Thursday, April 24, 2008

Palomar Observatory

Earth has witnessed many gigantic features: Dinosaurs, pyramids, sky scrapers...and telescopes. These gigantic features succumb to evolution, external circumstances, limitations of effectiveness, and better technology. The Palomar/Hale telescope is indeed the most massive and far reaching optical astronomy tool we have. The 200 inch precision ground Corning Pyrex glass has provided breathtaking images of the universe and accurate astronomical data. The telescope is a marvel of mid 1930 technology and technical insight in construction and has been updated for the computer age. Limited by earth’s atmosphere and other specialized astronomy instruments [terrestrial and in space], it still serves a useful function.

National Park Service:

Name: Palomar Hale Reflector (Hale Telescope)
Location: Palomar Mountain, California
Classification: Private, structure
Period: 1936-present
Areas of Significance: National Register: education, engineering, science, NHL: science, Subtheme: physical science, Facet: astronomy
Builder: Dr. Russell W. Porter


The 200-inch Hale reflector is the principal instrument at the Palomar Observatory of the California Institute of Technology. The telescope was placed in operation on June 3, 1948, and dedicated to the memory of George Ellery Hale, whose leadership and vision were responsible for its creation. [1]

The dome of the Palomar Observatory is 135 feet high and 137 feet in diameter and divided into two sections. The solid lower concrete portion is immovable while the upper aluminum and steel section can be rotated to permit the telescope to observe any section of the sky through the open shutters. The base section houses photographic dark rooms, telescope-control computers, library, lounge, storerooms, air-conditioning equipment, photographic plate storage vault, motor generators, massive switchboards, elevators, and an oil pumping system that supplies the main bearings of the telescope. The uppermost floor of the solid section houses the telescope. Adjacent to the telescope is a glassed-in observatory gallery from which visitors can view the instrument during daylight hours.

The movable upper section of the dome weighs 1000 tons and moves on tracks to permit the observation of any section of the sky. It rotates on 32 four wheel trucks which move so smoothly that no vibration is transmitted to the telescope. It is driven by two four-horsepower motors. The two shutters weigh 125 tons each and roll together at the end of each night's observation to seal the interior of the dome against the heat of the day and inclement weather.

The most important requirement in the design of the dome was good insulation. For this reason there is a four-foot gap between the concrete walls which form the base of the building. The interior wall is filled with aluminum foil insulation. There is also a four foot gap between the inner and outer walls of the steel dome. The inner face of the dome is made of aluminum panels built in the shape of boxes and filled with crumpled aluminum foil. The outside of the dome is composed of steel plates, 3/8-inch thick, butt-welded and molded to form a strong but smooth hemispherical dome. As the warm air rises through the double walls of the building and the double layers of the dome, cool air enters below, thus preventing the heating of the dome from the Sun's rays during the day.

The foundation of the dome is anchored to the mountain while the foundation for the telescope is separately built on a base of crushed granite to protect the telescope from jar and vibration in the event of an earthquake.

Operational Description of the Hale Telescope:

The main telescope tube carries the 200-inch mirror at its bottom end. This structure is supported on ball-bearing trunnions anchored in a large yoke, which consists of two 10-foot-diameter inclined tubular girders tied together at the south end by a cross member supported on a pivot bearing, and at the north end by a giant horseshoe bearing. The tube weighs 150 tons.

When in operation, light from the sky is concentrated by the 200-inch concave mirror into an image at the "prime focus" at the upper end of the tube. Longer focal lengths--and hence larger images--can be provided by inserting one or the other of two convex mirrors just ahead of the prime focus. The first of these, known as the Cassegrain secondary mirror, reflects the light down the tube through a hole in the center or the main mirror to a focus just below the bottom of the tube. If a still larger image is desired, the second convex mirror is substituted. With the aid of a diagonal flat mirror its sends the light down through the south polar-axis to a constant-temperature room in the stationary portion of the dome.

