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Astronomy and Astrophysics
Mount Wilson Observatory
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Mount Wilson Observatory
Mount Wilson Observatory — Mount Wilson, California.
100-inch Hooker Telescope Dome, circa 1970.
(Photo Credit: Mount Wilson Observatory)

Name:Mount Wilson Observatory
Location:Mount Wilson, California
Classification:Private/Public (federal), buildings/structure

Areas of Significance:National Register: education, engineering, science, NHL: science, Subtheme: physical science, Facet: astronomy

Builder:George Ellery Hale


Mount Wilson Observatory, placed in operation in 1904, was the second (after Lick) of the great astronomical research observatories to be established in the Far West. The observatory, at 5,710 feet altitude, is located in the Angeles National Forest on a 1,050-acre plateau at the summit of Mount Wilson. All of the major research telescopes described below, with the exception of the 100-inch Hooker reflector, are in operation. The 100-inch Hooker reflector was mothballed by the Carnegie Institution (operator of the observatory since 1904) in 1985 due to lack of funds. Mount Wilson Observatory is in Los Angeles County, 20 miles east of Los Angeles on State Highway 2. [1]

Snow horizontal solar telescope

The Snow horizontal solar telescope was built at a cost of $10,000 for the use of the Yerkes Observatory in Wisconsin, with funds provided by Helen Snow. Due to poor visual conditions at the Wisconsin site, Hale had the telescope shipped to the Mount Wilson Observatory, where it was carried up the mountain, piece by piece, on the backs of mules following a dirt trail only four feet wide. When assembled in 1905 the Snow horizontal solar telescope became the first permanent telescope to be placed on the mountain.

The Snow telescope contains a coelostat, or slowly revolving mirror, on a clock-driven mounting, that receives light from the sun and is reflected to a plane mirror 30 inches in diameter. From this mirror the beam is reflected nearly horizontally to a point 100 feet north, where it falls on a telescope with a 24-inch concave mirror of 60-foot focal length, thus forming a solar image about 6.5 inches in diameter. Because the path over which the beam of sunlight has to pass is very long, Hale raised the telescope high above the ground on a series of stone piers, to protect the beam of light from distortions caused by rising hot air. The telescope is covered by a long corrugated metal shed to protect its working parts from the elements.

The horizontal design of the Snow telescope was soon superseded by solar telescopes using a vertical design that provided a clearer image of the sun. The Snow telescope is only lightly used today for solar and night-time studies that require a stable platform and a large aperture (24-inch) mirror.

The 60-foot solar telescopy

The horizontal design of the Snow telescope proved unsatisfactory because of heat rising from the ground that caused mirror distortion and air turbulence. To solve this problem, Hale designed a vertical telescope in 1907--the 60-foot Solar telescope.

This instrument employs a 60-foot tower to house a 12-inch coelostat, a 22 x 12.5-inch elliptical flat mirror and a 12-inch objective doublet with a focal length of 60 feet. Both mirrors are 12 inches thick to prevent rapid distortion due to solar heating. A laboratory at the base of the tower contains a 30-foot underground spectroheliograph, sunk into the bedrock of the mountain, which produces a 6-inch solar image. The mirrors of the telescope are attached to motorized mountings in an assembly housed in a small white dome 60 feet above the ground.

In order to prevent the instrumentation from buffeting in the wind, the telescope has two towers for structural stability. The larger outer tower supports the dome, while the smaller inner tower supports the mirror and lenses. The siting of the spectroheliograph in a pit dug 30 feet into the bedrock of the mountain, below the telescope, provides a stable vibration-free location calculated to produce sharp images of the sun.

In the years since 1907, the 60-foot solar telescope has remained in constant use. While the basic design and structure of the telescope has remained intact, recent improvements include the installation of an automatic guiding system to provide a stable light feed to a tuneable magneto-optical filter that is mounted on the spectrograph table. A computer system for data handling and reduction has been installed in a specially constructed building next to the tower. The 60-foot solar telescope is still in use.

