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|THE STORY OF THE LENS||Telescope Mirrors|
|ROM the days of Galileo and Sir Isaac Newton astronomers have persistently sought to bring to perfection the two great types of telescopes—the refracting and the reflecting—and to demonstrate, if possible, the superiority of one or the other. After the immense six-foot reflecting mirror made by Lord Rosse and now in the|
British Museum, all the big telescopes for nearly a century were of the
refracting type, and not until the last decade has the reflector once more
gained the ascendency. The two largest reflecting mirrors ever ground
were recently completed, one for the Solar Observatory at Mount Wilson,
California, and the other for the Dominican Astronomical Observatory at
The making of a telescope mirror is a long and most exacting process. It begins with the manufacture of the glass and ends only when the mirror has been placed without mishap at the bottom of the great telescope tube. The order for the glass to be used in the mirror for the Mount Wilson observatory was given to a French firm in 1906 and the glass was delivered at Pasadena, California, in 1909. Because the rough block of glass seemed to reveal serious flaws, it was decided to make on the spot glass of a better quality. Accordingly a furnace capable of holding twenty tons of molten glass was built and a year wasted in demonstrating the futility of the proposition. The glass was either hopelessly defective or was broken in removing it from the furnace. Then in 1910, under the direction of Prof. George W. Ritchey of the Mount Wilson observatory staff, the grinding of the previously rejected piece of glass was begin. The block weighed 4½ tons and the grinding required three years. The completed mirror is 101 inches in diameter and 12¾ inches thick at the edge. Its greatest depth of curvature, i.e., departure from the horizontal surface, is only 1¼ inches. The surface is slightly parabolic and yet as its center deviates from a true spherical form by only one-thousandth of an inch. A deviation of more than one two-hundred-thousandth of an inch from the theoretical form calculated by the mathematician is positively not permissible. No wonder three years are required for grinding such a mirror!
Two other convex mirrors of approximately 29 and 55 inches aperture respectively were also ground to use in conjunction with the big 100-inch mirror. The focal length of the large mirror alone is 42
feet and when combined with the other two mirrors gives focal lengths
of 150 and 251 feet. The telescope may be used in the Cassegrain,
Coude or Newtonian combinations and with spectroscope or photographic
The big reflecting mirror for the Dominion Astronomical Observatory is 73 inches in diameter, 12 inches thick at the edge and is pierced by a hole 101/8 inches in diameter to permit of its use as a Cassegrain telescope. Its silvered surface is a parabola in form and reflects the light to a focus 30 feet distant. The mirror weights 2¼ tons but is supported so perfectly in its containing cell that not the slightest distortion can occur. As a Newtonian telescope in which the image produced by the mirror itself is viewed directly with an eye-piece the focal length is that of the mirror—30 feet. When used with the second convex reflecting mirror as a Cassegrain telescope the focal length is 108 feet. The mirror was made by the John A. Brashear Co. of Pittsburgh and the telescope was designed and constructed by the Warner and Swasey Co. of Cleveland.
The purpose of a large telescope mirror is not to magnify but to gather light and to produce an intensely bright real image which may be observed with a magnifying eye-piece. The eye-piece is really a very powerful compound microscope and yet the largest telescope is unable to magnify a single star, so distant are they. The larger the mirror the longer its focal length and the more light it will gather. The human eye gathers very little light and stars fainter than the sixth magnitude are invisible. Not more than 5,000 thousand can be counted without the aid of a telescope. Up to the building of the giant reflector at Mount Wilson, there existed approximately 219,000,000 telescopic stars; but it is now hoped that the population of the stellar universe may be increased by at least 100,000,000. These telescopes, too, have been especially designed for spectroscopic and photographic work. In these marvelous creations of astronomical engineering once more new standards have been set for human endeavor and attainment.