Saturday, October 21, 2006

20" Inside Diameter Stainless Steel Rings?

If you have spent much time looking through Scientific American's classic, Amateur Telescope Making, you will recall seeing lots of pictures of steel skeleton tubes for larger telescopes. A series of rings (often six or seven of them) were welded to a dozen or steel rods. They look really heavy--but were they?

I have a 17.5" Dobsonian that was apparently the first telescope that a bunch of Boise families made some years ago. I do not know what they were thinking. It uses a Sonotube--but then it had a massively heavy wooden box built around it, upon which the altitude bearings are mounted. I'm not sure of the exact weight, but with the mirror out, this beast easily weighs 100 pounds. Since 20" inside diameter Sonotube weighs about four pounds per foot, the wood is adding a huge amount of unneeded weight. My goal is to get this beast light enough to mount on a Losmandy G-11 mount, and something that can mount to a dovetail--which would seem to preclude any of the truss tube designs that I have seen.

So what if I started over, and built one of those steel skeleton tubes to hold everything in place? Stainless steel is about .29 pounds/cubic inch. If I've done the math right, four 1/8" thick, 1" wide flats 90" long bolted (and lock washered) to five rings that 20" inside diameter, 1" wide, and 1/8" thick, totals less than 19 pounds. Even if I go up to 1/4" thick steel flats (which seems excessive), the weight is 32 pounds. The total stiffness of four 1/8" thick pieces of stainless, mounted to five rings, is quite extraordinary.

The flats are easy to get. But is there a source for 20" inside diameter stainless steel rings? Or can someone suggest an alternative material that is readily available? Would epoxying 1" sections of Sonotube add enough stiffness to the flats without the full weight of Sonotube? Perhaps I just should just stick with Sonotube.

Alternatively: instead of rings, I use construct this as a skeleton box, with the four flats held in position by 20" long flats of stainless steel. Using a total of five sets of four flats (the equivalent of five impossible to find 20" stainless steel rings), the total weight, if all of these are 1/8" stainless steel, is only 28 pounds (plus the weight of the bolts holding everything together). Add 37 pounds for the primary mirror, and about ten pounds for the secondary mirror, holder, and eyepiece, and this is 75 pounds. The bottom flat could even be machined with a built-in dovetail to fit the Losmandy mount.

This is still a bit too heavy for the G-11 mount--but I am wondering if there is some way to calculate the amount of flex that these parts will experience. Perhaps I don't need five rings. Perhaps the 90" long flats don't need to be quite so thick. It makes me wish that I had taken mechanical engineering classes. I know that Young's modulus describes deformation, but I'm not quite sure how to apply all this a real world problem. Perhaps it is time to go hit the library on this exciting subject.

No comments:

Post a Comment