The horseshoe bearing mounting permits the main telescope tube to see the North Pole without interference. Oil is pumped though the pads supporting the horseshoe bearings at a pressure of 300 pounds per square inch, sufficient to lift the horseshoe and its load a few thousandths of an inch. Between these two bearings the entire 530 tons of moving parts of the telescope are "floated" on films of oil, providing virtually friction-free operation.

The right ascension drive consists of two large gears that are used to move the telescope; one gear slews the telescope east and west into the required position, while the other makes it follow the stars. A computer system is connected to the telescope to aim this system.

The east and west declination trunnions are the pivotal bearings on which the telescope tube tilts north and south in declination. Located in each trunnion is an array of small motors and gear trains that provide slow motion fine adjustments of the declination setting.

The 200-inch primary mirror is the heart of the Hale Telescope. All of the other parts of the telescope have only one purpose--to make possible for the mirror to perform its light-gathering function as efficiently as possible. The mirror is supported at the bottom of the telescope tube on 36 delicate counterbalance supports to maintain rigidity. The delicate reflecting surface of the mirror is protected by covers that close like the petals of a flower over the glass disk.

The aluminizing chamber is a large steel tank located on the main floor of the dome next to the telescope. When necessary, the mirror is removed from the telescope and placed in the aluminizing chamber to replace the aluminum coating that provides the reflectivity for the mirror. The prime-focus observing capsule is located at the top of the telescope tube. The capsule contains a seat for the use of the observing astronomer. On the bottom of the observer's cage are hyperboloidal convex mirrors capable of changing the focal length of the telescope to suit various observing requirements.

The Cassegrain and Coude secondary mirrors are used to change the focal length of the telescope. With the use of this system, the focal length of the 200-inch mirror can be changed from 660 inches to an effective focal length of 6000 inches.

The Cassegrain-Coude diagonal mirror is mounted in line with the declination trunnions. This mirror reflects the image formed by the main mirror either into the yoke girder or down the south polar-axis to the Coude-spectrograph room.

The Coude-spectrograph room contains four spectrograph mirrors of different focal lengths. The room is light-tight and serves as a camera. The astronomer chooses the mirror with the focal length he requires and rolls it into the light beam on its carriage to make his observations.

New auxiliary equipment for the 200-inch Hale reflector is continually under development to enhance the light-gathering capabilities of the instrument and to keep it as one of the foremost research instruments in astronomy and astrophysics in the world today.


The construction and delivery of the Pyrex glass disk for the Palomar 200-inch reflector in 1936 marked a watershed in the history of astronomy. With the successful casting of this large mirror, the Palomar project, conceived by George Ellery Hale (1868-1938), and funded with a grant of $6 million by the Rockefeller Foundation, moved toward the completion of the largest reflecting telescope in the world by 1948. In the 40 years since the completion of the Palomar project, the 200-inch reflector remains at the leading edge of research in the sciences of astronomy and astrophysics and stands today as a monument to George Ellery Hale and his efforts to produce the finest instruments in the world to answer the fundamental questions concerning the origin and nature of the universe.


Like buried treasures, the outposts of the universe have beckoned to the adventurous from immemorial times. Princes and potentates, political or industrial, equally with men of science, have felt the lure of the unchartered seas of space, and through their provision of instrumental means the sphere of exploration has rapidly widened. [2]

With this statement, astronomer George Ellery Hale opened his article in the April 1928 issue of Harper's Magazine to set forth the case for the building of what was to become the 200-inch Palomar reflector. The purpose of this article was to inform the American public about his proposal to construct the largest telescope in the world to answer questions relating to the fundamental nature of the universe. Hale hoped that the American people would understand and support his project.

Hale followed this article with a letter to the International Education Board (later absorbed into the General Education Board) of the Rockefeller Foundation dated April 28, 1928, in which he requested funding for this project. In his letter, Hale stated:

No method of advancing science is so productive as the development of new and more powerful instruments and methods of research. A larger telescope would not only furnish the necessary gain in light space-penetration and photographic resolving power, but permit the application of ideas and devices derived chiefly from the recent fundamental advances in physics and chemistry. [3]

Hale was successful beyond his dreams when the Rockefeller Foundation voted to support the project with a grant of $6 million. The Palomar project (named after Palomar Mountain that was to be the site of the new observatory) was now under way.

