![]() The mirrors were made from Zerodur glass-ceramic by the German company Schott AG. Each segment is 1.8 meters wide, 7.5 centimeters thick, and weighs half a ton. In the Keck telescopes, each primary mirror is made of 36 hexagonal segments that work together as a unit. A mirror of similar size cast of a single piece of glass could not be made rigid enough to hold its shape precisely it would sag microscopically under its own weight as it was turned to different positions, causing aberrations in the optical path. The key advance that allowed the construction of the Keck telescopes was the use of active optics to operate smaller mirror segments as a single, contiguous mirror. The Keck II telescope showing the segmented primary mirror ![]() The Keck I telescope began science observations in May 1993, while first light for Keck II occurred on October 23, 1996. With construction of the first telescope well advanced, further donations allowed the construction of a second telescope starting in 1991. Keck Foundation gave $70 million to fund the construction of the Keck I telescope, which began in September 1985, with first light occurring on November 24, 1990, using nine of the eventual 36 segments. With a design in hand, a search for the funding began. With a concept first proposed in 1977, telescope designers at the University of California, Berkeley (Terry Mast) and Lawrence Berkeley Laboratory ( Jerry Nelson) had been developing the technology necessary to build a large, ground-based telescope. They are currently the 3rd and 4th largest. Both telescopes have 10 m (33 ft) aperture primary mirrors, and when completed in 1993 (Keck 1) and 1996 (Keck 2) were the largest optical reflecting telescopes in the world. Keck Observatory is an astronomical observatory with two telescopes at an elevation of 4,145 meters (13,600 ft) near the summit of Mauna Kea in the U.S.
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