Mount Wilson Observatory is located east of Los Angeles in the San Gabriel Mountains at a height of 1 mile above sea level. Established in 1904 by George Ellery Hale, Mount Wilson today continues to utilize a wide array of solar telescopes, large reflectors and experimental optical interferometers. The largest telescope is the Hooker Reflector, opened in 1918 with a mirror 100 inches in diameter, the largest in the world for 30 years. There is also a 60-inch reflector, completed in 1908, which was the largest of its type at the time. Observing facilities, living quarters and shops dot the mountain ridge. Some of Mount Wilson's tower telescopes are easily visible from a wide portion of the Los Angeles Basin.
Mount Wilson Observatory is owned by its original patron, the Carnegie Institution of Washington, and is operated by the Mount Wilson Institute which manages the remaining instruments for a wide variety of purposes, inviting astronomers from many institutions to participate in solar, stellar and galactic astronomy.
Hale built the Observatory in stages, starting out as the Mount Wilson Solar Observatory. Administrative offices, shops and laboratories were built in Pasadena as telescope after telescope was built on the mountain site. Hale regarded his observatory as a test bench, where new technologies and new practices could be applied to astronomy and where physics and astronomy could join in the pursuit of the structure of the physical nature of the universe, the lives of stars, and the structure of atoms. The Carnegie Institution of Washington managed the observatory, and fostered many of its most important research programs in solar, stellar, galactic and extragalactic astronomy.
Spectroscopy has always been the chief mode of investigation at Mount Wilson, starting with Hale's first solar tower telescopes in 1904. Hale emphasized the physical meaning of celestial spectra -- what photographic spectra revealed about the physical and chemical structure, evolution (lives) and composition of the sun and stars. Hale was not the first to link spectroscopic studies of the chemical elements in terrestrial laboratories to what was seen with his telescopes, but he did foster closer ties between the two disciplines by combining physical laboratory data with celestial data and hiring physicists for his astronomical staff.
Hale's observatory was always preeminent in the physical study of laboratory and celestial spectra, but with the 100-inch completed, the observatory also had by far the most powerful and sensitive instruments for probing the deepest reaches of the universe. Naturally, the staff designed and built fast and highly efficient spectrographs for the various focus points of the 100-inch (Newtonian or Prime focus for the faintest objects; the modified Cassegrain or Nasmyth focus for higher dispersion spectra of brighter objects, and very high disperson for the brightest stars, planets and nebulae at the subterranean Coude focus). By the end of the 1920s, astronomers like Milton Humason, working with Edwin Hubble and others, had perfected techniques for obtaining the spectra of extremely faint galaxies using photographic exposure times sometimes spanning many successive nights of observing. As a result, Hubble had available spectroscopic data that he could combine with his careful photometric studies of the structure and form of extragalactic nebulae (or galaxies as they are now called) and the variable stars they contain. He combined these data, available at no other observatory at the time (1929), in quality or quantity, to show that the universe appeared to be expanding.