The 150-foot solar telescope

With the success of the 60-foot solar telescope, Hale proposed a 150-foot solar telescope of a similar design to achieve an even larger image. The Carnegie Institution provided the funds and the telescope was completed by 1912

The tower is of double construction. The outer tower supports the dome and protects the inner optical support tower from wind and vibration. The mirrors are fitted with water jackets for cooling, and the 12-inch objective is a triplet designed to minimize the secondary spectrum. The instrument pit containing the spectroheliograph is located 75 feet into the bedrock of the mountain. The motorized mirrors in the small dome 150 feet above the ground focus an image of the sun 17 inches across onto a table in the building at the base of the telescope.

The 150-foot solar telescope has been expertly maintained and upgraded over the years. Recent modifications include the installation of a computerized data handling system, a new grating and new exit and entrance slit assemblies. The 150-foot solar telescope is in use and remains one of the best facilities in the world for solar observation.

The 60-inch reflector

The 60-inch reflector at Mount Wilson was constructed in 1908. Hale used the 60-inch glass blank that his father purchased for him in 1896. George Ritchey finished the glass blank into a mirror of the proper size in the Mount Wilson optical shops in Pasadena, California. Ritchey also designed the tube and mounting for the telescope, which were built by the Union Iron Works in San Francisco. The design drew heavily on experience gained with the use of the 36-inch Crossley reflector at the Lick Observatory.

The telescope is supported by a 15-foot tube, which contains eight separate steel tubes and cross-braces designed to provide a stiffer truss and support system than was originally found in the Crossley reflector. The mirror is supported by a system of levers in a steel housing attached to the bottom of the tube and is fork-mounted on the polar axis. Just below the fork is a 10-foot diameter mercury float-bearing system designed to carry the weight of the telescope. The telescope is moved with electric motors. The 58-foot dome of the telescope is built from steel, on a concrete foundation, with double walls for the free circulation of air. This design is necessary to minimize temperature variations which could alter the shape of the mirror.

Hale designed the optical system of the 60-inch reflector so that the instrument could be used for a variety of purposes. As a Newtonian telescope it was an f/5 instrument for photography and low-dispersion spectroscopy. In a modified Cassegrain configuration, using a convex hyperboloidal mirror before the prime focus and a plane mirror at the lower end of the tube to reflect light to the side of the tube, it could be used at f/16 for spectrography and an f/20 for photography. Finally, as an f/30 Coude, light was reflected by an appropriately geared mirror through the hollow polar axis into a constant-temperature room housing a large spectrograph. This flexible optical system, which allowed the telescope to be used for photographic and spectrographic purposes, was a model for future large reflectors.

The 100-inch Hooker reflector

The 60-inch reflector was upgraded mechanically and electrically in 1967. In 1977 the telescope was equipped with a spectrometer for the study of stellar magnetic activity; in 1980 a minicomputer was added to record data from this spectrometer. The 60-inch reflector is in use as an important research telescope.

While the 60-inch reflector was under construction, George Ellery Hale began to plan an even larger telescope. In 1906 with the support of a $45,000 grant from wealthy Los Angeles industrialist J.D. Hooker, Hale ordered a 100-inch blank disk, made from wine-bottle glass, from the St. Gobain glass works in France.

After several failures, a satisfactory disk was cast in 1910 and shipped to Pasadena for polishing. Despite some flaws in the glass, Hale persuaded his friend George Ritchey to begin the long process of grinding the glass into the required shape. When Ritchey finished, the 100-inch mirror weighed about 9,000 pounds.

In 1910 Hale and Hooker persuaded the Carnegie Institution to grant $500,000 to complete the construction of the telescope. The preliminary design of the dome was drawn up by members of the observatory staff and turned over to the firm of Daniel H. Burnham, in Chicago, for completion. The English yoke mounting for the telescope was designed by Ritchey and then modified by Hale and another member of the Mount Wilson staff, Francis Pease. The use of the English yoke mounting to provide much stiffer support for the telescope meant that the instrument could never see the stars near the north celestial pole.

In 1915 the Fore River Shipbuilding Company of Quincy, Massachusetts, began sending the various components of the mounting to Pasadena. Since the parts were too large and heavy for the old road to the top of the mountain, a new road, widened to 12 feet, had to be built.

To support the telescope, a concrete pier 33 feet high was built into the bedrock of the mountain. At the bottom the pier is a rectangle 20 x 45 feet across, while the top mushrooms out to a circular observing floor, 54 feet in diameter, supported by two massive concrete support brackets. Within the hollow structure of the pier are several floors used for storage and darkroom space. Equipment to re-silver the huge mirror is found just under the telescope.

