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The Tripod
The combination of tripod and pillar (frontis-piece) was devised in order that the telescope could be swung through the meridian without fouling the legs. The only other way in which this could be accomplished would be to extend the declination axis several inches beyond its bearing support, thereby increasing the bending moment.
No weakness was introduced by the pillar, which proved to be exceedingly rigid. In construction, it consists of a 9" length of aluminum tubing, 4½// outside diameter, with a 1/8" wall. Tightly fitting caps at each end were built up of three thicknesses of 5/8" plywood, glued together, then turned in the lathe and perforated to admit the pipe that is screwed to the mounting. This pipe passes all the way through the pillar and is made up tight at the bottom by means of a lock nut. The three 2" blocks of maple to which the legs are attached were first glued to the lower cap, and included
in the turning and boring operation.
For the f/8 telescope, the pillar construction may prove every bit as rigid if it consists simply of, a 10" length of pipe, extending from the 45° elbow, and a heavy flange screwed onto its lower end. The flange in turn can be bolted to the wooden tripod head, or the pipe can be screwed directly into the tripod head of metal
shown in Fig. 76. If this form of construction is adopted, the tee that houses the polar axis should then be of the "bullhead" type shown in Fig. 73, with a 2" branch opening.
The legs of the tripod are made of maple, although any hardwood will do. The slats are 0.4" thick, l½" wide, and 21" long. The oak spacers are l 1/8" thick, the upper ones being the same width as the head blocks (2"), and the lower two tapered, narrowing down to 7/8" at the bottom. The pegs or feet are made from 1" bar stock, 31/4" long. One half of this length is reduced to a neck with a diameter of 3/8"; the other half is tapered to a point.
The neck is pushed into a 3/8" hole drilled into the lower wooden spacer. When glued and bolted together, the leg assembly (Fig. 74) makes an extremely rigid truss member. A synthetic resin glue, such as weldwood or cascamite, mixed to a thick creamy consistency, is recommended. The overall height is 31" from the floor to the top of the pillar with the legs spread at an angle of 60° to the floor.
The legs can also be fashioned from a solid board, as suggested in Fig. 75, but the board should be not less than l1/4" thick and 3" wide. As in the design in Fig. 74, brass bushings should be inserted into the holes to take up the wear against the
joining bolts. See page 200 on how to tie tripod legs together.
A wooden pattern for the tripod head, similar to Fig. 76, is easily made, and a casting in iron, brass, or aluminum made up at a local foundry. The diameter is a matter of choice, the larger the better, but 5½" is about the limit if the tube is not to tangle with the legs. A good thickness is about l½ ". Make the center hole about 1¾" in diameter, and very slightly taper it and all the sides of the pattern in the same direction, so that the foundry man can remove it from the sand.
To accommodate the 2" pipe, the center hole should later be bored out to a diameter of 2.236", and then threaded squarely with a 2" pipe tap, sufficient threading being cut to admit all but about the last thread on the pipe when it is made up tight. The only other machine work required on the casting is the drilling of the three holes (5/16" or 3/8") for attaching the Fig. 76. Plan, laid out in a Sy2" circle, for a tripod head. legs. The sides there should, of course, be filed until they are quite parallel.
Machining the Parts
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