Thursday, November 2, 2006

Retubing Big Bertha

Big Bertha, my 17.5" reflector, has an unnecessarily heavy framework. I've been thinking of ways to put her on a diet, perhaps with the possibility of getting her onto an equatorial mount.

I think the biggest part of why Big Bertha is so heavy is that the people that built her put a far heavier wooden structure around the tube than was really needed. I don't see how the tube, mirror, secondary mirror, holder, focuser, and finder weigh more than 120 pounds--and yet I am pretty sure that with this massive wooden box around the tube, which carries the altitude bearings, it may be closer to 200 pounds.

One possibility is to replace the existing Sonotube (which is delaminating because of many years of less than perfectly dry storage, before I received it). Sonotube that diameter is not hideously expensive, and surprisingly light. The 20" diameter Sonotube (the right size for Big Bertha) is only four pounds/linear foot, and I think that I only need six feet of it. (The current tube is longer than it needs to be, I think.)

But it turns out that you can fiberglass the outside of Sonotube, and get a reasonably waterproof and probably more attractive finish than just painting the Sonotube directly. The instructions sound messy, and probably require warmer weather than we will see here until May. The fiberglass will also make the tube stiffer than Sonotube is alone--which is traditionally one of the problems with Sonotube--it tends to deform when you tighten down screws to it.

A stiffer tube might also allow use of lightening holes as well--knocking a few pounds off the total weight of the tube. There's probably a book that explains how many holes and of what size give you the greatest savings of weight without compromising stiffness.

The temptation is strong to ditch the existing base, and start from scratch on a Dobsonian mount (or perhaps something more exotic, such as a yoke mount). Still, a good starting point would be to reattach the existing altitude bearings to the new tube, perhaps attaching them to something that clamps to the tube, so that I don't have to drill any unsightly holes in the tube.

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