By July 1917 the mounting was assembled and ready for the installation of the 100-inch mirror. By 1919 the Hooker reflector was in regular operation and producing useful scientific data. The final cost of the telescope to Carnegie was $600,000, not including the labor of the Mount Wilson staff, or the original contribution by Hooker.

Optically, the Hooker telescope is very similar to the 60-inch telescope. It can be used as an f/5 Newtonian, f/16 modified Cassegrain, or an f/30 Coude. In the latter case the beam is brought through the hollow polar axis into a constant temperature room in the south pier for high-dispersion spectrography.

The polar axis of the telescope, on which the 87-ton instrument must turn smoothly to counteract the rotation of the Earth, is defined by self-aligning journal bearings, while the bulk of the load is carried by two steel drums that float in mercury. The resulting friction is so low that the telescope, with 200,000 pounds of moving parts, can be rotated by the force of one hand at the end of the tube.

To rotate the telescope, a large worm gear 18 feet in diameter is mounted on the south end of the polar axle. The gear, of cast iron with hollow spokes, is made in two halves bolted together. It meshes with a tool-steel worm that is rotated by a mechanical driving clock at one turn per minute. Because of the accuracy required in tracking the stars as they move across the night sky, great precision was required in cutting the teeth of the worm gear. This operation was done by the Italian instrument maker, C. Jacomini, with the gear in place on the telescope. Using a microscope and a diamond scriber, Jacomini, divided the gear into 1440 equal segments, one for each tooth, scribing the marks on an inserted brass ring near the edge of the gear. Later he gashed the teeth individually. The gear and worm were "run in" with rouge and oil.

The rotating 100-foot, 500-ton double skin dome is similar to that found in the 60-inch telescope. The dome is supported by railroad trucks on a precision-ground double circular railroad track and is controlled by electric motors.

In 1981 the 100-inch Hooker reflector was designated an International Historic Mechanical Engineering Landmark by the American Society of Mechanical Engineers. In 1985, due to the lack of operating funds, the Carnegie Institution mothballed the Hooker reflector.

Other Features

In addition to the five major telescopes described above, there are multiple reflectors, refractors and cameras housed in smaller domes around the mountain. There are also many support facilities including garages, warehouses, water tanks, individual houses and dormitories on the mountain. Only the five major telescopes described in this nomination contribute to the national significance of the Mount Wilson Observatory.


The establishment of the Mount Wilson Observatory in 1904, by American astronomer George Ellery Hale, brought a new era to the science of astronomy. The Snow horizontal telescope and the two solar tower telescopes were the first major instruments placed on Mount Wilson. Completion of the 60-inch reflector in 1908 and the 100-inch Hooker reflector in 1917 made Mount Wilson the home of the two largest telescopes in existence and the center of the astronomical world. These telescopes represented a quantum leap in mechanical and optical engineering capability. They laid the technological foundation for all large modern telescopes. Many of the major advances and greatest names in 20th-century astronomy are associated with the Mount Wilson Observatory, including Edwin P. Hubble, who in 1929 used the 100-inch Hooker reflector to gather data that showed the universe to be in a regular state of expansion thereby providing the first clues to the origin of the universe. [2]


The decision of the Carnegie Institution of Washington, DC, to build the Mount Wilson Solar Observatory in the San Gabriel Mountains near Los Angeles was made at the urging of Dr. George Ellery Hale, the organizer and director of the Yerkes observatory at William's Bay, Wisconsin.

The first permanent telescope on Mount Wilson was the Snow horizontal solar telescope that Hale brought from the Yerkes observatory in 1904. The Snow telescope was followed by the 60-foot solar telescope in 1907 and the 60-inch reflector, for deep sky observations, in 1908. In 1912 Hale added the 150-foot solar telescope and in 1917 the 100-inch Hooker reflector. In 1920, after the completion of the 100-inch Hooker reflector, the Carnegie Institution, changed the name of the observatory from the Mount Wilson Solar Observatory to the Mount Wilson Observatory.

