In recent years the mounting of great equa torials has passed from the domain of the in strument-maker to that of the engineer, who finds abundant scope for ingenuity and technical expertness in combining very massive and so, rigid, construction with very delicate mechanism. At the present time the largest telescopes in the world are owned and made in America.
The table on following page gives a list of the larger refracting telescopes in the equipment of the more important American observatories.
Whilt the refracting telescope still holds its advantages for photographic astronomical work the tendency of late years has reverted to the reflecting telescope for visual work. Astrono mers prefer it because of the much clearer images obtained, due in large part to the entire absence of the secondary spectrum. The images formed by the best refractors have a turbid quality as compared with the clean crispness of the reflector. As in the case of the refracting instruments, America boasts the largest reflectors. The largest of all is the Hooker reflecting telescope of the Mount Wil son Observatory, Pasadena, Cal., with a clear aperture of 100A inches (2.549 meters) and a primary focal length of 507.5 inches (12.891 meters) ; it can be used directly on the axis either (a) as a focal plane instrument, or (b) in the Newtonian form; at its secondary foci it may be used (c) as a Cassegrain instrument with a focal length of 1,606 inches (40,792 meters), or (d) as a Coil& with focal length of 3,011 inches (76.480 meters). Celestial objects from 65° north to 53" south declination can be observed at the principal focus. The telescope is mounted after the English fashion, the skeleton tube containing the mirror at its lower end swinging between the sides of the open polar axis yoke which has a bearing at either end resting on cast-iron pedestals built up from the main concrete pier below. The mounting was designed by Mr. Pease and other members of the observatory staff, assisted by Prof. Peter Schwamb of the Massachusetts Institute of Technology. The pier is 33 feet high while the intersection of the axes is 50 feet above ground. Both the declination and right ascension bearings are composite; in part, of spherical type, serving to define the axes, while the remainder carries the load. The declination load is carried by means of counter weight systems while the polar axis is sup ported by means of steel drums built integral with the axis and floating in cast-iron tanks filled with mercury. The tube complete weighs 35 tons and the total moving parts weigh 100 tons. They are constructed of cast and struc tural steel throughout and in as large units as could be mac.hined as a whole. The larger por
tions of the mounting, some weighing 10 tons, were made in Quincy, Mass., and their trans portation up Mount Wilson to an devotion of 5,700 feet was a difficult feature in the erection of this instrument. The mirror is 101.2 inches diameter, 1234 inches thick and weighs about 9,000 pounds. It was cast in Saint Gobain, France, and figured by G. W. Ritchey in the 'obsenratory optical shops. It is mounted on a counterweight support system in the rear and on four edge arcs at the sides. To control its temperature it is surrounded with a lining of cork board built integral with its cell, and a system of piping is installed within this through which. liquid at any desired temperature may be circulated from tanks in the pier below. Fans serve to equalize the temperature around mirror and coils. When it becomes necessary to resilver the mirror it is removed from the tube and lowered in its cell by an electric elevator into the pier, where all apparatus and material is at hand and where the temperature may be controlled. The telescope has motor driven fast and slow motions, while the diurnal motion is supplied by the typical form of driv ing clock having a conical pendulum isochro nously governing a falling weight. This operates. through a worm and a worm wheel (17 feet in diameter) cut and ground with high precision.. The instrument is controlled, settings are made and the dome is turned from a station on the pier at which the readings of the circles are made; the °remote') system of electrical con trol is used throughout, and most of the opera tions are duplicated by auxiliary controls at the three observing stations or foci. To reach the principal focus an observing platform is pro vided which travels along the shutter opening. The dome is of structural steel throughout, 100 feet in diameter, 100 feet high and weighs 730 tons. The rotating portion 'Neighing 500 tons is carried on 28 trucks and is traction, driven by two motors at opposite sides. The trucks and rails are carefully machined to coni cal surfaces to eliminate vibration at the tele scope when the dome is moved. The shutter is in halves which open sideways and permit free working of the telescope from the zenith to the horizon. The walls and dome are of double construction and are ventilated at the top to prevent excess heating during the sum mer. A 10-ton crane is provided to assist in the erection and in the transfer of the various auxiliary sections of the tube.