The effort to build the 200-inch telescope was easily the most famous scientific undertaking of the 1930s. From the beginning, everyone associated with the project realized that the work must be done right or not at all. Every task associated with the Palomar project required a considerable extension of the technology of the day. [4]

For example, no 200-inch mirror had ever been cast before. The largest reflector in 1928, at the time Hale proposed the Palomar project, was the 100-inch mirror in the Hooker telescope at Mount Wilson, California. The 100-inch Hooker had also been conceived and brought to completion by Hale, but the Hooker telescope raised at least as many questions as it answered. The 200-inch mirror was to be the largest and most difficult telescope Hale ever constructed.

The Mount Wilson 100-inch mirror, a five-ton instrument, was a midget when compared to the estimated 40 tons necessary for a 200-inch mirror. Hale led the search for new materials to construct the 200-inch mirror investigating metal alloys, fused quartz, and a new glass technology called "Pyrex." After much experimentation and failure, Pyrex was selected as the best material. The contract for the mirror was given to the Corning Glass Works. Borrowing from then current construction technology, a solid sheet of glass was rejected in favor of a hollow reinforced ribbed disk. After a successful trial run which led to the casting of a 120-inch disk, Corning was ready to cast the 200-inch mirror. The first attempt was a failure when the molds broke loose and floated to the top. The problem with the mold was corrected and the mirror was finally cast in 1936. By April the disk was delivered safely to Pasadena, California, for grinding and polishing--a task that would eventually take twelve years to complete.

Other tasks, as great as the grinding of the blank glass disk, involved the designing of the tube that was to house the mirror and the auxillary optical equipment, the engineering of methods that would make the huge instrument responsive to delicate adjustments, and the design and construction of the huge dome to house the telescope.

For the mechanical design of the telescope Hale decided to use oil-pad bearings and the Serrurier truss. This was to overcome known limitations of a more traditional fork type mount that would not permit the telescope to observe the north pole and placed undue stress on the telescope's bearings in certain observing positions. The use of the Serrurier truss, in which the deflections of the primary and secondary mirror-support system are matched to maintain correct mirror alignment, was a convincing solution to the problem of telescope flexing. This system permitted the placement of the primary and secondary mirrors 43 feet apart at opposite ends of a 140-ton tube and allowed the mirrors to remain in alignment to within 1/100-inch. This new method also allowed the telescope to reach the entire sky. The use of oil-pad bearings with the Serrurier truss made the weight of the entire structure unimportant since the bearings would carry almost any load and, if the weight made the telescope bend, the mirror alignment was still under control. [5]

George Ellery Hale died in 1938 and did not live to see the completion of his last telescope. In June 1948 the 200-inch reflector was dedicated to his memory. The Hale reflector made it possible to photograph and resolve distant objects as dim at the 26th magnitude--objects only 1/40,000,000 as bright as the dimmest object visible to the naked eye. It detected faint galaxies that were billions of light years from the earth. It enabled astronomers to detect and resolve astronomical objects much better than was possible with the Hooker reflector, the largest telescope in the world prior to 1948. [6] Speaking at the dedication of the 200-inch reflector Dr. Lee Du Bridge, President of the Carnegie Institute said:

This great telescope before us today marks the culmination of over two hundred years of astronomical research. And for generations to come it will be the key instrument in Man's search for knowledge. [7]

The superior optical qualities of the 200-inch Hale reflector were demonstrated by astronomer Maarten Schmidt who came to the California Institute of Technology in 1959. Schmidt was interested in certain radio sources that astronomer Allan Sandage had managed to pinpoint to what looked like individual stars. The spectra of these radio-emitting stars were not familiar and astronomers were not able to make any sense of them. In 1963, using the 200-inch Hale reflector, Schmidt realized that the unfamiliarity of the spectra was the result of an enormous red shift and that the lines were familiar ones that ought to be in the ultraviolet section of the spectrum. This turned out to be correct and the enormous red shift indicated the objects to be very distant, a billion lights years away and more. Since the objects were too distant to be stars, or even galaxies, Schmidt concluded, they must be something else not previously seen in the history of astronomy. They were called "quasi-stellar objects"; that is, objects with a star like appearance or "quasars" for short. [8]

