Using a new technology of mirror construction and computer alignment, the Keck telescope provided a quantum leap in the abilities of ground-based, visible-light astronomy.
In 1985, a newly organized consortium called the California Association for Research in Astronomy
Since the mid-1970’s, when Soviet engineers decided to build a reflecting telescope with a mirror diameter of almost 600 centimeters (about 236 inches), prevailing opinion had been that the size of ground-based optical telescopes had reached practical limits. The Soviet mirror was not very successful. Conventional methods of mirror preparation, using fused silicates for material and precision machinery to grind the parabolic surface on the mirror, impose problems that compound themselves as mirror diameter increases. Such methods simply became impossible for the ponderous Soviet telescope, which weighed more than 50 tons. Construction of the previous record holder for size, the 508-centimeter (200-inch) Hale reflector at Mount Palomar Observatory near San Diego, California, completed in 1948, had demanded many technical innovations, including patterns of wafflelike indentations to reduce the enormous weight of the single piece of Pyrex constituting the mirror. Several attempts were necessary to manufacture a usable blank mirror disk for Hale, and months of grinding and polishing of the parabolic surface resulted in wastage of tons of extremely expensive Pyrex.
The engineers of the Keck telescope promised to overcome the physical and financial obstacles to larger mirrors with new engineering. Instead of a single mirror, the Keck instrument would align thirty-six hexagonal mirrors, each approximately 1.8 meters (5.9 feet) in diameter, into a single array with light-gathering power equal to a single mirror with a diameter of 10 meters (32.8 feet, roughly four times the gathering power of the Hale telescope). The mirrors would be arranged in a mosaic pattern reminiscent of a honeycomb (or, as one engineer noted, old-fashioned bathroom floor tiles). Positioning of each mirror segment would be controlled by computers throughout observing sessions, so that together the segments would focus their reflected light on a single, secondary mirror as a composite image. Computers would also continuously adjust thirty-six support points under each mirror segment, as well as alignment of the telescope support structure, to minimize thermal and gravitational deformation. This technology would permit each segment to be a scant 7.5 centimeters (2.9 inches) in thickness, almost waferlike by the standards of earlier mirrors of comparable diameter. The weight of all thirty-six segments would be about equal to that of the single primary mirror in the Hale telescope.
The hexagonal shape of the mirror segments raised problems with regard to conventional fabrication, grinding, and polishing techniques designed for circular mirrors. Surfaces of the mirror segments had to be configured for specific locations in the array, and differing surface and grinding formulas had to be used for particular positions. Engineers met this challenge by developing a process called stressed mirror polishing, which applies predetermined forces to the mirror edges to achieve appropriate levels of distortion during polishing. When the forces are released, the parabolic surface of the mirror relaxes into the desired shape.
The construction problems associated with segmented mirrors are more than offset by the advantages of segmented mirrors over single mirrors. In addition to overcoming the problem of excessive weight, segmented mirrors allow damage to the mirror surface to be repaired, or a mirror segment to be replaced, fairly inexpensively, whereas serious damage to a primary mirror such as the one in the Hale instrument would be a major scientific disaster and almost unthinkably expensive to repair. Equipment for aluminizing and polishing small mirrors is relatively inexpensive and easy to obtain; in contrast, customized machinery had to be built to aluminize and polish the giant primary mirrors of the Hale instrument. In addition, with a segmented mirror, analysis of the mechanical and structural characteristics of the telescope frame and mounting assembly may be completed, and changes made if required, with only a few of the mirror segments installed.
The technology used in constructing the Keck telescope borrowed from an earlier experiment called the Multi-Mirror Telescope
Traditionally, large telescope mirrors are made from fused quartz or silicate glass. Angel’s team decided to use borosilicate, a Pyrex-like material, in molten form. Ventilating devices were installed in the furnace to remove fumes as the material heated and thus minimize the danger that impurities would turn up in the finished mirror blank, a problem that had turned others away from borosilicate material. The first attempt at a borosilicate mirror was a 3.5-meter (11.5-foot) experiment in which a revolving furnace heated the material to molten form over a honeycomb support structure of carefully machined fiberboard hexagonal cores. Spinning the furnace as it held molten material allowed the team to create the parabolic surface for the mirror through centrifugal force. (This process also allowed the creation of deeper and more accurate parabolic surfaces than could be obtained through conventional grinding processes. Because a deeper parabolic curve means a shorter telescope focal length, the technique was expected to produce larger mirrors that could be installed in observatories of modest size.)
