Diagonal Holder
At this point a full-scale sectional drawing of the eyepiece end of the tube should be made, similar to Fig. 59, showing in detail the diagonal, holder, spider supports, and so forth. The block on which the diagonal rests, B, is a prism-shaped piece
of hardwood cut from a square stick of the same width as the diagonal. A ⅜" square-sectioned strip of wood is glued across the base of the hypotenuse face of B, and later one half of it is planed away, making a neat-fitting seat, C, for the diagonal A.
The latter is held in place with three pieces of thin sheet brass (Fig. 60), one of which is tacked to the back and the others to the sides of the wooden prism. These brass pieces should first be annealed by heating them over a flame to a red heat and quenching in water. Use a hardwood stick to bend the "ears" of the pieces over the corners of the diagonal. Do not pinch it tightly—there should be a barely perceptible shake to it.
A 1/16" thick brass plate, D, is fastened to the base of the prism with small wood screws. The small hinge F joins that plate to the 1/g" thick plate E; use either small metal screws or solder in attaching F. The hinge, which can be purchased at any hardware store, may be so stiff as to resist the action of the compression spring G, but it can be completely loosened by applying a pinch of rotten stone or emery and oil to the hinge joint, and working between the fingers for some minutes. A No. 4-36 screw and a compression spring between the plates, opposite the hinge joint,
provide adjustment in the angle of deflection.
The plate E screws onto a length of 1/4-20 rod which has been threaded at both ends. A study of Fig. 47 will disclose the fact that the rod, although centered in the lube, should not be centered in the plate, but offset slightly. Failure to take this into account means only that the fully illuminated field of view, circular in outline, will not be concentric about the optical axis, but as the offset in the 6-inch f/8 telescope amounts to only about 1/25" it can be ignored.
However, if a totally reflecting prism is used in place of the diagonal, it is correct to center it. The nuts I and K should first have been screwed on, and the joint at I should be soldered. The nuts K and L lock against the central support I, a small longitudinal adjustment being provided to compensate for error in locating the holes for the spider in the tube. N is a small counterweight (that might be omitted)
used to remove torque from the vanes.
If ordinary lock washers (not shown in the diagram) are inserted between the nuts K and L and the support I, then after the longitudinal adjustment is effected and the nuts moderately tightened, rotation of the rod H is possible without disturbing the locking mechanism. This is of decided assistance in making alignment
of the optical axis with the eyepiece.
Next- Spider Support |
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