Friday, May 3, 2013

Mounting Flat Bases to Round Tubes

A fairly common problem for amateur telescope makers is how to mount a flat base to a round tube.  Especially as Dobsonian mounts have become more common, which usually have rectangular upper and lower cages, makers of telescope parts have become increasingly focused on flat bottoms.  Even before Dobsonians were all the rage, how to mount items such as eyepiece focusers and finder scope rings was sometimes a problem.  At best, manufacturers had to offer several different radii of base for each accessory.

I have decided that the Moonlite Telescopes truss connector system isn't going to work for my situation.  This is not because there is anything wrong with it; it is well-designed and beautifully made.  The problem is that it is really intended for telescopes with much stiffer lower cages than I have.  (The truss is very strong and very stiff -- and thus exposed a flaw in using it on a .125" aluminum wall tube.)  I could probably make it work, but at this point, it is simpler, cheaper, less frustrating, and to my surprise, less weight, to use a solid tube for the rebuild of Big Bertha, the ill-behaved 17.5" reflector.  I'll probably be able to resell at least the truss blocks; I suspect that unless I find someone building a truss 13" or 15" f/4 reflector, the tubes now cut to 55" length will end up being recycled as scrap aluminum.

Along the way, however, I discovered a very useful trick for how to mount flat bases on round tubes.

The trick is to cut round rod of the right diameter to prevent rocking.  Here's what I did using 1/8" diameter steel rod:

In this case, I epoxied the rod in place to the round tube, creating a plane upon which the truss block can stably rest.  (No matter how you do this: there is one point of contact between a plane and a cylinder.)  The epoxy is just to hold the rods in place like enough to bolt the truss block in place; once the bolts are locked down, those rods are not going anywhere.

What diameter of rods do you need?  Remember that the formula for calculating the equation of a circle is x2 + y2 = r2.  In this case, the outer diameter of the round tube was 20.25"; if you want the supports 1" from the center contact point, solve for y to determine how much elevation (diameter of round rod) you need to clear the space between the tube and the plane (the y axis, assuming that you remember high school geometry).

y = square root of (r2 - x2)

For this example:  y = 10.075"; subtract from the radius of the tube,  meaning that you need a rod 0.0495" thick.  Your local hardware store will have a variety of diameters of steel and aluminum rod available; by cutting it slightly shorter than the base it will be supporting, it will be invisible, and add effectively no weight to your telescope.


  1. I still think a self-cast epoxy spacer is a very good solution, especially if you've got a bunch of resin (and filler) around from a boat project anyway. :-)

    And about this: "...the tubes ... will end up being recycled as scrap aluminum."

    Don't you have a garage??? Stick them away, you never know when you'll need some Al tubing!

  2. Personally, I might have used a parting agent and epoxy with a good, solid filler (Epoxy and Microballoons, for example) to make a perfectly molded filler. The parting agent would prevent the epoxy from sticking to the tube in the meantime.

  3. Microballoons take up space nicely, but are not recommended for high-strength applications. How strong does the molded filler need to be, you ask? Well, how thin does it get out toward the edge? I reply.

    Colloidal silica would be a better choice for filler, probably.