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Astronomy and Astrophysics Lick Crossley 36-inch Reflector

Lick Observatory — Mount Hamilton, California.
Exterior View of the Crossley 36-inc Reflector Building, 1988.
(Photo Credit: Lick Observatory)

DESCRIPTION

The Crossley 36-inch reflector at the Lick Observatory was built by British amateur astronomer Andrew Ainslie Common in England in 1876. Common planned the telescope, mounting, and housing himself while the 36-inch silver-on-glass reflector mirror was designed and built by the English telescope maker G. Calver. [1]

To minimize convective thermal disturbances of the image from any large masses of metal near the optical path, Common placed the primary mirror above the declination axis, within a rigid but lightweight open framework, supported by an equatorial fork mount. Counterweights were carried in boxes below the mirror and below the declination axle. The polar axis was partially floated in mercury to reduce friction, an innovation which was applied years later to both the 60-inch and 100-inch telescopes at the Mount Wilson Observatory. The telescope was stored horizontally in a small wooden house with a sliding roof. The house had an exterior platform on inclined ways to give access to the Newtonian focus. The housing and platform could be rotated as a unit to the desired azimuth.

In 1885 Common sold the 36-inch reflector to another British amateur astronomer, Edward Crossley, who moved it to his estate near Halifax in Yorkshire, England. The two 36-inch mirrors Common ordered for the telescope were included in the sale. Mirror A, the one used by Common, was installed in the telescope, while mirror B, also made by Calver, was sent to an optical firm in Dublin, Ireland, to be refigured.

Since the observer's working area on the platform for the telescope was too exposed to the weather, Crossley designed and constructed a substantial dome to house the telescope. The new dome was 39 feet in diameter and covered with 1/12-inch galvanized iron. The dome weighed 15 tons and was driven by a water engine that turned it through a complete revolution in five minutes. Pipes under the floor carried hot or cold water to heat and cool the dome when not in use. The observer's platform was suspended inside the dome and rotated with it.

By 1893 Crossley became dissatisfied with the climate of England for astronomical work and decided to dispose of the 36-inch reflector and dome. After an exchange of letters with Edward S. Holden, the director of the Lick Observatory, Crossley agreed to donate his telescope to Lick.

During the summer of 1895 the large 36-inch reflector was taken down and shipped to California. The massive dome built to house the telescope was also sent to Lick. By June 1896 the telescope was installed on Mount Hamilton and ready for operation. Since it was not possible to use Crossley's water engine to turn the dome due to the lack of running water on the mountain, the dome was turned instead by a rope and pulley system. One arm's-length pull of the rope was necessary to move the dome one inch.

The telescope was equipped with the f/5.8 newly refigured mirror B, which proved to be of excellent quality. However, due to problems in the original mounting, it proved difficult to take satisfactory long exposures.

On January 1, 1898, James E. Keeler assumed the directorship of the Lick Observatory and began to work with the Crossley reflector. Keeler immediately began to make modifications to the telescope to improve its optical qualities and working characteristics. Keeler cut down the pier that mounted the telescope by two feet, thus lowering the telescope by a similar amount to provide more clearance between it and the dome. At the same time the top of the pier was finished off with a slight bevel, so that the polar axis was parallel to the axis of the rotation of the Earth. These changes in the mounting of the telescope were necessary to compensate for the difference in latitude between Common's observatory in England and Mount Hamilton. Other modifications included the addition of a windscreen, a new and smoother drive clock and improvements to the drive train and double-sided plate holder. Keeler also adjusted the mirror of the telescope so that its optical axis was accurately aligned with the center of the tube, and added a new low-power finder telescope, for picking the right area of the sky, to work with the existing high-power telescope. The latter was given a new and lighter-weight mounting.

Although Keeler was able to work successfully with the telescope and produce exposures up to four hours by 1899 the instrument still proved difficult to handle and inadequate for longer observations. The major problem was the insufficiently rigid mounting, which failed to hold the telescope steady in high winds and flexed excessively at large zenith distances. Adding to these problems was the occasional slippage of the mirror in its cell.

James Keeler produced a large number of scientific papers based on his work with the Crossley before he died on August 12, 1900.

The next astronomer to work with the Crossley was Charles Dillon Perrine, who in 1900-1901, used the telescope to take nearly one thousand photographs of the minor planet Eros as it approached the Earth. Although Perrine continued to use the Crossley with good results, he was dissatisfied with its performance and operation. Perrine was determined to improve the telescope and in the years from 1902 to 1905, oversaw a reconstruction of the telescope, which brought it into its modern form.

