Mount Wilson Observatory

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Mount Wilson Observatory
The 100 inch Hooker telescope that Edwin Hubble used to discover the general expansion of the universe.
Organization:	Carnegie Institution of Washington
Location:	Mount Wilson, Los Angeles County, California
Coordinates:	34°13′21.2873″N, 118°03′42.0624″W
Altitude:	1,742 m (5,715 ft)
Website:	http://www.mtwilson.edu/vis/
Telescopes
Hale Telescope:	60" reflector
Hooker Telescope:	100" reflector
Infrared Spatial Interferometer:	3 65" reflectors
CHARA array:	6 40" reflectors

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 had built the 40 inch (1 m) telescope at 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, but that foundation is no longer a financial supporter of the observatory.^{[citation needed]}

Contents 1 60 inch (1.5 m) Hale telescope 2 100 inch (2.5 m) Hooker telescope 3 Solar telescopes 4 Interferometry 5 Miscellaneous 6 External links

[edit] 60 inch (1.5 m) Hale telescope

George Ellery Hale received the 60 inch (1.5 m) blank mirror, cast by Saint-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) 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 60 inch 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 Strategic Defense Initiative system, and the National Science Foundation funded the civilian conversion.

Today the 60 inch telescope is used for public outreach. Eyepieces are fitted to its focus instead of instruments. It is arguably the largest telescope in the world which the general public can look through freely.

The George Hale Solar Telescope is a National Historic Landmark. Another telescope, the 200-inch telescope at the Palomar Observatory, is also called the "Hale Telescope".

[edit] 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 2, 1917.

The mechanism incorporates a mercury float to provide smooth operation. The Hooker telescope was equipped in 1919 with a special attachment, an optical astronomical interferometer developed by Albert Michelson, much larger than the one he had used to measure Jupiter's satellites. 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. It was restarted in 1992 and outfitted with adaptive optics, and 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.

[edit] 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. The Snow solar telescope, built in 1904 is used for educational demonstrations. The telescopes are used to study helioseismology and other changes in the sun's nature.

[edit] Interferometry

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. Michelson performed the first measurements of other stars in the history of astronomical interferometry on the Hooker telescope in 1919.

The Infrared Spatial Interferometer (ISI) is an array of three 65 inch (1.65 m) telescopes operating in the mid-infrared. The telescopes are fully mobile and their current site on Mount Wilson allows for placements as far as 70 m apart, giving the resolution of a telescope of that diameter. The signals are converted to radio frequencies through heterodyne circuits and then combined electronically using techniques copied from radio astronomy. ISI is run by an arm of the University of California, Berkeley. The longest (70m) baseline provides a resolution of 0.003 arcsec at 11 micrometers. On July 9, 2003, ISI recorded the first closure phase aperture synthesis measurements in the mid infrared.

The Center for High Angular Resolution Astronomy (CHARA) array is an interferometer formed from six 1 m (40-inch) telescopes arranged along three axes with a maximum separation length 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 and began "routine operations" in early 2004. In infrared the integrated image can resolve down to 0.0005 arcseconds. As of 2005 four of the six telescopes have been commissioned for interferometric observations.

These and other astronomical interferometers are included in the List of astronomical interferometers at visible and infrared wavelengths. The history of the development of these instruments is given in History of astronomical interferometry.

[edit] Miscellaneous

• Letters to the Mount Wilson Observatory are the subject of a permanent exhibition at the Museum of Jurassic Technology in Los Angeles, California. A small room is dedicated to a collection of unusual letters and theories received by the observatory circa 1915–1935. These letters were also collected in the book No One May Ever Have the Same Knowledge Again: Letters to Mt. Wilson Observatory 1915-1935 (ISBN 0-9647215-0-3).

[edit] External links

• Official website
  • The 100-inch Hooker Telescope
  • 150-Foot Solar Tower Webcam Image
• CHARA
• The ISI Array
• Mount Wilson Observatory Clear Sky Clock
• Letters to the MWO, 1915-1935