Fig. 74.—Short’s Equatorial Mount.
In the instrument as shown there is first an azimuth circle A A supported on a base B B B B having levelling screws in the feet. Immediately under the azimuth circle is mounted a compass needle for approximate orientation, and the circle is adjustable by a tangent screw C.
Carried by the azimuth circle on a bearing supported by four pillars is a latitude circle D D for the adjustment of the polar axis, with a slow motion screw E. The latitude circle carries a right ascension circle F F, with a slow motion G, and this in turn carries on four pillars the declination circle H H, and its axis adjustable by the slow motion K.
To this declination circle is secured the Gregorian reflector L L which serves as the observing telescope. All the circles are provided with verniers as well as slow motions. And the instrument is, so to speak, a universal one for all the purposes of an equatorial, and when the polar axis is set vertical equally adapted for use as a transit instrument, theodolite, azimuth instrument, or level, since the circles are provided with suitable levels.
This mount was really a very neat and complete piece of work for the purpose intended, although scarcely suitable for mounting any but a small instrument. A very similar construction was used later by Ramsden for a small refractor.
It is obvious that the reach of the telescope when used as an equatorial is somewhat limited in the mount just described. In modern constructions the telescope is so mounted that it may be turned readily to any part of the sky, although often the polar axis must be swung through 180° in order to pass freely from the extreme southern to the extreme northern heavens.
The two motions necessary are those in right ascension to follow the heavenly bodies in their apparent course, and in declination to reach an object at any particular angular distance from the pole.
There are always provided adjustments in azimuth and for latitude over at least a small arc, but these adjustments are altogether rudimentary as compared with the wide sweep given by Short.