Trusses and Trapezoids


“Truss System”

2014 November 1. I’ve been going back and forth on the whole strut/truss thing. The idea of lugging around and keeping track of eight polls for a typical open-tube ATM Newtonian has bothered me from the beginning. I’ve kept circling around the idea of using a four-strut system. Highe describes both and identifies complementary advantages. Kriege/Berry comes down solidly in favor of truss systems. At my ATM workshop the incorporation of triangles into the support system was strongly advocated as a far more stronger system, coupled with the concern that four parallel poles would eventually and quickly twist.

A first alternative from the workshop was in each corner to use two or three poles of smaller diameter, held in parallel at intervals by blocks. This would give the structure the desired for triangular stability. A second alternative was to cut plywood into solid triangles with the lower base running along the mirror box sides and the top angle attached to the lower ring of the upper tube. Elaborate suggestions – involving fretsaws and carefully conceptualized silhouettes to carve out – were made regarding removing wood in the center of the triangle to lower the weight of the plywood, while not violating the integrity of the truss.

I’ve decided to run with a modified version of the second alternative. The parallel strut system doesn’t allay concerns I’ve had about how and where to attach struts to the upper tube. The two modifications I’ve made to the triangle system are (a) use trapezoids rather than triangles and (b) as the fret sawing of intricate designes, especially in 3/4″ plywood, seems to me more trouble than it’s worth, I’m thinking in terms of removing an interior area (one smaller trapezoid, or multiple smaller shapes) and replacing it with a designed 1/4″ insert.

I realized after some sketching that I couldn’t get the trapezoids to be less than 48″ (length of plywood). Also with all four trapezoids’ bases parallel, it would be impossible to fit all four on a single piece of plywood; and there would be no way to make a straight cut across a 4’x8′ sheet. With a borrowed pick up, I picked up some new uncut sheets of 4×8 (for this and other projects). I designed a pattern and cut it out of 1/4″ ply. The pattern allows me to skew the layout of the four trapezoids across the 3/4″ and leave the remainder more intact. I then rough cut the four trapezoids out with the intent of getting everything more precisely cut with a router and pattern bit.

Attaching the trapezoids to the mirror box and UTS. The four trapezoids sit three inches down in the mirror box on a 1/4″ plywood “top” to the mirror box. That plywood top sits on four blocks of 2×4 that strengthen the corner joints of the mirror box. The four trapezoids slide in with no room to spare. I made a notch at the base of each trapezoid on the right hand side. At the corresponding point on the mirror box, I drilled holes through the mirror box and inserted my “clamps.” The clamp is four inches or so of threaded rod, with a wooden block at the end inside the mirror box, and a fly nut on the outside of the mirror box. The bold helps position the trapezoid when the tube is being assembled. By tightening the fly nut – and thus drawing the wooden block in toward the side of the mirror box, the trapezoid is held fast.

On the bottom of the UTS are four rectangular wood blocks, 1x1x6. These are the braces against which the upper side of the trapezoid will be fastened. I had four thumbscrews with knurled heads left over from a different project. These slide through holes I drilled in the rectangular blocks and screw into internally threaded inserts I placed in the trapezoids.

I can assemble the telescope in less than ten minutes this way. The clamping holds the secondary mirror stable in relation to the primary mirror throughout all azimuthal and altitudinal movements of the scope. I hope to upgrade the bolts and nuts etc at some time. But the system itself works very well.



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