Mount Fowlkes near Fort Davis, Texas--The Hobby-Eberly Telescope represents a new era in the design and construction of telescopes. It is the first modern large optical telescope to use an innovative, cost-saving fixed-mirror design. It is also the first telescope ever designed and built to be optimized for spectroscopic surveys of the sky.
The primary mirror of the Hobby-Eberly Telescope is composed of 91 separate one-meter (39.37 inch) hexagonal mirrors, aligned by small computer-controlled motors to act as a single 11-meter (433-inch) light-gathering surface. The light-gathering power of large telescopes is determined by the area of their primary mirrors. The Hobby-Eberly Telescope will be able to gather light from objects close to 100 million times fainter than the unaided human eye can see.
Because of the way the Hobby-Eberly Telescope will be used, 9.2 meters (362 inches) of its surface will be accessible at any given time. Thus, the Hobby-Eberly Telescope is effectively the third-largest telescope in the world, after the twin 10-meter (393-inch) Keck I and Keck II telescopes in Hawaii.
Based on a modification of the concept used in the large radio telescope at Arecibo in Puerto Rico, the Hobby-Eberly Telescope has a primary mirror that does not move up and down from the horizon to the zenith to track stars as they move across the sky, as the primary mirrors of most telescopes do. The Hobby-Eberly Telescope is set in place to point at a given area of the sky prior to an observation period. A much lighter tracker assembly, mounted at the top of the telescope above the primary mirror, then moves to track the astronomical object being observed. Exposure times on a given object of up to two-and-a-half hours are possible.
The innovative design and the use of cost-effective, off-the-shelf technology made it possible to construct the Hobby-Eberly Telescope for a total price of $13.5 million. This is only a fraction of the cost of each of the only comparable telescopes in the world, the twin Keck I and Keck II telescopes in Hawaii. The fixed-mirror design of the Hobby-Eberly Telescope means that it cannot be pointed close to the horizon or the zenith, but scientists using it will be able to cover 70 percent of the sky available above McDonald Observatory over the course of a year, providing first-class scientific capability at a bargain price.
The Hobby-Eberly Telescope is designed to make substantial contributions to many areas of astronomical research. For example, astronomers will measure the chemical compositions of stars that were too distant to study previously, search for planets in orbit around distant stars, identify and measure activity on the surfaces of stars, learn more about "dark matter" located around galaxies, monitor violent activity in the hearts of some galaxies, and refine theories about star formation and evolution.
The Hobby-Eberly Telescope's primary mirror is curved in the shape of a partial sphere, so each of the Hobby-Eberly Telescope's mirror segments is identical in shape and curvature. They were produced by the same assembly-line method and any segment can be used to replace any other, a key cost-saving aspect of the telescope. Spherical mirrors do not produce images as sharp as those produced by the parabolic mirrors found in other telescopes. For this reason, the Hobby-Eberly Telescope uses a "spherical aberration corrector" that allows it to make sharp images over a small area. Light is transmitted from the spherical aberration corrector through fiber-optic cables to spectrographic instruments located in a temperature-controlled room beneath the telescope, where the spectra can be captured for later analysis by computer.
The Hobby-Eberly Telescope stands on Mount Fowlkes at McDonald Observatory, adjacent to Mount Locke, the site of McDonald Observatory's other research telescopes. McDonald Observatory, in far West Texas, has the darkest skies of any major observatory in North America. A new gallery, which will allow visitors a close-up view of the new telescope, will open in December, 1997. Currently more than 120,000 visitors per year visit McDonald Observatory. That number is expected to double in the next five years. McDonald Observatory is expanding its visitors' center facilities to keep pace with growing public interest.
Ground-breaking for the Hobby-Eberly Telescope occurred in March of 1994. "First light," the first engineering proof that the telescope works, was achieved in December 1996. "First Spectrum," the demonstration that the telescope and its commissioning instrumentation package work as planned, occurred in early September 1997, as did the initiation of limited regular scientific use of the telescope. The telescope is currently in its "commissioning" phase, similar to a "shake-down" cruise for a ship, in which its components are brought into optimal performance. The final instrumentation packages for the telescope are being constructed elsewhere and are expected to be installed throughout 1998.
The above story is based on materials provided by Penn State University. Note: Materials may be edited for content and length.
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