Fig. 69.—Table Mount with Slow Motion.
Thus arranged the mount is a very serviceable one for small instruments. Its stability, however, depends on the base upon which it is set. The writer once unwisely attempted to gain convenience by removing the legs of the stand and screwing its body firmly upon a very substantial tripod. The result was a complete failure in steadiness, owing to the rather long lever arm furnished by the height of the pillar; and the instrument, which had been admirably steady originally, vibrated abominably when touched for focussing.
The particular stand here shown is furnished with a rack motion in altitude which is a considerable convenience in following. More rarely adjustable steadying rods attached to the objective end of the instrument are brought down to its base, but for a telescope large enough to require this a better mount is generally desirable.
Now and then an alt-azimuth head of just the sort used in the pillar-and-claw stand is actually fitted on a tall tripod, but such an arrangement is usually found only in cheap instruments and for such tripod mountings other fittings are desirable.
Fig. 70.—Alt-azimuth Mount, Clark Type T.
The second form of alt-azimuth mount, is altogether of more substantial construction. The vertical axis, usually tapered and carefully ground in its bearings, carries an oblique fork in which the telescope tube is carried on trunnions for its vertical motion. The inclination of the forked head enables the telescope to be pointed directly toward the zenith and the whole mount forms the head of a well made tripod.
Figure 70 shows an excellent type of this form of mount as used for the Clark Type T telescope, designed for both terrestrial and astronomical observation. In this particular arrangement the telescope lies in an aluminum cradle carried on the trunnions, from which it can be readily removed by loosening the thumb screws and opening the cradle.