The fundamental construction of the equatorial involves two axes working at right angles positioned like a capital T.
The upright of the T is the polar axis, fitted to a sleeve and bearing the cross of the T, which is hollow and provides the bearing for the declination axis, which again carries at right angles to itself the tube of the telescope.
When the sleeve which carries the upright of the T points to the pole the telescope tube can evidently be swung to cover an object at any altitude, and can then be turned on its polar axis so as to follow that object in its apparent diurnal motion. The front fork of a bicycle set at the proper angle with a cross axis replacing the handle bars has more than once done good service in an emergency. Figure 75 shows in section a modern equatorial mount for a medium sized telescope.
Fig. 75.—Section of Modern Equatorial.
The mounting shown in Fig. 75, by Zeiss, is thoroughly typical of recent practice in instruments of moderate size. The general form of this equatorial comes straight down to us from Fraunhofer’s mounting of the Dorpat instrument. It consists essentially of two axes crossed exactly at right angles.
P, the polar axis, is aligned exactly with the pole, and is supported on a hollow iron pier provided at its top with the latitude block L to which the bearings of P are bolted. D the declination axis supports the telescope tube T.
The tube is counterpoised as regards the polar axis by the weight a, and as regards the declination axis by the weights b b. At A, the upper section of the pier can be set in exact azimuth by adjusting screws, and at the base of the lower section the screws at B. B. allow some adjustment in latitude. To such mere rudiments are the azimuth and altitude circles of Short’s mount reduced.
At the upper end of the polar axis is fitted the gear wheel g, driven by a worm from the clockwork at C to follow the stars in their course. At the lower end of the same axis is the hour circle h, graduated for right ascension, and a hand wheel for quick adjustment in R. A.
At d is the declination circle, which is read, and set, by the telescope t with a right angled prism at its upper end, which saves the observer from leaving the eye piece for small changes.