Design InnovationsFor several decades the 200-in (5.1-m) reflector at Palomar Observatory in southern California remained
the world's largest
telescope. Larger single mirrors have since been built, but such mirrors present problems of production and of keeping them from sagging or otherwise being distorted. One innovative approach has been used by an American astronomer, Roger Angel, at the University of Arizona, whose team has developed a technique for spin-casting mirrors of molten borosilicate glass rather than starting with giant slabs of cooled glass. The cooled mirror surface thus produced requires only a fraction of the polishing that was needed for mirrors produced in the traditional way.Another innovation is seen in the segmented-mirror design of the two Keck
Telescopes built at the Mauna Kea Observatory, Hawaii. The main mirror of each, 400 in (10 m) wide, consists of 36 hexagonal mirrors, each about 72 in (1.8 m) wide. The mirrors were placed under stress and ground and polished into shape. When the stress was relieved, the mirrors assumed their desired curvatures for the final, computer-controlled array.Optical InterferometryAnother design innovation is that of the Multiple Mirror Telescope (MMT) on Mount Hopkins, Ariz. The MMT, which began operating in 1979, consists of six telescopes, each 72 in (1.8 m) in diameter, that are computer-guided to perform as a single light-gathering mirror 176 in (4.5 m) in diameter. When the MMT functions in this way as an optical interferometer, its resolving power is significantly increased beyond that of the individual mirrors. Several projects have adopted this approach, such as the planned Very Large Telescope (VLT) of the European Southern Observatory in Chile. The VLT is to consist of four 323-in (8.2-m) telescopes aligned to function as a single telescope twice as wide. The first unit telescope, VLT-UT1, began operating in 1998.Optical interferometry makes it possible to measure extremely small angles by comparing light waves arriving at different locations. Extraordinary precision is required, along with clever strategies to overcome atmospheric distortion, but in recent years astronomers have thereby succeeded in measuring the diameters of nearby stars and distances between close binary stars. In the future the technique should be able to locate stars to accuracies 100 times better than in current star catalogs. Interferometry would also benefit greatly from being based in space, thus overcoming the limitation that atmospheric turbulence imposes on aperture size.