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4 - Competing Primary Mirror Designs

from Part 1 - Optical Observatories

Published online by Cambridge University Press:  15 December 2016

David Leverington
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Summary

Spun Honeycomb Mirrors

The second half of the twentieth century saw the development of three different designs of primary mirror for large optical reflectors. These were spun honeycomb mirrors as used in the Vatican Advanced Technology Telescope on Mount Graham, segmented mirrors (see Section 4.2) as used in the Keck Telescope on Mauna Kea, and thin meniscus mirrors (see Section 4.3) of the sort used in ESO's New Technology Telescope on La Silla. A fourth type, a liquid mirror (see Section 4.5), was also developed, but this had more limited applications. This present section outlines the development of spun honeycomb mirrors as produced at the University of Arizona's Steward Observatory Mirror Laboratory.

The Multiple Mirror Telescope (MMT) had been completed at the Whipple Observatory on Mount Hopkins, Arizona in 1979 with six 72 inch (1.8 m) honeycomb mirrors. These mirrors, made of low expansion, fused silica glass, had been obtained as Air Force surplus equipment. The design of this MMT (see Chapter 10) was innovative in many ways. For example, the observatory building had been designed to allow rapid air movement across both the mirror and telescope structure, allowing them to achieve ambient temperatures quickly and stay there. The honeycomb structure of the mirrors also assisted this.

Neville Woolf of the University of Arizona suggested to his colleague Roger Angel that he look into the possibility of building a larger telescope than the MMT of the same general design.(1) Angel was impressed by the performance of the MMT's honeycomb mirrors and decided to look into the possibility of building larger ones without using expensive low-expansion glass. Instead, Angel and his graduate student John Hill decided to use borosilicate glass, as not only was it cheaper but it had a much lower melting temperature than the low-expansion alternatives, making it easier to cast.

Honeycomb backing for mirrors had been around for some time. The design, which was then called cellar or ribbed, had first been suggested by George Ritchey in the 1920s,(2) and had been used in a modified form for the 200 inch (5 m) Palomar Telescope that saw first light in the late 1940s.

Type
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Observatories and Telescopes of Modern Times
Ground-Based Optical and Radio Astronomy Facilities since 1945
 , pp. 67 - 95
Publisher: Cambridge University Press
Print publication year: 2016

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