Two Large Telescopes

TWO LARGE TELESCOPES The following are some details of the two largest telescopes at present existing. (See photographs in article OBSERVATORY.) The Mount Wilson Telescope.—The diameter of the mirror is 100.4 in. (255 cm.), the focal length 42.3 ft. (12.89 metres). The tube of the telescope is swung in a split polar axis, the cross pivots on which the tube rotates serving as the declination axis. The great weight of the telescope (1 oo tons) is carried by large drums situated at the upper and lower ends of the polar axis, which float in troughs of mercury, while the actual direction of the .axis is determined by self-aligning spherical bearings near its two extremities. The telescope can be used either in the Newton ian or Cassegrain form, separate terminal sections of the tube being provided as carriers for the auxiliary mirrors of each. These car riers, or "cages" as they are called, are very massive, and are removed and exchanged with the aid of an electric crane. When it is necessary to remove the mirror for silvering, which is about twice a year, it is lowered by means of a large hydraulic jack into a laboratory just beneath the floor. Two large tubes lying parallel to the telescope contain weights that can be moved longitudinally to perfect the balance. The instrument is provided with quick and slow motion devices operated electrically, and essentially all movements of it and of the dome are accomplished by motors under independent control by the observer at the eye end and the assistant at the desk. Forty motors are used in the dome. The mirror is supported by a counterpoise system. In order to mini mize the effect on the mirror of changes in the temperature of the air, it is surrounded by an insulating sheath of cork-board. Means were originally provided for keeping the mirror cell jacketed with brine, which was automatically held at a constant temperature, but they were eventually discarded and reliance was placed on the cork insulation as well as on the protective covering of the dome.

The Victoria Telescope (British Columbia) .

The diameter of the mirror is 72.5 in. (184.15 cm.) the focal length 30.1 ft. (9.18o metres). Critical features of this telescope are the polar axis bearings and the construction of the tube. The polar axis rotates in ball bearings of large size, unrelieved by supporting or flotation devices. This departure from usual practice in the con struction of large telescopes has proved very successful. The lattice-work part of the tube is built as a unit, without removable end pieces such as those of the Mt. Wilson telescope, provision being made for the direct interchange of the Cassegrain mirror, the Newtonian mirror and the focal plane apparatus without the removal of a terminal section of the tube. The telescope is electrically operated.

Telescopes under Construction.

Several American univer sities are undertaking the erection of great telescopes, among them the University of Texas with an 82-inch reflector and the Uni versity of Michigan with one of 85 inches. But the successful casting and cooling of a 200-inch mirror by the Corning Glass Works in 1934 and 1935 has been the most dramatic development. The telescope for which this mirror has been cast is being erected (1937) by the California Institute of Technology with the aid of Carnegie and Rockefeller funds. It will be double the size and have four times the light-gathering power of the reflector on Mount Wilson, which is still (1937) the largest in the world. The new telescope will facilitate research in spectrum photography.

forms of telescope (other than equatorials) are treated under TRANSIT CIRCLE and ZENITH TELESCOPE. See also OB SERVATORY, where illustrations are given. For a survey of the technical details of construction of telescopes and general instrumental equip ment of a large observatory, see Gell, History and Description of the Cape Observatory (1913). (D. GI.; A. S. E.)