The New Heavens | Page 7

in the constellation of the Hunting Dogs (Pease).
Photographed with the 60-inch telescope.]
The parts of the telescope which are moved by the driving-clock weigh about 100 tons, and it was necessary to provide means of reducing the great friction on the bearings of the polar axis. To accomplish this, large hollow steel cylinders, floating in mercury held in cast-iron tanks, were provided at the upper and lower ends of the polar axis. Almost the entire weight of the instrument is thus floated in mercury, and in this way the friction is so greatly reduced that the driving-clock moves the instrument with perfect ease and smoothness.
The 100-inch mirror rests at the bottom of the telescope tube on a special support system, so designed as to prevent any bending of the glass under its own weight. Electric motors, forty in number, are provided to move the telescope rapidly or slowly in right ascension (east or west) and in declination (north or south), for focussing the mirrors, and for many other purposes. They are also used for rotating the dome, 100 feet in diameter, under which the telescope is mounted, and for opening the shutter, 20 feet wide, through which the observations are made.
A telescope of this kind can be used in several different ways. The 100-inch mirror has a focal length of about 42 feet, and in one of the arrangements of the instrument, the photographic plate is mounted at the centre of the telescope tube near its upper end, where it receives directly the image formed by the large mirror. In another arrangement, a silvered glass mirror, with plane surface, is supported near the upper end of the tube at an angle of 45°, so as to form the image at the side of the tube, where the photographic plate can be placed. In this case, the observer stands on a platform, which is moved up and down by electric motors in front of the opening in the dome through which the observations are made.
[Illustration: Fig. 14. Spiral nebula in Andromeda, seen edge on (Ritchey).
Photographed with the 60-inch telescope.]
Other arrangements of the telescope, for which auxiliary convex mirrors carried near the upper end of the tube are required, permit the image to be photographed at the side of the tube near its lower end, either with or without a spectrograph; or with a very powerful spectrograph mounted within a constant-temperature chamber south of the telescope pier. In this last case, the light of a star is so reflected by auxiliary mirrors that it passes down through a hole in the south end of the polar axis and brings the star to a focus on the slit of the fixed spectrograph.
ATMOSPHERIC LIMITATIONS
The huge dimensions of such a powerful engine of research as the Hooker telescope are not in themselves a source of satisfaction to the astronomer, for they involve a decided increase in the labor of observation and entail very heavy expense, justifiable only in case important results, beyond the reach of other instruments, can be secured. The construction of a telescope of these dimensions was necessarily an experiment, for it was by no means certain, after the optical and mechanical difficulties had been overcome, that even the favorable atmosphere of California would be sufficiently tranquil to permit sharply defined celestial images to be obtained with so large an aperture. It is therefore important to learn what the telescope will actually accomplish under customary observing conditions.
Fortunately we are able to measure the performance of the instrument with certainty. Close beside it on Mount Wilson stands the 60-inch reflector, of similar type, erected in 1908. The two telescopes can thus be rigorously compared under identical atmospheric conditions.
The large mirror of the 100-inch telescope has an area about 2.8 times that of the 60-inch, and therefore receives nearly three times as much light from a star. Under atmospheric conditions perfect enough to allow all of this light to be concentrated in a point, it should be capable of recording on a photographic plate, with a given exposure, stars about one magnitude fainter than the faintest stars within reach of the 60-inch. The increased focal length, permitting such objects as the moon to be photographed on a larger scale, should also reveal smaller details of structure and render possible higher accuracy of measurement. Finally, the greater theoretical resolving power of the larger aperture, providing it can be utilized, should permit the separation of the members of close double stars beyond the range of the smaller instrument.
CRITICAL TESTS
The many tests already made indicate that the advantages expected of the new telescope will be realized in practice. The increased light-gathering power will mean the addition of many millions of stars to those already known. Spectroscopic observations now in regular progress have carried the range of these investigations far