Hale, who was appointed the director of Mount Wilson in 1904 long dreamed of establishing a mountaintop observatory that would combine a solar telescope and a large reflecting telescope. Hale's experience at Yerkes, with its large 40-inch refractor, convinced him that the future of astronomy resided with the large reflecting telescope that would prove a suitable instrument for the study of astrophysics--the application of the principles of physics to astronomical objects beyond the earth. He justified the combination of a solar observatory with a large reflecting telescope, designed for stellar astronomy, when he wrote: "The story of the origin of the sun and its development is illustrated in stars of many types which are no less important to a thorough understanding of its physical constitution than is a direct investigation of solar phenomena."

At Mount Wilson, Hale would put his theory of the modern observatory into practice. The Mount Wilson Observatory soon set the style for modern observatories. Whereas Yerkes was built on the traditional pattern of an imposing central building with domes for the various telescopes and separate houses for staff on the grounds, at Mount Wilson Hale built only the observing instruments and a "monastery" with temporary accommodations for the staff actually engaged in observing. The staff lived in Pasadena, where the headquarters of the observatory was located.

The concentration of these modern instruments and the favorable location of the observatory on a high mountain with clear skies soon attracted the most famous astronomers in the world to work with and use these telescopes. Hale encouraged this practice by helping many of them secure funds from the Carnegie Institution through its Research Associate Program. These astronomers included Jacobus Kapteyn from the Netherlands, Ejnar Hertzsprung from Germany, Bertil Lindblad from Sweden, and Jan Oort from the Netherlands. Among the famous American astronomers who came to Mount Wilson were Edward Emerson Barnard from the Lick Observatory, Albert Michelson from the University of Chicago, Henry Norris Russell from Princeton University, and Joel Stebbins from the University of Wisconsin.

The success of the Mount Wilson telescopes was soon spread through the influence of these men to the wider astronomical community and led to the construction of a whole new series of big reflectors between the two World Wars. While no one telescope was as large as the 100-inch Hooker reflector, many smaller reflectors were built using many of the design principles first tested at Mount Wilson.

Perhaps the most famous instrument at Mount Wilson was the 100-inch Hooker reflector, which from 1919 until it was moth-balled in 1985, never ceased to make significant contributions to the science of astronomy.

The 100-inch Hooker reflector was responsible for the first detailed photographs of "spiral nebulae." According to an early estimate by Hale, the Hooker reflector could photograph at least two million nebulae. In addition, these photographs not only showed structure, but they also showed fainter stars than anyone had seen before. Astronomer Edwin P. Hubble's detection of the first Cepheid variable star in the Andromeda Galaxy in 1923 ended all arguments on the nature of the spirals. This discovery provided positive proof that the spiral nebulae were each a stellar system, external to our own galaxy, the Milky Way. Later, Hubble and his assistant Milton Humason, as a result of their observations with the Hooker reflector, discovered that most galaxies were moving at high speeds away from the Earth. Using these observations, Hubble showed that the universe expands according to the following law: The greater the galaxy's distance from us, the greater its velocity, in direct proportion. This means that a galaxy twice as far from us as another is receding twice as fast. Hubble soon realized that such an expansion has two curious properties. First, every galaxy appears to be the center of the expansion as seen from that particular galaxy. Second, at one time in the distant past, all the matter in the universe must have been collected together at one place and time. Hubble's observations pointed clearly to a beginning. Here for the first time was direct evidence for a unique creation event, what cosmologists call the "Big Bang" theory.

Another important visitor to Mount Wilson was the Nobel Prize-winning physicist Albert Michelson, of the University of Chicago, who came to the observatory in 1919 and returned frequently as a guest investigator. In historic experiments on Mount Wilson in the 1920s Michelson accurately measured the speed of light. Michelson mounted a rotating octagonal mirror on Mount Wilson, and used it to reflect a beam of light to a flat mirror mounted on nearby Mount Baldy. The Mount Baldy mirror in turn reflected the beam back to another facet of the spinning mirror on Mount Wilson, where it was reflected to the observer's eye. By adjusting the rotation speed so that the 8-sided mirror would make exactly 1/8th of a turn as the light traveled from Mount Wilson to Mount Baldy and back, Michelson measured the speed of light with great accuracy.

