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

[Tom Fredrickson <newsforfred@comcast.net>]

Hi Guy,

I made some split rings for my 4 1/4" that work nicely. I used plywood.
The two ends are the size of the tube + thickness of wood * 2 + 1/4".
I routed out the ends and lined them with light weight leather from a
welding jacket. The hole should be radius of tube + thickness of
leather.

I use plywood spreader on the side to attach the bearings. And solid
wood on top and bottom to attach spring latches and piano hinge. 

Route the ends then build the cradle attach the hinge and remove it.
Then set you saw to rip in the center of the top and bottom pieces
leaving enough material to hold it all together. split it with a hand
saw and reattach the hinge.

install the leather clamp it on the tube and adjust the spring lathes on
the top. with the top piece about 2" wide it makes a handle to Carry the
scope. pop open the spring clamps and the tube can be rotated.

Hope this will help.

Tom


On Mon, 5 Nov 2007 18:36:47 -0800 (PST)
Guy Brandenburg <gfbrandenburg@yahoo.com> wrote:

> 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
> 
> =============================
> "Does anyone seriously believe that the reason U.S. companies are
> shipping jobs to Mexico and Asia is that they believe those
> countries' schools are better?" --  asks Alfie Kohn
> _______________________________________________ ATM mailing list
> http://www.atmlist.net/
> 
_______________________________________________
ATM mailing list http://www.atmlist.net/