Casting the first 8.4 meter borosilicate honeycomb mirror for the large binocular telescope

J. M. Hill, J Roger P Angel, R. D. Lutz, B. H. Olbert, Peter A Strittmatter

Research output: Chapter in Book/Report/Conference proceedingConference contribution

12 Citations (Scopus)

Abstract

We report on the casting of the first 8.4 meter diameter borosilicate honeycomb mirror at the Steward Observatory Mirror Laboratory. This blank will become the world's largest monolithic glass telescope mirror, and is the first of two mirrors for the Large Binocular Telescope Project. The honeycomb 8.4 meter mirror was cast from 21 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 1662 hexagonal voids that form the honeycomb structure are produced by ceramic fiber boxes bolted to the bottom of the mold with SiC bolts. The furnace rotates at 6.8 rpm during the casting process to produce the F/1.14 paraboloid on the front surface. This shaping 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 and the back faceplate will be 25 mm. The overall thickness of the finished honeycomb blank is 89 cm at the outer edge and 44 cm at the central hole. The first 8.4 meter mirror blank was cast in January 1997. During the casting, two tons of glass leaked from the mold inside the spinning furnace. After a three month annealing cycle the furnace was opened for inspection. As a result of the leakage about 2 square meters of the faceplate near one edge of the mirror was too thin to be polished. In April 1997, an additional two tons of glass was loaded on top of the intact honeycomb structure. In June 1997, after heating slowly back to the annealing temperature, this extra glass was flash melted onto the front of the blank to assure that the faceplate was of sufficient thickness. After a further three month annealing cycle, the furnace was re-opened to reveal a superb casting with low bubble content and little trace of the fusion boundary. The blank has been removed from the furnace using a fixture glued to the upper surface of the blank. It will soon be stripped of its mold material in preparation for polishing.

Original languageEnglish (US)
Title of host publicationProceedings of SPIE - The International Society for Optical Engineering
EditorsL.M. Stepp
Pages172-181
Number of pages10
Volume3352
DOIs
StatePublished - 1998
EventAdvanced Technology Optical/IR Telescopes VI - Kona, HI, United States
Duration: Mar 23 1998Mar 25 1998

Other

OtherAdvanced Technology Optical/IR Telescopes VI
CountryUnited States
CityKona, HI
Period3/23/983/25/98

Fingerprint

honeycomb mirrors
Binoculars
Telescopes
blanks
Casting
Mirrors
telescopes
mirrors
furnaces
Furnaces
Honeycomb structures
honeycomb structures
Glass
glass
Annealing
annealing
casts
ceramic fibers
bolts
Ceramic fibers

Keywords

  • Borosilicate
  • Honeycomb
  • Primary mirror
  • Telescope

ASJC Scopus subject areas

  • Electrical and Electronic Engineering
  • Condensed Matter Physics

Cite this

Hill, J. M., Angel, J. R. P., Lutz, R. D., Olbert, B. H., & Strittmatter, P. A. (1998). Casting the first 8.4 meter borosilicate honeycomb mirror for the large binocular telescope. In L. M. Stepp (Ed.), Proceedings of SPIE - The International Society for Optical Engineering (Vol. 3352, pp. 172-181) https://doi.org/10.1117/12.319295

Casting the first 8.4 meter borosilicate honeycomb mirror for the large binocular telescope. / Hill, J. M.; Angel, J Roger P; Lutz, R. D.; Olbert, B. H.; Strittmatter, Peter A.

Proceedings of SPIE - The International Society for Optical Engineering. ed. / L.M. Stepp. Vol. 3352 1998. p. 172-181.

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Hill, JM, Angel, JRP, Lutz, RD, Olbert, BH & Strittmatter, PA 1998, Casting the first 8.4 meter borosilicate honeycomb mirror for the large binocular telescope. in LM Stepp (ed.), Proceedings of SPIE - The International Society for Optical Engineering. vol. 3352, pp. 172-181, Advanced Technology Optical/IR Telescopes VI, Kona, HI, United States, 3/23/98. https://doi.org/10.1117/12.319295
Hill JM, Angel JRP, Lutz RD, Olbert BH, Strittmatter PA. Casting the first 8.4 meter borosilicate honeycomb mirror for the large binocular telescope. In Stepp LM, editor, Proceedings of SPIE - The International Society for Optical Engineering. Vol. 3352. 1998. p. 172-181 https://doi.org/10.1117/12.319295
Hill, J. M. ; Angel, J Roger P ; Lutz, R. D. ; Olbert, B. H. ; Strittmatter, Peter A. / Casting the first 8.4 meter borosilicate honeycomb mirror for the large binocular telescope. Proceedings of SPIE - The International Society for Optical Engineering. editor / L.M. Stepp. Vol. 3352 1998. pp. 172-181
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abstract = "We report on the casting of the first 8.4 meter diameter borosilicate honeycomb mirror at the Steward Observatory Mirror Laboratory. This blank will become the world's largest monolithic glass telescope mirror, and is the first of two mirrors for the Large Binocular Telescope Project. The honeycomb 8.4 meter mirror was cast from 21 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 1662 hexagonal voids that form the honeycomb structure are produced by ceramic fiber boxes bolted to the bottom of the mold with SiC bolts. The furnace rotates at 6.8 rpm during the casting process to produce the F/1.14 paraboloid on the front surface. This shaping 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 and the back faceplate will be 25 mm. The overall thickness of the finished honeycomb blank is 89 cm at the outer edge and 44 cm at the central hole. The first 8.4 meter mirror blank was cast in January 1997. During the casting, two tons of glass leaked from the mold inside the spinning furnace. After a three month annealing cycle the furnace was opened for inspection. As a result of the leakage about 2 square meters of the faceplate near one edge of the mirror was too thin to be polished. In April 1997, an additional two tons of glass was loaded on top of the intact honeycomb structure. In June 1997, after heating slowly back to the annealing temperature, this extra glass was flash melted onto the front of the blank to assure that the faceplate was of sufficient thickness. After a further three month annealing cycle, the furnace was re-opened to reveal a superb casting with low bubble content and little trace of the fusion boundary. The blank has been removed from the furnace using a fixture glued to the upper surface of the blank. It will soon be stripped of its mold material in preparation for polishing.",
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