Hale telescope

The 200 inch (5 m) Hale telescope.

The Hale Telescope is a 200-inch (5.1 m), f/3.3 reflecting telescope at the Palomar Observatory in California, named after astronomer George Ellery Hale. The telescope was the largest operating telescope in the world from its completion in 1948 until the BTA-6 became operational in 1975.

History

Hale supervised the building of the telescopes at the Mount Wilson Observatory with grants from the Carnegie Institution of Washington: the 60-inch (1.5 m) telescope in 1908 and the 100-inch (2.5 m) telescope in 1917. These telescopes were very successful, leading to the rapid advance in understanding of the scale of the Universe through the 1920s, and demonstrating to visionaries like Hale the need for even larger collectors.

In 1928 Hale secured a grant of US$6 million from the Rockefeller Foundation for "the construction of an observatory, including a 200-inch (5.1 m) reflecting telescope" to be administered by the California Institute of Technology (Caltech), of which Hale was a founding member. In the early 1930s, Hale selected a site at 1,700 m (5,600 ft) on Palomar Mountain in San Diego County, California, USA as the best site, and less likely to be affected by the growing light pollution problem in urban centers like Los Angeles. The Corning Glass Works was assigned the task of making a 200-inch (5.1 m) primary mirror. Construction of the observatory facilities and dome started in 1936, but because of interruptions caused by World War II, the telescope was not completed until 1948 when it was dedicated.^[1] Due to slight distortions of images, the telescope spent 1949 being corrected, before it could be used for research in 1950.^[1]

The 200-inch Hale saw first light on January 26, 1949 under the direction of American astronomer Edwin Powell Hubble, targeting NGC 2261, an object also known as Hubble's Variable Nebula. ^[2][3] The photographs made then were published in the astronomical literature and in the May 7, 1949 issue of Collier's Magazine. ^[4]

The Art Deco dome housing the Hale Telescope opening at dusk

The telescope continues to be used every clear night for scientific research by astronomers from Caltech and their operating partners, Cornell University, the University of California, and the Jet Propulsion Laboratory. It is equipped with modern optical and infrared array imagers, spectrographs, and an adaptive optics^[5] system. It has also used Lucky cam, which, in combination with adaptive optics pushed the mirror close to its theoretical resolution for certain types of viewing. ^[5] (see Lucky imaging)

Components

200 inch mirror

The primary mirror for the Hale telescope was cast in 1934 at Corning Glass Works in New York State using Corning's then new material called Pyrex (borosilicate glass).^[6] Pyrex was chosen for its low expansion qualities so the large mirror would not distort the images produced when it changed shape due to temperature variations (a problem that plagued earlier large telescopes). The mirror was cast in mold with 36 raised mold blocks (similar in shape to a waffle iron). This created a honeycomb mirror that cut the amount of Pyrex needed down from over 40 tons to just 20 tons, making a mirror that would cool faster in use and have multiple "mounting points" on the back to evenly distribute its weight.^[7] While the glass was being poured into the mold during the first attempt to cast the 200 inch mirror, the intense heat caused several of the molding blocks to break lose and floated to the top, ruining the mirror. The defective mirror was used to test the annealing process. After the mold was re-engineered a second mirror was successfully cast and after cooling several months it was transported by rail to Pasadena, California.^[8][9] In Pasadena the mirror was ground and polished and a central hole was cut in it so light could pass through when the telescope was used in a cassegrain configuration, reducing the weight of the mirror to 14.5 tons. The main mirror is re-coated every 18-24 months, using the same aluminum vacuum-deposition process invented in the 1930 by CalTech phycist and astronomer John Strong.^[10]

The Hale's 200 inch mirror was near the technological limit of a primary mirror made of a single rigid pieces of glass.^[11][12] Using a monolithic mirror much larger than the 5 meter Hale or 6 meter BTA-6 is prohibitively expensive due to the cost of both the mirror, and the massive structure needed to support it. A Mirror beyond that size would also sag slightly under its own weight as the telescope is rotated to different positions,^[13][14] changing the precision shape of the surface, which must be accurate to within 2 millionths of an inch (25 nm). Telescopes over 6 meters use a different mirror design to solve this problem, with either a single thin flexible mirror or a cluster of smaller segmented mirrors, whose shape is controlled by a computer controlled active optics system using actuators built into the mirror's mount.

Mounting structures

The Hale telescope uses a unique type of Equatorial mount called a "Horseshoe mount", a modified yoke mount that replaces the polar bearing with an open "horseshoe" structure that gives the telescope full access to the entire sky including Polaris and stars near it. The optical tube assembly (OTA) uses a Serrurier truss, then newly invented by Mark U. Serrurier of CalTech in Pasadena in 1935, designed to flex in such a way as to keep all of the optics in alignment. ^[15]

See also

• Serrurier truss
• List of largest optical reflecting telescopes
• List of largest optical telescopes historically
• List of largest optical telescopes in the 20th century

External links and references

• The 200-inch (5,100 mm) Hale Telescope

Footnotes

Coordinates: 33°21′22.7″N 116°51′53.6″W / 33.356306°N 116.864889°W / 33.356306; -116.864889