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Progress on Mount Graham.

Two pacesetting telescopes were dedicated last September 18th at the University of Arizona's Mount Graham International Observatory. One is the Submillimeter Telescope (SMT), a co-operative project with Germany's Max Planck Institute for Radio Astronomy; the other is the Vatican Advanced Technology Telescope (VATT). Both are presently undergoing final alignment and calibration and will begin science programs this year.

After years of contentious debate and judicial wrangling, the opening of these instruments is welcome news for university astronomers who had to overcome objections from such diverse groups as the Sierra Club, Native Americans, and government agencies. Mount Graham had been selected in the early 1980s because of its superb seeing, dark skies, high elevation, and proximity to Tucson. Yet the development of an observatory there was threatened because an endangered subspecies of red squirrel lives on the mountain. Compliance with U. S. Forest Service stipulations, to insure minimal impact on the environment and the squirrels, forced many changes in site size and location.

The VATT sports a lightweight, 1.8-meter, honeycomb mirror that was spin-cast and figured at the university's mirror laboratory (S&T: May 1992, page 491). It features a Gregorian optical system, which is similar to a Cassegrain except the 0.4-meter secondary is concave rather than convex. According to Christopher Corbally, a Vatican astronomer and driving force behind the telescope project, this design best utilizes the extremely fast (f/1.0) primary, facilitates mirror testing, and provides for a very squat, rigid telescope. Other advanced features are adaptive optics and a primary that can be thermally controlled to match the temperature of the surrounding air. The anticipated resolution is 0.06 arc second at the near-infrared wavelength of 2 microns. The SMT is designed to operate between 1 millimeter and 300 microns, a largely unexplored portion of the electromagnetic spectrum. One of the major problems associated with these so-called submillimeter wavelengths is the need to maintain an extremely accurate dish contour. For telescopes operating at longer wavelengths, steel or aluminum structural elements can hold adequate tolerances. But for submillimeter work, control of thermal expansion and contraction requires a high- tech solution. As in golf clubs, a graphite-epoxy composite was used to fabricated large portions of the 10-meter primary dish, the secondary reflector, and much of the support structure. The face of the primary is expected to hold its paraboloidal shape to within 0.001 inch over the temperature extremes encountered at the 10,400- foot site. Over the next year groundbreaking will take place for Mount Graham's next major facility, the Large Binocular Telescope (LBT, formerly called the Columbus Project). A site 500 feet east of the SMT has been approved by the U. S. Forest Service. The LBT, a joint project of the University of Arizona, Italy's Arcetri Astrophysical Observatory, and the Research Corporation, will feature twin, 8.4- meter, honeycomb mirrors.

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Title Annotation:Mount Graham International Observatory
Publication:Sky & Telescope
Date:Mar 1, 1994
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