Casting 6.5 meter mirrors for the MMT Conversion and Magellan

B. Olbert, J. R.P. Angel, J. M. Hill, S. F. Hinman

Research output: Contribution to journalConference articlepeer-review

28 Scopus citations


We report on the casting of two 6.5 meter diameter borosilicate honeycomb mirrors at the Steward Observatory Mirror Laboratory. These F/1.25 primary mirror blanks are for the MMT Conversion Project in Arizona and the Magellan Project in Chile. The first 6.5 meter mirror blank was cast in April 1992. After a three month annealing cycle it was removed from the furnace using a fixture glued to the upper surface of the blank. It has since been stripped of its mold material in preparation for polishing. The mold material is broken up with a high pressure water spray. The second identical 6.5 meter was cast in February 1994. Each honeycomb 6.5 meter is cast from over 10 tons of E6 borosilicate glass manufactured by Ohara. This glass is melted into a mold constructed of aluminosilicate fiber to produce a honeycomb structure with roughly 20% of solid density. The hexagonal voids in the honeycomb are produced by ceramic fiber boxes bolted to the bottom of the mold with SiC bolts. The furnace rotates at 7.4 rpm during the casting process to produce a F/1.25 parabola on the front surface. This rough parabola minimizes the amount of glass which must be removed during the grinding process. The front faceplate of the mirror will be 28 mm thick after generating. After the second 6.5 meter blank has been cast, minor modifications will be made to the furnace to allow casting of the first 8.4 meter mirror for the Large Binocular Telescope.

Original languageEnglish (US)
Pages (from-to)144-155
Number of pages12
JournalProceedings of SPIE - The International Society for Optical Engineering
StatePublished - Jun 1 1994
EventAdvanced Technology Optical Telescopes V 1994 - Kailua, Kona, United States
Duration: Mar 13 1994Mar 18 1994

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics
  • Computer Science Applications
  • Applied Mathematics
  • Electrical and Electronic Engineering


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