Hale Telescope
A 200-inch reflecting telescope at Palomar Observatory, completed in 1949.
The Hale Telescope is a 200-inch (5.1 m), f/3.3 reflecting telescope located at Palomar Observatory in San Diego County, California. It is named after astronomer George Ellery Hale, who secured a $6 million grant from the Rockefeller Foundation in 1928 to plan, design, and build the observatory. The project took 20 years, and Hale died before it was completed. When finished in 1949, the Hale was twice the diameter of the next largest telescope and introduced several new technologies, including a specialized mount design and a large, aluminum-coated "honeycomb" mirror made of low-thermal-expansion Pyrex. It remains in active use today.
For over 30 years, the Hale Telescope represented the upper limit of large optical telescope construction. It was the world’s largest telescope from 1949 until the Soviet BTA-6 was built in 1976, and the second largest until the Keck 1 telescope at the Keck Observatory in Hawaii was completed in 1993.
Hale had previously overseen the building of the 60-inch (1.5 m) and 100-inch (2.5 m) telescopes at Mount Wilson Observatory, funded by the Carnegie Institution of Washington. Those instruments greatly advanced understanding of the universe’s scale in the 1920s and convinced Hale that an even larger telescope was needed. The chief optical designer for the 100-inch telescope, George Willis Ritchey, wanted the new telescope to use a Ritchey–Chrétien design, which would have provided sharper images over a wider field of view than a standard parabolic primary. However, Hale and Ritchey had a falling-out. With the project already late and over budget, Hale refused the new design, and Ritchey left. The Hale Telescope became the last world-leading telescope to use a parabolic primary mirror.
In the early 1930s, Hale chose a site at 1,700 m (5,600 ft) on Palomar Mountain in San Diego County, away from growing light pollution in cities like Los Angeles. The Corning Glass Works was tasked with making the 200-inch primary mirror. Construction of the observatory and dome began in 1936, but World War II caused delays, and the telescope was not completed until 1948, when it was dedicated. Corrections for slight image distortions were made throughout 1949, and it became available for research in 1950. The U.S. Post Office issued a 3-cent stamp in 1948 commemorating the telescope and observatory.
Quick Facts
- First Light
- January 26, 1949, 10:06pm PST
- Discovered
- Caliban, Sycorax, Jupiter LI, Alcor B
Facts from the source article.
Lore & Background
Hale supervised the building of telescopes at Mount Wilson Observatory, including the 60-inch in 1908 and the 100-inch in 1917, which led to rapid advances in understanding the scale of the Universe. In 1928 he secured a $6 million grant from the Rockefeller Foundation for a 200-inch reflecting telescope, to be administered by Caltech. The chief optical designer for the 100-inch, George Willis Ritchey, intended the new telescope to be of Ritchey–Chrétien design, but after a falling-out Hale refused to adopt it, and the Hale became the last world-leading telescope with a parabolic primary mirror.
The primary mirror was cast in 1934 at Corning Glass Works in New York using Pyrex. The first casting attempt failed when molding blocks broke loose, but a second mirror was successfully cast. The mirror blank was transported by rail to Pasadena, then by specially designed semi-trailer to the optical shop. Over 13 years, almost 10,000 lb of glass was ground and polished away. The telescope uses a horseshoe equatorial mount and a Serrurier truss, invented by Mark U. Serrurier in 1935, to keep optics aligned. The dome is made of welded steel plates about 10 mm thick, with a moving weight of about 1000 US tons.
Reader's Guide
The Hale Telescope represented the technological limit in building large optical telescopes for over 30 years. It was the largest telescope in the world from 1949 until the Soviet BTA-6 was built in 1976, and the second largest until the Keck 1 in 1993. Its 200-inch mirror was near the technological limit of a single rigid piece of glass; larger monolithic mirrors are prohibitively expensive and would sag under their own weight. Modern telescopes over 9 meters use thin flexible or segmented mirrors with active optics.
During its first 50 years, the Hale made significant contributions to stellar evolution, cosmology, and high-energy astrophysics. Quasars were first identified as high-redshift sources by spectra taken with the Hale. Halley's Comet's 1986 approach was first detected in 1982 using the Hale with a CCD camera. Two moons of Uranus were discovered in September 1997 using the telescope. It continues to be used every clear night for scientific research by Caltech and partners, equipped with modern imagers, spectrographs, and adaptive optics. The US Post Office issued a 3c stamp in 1948 commemorating the telescope.
Did You Know?
- Its primary mirror is a honeycomb design made of Pyrex, cast at Corning Glass Works in 1934.
- First light occurred on January 26, 1949, targeting NGC 2261 (Hubble's Variable Nebula).
- The telescope uses a horseshoe equatorial mount, giving full access to the sky including Polaris.
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