Mount Wilson Observatory

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Mount Wilson Observatory
100 inch Hooker Telescope 900 px.jpg
The 100-inch (2,500 mm) Hooker telescope that Edwin Hubble used to discover the general expansion of the universe
Organization Mount Wilson Institute
Code 672  
Location Mount Wilson, Los Angeles County, California
Altitude 1,742 m (5,715 ft)
60-inch Telescope 60-inch reflector
Hooker Telescope 100-inch reflector
Infrared Spatial Interferometer 3 65-inch reflectors
CHARA array 6 40-inch reflectors
Mount Wilson California Map.PNG

The Mount Wilson Observatory (MWO) is an astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson, a 5,710-foot (1,740 m) peak in the San Gabriel Mountains near Pasadena, northeast of Los Angeles. The observatory contains two historically important telescopes: the 60-inch (1.5 m) Hale telescope built in 1908 and the 100-inch (2.5 m) Hooker telescope, which was the largest telescope in the world from its completion in 1917 until the 200-inch Hale telescope at Palomar Observatory was built in 1948.


The observatory was conceived and founded by George Ellery Hale, who had built the 40-inch (1.0 m) telescope at the Yerkes Observatory. The Mount Wilson Solar Observatory was first funded by the Carnegie Institution of Washington in 1904, leasing the land from the owners of the Mount Wilson Hotel in 1904. Among the conditions of the lease was that it allow public access.[2]

The Observatory was a filming location in a space themed episode of Check It Out! with Dr. Steve Brule

60-inch (1.5 m) telescope[edit]

Mount Wilson Observatory with the smaller 60 inch telescope cupola on right, the large 100 inch cupola on the left

George Ellery Hale received the 60-inch (1.5 m) mirror blank, cast by Saint-Gobain in France, in 1896 as a gift from his father, William Hale. It was a glass disk 7 12 inches (190 mm) thick and weighing 1,900 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. "First light" was December 8, 1908. It was at the time the largest operational telescope in the world.[1] The 60 inch telescope is a reflector telescope of Cassegrain design.



Today, the 60 inch telescope is used for public outreach. Eyepieces are fitted to its focus instead of instruments. It is the largest telescope in the world devoted to the general public. As of June 2014, the cost for a half-night of observation is $900, $1700 for a full night for a group of up to 25.[3]

100 inch (2.5 m) Hooker Telescope[edit]

The 100-inch (2,500 mm) mirror on its way up the Mount Wilson Toll Road on a Mack Truck in 1917.
Comparison of nominal sizes of primary mirrors of the 100" Hooker Telescope and some notable optical telescopes (click for detail)

Once the 60 inch telescope project was well underway, Hale immediately set about creating a larger telescope. John D. Hooker provided crucial funding for it, along with Carnegie.[4] 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 inches (2.5 m) telescope was completed and saw "first light" on November 2, 1917. The blank started with over two tons of fused glass which was melted in a furnace into one piece. The blank once melted into one piece took over a year to cool without cracking.[5] As with the 60 inch telescope, the entire telescope is suspended on a mercury bearing, to allow it to turn smoothly.

In 1919 the Hooker telescope was equipped with a special attachment, an optical astronomical interferometer developed by Albert A. 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 diameter of stars, such as Betelgeuse, the first time the size of a star had ever been measured. Henry Norris Russell developed his star classification system based on observations using the Hooker.

In 1935 the silver coating used since 1917 on the Hooker 100 inch mirror was replaced with a more modern and longer lasting aluminum metallic coating that reflected 50% more light than the older silver method of coating. The newer method of coating for the telescope mirrors was first tested on the older 60 inch mirror telescope.[6]

Workmen assembling the polar axis of the Hooker telescope.

Edwin Hubble performed many critical calculations from work on the 100 inches (2.5 m) telescope. He determined that some nebulae 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.[1] Hubble also discovered Cepheid variables using the 100 inch telescope.

The Hooker's reign of three decades as the largest telescope came to an end when the Caltech-Carnegie consortium completed its 200-inch (5.1 m) telescope in 1948 at Palomar Observatory, 90 miles (140 km) south, in San Diego County, California.

By the 1980s, the focus of astronomy research had turned to deep space observation, which required darker skies than what could be found in the Los Angeles area, due to the ever-increasing problem of light pollution. In 1986, the Carnegie Institution, which ran the observatory, handed it over to the non-profit Mount Wilson Institute. At that time, the 100 inches (2.5 m) telescope was deactivated, but it was restarted in 1992 and outfitted with adaptive optics. The Hooker telescope remains one of the pre-eminent scientific instruments of the 20th century.

The telescope has a resolving power of 0.05 arcsecond.

Solar telescopes[edit]

There are three solar telescopes, two of which are now used for astronomical research, also known as solar towers due to their construction. 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.


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


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 "routine operations" in early 2004. In the 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.


Other telescopes[edit]

A 24-inch (610 mm) telescope fitted with an infrared detector purchased from a military contractor was used by Eric Becklin in 1966 to determine the center of the Milky Way for the first time.[7]

In 1968, the first large-area near-IR (2.2 µm) survey of the sky was conducted by Gerry Neugebauer and Robert B. Leighton using a 62-inch (1.6 m) reflecting dish they had built.[8] The instrument is now in the Smithsonian.[9]


Old photo of the 60-inch reflector dome
  • The historic monument came under threat during the August 2009 California wildfires.[10]
  • The poet Alfred Noyes was present for the "first light" of the Hooker telescope on November 2, 1917. Noyes used this night as the setting in the opening of Watchers of the Sky, the first volume in his trilogy The Torchbearers, an epic poem about the history of science. According to his account of the night, the first object viewed in the telescope was Jupiter and Noyes himself was the first to see one of the planet's moons through the telescope.[11]

See also[edit]


External links[edit]