By 1989, the Angel team had cast a 6.5-meter (21.3-foot) blank destined to replace the six-mirror MMT array on Mount Hopkins. Encouraged by these early results, the National Science Foundation elected to provide major funding, and the completed telescope was in operation by May, 2000.
Another initiative came from the Dutch-born astronomer Jacques Beckers, founding director of the MMT project at Mount Hopkins. Under his direction, plans developed for a massive project called the National New Technology Telescope
The ESO project borrowed yet another innovation in telescope design developed in the 1980’s by a research team at the University of Texas. Known as “active optics,” the system depended on a flexible rather than a rigid mirror a useful analogy is the difference between a soft contact lens and a hard contact lens with very extensive computer mediation controlling large numbers of adjustable pads at frequent intervals on the back surface of the mirror. The flexible mirror, like the Angel mirrors, can be thinner and lighter than conventional mirrors of the Hale variety.
In the last quarter of the twentieth century, ground-based visual astronomy, widely perceived in 1975 as a technological dead end and an increasingly marginal pursuit, instead embarked on a period of unprecedented technological progress and daring engineering. The Keck telescope proved capable of detecting phenomena 15 billion light-years or more from Earth, helping to bring astronomers to the very frontiers of the universe, where the light observed from immensely distant sources was emitted at around the time of the “big bang,” the beginning of the universe. Many of Keck’s new technologies were to be used in orbiting platforms, such as the Hubble Space Telescope, in the 1990’s.
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Brunier, Serge, and Anne-Marie Lagrange. Great Observatories of the World. Richmond Hill, Ont.: Firefly Books, 2005. Oversize volume presents profiles of thirty-six of the world’s leading observatories, ten space-based telescopes, and eleven “observatories of the future.” Focuses on telescope technology. The many illustrations include photographs of the observatories themselves as well as of the celestial objects seen through their telescopes. -
Fisher, Arthur. “Spinning Scopes: Making Larger Reflecting Telescope Mirrors in a Rotating Furnace.” Popular Science 231 (October, 1987): 76-81. Summarizes the Angel technology for mirror fabrication and discusses how the process revolutionized the outlook for firm mirrors. Also speculates about possible future projects. -
Henbest, Nigel. “The Great Telescope Race.” New Scientist 119 (October 29, 1988): 52-59. General discussion of the profusion of large-mirror telescope technology in the 1980’s. -
Krisciunas, K. Astronomical Centers of the World. New York: Cambridge University Press, 1988. Informative survey of the large number of new or rapidly expanding observatories in the late twentieth century includes extensive coverage of developments in the Southern Hemisphere. Discusses the “seeing” conditions at such locations as Cerro Tololo and Mauna Kea and the growing threat of light and atmospheric pollution near older centers in the American Southwest. -
McCoy, Jan. “Angel Builds ’Em Bigger.” Sky and Telescope 76 (August, 1988): 128-129. Provides an excellent illustrated synopsis of the Angel process for mirror fabrication using a spinning furnace and honeycombed compartments. -
Nelson, Jerry. “The Keck Telescope.” American Scientist 77 (March/April, 1989): 170-176. Progress report on the great segmented-mirror telescope at the Keck Observatory in Mauna Kea. -
Sinnott, Roger W. “The Keck Telescope’s Giant Eye.” Sky and Telescope 80 (July, 1990): 15-22. Summary article published as the telescope neared completion provides numerous excellent illustrations. Includes discussion of preliminary testing using four mirror segments. -
Watson, Fred. Stargazer: The Life and Times of the Telescope. New York: Da Capo Press, 2005. History of the telescope’s development includes discussion of the impacts on society of the discoveries the instrument has made possible. Presents the stories of the astronomers and other scientists responsible for advances in telescope technology. Includes glossary and index.
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