W. M. Keck Observatory
The W. M. Keck Observatory is a two-telescope astronomical observatory at an elevation of 4,145 meters (13,600 ft) near the summit of Mauna Kea in the U.S. state of Hawaii. Both telescopes have aperture primary mirrors, and when completed in 1993 (Keck 1) and 1996 (Keck 2) were the largest astronomical telescopes in the world. They are currently the 3rd and 4th largest. Overview. 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. With a design in hand, a search for the funding began. In 1985, Howard B. Keck of the W. M. Keck Foundation gave $70 million to fund the construction of the Keck I telescope, which began in September 1985, with first light occurring on 24 November 1990 using nine of the eventual 36 segments. With construction of the first telescope well advanced, further donations allowed the construction of a second telescope starting in 1991. The Keck I telescope began science observations in May 1993, while first light for Keck II occurred on October 23 , 1996. 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. 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. In the Keck telescopes, each primary mirror is made of 36 hexagonal segments that work together as a unit. Each segment is 1.8 meters wide, 7.5 centimeters thick , and weighs half a ton. The mirrors were made from Zerodur glass-ceramic by the German company Schott AG. On the telescope, each segment is kept stable by a system of active optics, which uses extremely rigid support structures in combination with three actuators under each segment. During observation, the computer-controlled system of sensors and actuators dynamically adjusts each segment's position relative to its neighbors, keeping a surface shape accuracy of four nanometers. As the telescope moves, this twice-per-second adjustment counters the effects of gravity and other environmental and structural effects that can affect mirror shape . Each Keck telescope sits on an altazimuth mount. Most current 8–10 m class telescopes use altazimuth designs due to their reduced structural requirements compared to older equatorial designs. Altazimuth mounting provides the greatest strength and stiffness with the least amount of steel, which, for Keck Observatory, totals about 270 tons per telescope, bringing each telescope's total weight to more than 300 tons. Two proposed designs for the next generation 30 and 40 m telescopes use the same basic technology pioneered at Keck Observatory: a hexagonal mirror array coupled with an altazimuth mounting. Each of the two telescopes has a  primary mirror of 10 meters (32.8 ft or 394 in), slightly smaller than the Gran Telescopio Canarias. However, all of the light collected by the Keck primary mirrors (75.76 m) is sent to the secondary mirror and instruments, compared to GTC's primary mirror, which has an effective light-collection area of 73.4 m, or less than each of the Keck primary mirrors. Because of this fundamental design difference, the Keck telescopes arguably remain the largest steerable, optical/infrared telescopes on Earth. The telescopes are equipped with a suite of cameras and spectrometers that allow observations across much of the  visible and near-infrared spectrum. Management. The Keck Observatory is managed by the California Association for Research in Astronomy, a non-profit 501(c)(3) organization whose board of directors includes representatives from Caltech and the University of California. Construction of the telescopes was made possible through private grants of over $140 million from the W.M. Keck Foundation. The National Aeronautics and Space Administration (NASA) joined the partnership in October 1996 when Keck II commenced observations. Telescope time is allocated by the partner institutions. Caltech, the University of Hawaii System, and the University of California accept proposals from their own researchers ; NASA accepts proposals from researchers based in the United States. Jerry Nelson, Keck Telescope project scientist, contributed to later multi-mirror projects until his death in June 2017. He conceived one of the Kecks's innovations: a reflecting surface of multiple thin segments acting as one mirror. Instruments. Both Keck Observatory telescopes are equipped with laser guide star adaptive optics, which compensates for the blurring due to atmospheric turbulence. The first AO system operational on a large telescope, the equipment has been constantly upgraded to expand the capability.