Perrine replaced Common's original tube and mount with a much more rigid closed tube on an English equatorial mounting. The Newtonian flat mirror, which brought the light out to a focus at the side of the tube, was removed and, in its place, Perrine introduced a plateholder directly at the prime focus of the telescope in the middle of the upper end of the tube.

Perrine also introduced a system of prisms and transfer lens so that the observer could "guide" or accurately follow the motion of the stars during the exposure from an eyepiece just outside of the tube. With these modifications, the Crossley became a faster and more efficient telescope for photographing nebulae and star fields.

The Crossley remained unchanged until 1934 when the large 36-inch mirror was coated with aluminum, thus increasing its light-gathering capacity. In the early 1950s, the drive mechanism of the telescope was replaced. The polar axis was turned end for end so that a worm gear could now be used to drive the telescope from the south polar axle housing, and an electronic clock replaced the old mechanical clock. In the late 1960s Selsyn telescope-position readouts were installed at the observing end of the instrument, and the observer's platform was enlarged and strengthened to carry the additional electronic equipment required by modern observational techniques. A large bearing was installed to ease rotation of the top section of the tube. Finally, a modern darkroom was built.

The Crossley 36-inch reflector is found a few hundred yards southwest of the Main Observatory Building of the Lick Observatory and is still in use as an operational scientific instrument for the study of the stars and galaxies.

SUMMARY

The Crossley 36-inch reflector at the Lick Observatory was the first of a long line of metal-film-on-glass modern reflecting telescopes that have dominated major astronomical advances for the past century. In addition, the Crossley has produced more scientific results than any other telescope of its size, including several historically important studies in stellar evolution, the structure and spectra of planetary nebulae, and the discovery and spectral analysis of faint variable stars in young clusters. The Crossley also contributed to studies that confirmed the expansion of the universe

HISTORY

The Crossley 36-inch reflecting telescope, at the Lick Observatory, marked the first modern application of a reflecting telescope to astronomical studies. [2]

In 1789 Sir William-Herschel built a reflecting telescope with a 48-inch polished mirror, but the telescope was difficult to point and the mirror needed constant polishing. Shortly thereafter, the two-component lens was developed and refractors became the telescope of choice. Despite the many advantages of reflectors over refractors, it was not until 1880, when the technique of making concave silver-surfaced glass mirrors was perfected, that reflectors again assumed importance in astronomy. [3]

One of the earliest such telescopes was the 36-inch reflector built by British amateur astronomer Andrew Common. Common's telescope was built around a 36-inch silver-on-glass mirror that was mounted on an equatorial fork and used as a photographic telescope. The chief innovations in Common's telescope were the achievement of a smooth drive by relieving the bearings of almost the entire weight of the telescope, and the invention of an adjustable plate holder. The bearing load was diminished by submerging a hollow steel float in mercury, while the plateholder had an eyepiece and crosswire attached so that a star just off the edge of the plate could be watched and, if it drifted away from its starting position, be brought back by moving the plateholder. The end result showed that it was possible to build a telescope that was sufficiently smoothly and accurately enough driven to allow very good photographs to be taken. [4] By 1881 Common published a description of his 36-inch reflector, introducing the paper with a list of ten conditions vital for the successful operation of a large reflecting telescope. The last of these conditions was a suitable location for the erection of the telescope. [5]

In 1885 Common sold his 36-inch reflecting telescope to Edward Crossley of Halifax, Yorkshire, England, who donated the telescope to the Lick Observatory shortly after his retirement from astronomy in 1893.

Crossley's donation of his telescope to Lick was fortuitous. For the first time a large reflecting telescope was located on a suitable mountain site where its large aperture could be used to its fullest advantage.

Within a short time the Crossley reflector was put to good use when James E. Keeler initiated a program of nebular photography with it. Keeler's photographs showed the existence of hundreds of spiral nebulae that are now known as galaxies. Neither Keeler nor anyone else at the time realized that nebulae were predominantly extragalatic, but Keeler, using Crossley photographs, was the first to realize that these objects were a major constituent of the universe. After Keeler's death, astronomer C.D. Perrine completed Keeler's observational program, and in 1908 published a remarkable selection of Crossley photographs in memory of Keeler. Keeler's and Perrine's success with the Crossley reflector was probably more influential than any other single factor in convincing professional astronomers of the practical effectiveness of large reflectors. [6]

By the early 1900s, as a result of Keeler's and Perrine's work with the Crossley, it was apparent that the future of large telescopes lay with mirrors rather than lenses. A few years later, when George Ellery Hale began to plan for the establishment of a large observatory on Mount Wilson in California, the use of a large refracting telescope was not even considered. The Crossley had shown the way to the future of astronomy. Large reflecting telescopes would now dominate 20th-century astronomy.

