In Fig. 173, Chap. X a pair of these same mounts are shown at Harvard Observatory. The nearer one, carrying a celestial camera, is exposed to view. It is provided with a slow motion and clamp in declination, and with an electric drive in R. A., quickly unclamped for swinging the camera. It works very smoothly, its weight is taken by a very simple self adjusting thrust bearing at the lower end of the polar axis, and altogether it is about the simplest and cheapest equatorial mount of first class quality that can be devised for carrying instruments of moderate length.
Several others are in use at the Harvard Observatory and very similar ones of a larger growth carry the 24 inch Newtonian reflector there used for stellar photography and the 16 inch Metcalf photographic doublet.
Fig. 82.—Mounting of Mt. Wilson 60-inch Reflector.
Fig. 83.—The 60-inch as Cassegrainian, F = 100′.
In fact the open fork mount, which was developed by the late Dr. Common, is very well suited to the mounting of big reflectors. It was first adapted by him to his 3 ft. reflector and later used for his two 5 ft. mirrors, and more recently for the 5 ft. instrument at Mt. Wilson, and a good many others of recent make. Dr. Common in order to secure the easiest possible motion in R. A. devised the plan of floating most of the weight assumed by the polar axis in mercury.
Figure 82 is, diagrammatically, this fork mount as worked out by Ritchey for the 5′ Mt. Wilson reflector. Here A is the lattice tube, B the polar axis, C the fork and D the hollow steel drum which floats the axis in the mercury trough E. The great mirror is here shown worked as a simple Newtonian of 25 ft. focal length. As a matter of fact it is used much of the time as a Cassegranian.
To this end the upper section of tube carrying the oblique mirror is removed and a shorter tube carrying any one of three hyperboloidal mirrors is put in its place. Fig. 83 is the normal arrangement for visual or photographic work on the long focus, 100 ft. The dotted lines show the path of the rays and it will be noticed that the great mirror is not perforated as in the usual Cassegrainian construction, but that the rays are brought out by a diagonal flat.
Figure 84 is a similar arrangement used for stellar spectroscopy with a small flat and an equivalent focus of 80 ft. In Fig. 85 a radically different scheme is carried out. The hyperboloidal mirror now used gives an equivalent focus of 150 ft., and the auxiliary flat is arranged to turn on an axis parallel to the declination axis so as to send the reflected beam down the hollow polar axis into a spectrograph vault below the southern end of the axis. Obviously one cannot work near the pole with this arrangement but only through some 75° as indicated by the dotted lines. The fork mount is not at all universal for reflectors, as has already been seen, and Cassegrainian of moderate size are very commonly mounted exactly like refractors.