History of Mount Wilson Observatory
Hooker 100" Telescope, Expanding Universe and Scientific Discoveries
Sep 6, 2009
In early September 2009, the Station wildfire raged through the San Gabriel Mountains nearly engulfing historic Mount Wilson Observatory. Heroic firefighting crews and observatory personnel worked frantically to save the historic observatory from the conflagration. They succeeded. Why was their effort to save Mount Wilson Observatory so important?
Mount Wilson Telescopes
In 1897 George Ellery Hale leased the summit of Mount Wilson to build an astronomical observatory, which was officially founded on December 20, 1904. Hale originally constructed large tower solar telescopes to study the Sun, but soon added large general purpose reflecting telescopes.
When the newly completed 60" telescope saw its first light on December 8, 1908, it took the title as the world's largest telescope from the Yerkes Observatory 40" refractor. It soon lost the title.
The 100" Hooker telescope saw first light atop Mount Wilson on November 1, 1917. With its 4.5-ton 100 inch diameter primary mirror the Hooker 100" telescope became the world's largest telescope, until the 200" Mount Palomar telescope opened in 1948.
These telescopes were engineering marvels built under extremely challenging conditions. The 87-ton Hooker telescope is housed in a500-ton 100-foot diameter dome. Imagine hauling parts for this immense but precise scientific instrument up a nine mile narrow winding dirt mountain road. Think mules! The trucks of the day were not always up to the task, so the construction engineers often resorted to mules.
Newer Mt Wilson Telescopes
Modern telescopes on Mt Wilson include the six 1-meter telescopes in the CHARA (Center for High Angular Resolution Astronomy) array. The resolving power (smallest detail a telescope sees) depends on the diameter of the telescope. A technique called interferometry allows an array of telescopes to have the resolving power of a single telescope as large as the entire array. The CHARA array resolves details as if it were a 330 meter diameter telescope.
The Infrared Spatial Interferometer, ISI, on Mt Wilson is similar to the CHARA array, but it works at infrared rather than optical wavelengths.
Scientific Discoveries Made at Mt Wilson
Edwin Hubble used the Hooker 100" telescope to make what were among the most important astronomical discoveries of the 20th century. In 1924 Hubble's observations of Cepheid variable stars in the Andromeda galaxy proved conclusively that it is an external galaxy outside our Milky Way. (Hubble, Publications of the American Astronomical Society 5, 261, (1925)) Hubble then studied external galaxies and devised the galaxy classification scheme that astronomers still use.
Most importantly, Hubble found that there was a correlation between the distances to galaxies and their recessional velocities (redshifts). This velocity-distance relation told Hubble that the universe is expanding. (Hubble, Proceedings of the National Academy of Sciences 15, 168, (1929))
Rebirth of Mt Wilson
Growth and light pollution in Los Angeles nearly destroyed Mt Wilson Observatory. The 100" Hooker telescope closed from 1986 to 1994. Astronomers and friends of the observatory managed to save Mount Wilson by devising astronomical research projects that did not depend on completely dark skies.
Laird Thompson devised adaptive optics at Mount Wilson. He used the 60" telescope from 1992 to 1995 and the Hooker 100" after 1995. Thompson's adaptive optics technique uses a powerful laser to simulate a point of light in the sky. Computers process astronomical images from the telescope to make the laser image look like a point. The resulting image shows clear details in real celestial objects because the processing removes the atmospheric distortion effects for both the laser and the celestial objects.
In the 1960s Olin Wilson started monitoring spectral features of stars like the Sun, at Mt Wilson. Sallie Baliunas continued this work during the 1980s when the observatory nearly closed. She found that many solar type stars have brightness changes related to cycles in their spot activity. This work helps astronomers understand the connection between solar activity cycles and Earth's climate changes.
Losing Mount Wilson Observatory to either wildfires or light pollution would have been tragic. Its telescopes first allowed mankind to probe the boundaries of the universe where, in Hubble's words:
"With increasing distance, our knowledge fades, and fades rapidly. Eventually, we reach the dim boundary-the utmost limits of our telescopes. There, we measure shadows, and we search among ghostly errors of measurement for landmarks that are scarcely more substantial." (Hubble, Realm of the Nebulae, Dover 1958)
Further Reading
Mount Wilson Observatory Association
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