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ASTRONOMICAL PHOTOGRAPHY 285
of stars have been discovered, which are too dim to be seen even with the most powerful telescopes.
Telescopes employed in Astronomical Work. — The telescopes used in astronomy are of two distinct kinds, Reflecting and Refracting. The former is generally known as a reflecting telescope or a reflector ; this is much the simpler of the two, and as a rule much less expensive. It consists essentially of a concave mirror turned towards the object, and this mirror forms in the air space in front of itself an inverted image of the object.
To use this for photographic purposes it is apparent that in order to record the image, either the plate must be somehow or other supported in front of the reflector, or else a small reflector must be arranged so as to throw the rays out to one side, or to reflect them back through a hole in the centre of the large mirror.
In the former instance the telescope is known as a Newtonian Reflector, and in the latter as a Cassegrain Reflector.
With both types the centre of the main reflector is of course quite useless so far as the formation of the image is concerned. One of the largest reflectors yet constructed is the 60-inch at the Mount Wilson Solar Observatory, U.S.A.
The Mount Wilson ^0-inch Reflector. — This instrument can be arranged as a Newtonian and as a Cassegrainian for purposes of direct photography ; and as a modified Cassegrain type — Cassegrain-coude — for spectroscopic work.
When arranged as a Newtonian, it can also be used for spectroscopic work, with a spectrograph carried at the Newtonian focus. The focal length is then 29& inches. When used as a Cassegrainian for direct photography, it has an equivalent focal length of about 100 feet, and when used for spectroscopic work its equivalent focal length is about 80 feet.