[ATM] holding a tube in place in an inexpensive dobnewt

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

I designed, and sawed out, and had kids assemble, five dob-newts (anywhere from 4,4 to 6 inch diameter, and f/ ratios from f/5 to f/10) that either I or another local ATMer had ground and/or figured. They work OK except that the tubes have an excessive tendency to slide around in the cradle. I had tried routing out and then sanding/filing out a circular disk to fit just above the cradle, to prevent this, but it took an excessive amount of time to do this, especially since I discovered that some of the Home-Depot-style non-Sonotubes were actually not cylindrical in cross-section, but slightly conical.

I am now considering two possible remedies:

(1) a split ring made out of either plywood or metal strapping material or a combination of the two, that sits just above the cradle, and can be clamped as tight as one wants, so that the tube can be rotated if there are really little'uns trying to view somthing or if there are tall teenagers / adults. This will require a lot of routing and fiddly stuff where the gap in the circle is located. For five telescopes.

(2) An entirely different sort of clamp that fits into one of the corners of the cradle and uses some sort of a large-handled screw to wedge it onto the tube. The cradles have trunnion bearings on their sides made out of PVC pipe, and they follow the length of the tube for about 2 feet or so. The screws for the clamps would be held in place by threaded inserts, and would press against a piece of wood that is parallel to the tube and is as long as the cradle. Not sure how to prevent the screw from eventually passing through the wood clamping piece. Disadvantage of this method is that the tube could not be easily turned without unclamping it.

I also found that a Richard Berry type of trunnion tends to move too easily. I think I have a plan for fixing that.

Any thoughts and suggestions will be read with interest, though not necessarily followed.

Guy



Guy Brandenburg, Washington, DC
My home page on astronomy, mathematics, education:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
or else 
http://tinyurl.com/r6fh2

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