In the years since the early 1900s astronomers at Lick have used the Crossley for several historically important studies in stellar evolution, including the structure and spectra of planetary nebulae and the discovery and spectral analysis of faint variable stars in young clusters. The Crossley has also contributed to studies confirming the expansion of the universe.

In 1908 astronomer Edward A. Fath, using data collected with the Crossley reflector, established that none of the spirals observed had continuous spectra, but were in his opinion, clusters of individual stars. His conclusion was that spiral nebulae must be very distant and composed of very faint stars.

Building upon the work of Keeler, Perrine, and Fath, astronomer Heber D. Curtis devoted his efforts with the Crossley toward a better understanding of the nature of spiral nebulae. In 1912 and 1913 Curtis published extensive lists and descriptions of the brighter nebulae and clusters that he photographed using the Crossley. Curtis' observations of numerous faint novae in the nebulae he photographed with the Crossley led to his conclusion that the nebulae were far more distant from the Earth than had previously been thought. Curtis concluded the nebulae were beyond our own galaxy. Curtis led the way in pushing forth the understanding of spiral nebulae as galaxies, giant island universes, external to our galaxy. In arriving at this view Curtis collided head on with astronomer Harlow Shapley, who held the opposite point of view in the famous 1920 Curtis Shapley debate held before the National Academy of Sciences, in Washington, DC.

In 1930 astronomer Robert J. Trumpler used data obtained from the Crossley to publish his classic paper in which he examined the distances, dimensions and space distributions of open star clusters. Trumpler proved conclusively that our galaxy does contain a layer of absorbing gas and dust which attenuates light of different colors by different amounts. By properly taking into account this attenuation, Trumpler's work led to considerably smaller dimensions for our galaxy than had previously been considered.

Continued contributions made by astronomers using the Crossley telescope in this century are almost endless. Comets, asteroids, and satellites of the planets were regularly discovered. There were many studies of novae, planetary nebulae and their central stars, star clusters, and interstellar medium. Variable stars received attention, especially after photoelectric photometry became popular. In more recent years the Crossley has continued to be used as a scientific instrument, producing important new information in the study of astronomy.

The Crossley reflector was the first large reflecting telescope put into operation and used by astronomers at a first-class site. Using the Crossley, James Keeler and other astronomers at Lick determined the success of the reflector design for all subsequent large telescopes. Once the Crossley was in operation on Mount Hamiliton, the future path of astronomical research in the 20th century, based on large reflecting telescopes, was determined. While larger and more powerful reflecting telescopes were built, all would be, in a sense, the technological descendants of the Crossley.

FOOTNOTES

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

Donald E. Osterbrock, et al., Eye on the Sky: Lick Observatory's First Century (Berkeley, California: University of California Press, 1988).

Donald E. Osterbrock, James E. Keeler: Pioneer American Astrophysicist and the Early Development of American Astrophysics (Cambridge, England: Cambridge University Press, 1984).

Remington P. S. Stone, "The Crossley Reflector: A Centennial Review - I," Sky and Telescope Magazine, October 1979, pp. 307-311.

____________________, "The Crossley Reflector: A Centennial Review - II," Sky and Telescope Magazine, November 1979, pp. 396-400.

2. Leslie Sweeney and John Gustafson, ed. "Lick Observatory" (University of California, 1984), p. 16.

3. Ibid.

4. Richard Learner, Astronomy Through the Telescope (New York : Van Nostrand Reinhold Company, 1981), p. 109.

5. Ibid.

6. Stone, I, op. cit., p. 311.

7. Stone, II, op. cit., p. 398.

BIBLIOGRAPHY

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.

Gustafson, John R. "Pioneering Astronomy at Lick Observatory," Astronomy Magazine, May 1988, pp. 6-15.

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, no date.

Osterbrock, Donald E. James E. Keeler: Pioneer American Astrophysicist. Cambridge, England: Cambridge University Press, 1984.

Osterbrock, Donald E., et al. E y e on the Sky: Lick Observatory's First Century. Berkeley, California: University of California Press, 1988.

Stone, Remington, P. S. "The Crossley Reflector: A Centennial Review - I," Sky & Telescope Magazine, October 1979, pp. 307-311.

_______________________. "The Crossley Reflector: A Centennial Review - II," Sky & Telescope Magazine, November 1979, pp. 396-400.

Sweeney, Leslie and John Gustafson, ed. Lick Observatory. Berkeley, California: University of California, 1984.

PHOTOGRAPHS

(click on the above photographs for a more detailed view)

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