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Hobby-Eberly Telescope
ITT Fabricates World's Largest Segmented Mirror for Hobby-Eberly Telescope 
In September 1997, ITT completed delivery of the world's largest segmented mirror to the new Hobby-Eberly Telescope at the McDonald Observatory in west Texas.
The 11m-diameter mirror is a key component of the world's first major telescope designed specifically for spectroscopy — the study of individual wavelengths of light from celestial objects. Comprised of 91 hexagonal glass segments, the mirror will gather light from objects 100 million times fainter than the unaided human eye can see. Captured data will reveal detailed information about the composition, structure, temperature and motion of stars, galaxies and other astronomical objects. "The telescope and its 11m primary mirror represent great engineering accomplishments at a low price, and ITT has played a significant role with hardware design, testing assistance and in optical fabrication work," said Victor L. Krabbendam, chief engineer and manager with the Hobby-Eberly Telescope project. Mirror Segments  Made from Zerodur, a glass/ceramic material, each hexagonal segment measures 1m from flat to flat and is 50mm thick. The blank segments were delivered to ITT's optics manufacturing facility, where they were shaped, ground, polished and final figured to their specified surface shape and radius of curvature.
Unlike the 10m Keck I Telescope in Hawaii, whose 36 mirror segments were ion polished by ITT to yield a different aspheric curvature on each segment, each of the Hobby-Eberly Telescope's 91 segments is formed as a 26m radius partial sphere. This enabled ITT to make each segment identical in size and shape. In fact, no segment radius deviates from any another by more than 0.002%. Final Figuring 
To meet this and other demanding requirements of the mirror segment design, ITT employed innovative optical metrology techniques, and a proprietary polishing technique known as
ion figuring.
The ion figuring process finishes the final shape of an optical surface at the atomic level using an Argon ion beam in a vacuum chamber. Computer-controlled, using proprietary software, the system enables opticians to predict with great certainty how the optic will be figured without the lengthy "stop-and-start" process required of traditional polishing techniques. For segmented optics, the process provides exceptional edge-of-aperture control. Ion figuring each segment consisted of two iterative steps: the first step to improve the surface quality, followed by a test, followed by a final figuring step. Total time to ion-figure each segment took two weeks, a dramatic time reduction over traditional polishing techniques. Cost Reduction The speed of the ion figuring process not only saved time, it dramatically improved performance of the optical segments while lowering costs. Ion figuring was one factor that enabled the consortium of five universities to construct the Hobby-Eberly Telescope for a total price of $13.5 million—about one—sixth the cost of other modern telescopes of comparable size. Other factors included a combination of special-purpose design, innovative engineering concepts, and use of off-the-shelf technology. Another key cost-saving factor was that all the segments are alike, and were produced by the same method. Consequently, any segment can be used to replace another. Besides optics fabrication and testing, engineers from ITT's optics manufacturing facility worked collaboratively with telescope officials to design and test the primary mirror segment support structure. ITT engineers also were involved extensively in other design and analysis work related to the telescope. |
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