THE ALT-AZIMUTH STAND.
Fig. 3.
The Telescope, being balanced on trunnions, can be moved from an elevation approaching the zenith to an almost horizontal position. In order that it may be secured anywhere between these extremes, attached to the upper part of the telescope is an iron rod, which, sliding through the end of the arm of the stand, can be there clamped. The telescope will now be clamped in Altitude. As the progression of celestial objects will apparently be very slow, resource must be had to the smoothness of motion obtained by a screw. The upper end of the Altitude rod is therefore tapped to receive a long screw with a large milled head, jointed to the telescope body; by revolving this head the telescope is raised or depressed accordingly as the screw is unscrewed, or the reverse. It is necessary that the screw should be withdrawn some way from the rod before clamping it, preparatory to following an object which has passed the meridian, or is setting; as perhaps, just when the clearest vision is obtained, the observer may be annoyed by the screw action being suddenly stopped by the milled head coming in contact with the top of the rod.
When viewing objects near the zenith, and the focus long, the handle attached to the clamp will be found useful, as it can thereby be reached without leaving the finder. The handle may be so placed that a downward push should clamp, and an upward pull release.
The second motion in the Alt-azimuth Stand, namely, Azimuth, is obtained as follows:—The strong iron disc which forms the upper fitting of the legs has its surface accurately turned. On this revolves an iron disc, rather less in diameter, to which the trunnions which support the telescope are attached. The main axis of this disc passes through the centre of the lower disc, and then through a hollow bearing tube, a continuation of it. All these fittings having been most carefully turned and ground together, great steadiness, combined with facility of horizontal movement, is ensured. In order that this motion can be communicated as evenly as possible, resource must be again had to a screw which is thus applied. Just within the circumference of the lower disc is a narrow groove, turned to such a depth that the ring which is thus separated from the main disc is still firmly held to it by the uncut portion. An iron clamp grooved to this ring holds the nut of a long screw, the plain end of which is jointed to the upper disc. When this clamp is fixed to the ring, any motion given to the screw will act on the upper disc, and cause it to revolve, and thus the whole telescope will be slowly moved in Azimuth. The advantages of this plan are many, the most important being the rapidity and ease with which the telescope can be shifted from one object to another, even to those in contrary directions; all that is necessary being to release the clamp and turn the telescope to the object required. The clamp being carried round with the upper disc, can be fixed directly the desired position is obtained, when the screw is at once in action. Should it happen, whilst following an object, that the screw becomes exhausted from the joint and clamp coming together, the clamp should be released, and the screw turned sufficiently in the reverse way to bring it into action, the weight on the upper disc keeping the telescope meanwhile in position. If this operation be rapidly performed, the whole length of screw can be brought in play before the object has left the field of view of the finder, and can thus be easily refound with the higher power of the telescope. Motion is applied to the screw by means of a Hook’s joint, named thus from its inventor. This joint being furnished with a long handle, enables the observer, by means of it, to move the Telescope in Azimuth at any rate, and without removing his eye from the eye-piece.
It will be seen from the preceding remarks that by means of the vertical and horizontal screw motions, the telescope, when clamped, can be moved in any direction with the greatest facility, permitting a celestial object to be observed with high powers for a considerable time, and with the greatest pleasure and comfort to the observer.
