This mount strongly suggests the original one of Hadley, Fig. 16, and is most firm and serviceable. A reflector thus mounted is remarkably convenient in that the eyepiece is always in a most accessible position, the view always horizontal, and the adjustments always within easy reach of the observer.
Fig. 72.—Alt-azimuth Newtonian Reflector.
Whenever it is necessary to follow an object closely, as in using a micrometer and some other auxiliaries, the alt-azimuth mount is troublesome and some modification adjustable by a single motion, preferably made automatic by clockwork, becomes necessary.
The first step in this direction is a very simple one indeed. Suppose one were to tilt the azimuth axis so that it pointed to the celestial pole, about which all the stars appear to revolve. Then evidently the telescope being once pointed, a star could be followed merely by turning the tube about this tilted axis. Of course one could not easily reach some objects near the pole without, perhaps, fouling the mount, but in general the sky is within reach and a single motion follows the star, very easily if the original mount had a slow motion in azimuth.
Fig. 73.—Parallactic Mount for Reflector.
This is in fact the simplest form of equatorial mount, sometimes called parallactic. Figure 73 shows the principle applied to a small reflector. An oblique block with its angle adjusted to the co-latitude of the place drops the vertical axis into line with the pole, and the major part of the celestial vault is then within easy reach.
It may be regarded as the transition step from the alt-azimuth to the true equatorial. It is rarely used for refractors, and the first attempt at a real equatorial mount was in fact made by James Short F. R. S. in mounting some of his small Gregorians.[15] As a matter of record this is shown, from Short’s own paper before the Royal Society in 1749, in Fig. 74.
A glance shows a stand apparently most complicated, but closer examination discloses that it is merely an equatorial on a table stand with a sweep in declination over a very wide arc, and quite complete arrangements for setting to the exact latitude and azimuth. The particular instrument shown was of 4 inches aperture and about 18 inches long and was one of several produced by Short at about this epoch.