Mount Wilson Observatory

The Mount Wilson Observatory (MWO) is an astronomical observatory in Los Angeles County, California. The MWO is located on Mount Wilson, a 5,715 foot (1,742 m) peak in the San Gabriel Mountains near Pasadena, northeast of Los Angeles.

Mount Wilson has naturally steadier air than any North America location, making it ideal for astronomy and in particular for interferometry. The growth of greater Los Angeles has limited the ability of the observatory to engage in deep space astronomy, but it remains a productive center with many new and old instruments in use for science.

It was first directed by George Ellery Hale, who brought the 40 inch (1 m) telescope from the Yerkes Observatory. The Mount Wilson Solar Observatory was first funded by the Carnegie Institution of Washington in 1904, two years after its own founding, and that foundation is still the main supporter of the observatory.


60 inch (1.5 m) Hale telescope

George Ellery Hale received the 60 inch (1.5 m) blank, cast by St. Gobain in France, in 1896 as a gift from his father, William Hale. It was a glass disk 7 1/2 inches (191 mm) thick and weighing 1900 pounds (860 kg). However it was not until 1904 that Hale received funding from the Carnegie Institution to build an observatory. Grinding began in 1905 and took two years. The mounting and structure for the telescope was built in San Francisco and barely survived the 1906 earthquake. Transporting the pieces to the top of Mount Wilson was an enormous task, and a special electric truck was built to carry the material up. 'First light' was December 8, 1908. It was at the time the largest operational telescope in the world.

The 60 inch (1.5 m) Hale reflector became one of the most productive and successful telescopes in astronomical history. Its design and light-gathering allowed the pioneering of spectroscopic analysis, parallax measurements, nebula photography, and photometric photography. Though surpassed in size by the Hooker telescope nine years later, the Hale telescope remained one of the largest in use for decades.

In 1992 the Hale telescope was fitted with an early adaptive optics system, the Atmospheric Compensation Experiment (ACE). The 69-channel system improved the potential resolving power of the telescope from 0.5-1.0 arc sec to 0.07 arc sec. ACE was developed by DARPA for the SDI system, and the National Science Foundation funded the civilian conversion.

The George Hale Solar Telescope is a National Historic Landmark.

100 inch (2.5 m) Hooker telescope

Hale immediately set about creating a larger telescope. John D. Hooker provided crucial funding for it, along with Carnegie. The Saint Gobain factory was again chosen to cast a blank in 1906, which it completed in 1908, After considerable trouble over the blank (and potential replacements), the 100 inch (2.5 m) telescope was completed and saw "first light" on November 1, 1917.

The mechanism incorporates a mercury float to provide smooth operation. The Hooker telescope was equipped in 1919 with a special attachment, an optical interferometer developed by Albert Michelson the first time such an instrument had been used in astronomy. Michelson was able to use the equipment to determine the precise size and distance of stars, such as Betelgeuse. Henry Norris Russell developed his star classification system based on observations using the Hooker.

Edwin Hubble performed his critical calculations from work on the 100 inch (2.5 m) telescope. He determined that nebula were actually galaxies outside our own Milky Way. Hubble, assisted by Milton L. Humason, discovered the presence of the redshift that indicated the universe is expanding.

The Hooker's long reign as the largest telescope came to an end when the Caltech-Carnegie consortium completed its 200 inch (5 m) telescope in 1948 at Mount Palomar, 90 miles (150 km) south, in San Diego County, California.

In 1986 the 100 inch (2.5 m) telescope was inactivated. But it was restarted in 1992 and outfitted with adaptive optics. For a couple of years the Hooker telescope was again the sharpest (highest acutance) instrument in operation, including the Hubble Space Telescope. While it no longer holds that claim either, the Hooker telescope remains one of the pre-eminent scientific instruments of the 20th century.

The telescope has a resolving power of 0.05 arcsec.

Solar telescopes

There are three solar telescopes, two of which are now in use for science. The 60 foot (18 m) tower telescope was completed in 1908, and the 150 foot (46 m) tower telescope was completed in 1912. A 1904 telescope is used for educational demonstrations. The telescopes are used to study helioseismology and other changes in the sun's nature.


The extremely steady air over Mount Wilson is well suited to interferometry, the use of multiple viewing points to increase resolution enough to allow for the direct measurement of the size of details such as star diameters. Inteferometry was pioneered on the Hooker telescope by Michelson, and new telescopes have furthered the technology.

The Infrared Spatial Interferometer (ISI) is an array of three 65 inch (1.7 m) telescopes. They can be placed as far as 70 m apart, giving the resolution of an instrument of that size. The signals are converted into radio waves thorugh a heterodyne circuit and then combined electronically. ISI is run by the an arm of the University of California, Berkeley. It can resolve .002 arcsec, the theoretical limit of earth-bound sources. On July 9, 2003, it recorded the first measurements of three observations of a stellar source in the 11.15 micrometre range.

The Center for High Angular Resolution Astronomy (CHARA) is an array of six 40-inch telescopes arranged along three axes to make an instrument with a diameter of 330 m. The light beams travel through vacuum tubes and are combined optically, requiring a building 100 meters long with movable mirrors to keep the light in phase as the earth rotates. CHARA is operated by the Georgia State University and began scientific use in 2002.

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