The Crossley Reflector

Amateur astronomer Andrew A. Common built the Crossley Reflector in Great Britain in 1879, around the time that the Great Lick Refractor was built. Whereas the Great Refractor was one of the last large refractors built, the Crossley was one of the first large reflectors ever built. Large reflectors became practical after a new technology for making concave, silver coated glass mirrors was perfected in 1880. Prior to this, such mirrors required constant polishing to remain reflective.

Using this reflector, A.A. Common was the first to discover that stars too faint to be seen through a telescope with the eye could be seen in photographs taken through the telescope using a long exposure time. This discovery was the basis for all future astrophotography and spectroscopy. For these early astronomical photographs, Common was awarded the Royal Astronomical Society gold medal for Astronomy.

After deciding to build a larger telescope, Common sold the 36-inch reflector to Edward Crossley in 1885. Crossley built a new dome enclosure to protect the telescope and observers from the harsh Halifax (UK) weather, but this climate was far from ideal for observation. After about 10 years, Crossley generously donated both telescope and dome to Lick Observatory, where it was put into operation in 1896.

Crossley Research at Lick Observatory

Lick observer James Keeler used the Crossley to produce early photographs of nebulae, in the hopes of better understanding their nature. To his surprise, many fainter nebulae could be seen in the background of an Orion Nebula image. Previously, astronomers had assumed that nebulae were a rare phenomena, and that all nebulae were essentially the same. Keeler’s photos with the Crossley reflector not only proved that nebulae were common phenomena, but also illustrated a variety of types of "nebulae," some of which were spiral galaxies filled with billions of stars, rather than a single local object. Such detailed photographs were possible because this reflecting telescope focused light more effectively than a refractor. Thus Keeler’s photos also proved quite graphically the superiority of reflectors to refractors for modern astronomical research.

The 36-inch Crossley reflector was used for observation extensively before the 120-inch Shane reflector was built on Mt. Hamilton in 1959. The Crossley reflector is still used occasionally for research, although it has limited light-gathering ability and is not physically easy to use. Research over the past few years has included SETI (Search for Extraterrestrial Intelligence) projects, eclipsing binary star research, and some extrasolar planetary search observations.

Technical Difficulties

When the Crossley Reflector first arrived on Mt. Hamilton in 1896, William Hussey, who had been assigned the task of making the Crossley operational, called it a "pile of junk." This attitude was reflected in his work on the Crossley, and it was not until James Keeler arrived as Director in 1898 that serious rennovation of the Crossley was begun.

Keeler adjusted the inclination of the axis of the telescope to the correct angle for the Lick Observatory latitude, to align the telescope with the earth's axis. He then adjusted the drive clock, enabling the Crossley to follow star motions more accurately. He also removed the old mirror coating and applied a new reflective surface, and made various other improvements. By 1899, Keeler had taken a series of photos of nebulae impressive enough to cause a sensation at a meeting of the fledgling AAS (American Astronomical Society).

After Keeler's death in 1900, Charles Perrine completed Keeler's collection of nebulae photographs as well as overseeing dramatic improvements to the Crossley. Between 1902 - 1904, Perrine improved the mechanical mounting, drive, and gears such that the telescope tracked more accurately. He redesigned the way that photos were taken by removing a secondary mirror and installing a plateholder at prime focus (at the telescope top). Light would now be reflected off only one mirror before striking the photographic plate, increasing the amount of light available to expose each plate. Finally, as is apparent in the photo to the left, he replaced the minimal lens mounting structure with a rigid tube, preventing the mirror from slipping and the telescope from flexing under its own weight during observation at certain angles and under windy observing conditions.

The Crossley telescope has now been upgraded to the extent that no original telescope parts remain! Compare the photos on this page to see how the Crossley has changed (click on small images for enlargements). The top photo illustrates one of the fundamental user challenges of the Crossley: the observer must stand on a mezzanine 25 feet above the dome floor to look through the telescope. Observers must take extreme care to avoid falling while observing during the wee small hours of the night. Remarkably, considering the physical challenge of using this telescope, its age, and its limited light-gathering capacity, this telescope is still used occassionally for current research.

The photograph of the Orion Nebula on the right, taken with the Crossley reflector in 1928, illustrates the fine detail which the Crossley is capable of imaging (click on photo for enlargement). This photograph is frequently used in science educational materials and amateur astronomy publications.

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