed form of the Hale casting in its use of a honeycomb sandwich structure to reduce weight, enhance stiffness and control thermal distortion. The blank weighs 10 tons, less than the 5 m and most 4 m blanks, despite its 72 cm edge thickness. Chunks of Ohara E6 borosilicate glass were melted at 1180 C over 1020 ceramic cores to form the internal honeycomb. Wind-induced distortion, a serious issue for large telescopes aiming at matching the best atmospheric seeing, is eliminated because of the high stiffness. The honeycomb blank is an order of magnitude stiffer than the same mass of glass in a meniscus blank. Similarly mirror seeing, often a limitation when seeing is half an arcsecond or better, is eliminated by ventilation of the very open structure. Ventilation of the honeycomb with ambient air allows the mirror to track the nighttime temperature with a time constant of 30 -- 40 minutes. After three months of cooling and annealing, the furnace was opened in June 1992. At the time of this writing, the blank is about to be lifted for removal of the internal mold parts that form the honeycomb structure in the one-piece casting. Optical finishing of the F/1.25 parabolic surface by diamond generating and stressed lap polishing will begin in 1993.
Angel James Roger P.
Hill James M.
Woolf Nancy J.
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