Photographs of Nebulæ and Clusters Made with the Crossley Reflector
Common planned the telescope, mounting, and housing himself while the inch silver-on-glass reflector mirror was designed and built by the English telescope maker G. 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 inch and 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 Common sold the inch reflector to another British amateur astronomer, Edward Crossley, who moved it to his estate near Halifax in Yorkshire, England. The two 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.
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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 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.
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The observer's platform was suspended inside the dome and rotated with it. By Crossley became dissatisfied with the climate of England for astronomical work and decided to dispose of the 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 the large inch reflector was taken down and shipped to California.
[Heliogravure plates in Photographs of nebulae and clusters made with the Crossley reflector]
The massive dome built to house the telescope was also sent to Lick. By June 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. However, due to problems in the original mounting, it proved difficult to take satisfactory long exposures. On January 1, , 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.
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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 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, The next astronomer to work with the Crossley was Charles Dillon Perrine, who in , 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 to , 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 when the large inch mirror was coated with aluminum, thus increasing its light-gathering capacity. In the early s, 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.
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In the late s 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 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.
The Crossley 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. The Crossley inch reflecting telescope, at the Lick Observatory, marked the first modern application of a reflecting telescope to astronomical studies. In Sir William-Herschel built a reflecting telescope with a 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 , when the technique of making concave silver-surfaced glass mirrors was perfected, that reflectors again assumed importance in astronomy. One of the earliest such telescopes was the inch reflector built by British amateur astronomer Andrew Common.
Common's telescope was built around a 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.
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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.
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