Tubes? Tubes? We Don't Need No Stinkin' Tubes!
With apologies to The Treasure of Sierra Madre. The more I have been looking at the problem of how to reduce the weight of the tubes that carry the mirror and mirror cell on one end, and the diagonal mirror holder and eyepiece focuser on the other end, I find myself wandering back to my original concept--totally tubeless.
The original brilliant (or deranged) scheme was three square tubes to which the mirror cell will attach at one end. This has never been the big problem; there's three bolts to hold the mirror cell to the square tubes that provide the necessary stiffness.
At the other end, I have to attach a eyepiece focuser, a diagonal holder to position the secondary mirror in place, and a finder scope. But without a tube, how do I attach those items?
The green squares are the 1" (or perhaps less) tubes that provide the rigidity. With a three vane spider, the spider "legs" attach directly to the tubes. These have to be very rigid; probably steel, not aluminum, since you want them as thin as possible to reduce diffraction. But how do you attach the eyepiece focuser? With a piece of aluminum, probably 1/8" thick or more that forms half a hexagon, bolted to the square tubes at the 120 and 240 degree angles (where 0 degrees is the tube at the bottom). The finderscope can bolt directly to one of the square tubes.
This approach knocks eight to twelve pounds off the total weight, and even better, knocks that weight off the ends of the scope--where they produce the maximum load on the square tubes. Even if I have to increase the thickness of the tubes somewhat, and even if the increase in the weight of the square tubes were to cancel out the weight savings from the round tubes, this means no money having the round tubes made, and no struggles finding a vendor to make them.
The downside is that the diagonal spider legs have to be stronger than before, since they are providing some structural reinforcement at the far end. Alternatively, since I am already putting a shelf in place for the eyepiece focuser to mount, I could just complete this all the way around at both ends. I also have to come up with some way to cover the optics when not in use. I also have to completely wrap the telescope in black cloth when in use, but I was already expecting to have to do that.