The Tube
This may be made of almost any material, sheet metal, cardboard, plywood, aluminum, or bakelite tubing, or it may be of the skeleton type, although the latter is not recommended except for large telescopes. Rolled sheet metal is perhaps most
frequently used. Aluminum (1/16" wall) and bakelite 1/8" wall) are light and accurately round.
The cardboard tube around which rugs are rolled is excellent if first doused inside and out with two or three coats of shellac. A tube of any of these materials will provide all the strength and stiffness needed for the telescope.
The tube's length should he about 52", equal to the focal length of the mirror plus whatever is needed to enclose the cell. To locate the eyepiece opening, mark a point on the surface of the tube 41½" from the plane in which the surface of the mirror will lie, and with that point as a center describe a circle of ¾" radius. (The distance of 41½" assumes a focal length of 48", a tube diameter of 7", and a distance of 3" outside of the tube for the focal plane.
However, see paragraph on page 116 if the tube is accurately round.) Now drill numerous holes around this circle with a small drill, not larger than 1/8", and cut out the hole which, when smoothed up with a file, will be about 1⅝" in diameter.
Next- Eyepiece Support |
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