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Above it are the "Monastery" and other buildings used as quarters by the astronomers of the Mount Wilson Observatory while at work on the mountain. The dome of the 60-inch reflecting telescope is just below the 150-foot tower, while that of the 100-inch telescope is farther to the right.
Photographic plates, which reveal invisible stars and nebulæ when exposed for hours in modern instruments, were not then available. In any case they could not have been used, in the absence of the perfect mechanism required to keep the star images accurately fixed in place upon the sensitive film. Building and revolving dome, 100 feet in diameter, covering the 100-inch Hooker telescope.
Moreover, most of them are much too faint for examination with this instrument. At the 134-foot focus the 100-inch telescope gives a large-scale image of such clusters, and permits the spectra of stars as faint as the fifteenth magnitude to be separately photographed. Hubble's Variable Nebula. One of the few nebulæ known to vary in brightness and form.
But in spite of this decrease, the gain of a single additional magnitude may mean the addition of many millions of stars to the total of those already shown by the 60-inch reflector. Here is one of the chief sources of interest in the possibilities of a 100-inch reflecting telescope.
The approaching completion of the 100-inch telescope of the Mount Wilson Observatory led me to suggest to Professor Michelson, before the United States entered the war, that the method be thoroughly tested under the favorable atmospheric conditions of Southern California.
Twenty-foot Michelson interferometer for measuring star diameters, attached to upper end of the skeleton tube of the 100-inch Hooker telescope. In the practical application of this method to the measurement of star diameters, the chief problem was whether the atmosphere would be quiet enough to permit sharp interference fringes to be produced with light-pencils more than 100 inches apart.
In fact, when the holes are moved apart to the full aperture of the 100-inch Hooker telescope, the interference fringes are still visible even with the star Betelgeuse, though its angular diameter is perhaps as great as that of any other star. Thus, we must build an attachment for the telescope, so arranged as to permit us to move the openings still farther apart.
For the photography of nebulæ and the study of the fainter stars, the reflector has special advantages, illustrated by the work of such instruments as the Crossley and Mills reflectors of the Lick Observatory; the great 72-inch reflector, recently brought into effective service at the Dominion Observatory in Canada; and the 60-inch and 100-inch reflectors of the Mount Wilson Observatory.
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.
The conceptions of the stellar universe, except those that ignored the solid ground of observation, were limited by the small aperture of the human eye. But the dawn of another age was at hand. Photographed with the 100-inch telescope. This short-exposure photograph shows only the bright central part of the nebula.
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