The Principal Focus
As already indicated, the principal mirror when used alone forms an image of the star 30 feet above, at the centre of the upper end of the tube, and an eyepiece could be placed there for visual work or a photographic plate for direct photographs of nebulae, etc. However, it is generally more convenient to use a flat mirror at 45° forming the image at the side of the tube for photographs and so the telescope is only used in this form with a small spectrograph for the ultra-violet region of star spectra. The course of the parallel beam of light from the star to its image on the slit of the spectrograph is graphically shown in Fig. 5 A and also its passage through the slit prisms and lenses of the spectrograph. The position of the star image on the slit of the spectrograph can be observed by a guiding telescope extending to the edge of the tube and can be kept central by the portable aluminium switchboard already described. The elevating platform is of course used in work in this position.
The Newtonian Arrangement
For direct photography or visual observations at the focus of the 72-inch mirror, the reflected cone of star light from the mirror B, Fig. 5, is intercepted by a plane mirror also silvered on the front surface, 19·5 inches diameter and 3·25 inches thick placed at 45°. This form of reflecting telescope was first used by Newton, hence the name. The focus is then formed, as shown, at the side of the tube, and if a plate is placed there and accurately guided by small eyepieces with cross wires, photographs of any desired small region in the sky can be obtained, Fig. 4 being made in this position, or visual observations may be made. The oculars can easily be reached from the observing platform for any position of the telescope.
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