My project for Saturday, March 18, 2006, was to arrive at a final design for the truss tube sockets to connect the truss assembly to my mirror box. My initial intention was to use an assembly similar to that found on StarMaster telescopes as seen in my article on mirror box design. However, after I cut out the light baffle and placed it on the mirror box, the truss assembly base idea seemed awkward. The middle of the base would have to be narrow (to allow for the dust cover to be left in place while the truss assemblies are mounted). Perhaps the end result would have been acceptable, but it did not have a good quality feel in my estimation. So I went back to the drawing board. Literally.
This close up shows the L. S. Starrett label on a #251 steel beam trammel that belonged to my grandfather. Starrett still makes the same instrument. My grandfather’s trammel languished unused in an old tool box for decades. The first thing I did this morning was clean it up and get it operational. From now on it will take its rightful place in my precision measuring and layout kit.
I studied the truss pole sockets of Achim Strnad’s 20-inch Ursus scope and also Stathis Kafalis’s 24-inch Kyklopas. I wasn’t about to allow truss sockets in the mirror box itself — the idea of manipulating hardware in close proximity to the primary mirror (in the dark!) is not my idea of a fun or safe practice. In this respect, Achim’s design (which places the sockets above the light baffle where the truss can be assembled with the dust cover on) represents the better approach in my opinion. 
But the requirement that my mirror box be no more than 9-inches in height to fit in the external storage compartment on our RV meant that I would not have room to incorporate the truss pole sockets below the top edge of my mirror box.
I considered building a second, shallow box, say three or four inches in height, incorporating the four corner truss pole sockets that would bolt onto the main mirror box. The box would have a circular cutout large enough to leave the dust cover in place while mounting it on the primary box. This would be easy to remove for RV travel but could be left in place to simplify setup when using the scope in my backyard or going to dark sky sites with my pickup truck. After thinking about this option for a while, I thought I would try eliminating the box altogether and making the truss socket assemblies themselves bolt onto the mirror box. The shallow box design might be slightly stiffer, but independent socket assemblies would be easier to transport and ought to be stiff enough.
The above image shows the angle drilling jig I made for my drill press. The jig is required to drill the angled tube holes in the socket assembly. I made the jig on Monday and Tuesday evenings at the beginning of the week. Wednesday I had dinner with the Unnotables (astronomy buddies), Thursday I had my guitar lesson, and Friday Elly and I had pizza and a movie — so progress on the scope was halted until this morning.
This picture shows a full-scale layout drawing of the mirror box top on my workbench. I learned the value of such drawings when working on the tailgate assembly. I cut the paper from a large roll of brown Kraft paper (which is very easy to erase, has lots of different uses, and is inexpensive). The drawing consists of a box marking the outer edges of the mirror box, which is 28.25 inches per side. To that I added a 26-inch circle representing the dustcover. Then I started messing around with the truss socket assembly shape. I wanted something large enough to be stable on the mirror box when bolted in place. It also had to allow space for four mounting bolts, a central compression bolt, and the truss tube sockets themselves. Here is a close up of the socket assembly drawing.
One advantage of a full scale drawing like this is you can place the parts directly onto the drawing for layout, or cut pieces out for patterns. In this case, I mounted a scrap of 3/4-inch Baltic birch plywood over the center mark on the drawing, placed a piece of 3/4-inch stock in position on the corner, and used the steel beam trammel to scribe the correctly-sized arc on the stock. Incidentally, the corner scraps from the cage ring cutouts were plenty large enough for socket assembly stock. Since I did not have a pencil holder for the trammel, I chucked an ordinary wood pencil in my drill press and sanded it down to fit the trammel. I plan to order a Starrett pencil holder for the trammel.
At this point I interrupted work for a trip to Metals by the Foot to investigate socket assembly hardware — I wanted to look at various socket cap screws. When I returned, Elly and I ran some errands — a trip to Barnes and Noble on the Plaza (which was a waste of time) and then a trip to Sam’s Club for necessary kitchen supplies (cheese and wine!).
I resumed work in the late afternoon around 4:00 PM, cutting out three pieces of Baltic birch plywood matching my pattern layout. I also made a trip to Strasser for further hardware hunting and gathering. I have decided to use 5/16-inch by 3.5 inch socket cap screws for the mirror box truss assemblies and 2-inch 5/16 socket cap screws for the secondary cage sockets. Plastic knobs that snap onto socket cap screws are readily available for finger tightening, but rather than use these I plan to get a Tee-handle Allen wrench. I use a similar wrench when assembling my CI-700 mount and find it much easier to work with than finger knobs. Standard socket cap screws are also a lot less expensive than commercial finger knobs, which cost around $6 each! While at Strasser I picked up some stainless steel tee-nuts (they only had six, unfortunately, not enough for the whole project) and eight compression springs — enough for both the mirror box and secondary cage sockets.
This picture shows the socket assembly parts on one corner of the mirror box. I am thinking about recessing the four screws that attach the assembly to the box flush with the surface of the socket assembly but have not made up my mind on that score. One problem with doing so is that the recesses would be a perfect collecting point for dew.
Here is a shot of my work table at the end of the day with the hardware resulting from two hunting and gathering missions.
Work on Sunday
I had a lot of errands to run on Sunday, but I still got my first truss socket assembly prototype completed. Here are a couple pictures showing it in position on the mirror box.
It is not bolted down or anything. I just wanted to see how it would fasten into the corner bracing inside the mirror box (which I have not constructed yet). Gluing the pieces up turned out to be easier than I had thought. I used TiteBond III, an excellent water-proof wood glue that is a pleasure to work with. To hold the pieces in position and clamp them while the glue set I used the socket cap screws. I kept them clamped for about 45 minutes, and then let the assembly harden and dry for a couple more hours (when I was running around). In the late afternoon I cleaned up the edges on my stationary sander and an oscillating spindle sander. Then I drilled out the center hole, and cut the center section out on my band saw. Here are a couple more close up images of the assembly.
I used a compression spring to hold the center section open for each pole insertion and a stainless steel tee-nut for the compression screw. The overall design seems quite good to me. It is easy to construct, operates very reliably, and a small amount of tension on the clamping screw fixes the truss poles rigidly in place.
I decided that the assembly was too small, however. The hold down screws were positioned too near the mirror box side to allow for tee-nuts to be conveniently located in the interior corner bracing. In addition, the corner bracket screws interfere with the socket assembly hold down screws. So I drew a second, larger assembly design and put that together during the week. Here is a picture of the second prototype shown next to the first.
The second version solved the problem of the hold down screws being too close to the side of the mirror box, but I had to move the sockets in from the corner and space them farther apart to allow room for the corner hold down screws. I decided to build a third (and hopefully final!) prototype, eliminating the corner hold down screws so the pole sockets could be moved closer together and closer to the corner. I screwed the first and second prototypes to a scrap piece of board in approximately the same position they would be on the actual mirror box. I experimented with using just two hold down screws instead of four and decided two would be plenty rigid, especially with the larger socket assembly design.
This may seem like a lot of back and forth in designing the assemblies. A more experienced draftsman might have been able to work out all the details up front on paper. But the materials were inexpensive — just scraps from cutting out my cage rings, actually — and the fabrication was simple enough. Making the prototypes helped me work out some kinks in the design that I probably would not have resolved without the experimentation.
Here are a couple of pictures showing the third design layout.
