Thursday, June 6, 2013

The Dual Layer Ring Approach Did Not Work

At least the aluminum wasn't expensive to do an experiment.  The difficulty is that it is hard to bend two pieces of aluminum precisely enough on a brake to make one a close fit to the other.  Nor did I have any confidence that screwing them together would solve the problem, or produce an adequately stiff ring for such a heavy telescope.

I still think that a single 1/4" thick piece of aluminum might be sufficient, but there's no way to bend something that size with the equipment that I have, or that makes sense to buy, for a one time construction project.

In any case, until I can actually use the telescope, and verify that the optics are worth investing serious money in a carbon fiber composite tube, I am not going to spend more than $500 on tube rings.

Last night I discovered that trying to use a finderscope without crosshairs is nearly useless.  This is a University Optics 8x50mm finder that was part of the original Dobsonian telescope.  It looks like it was cobbled from parts; the diagonal looks like it might have been broken at one time, and been epoxied back together.  I have not seen any UO finders of this type with eyepieces that lack crosshairs, so it is possible that this is just a standard .965" eyepiece that someone found and put into this finder.  I have no idea what power this eyepiece gives with this finderscope, which may be why I am having trouble lining it up with the main scope -- perhaps it is a very narrow field of view.

New illuminated finders are pretty expensive, so I think rather than just abandon the Celestron 7x50mm straight-through illuminated finder, I will use a piece of aluminum rectangular tubing to raise the rings that hold it up several inches so that I can look through it without having to put my head through the tube.

If I can persuade myself that the optics are good enough, I'll spend the money on a carbon fiber composite tube.  My wife would rather that I spend the money to buy something off-the-shelf -- but the price of off-the-shelf telescopes this size is approaching $8000 -- and they are generally 70 pounds or more -- simply too heavy for the Celestron CI-700 mount that I have.  It makes more sense to buy a carbon fiber composite tube like this one, or from this maker in order to get the weight of the telescope between 60 pounds.


  1. You seem to be trying to reinvent the wheel as a do-it-yourself project. i am sure in the Boise area there is a metal fabricator that can make what you want for a reasonable cost. Just show up with clear drawings and specifications and they will give you a price.
    I have seen what modern well equipped shops can do and what you describe is not that complicated. Don't insist on the technique used. Welding may be easier then bending 1/4 inch Aluminum. They are the experts but you know what you need.

  2. I was trying to do it myself because I could, and because my experience with metal fabricators is that it won't be any cheaper than the $549 that Parallax charges for rings.

    I do wonder if cutting the pieces of a hexagon from 1" thick aluminum, then having someone weld them together, might be a solution.