The Cassegrain Arrangement
The most generally useful arrangement of the 72-inch telescope is, however, the Cassegrain form, so called from the French astronomer who first used it. About 7 feet below the focus, the conical reflected pencil from the 72-inch mirror is intercepted by a convex mirror of the same size as the Newtonian and of about 10 feet focal length as seen in C, Fig. 5, and also shown in Fig. 2 and 3. This mirror turns the light downward and, after passing through the central hole, forms the image of the star about two feet below the mirror surface on the slit of the spectrograph or on a visual attachment as shown. The significant property of this combination is that the focal length is increased from 30 to 108 feet without changing the tube length in a somewhat similar manner to the action of a telephoto lens. It has the same size of image and magnifying power as a refractor with a tube 108 feet long and has the decided advantage of a much shorter tube and smaller dome. Observations with this arrangement are made at the lower end of the tube from the observing floor and with much greater ease and convenience than at the upper end. Changes from the Cassegrain to the Newtonian or Principal Focus arrangements are readily effected by a device due to the genius of Mr. Swasey whereby only the mirrors and attaching tubes require to be handled, instead of the whole upper end of the tube as in previous reflectors.
Fig. 5.—COURSE OF LIGHT IN TELESCOPE FROM STAR TO FOCUS