BIGGER AND BETTER
One of the first giant telescopes was built in 1845, by William Parsons on his Irish estate. Its mirror spanned 72 inches (1.8 m), and until 1908, it was the largest telescope in the world. In 1917, the 100 inch (2.5 m) Hooker Telescope was built by American astronomer George Hale on Mount Wilson, California. Hale started work on an even bigger telescope, but died before it was finished in 1948.
The 200 inch (5 m) Hale Telescope on Mount Palomar, California, was the world's largest for nearly 30 years.
In 1977, the Russians completed the 240 inch (6 m) Bolshoi Telescope in the Caucasus Mountains. Although larger than the Hale, design flaws prevented it realizing its full potential. In 1992, the 400 inch (10m) Keck I Telescope on Mauna Kea, Hawaii, became the world's biggest. It was followed by Keck II (also 400 inches) in 1995. Because such large telescope mirrors are very difficult to make in one piece, each was created from 36 hexagonal segments.
The resolution capabilities of ground-based telescopes have been limited by the effects of atmospheric seeing. This, however, is becoming less of a problem. Computer-controlled regulators can rapidly adjust the shape of a telescope's optics to reduce the turbulent effects of Earth's atmosphere and produce sharper images. Known as adaptive optics, the technique is being applied to many of the world's large telescopes.