Michelson also used the 100-inch Hooker reflector to advantage. He developed an instrument called the stellar interferometer, a device for use on a telescope to allow the measurement of very small distances in the sky. With extra mirrors mounted on a 20-foot beam attached to the top of the Hooker reflector, Michelson made the first direct measurements of the sizes of stars other than the sun. In 1920, for example, Michelson determined the diameter of the star Betelgeuse to be 215,000,000 miles, a huge distance compared to that of our own sun. Other stellar diameters were measured, and they were able to prove the validity of astronomer Henry Norris Russell's theory of red super-giant stars.

For most of this century the Mount Wilson Observatory and its major telescopes have contributed mightily to our understanding of the science of astronomy and the universe we inhabit. Within a few years after the observatory was begun, five of the world's great telescopes, including the world's largest solar and stellar telescopes, were in operation on the mountain. Many fundamental problems in astronomy--the nature of sunspots, the temperature and composition of stars, the structure of the universe, and the most basic questions of all involving the very origin of the universe were addressed by the greatest astronomers in the world using the best equipment money could buy. George Ellery Hale's conception of the modern observatory was vindicated. The Mount Wilson model had forever changed the history of the science of astronomy.


1. The descriptive material in this section was taken from the following sources.

Helen Wright, Explorer of the Universe: A Biography of George Ellery Hale (New York: E.P. Dutton & Co., Inc. 1966), pp. 207-209.

Michael Simmons, "Reflections on Mount Wilson's Past: Building the 100-inch Telescope," Reflections from the Mount Wilson Observatory Association, Fall 1986, pp. 1, 12-16.

Owen Gingerich, ed., Astrophysics and twentieth century astronomy to 1950 (Cambridge, England: Cambridge University Press, 1984), IV: 137-44.

Richard Learner, Astronomy Through the Telescope (New York: Van Nostrand Reinhold Company, 1981), p. 112.

The American Society of Mechanical Engineers, The 100-Inch Telescope of the Mount Wilson Observatory (Northrop Corporation, 1981). (Brochure)

2. The historical background for this section was taken from the following sources:

Gingerich, op. cit., pp. 137-144

Mount Wilson Institute, The Mount Wilson Observatory: A View Toward the Future, Draft Proposal, October 1987.

Allan Sandage, "Inventing the Beginning," Science 84, November 1984, pp. 111-113.

Siegfried Marx and Werner Pfau, Observatories of the World (New York: Van Nostrand Reinhold Company, 1984), pp. 115-119.

Simmons, op. cit., pp. 14-16.


Abell, George O. Exploration of the Universe. 4th ed., Philadelphia: Saunders College Publishing, 1982.

American Society for Mechanical Engineers. The 100-inch Telescope of the Mount Wilson Observatory. Northrop Corporation, 1981. (Brochure)

Asimov, Isaac. Eyes On The Universe. Boston: Houghton Mifflin Company, 1975.

Gingerich, Owen, ed. Astrophysics and twentieth century astronomy to 1950. Cambridge, England: Cambridge University Press, 1984.

Kirby-Smith, H.T. U.S. Observatories A Directory and Travel Guide. New York: Van Nostrand Reinhold Company, 1976.

Learner, Richard. Astronomy Through the Telescope. New York: Van Nostrand Reinhold Company, 1981.

Marx, Siegfried, and Werner Pfau. Observatories of the World. New York: Van Nostrand Reinhold Company, 1984.

Mount Wilson Institute. The Mount Wilson Observatory: A View Towards Its Future. Draft Proposal, 1987.

Mount Wilson Observatory Association. "Mount Wilson Observatory: A Self-Guided Walking Tour." no date.

Sandage, Allan. "Inventing the Beginning." Science 84, November 1984, pp. 111-113.

Simmons, Michael. "Reflections on Mount Wilson's Past: Building the 100-inch Telescope" Reflections from the Mount Wilson Institute, Fall 1986, pp. 1, 12-16.

Wright, Helen. Explorer of the Universe: A Biography of George Ellery Hale. New York: E. P. Dutton & Co., Inc., 1966.


Mount Wilson Observatory Mount Wilson Observatory Mount Wilson Observatory

Mount Wilson Observatory Mount Wilson Observatory Mount Wilson Observatory

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Palomar Observatory 200-inch Reflector


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