74-in. Telescope for the University of Toronto


    THE issues of Engineering for March 9 and 30 and April 20 contain a fully illustrated description of the 74-in. reflecting telescope now being completed by Messrs. Sir Howard Grubb, Parsons and Co., at Newcastle for the David Dunlap observatory of the University of Toronto. An account of the instrument was published in NATURE of October 14, 1933. The observatory, which is being given as a memorial to the late David A. Dunlap, of Toronto, by his widow and son, is being erected on Richmond Hill, 800 ft. above sea-level, a few miles north of Toronto. The circular steel building and the 61-ft. dome for housing the telescope were made by Messrs. The Cleveland Bridge and Engineering Co., Ltd., at Darlington, and these together with the main parts of the telescope were sent to Canada last year. The polishing of the mirror is now in hand. The telescope, the largest in the British Empire and the second largest in the world, weighs about 50 tons, of which the moving parts account for about 35 tons. The polar axis is 22 ft. long and the declination axis 13 ft. long; the driving wheel on the former having a pitch diameter of 8 ft. with 960 teeth of 8 mm. pitch. The article gives details of the driving and controlling mechanisms. The disc for the mirror, of special Pyrex glass, was made by the Corning Glass Works, New York, and when received at Newcastle weighed 2 tons 6 cwt. For grinding and polishing the mirror a special machine has been made which allows the mirror to be tilted for testing purposes without being removed from the machine. The telescope, it may be added, may be used either as a Cassegrain or a Newtonian, for which two mirrors 19 in. and 20 in. in diameter respectively are provided. THE David Dunlap Memorial telescope is illustrated in the issue of the Sphere dated April 21, which also includes, photographs of the new 36-in. Yapp reflector at the Royal Observatory, Greenwich. Accompanying these illustrations is an article entitled “Studying the Sun in Calcium Light” and several lunar photographs taken with the 100-in. reflector at Mount Wilson, California.

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