I connected the truss tube to the mirror cell with three blocks of wood of dimension 1.5 x 2.5 x 3.5", cut right from a two-by-four (a misnomer, two-by-four's are actually 1.5 x 3.5"), and bolted to the mirror cell at a radius of 9" very close to 120 degrees apart. The inner diameter of the blocks is 8.25" which allows them to serve as bumpers for the mirror if it slides around accidentally. The blank in the photos is a 16.5" diameter piece of plywood left over from making the secondary ring, and fits nicely between the blocks. I still need to stabilize the blocks on the mirror cell, since now they are prone to rotating a little around the central bolt. I may do this by routing a pattern into the bottom so they will fit snugly on the metal frame.
I mounted three ball-and-socket joints from MoonLite Telescope Accessories (MTA) to the wooden blocks, and another three to the secondary cage ring. These joints greatly simplify the design of the whole telescope, since they make the complex angles of the truss tube possible, are are easy to dis/assemble. The tubes themselves are aluminum, 1.25" in outer diameter, 0.05 thick, and are also from MTA. "Do you know what the @#$% you can do with an aluminum tube?"
The distance from the mirror center to the ball-and-socket joint seats is approximately 10.4", and they are about 7.58 degrees apart at that distance. This makes the truss intrusion 8.55", just larger than the aperture of the secondary cage. By truss intrusion, I mean the distance of the truss tube to mirror center at its closest in projection. Picture a hexagon inscribed in a circle - the midpoints of the sides of the hexagon are closer to the center of the circle than the vertices of the hexagon are.
Right now the tubes are all 6' in length, but I will shorten them by about 1' after I order the mirror. I am planning to buy a 16" f/4.5 mirror but the exact focal length has to be measured after fabrication, so I won't chop the tubes until then. I also need to drill the tube caps to seat them a little more securely.
I also made a focuser board, dimension 6 x 8", with a 2 & 3/8" diameter hole centered at 5 inches from the end. The board made is 13-ply "import birch" 3/4" plywood from Dunn Lumber, same as the secondary ring. It will eventually hold the MTA focuser I bought, but I haven't attached it yet. I'm waiting until the spider and mirror holder arrive before I do any more with the focuser board. I've ordered these from astrosystems.biz and they should be here in a couple of weeks.
The fasteners for these components are (mostly) all from McMaster-Carr, which I again will recommend for their extensive but easily navigable catalog.
The assembled truss tube and focuser board have a very light but rigid feel. I am eager to see how things improve after the two improvements to the wooden blocks and tube caps that I mentioned. After I install the spider/secondary mirror holder, I will probably be able to get a good estimate of the weight of the entire secondary cage, which I need to calculate the diameter of the trunions that I will build next!