The discovery of quasars has had an enormous impact on modern astronomy. Most scientific discoveries either strengthen an existing scientific concept or result in the birth of a new theory. The discovery of quasars, however, has resulted in the bewilderment of astronomers since there is no easy way to explain their existence. The consequence of their discovery was that one either had to question the validity of the yardstick of the astronomer, the red shift, or to agree that there are processes out there for which we have no explanation. [9]

While new technologies have led to the construction of larger telescopes based upon techniques not known in 1948, the Hale reflector remains in the forefront of research in the fields of astronomy and astrophysics and is the largest successful reflecting telescope in the world today. With the construction of this telescope, Hale pushed the technology of the monolithic mirror reflecting telescope to its physical limits and created one of the finest research telescopes in the world. The technology of the giant Pyrex glass mirror, cast in 1936, remains the best in astrophysics. A 1947 CAL Tech publication referres to the final polishing of the mirror as ". . .the most daring optical job ever attempted." [10] The Hale reflector stands today as a monument to George Ellery Hale and his quest for better and more efficient instruments to answer the fundamental questions concerning the origin and evolution of the universe.


1. The description of the 200-inch Hale reflector was taken from the following sources:

Staff of the Palomar Observatory, Giants of Palomar (Hansen Planetarium, 1983).

Helen Wright, Palomar: The World's Largest Telescope (New York: Macmillan Company, 1952), pp. 154-57.

2. George Ellery Hale, "The Possibilities of Large Telescopes," Harper's Magazine, April 1928, p. 639.

3. Bob Clark, et al., "The Palomar Observatory Project" (Unpublished Report, Files of the Palomar Observatory, 1983), p. 1.

4. Ibid., 11.

5. Richard Learner, "The Legacy of the 200-Inch," Sky & Telescope, 71 April 1986), p. 350.

6. Isaac Asimov, Eyes On The Universe (Boston: Houghton Mifflin Company, 1975), p. 185.

7. Patrick Moore, Patrick Moore's History of Astronomy (London: Macdonald & Co. 1977), p. 154.

8. Isaac Asimov, Asimov's Biographical Encyclopedia of Science & Technology (New York: Doubleday & Company, 1982), p. 890.

9. Alexander Hellemans and Bryan Bunch, The Timetables of Science; A Chronology of the Most Important People and Events in the History of Science (New York: Simon and Schuster, 1988), p. 541.

10. Clark, pp. 16-17.


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

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

Asimov, Issac. Asimov's Biographical Encyclopedia of Science & Technology. New York: Doubleday & Company, 1982.

Clark, Bob. et al. "The Palomar Observatory Project." Unpublished Report, Files of the Palomar Observatory, 1983.

Di Cicco, Dennis. "The Journey of the 200-inch Mirror." Sky & Telescope, April 1986, pp. 347-48.

Hellmans, Alexander and Bunch, Bryan. The Timetables of Science: A Chronology of the Most Important People and Events in the History of Science. New York: Simon & Schuster, 1988.

Hale, George Ellery. "The Possibilities of Large Telescopes." Harper's Magazine, April 1928, pp. 639-646.

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.

_______________, "The Legacy of the 200-inch." Sky & Telescope, April 1986, pp. 349-353.

Moore, Patrick. Patrick Moore's History of Astronomy. London: Macdonald & Co., 1977.

Staff of the Palomar Observatory. Giants of Palomar. Hansen Planetarium, 1983.

Woodbury, David O. The Glass Giant of Palomar. New York: Dodd, Mead & Company, 1954.

Wright, Helen. Palomar: The World's Largest Telescope. New York: The Macmillan Company, 1952.

Many photographs from these two websites:

Timeline from Caltech archives covering the early history of Palomar Observatory.

Caltech Archives

The Journal of San Diego History

"Palomar, After 50 Years"

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