The Shane 3-Meter Reflector

THE SHANE TELESCOPE is the premier research instrument of Lick Observatory. Its name honors astronomer C. Donald Shane, who led the effort to acquire the necessary funds from the California Legislature, and who then oversaw the telescope's construction. In recognition of his efforts, the UC Regents voted in May 1977 to rename the 120-inch the C. Donald Shane telescope. The Shane reflector began collecting data in 1959, and it remains one of the world's most productive telescopes.

The 120-inch reflector took as long to build as James Lick's 36-inch refractor. The five-ton glass blank for the primary mirror was originally cast by the Corning Glass Company in 1933 as a test blank for the Palomar Observatory 200-inch Hale telescope. The blank remained in storage until 19SI, when Shane's efforts enabled Lick Observatory to buy it at a favorable price and move it to Mount Hamilton for grinding and polishing.

It then took eight years to grind and polish the unfinished glass blank into a mirror form. About 650 pounds of glass were slowly removed to make the final smooth "bowl," ten feet across but only an inch and a half deep in the center. Care and accuracy were needed because surface variations as small as a millionth of an inch can affect the mirror's performance.

The final preparation for the mirror was applying its reflecting coat of aluminum. For this operation, the glass disk is lowered into a vacuum chamber in the basement of the telescope building. Inside the evacuated chamber, a small amount of vaporized aluminum is allowed to settle on the scrupulously clean glass surface. The aluminum coating is about as thick as the moisture that breath leaves on a window--if the coating were too thick, it would settle with an uneven surface, and if it were too thin, it would transmit light. With proper protection the delicate coating can last five or more years.

Although the 300-squarefoot reflecting surface of the mirror weighs but a fraction of an ounce, a 150-ton structure is required to keep the mirror aimed. The four and a half-ton mirror itself must be properly cradled-while at any angle-to prevent distortion from its own weight. The backside of the mirror has "pockets" in which supports have been fitted. Force applied through these supports evenly distributes the mirror's weight and maintains its curvature, no matter what the position of the telescope.

Despite the telescope's great weight it turns easily under the power of a small motor because it is precisely balanced. The whole mass floats on a thin film of oil under high pressure to minimize friction. An operator can move the telescope and monitor its position from a control desk, or a computer can handle these tasks. A computer also moves the open slit in the 96-foot diameter, 260-ton dome as the telescope tracks.

The mirror and its supports are mounted at the lower end of a 50-foot-long steel framework. The open steel tube, balanced between the tines of a huge two-pronged fork, can sweep the sky from horizon to horizon. The handle of the fork, or polar axle, can rotate about its own axis, which is aligned parallel to the earth's rotation axis. These two motions-the telescope tube swinging between the fork tines and the rotation of the fork itself-allow the telescope to point to any direction in the sky. Once an object is located, the tube is locked in place and the polar axle turns in pace with the earth's rotation.

A major advance in the telescope's efficiency came in 1969 when the Cassegrain focus was first put into operation. Since the lowest point of the Cassegrain focus is an inconvenient fifteen feet above the dome floor, all instruments located there are operated remotely from the control room next to the telescope. Along with the light detecting instruments, a sensitive television camera is mounted at the focus. Observers sit in a warm, well-lit room watching celestial objects on a television screen while a computer displays and records incoming data instantaneously.

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