There is a modified equatorial mount sometimes known as the “English” equatorial in which the polar axis is long and supported on two piers differing enough in height to give the proper latitude angle, the declination axis being about midway of the polar axis. A bit of the sky is cut off by the taller pier, and the type is not especially advantageous unless in supporting a very heavy instrument, too heavy to be readily overhung in the usual way.
Fig. 77.—Universal Observatory Mount (Clark 9-inch).
In such case some form of the “English” mounting is very important to securing freedom from flexure and thereby the perfection of driving in R. A. so important to photographic work. The 72 inch Dominion Observatory reflector and the 100 inch Hooker telescope at Mt. Wilson are thus mounted, the former on a counterpoised declination axis crosswise the polar axis, the original “English” type; the latter on trunnions within a long closed fork which carries the polar bearings at its ends.
Fig. 78.—English Equatorial Mount (Hooker 100-inch Telescope).
Figure 78 shows the latter instrument, of 100 inches clear aperture and of 42 feet principal focal length, increased to 135 feet when used as a Cassegrainian. It is the immense stability of this mount that has enabled it to carry the long cross girder bearing the interferometer recently used in measuring the diameters of the stars. Note the mercury-flotation drum at each end of the polar axis. The mirrors were figured by the skillful hands of Mr. Ritchey.
Fig. 79.—English Equatorial Mount (72-inch Dominion Observatory Telescope).