The Mirror Box
On to the mirror box. I had decided to use ½ Baltic Birch for the majority of the project. This was my first experience with this material and I must say that I was thrilled with it. Wonderful clean faces, no voids, and took tooling well. I am a convert. The only issue was tracking some down. I called all of the local lumber yards and home improvement centers without any luck. I finally came on the idea of calling custom cabinetmakers shops as they often utilize this product in their work. Sure enough, I found a shop that had some and would be happy to sell me a sheet. A 5x5' sheet ( the standard size ) was $23, what a bargain.
I was lucky to have access to a nicely equipped shop throughout this project. My brother gave me the run of his place and I took full advantage of his generous offer. I showed up one Saturday morning with my plywood and a cutting plan and we went to work. With his help we made short work of the cutting. Using the table saw ensured straight square cuts that got the woodworking portion of the project off to a good start.
We then gathered the parts for the mirror box, some bar clamps and did a dry assembly, checking for square. Happy with what we saw, we assembled the box using glue and 1.25" gun fired finish nails. We clamped everything down and left it to dry.
Once the box dried, I installed the mirror cell. I used bolts evenly spaced on all four sides, careful not to deform the box by over tightening them. I counter sunk the heads to make for a sleek appearance.
The Tubes and Clamps
The next consideration was the tubes and their clamps. I learned from the book, that to maintain the stiffness in the tubes I needed to pull off the two tube design, the tubes would need to be of a relatively large diameter. In exchanging emails with Peter, he advised that his tubes were of 2" OD and .065 wall thickness. As his scope had about the same focal length as mine, I adopted those as the specs for my tubes.
In order to provide the repeatability necessary to avoid big collimation challenges upon each reassembly, the tube holders had to provide a positive and repeatable seat for the tubes. Because of the diameter of the tubes, split blocks as used on many truss designs would not work. Instead, I used the channel approach advocated by Ravneberg on his Alice scope. Instead of wood channels however, I used 1.5" aluminum C channel. The tubes rest in these channels and are pulled down to a positive seating by a single bolt running through the side of the mirror box, channel and tube. It is very strong, stable and quick to assemble. The bolts are kept captive by a c clip on the threaded end of the bolt. I have a set of channels at each end of the tube, making for only 4 bolts and knobs to deal with during assembly.
I attached the channels to the inside of the mirror box with screws.
The Focuser Board
The focuser board is about as simple as it gets. The design called for the board to serve as a cover for the mirror box during storage and transport, so it is a simple square of ½' Baltic Birch sized to the inside dimensions of the mirror box (15"x15"). The focuser board also was equipped with a set of channels.
The Secondary Support
The unique nature of the upper assembly on this scope calls for a compromise on the design of the secondary spider. As we have no opposing wall to attach to, the spider must be stout enough to hold its shape and position with just two attachment points on the same plane. This calls for a thicker material than traditionally is used for a spider. In this case I used 1.5" by 1/8th" aluminum. In order to make the support to the proper size, I drew it out full size on a sheet of cardboard. I then took the material and using a vise and some handwork made the bends in the stock. Throughout the process I repeatedly removed it and compared it to the full size drawing in order to get the proper angles and shape. I was able to complete the shaping in about an hour.
To attach it to the focuser board, I inserted one bolt through from the face side of the focuser board, countersunk for appearance. I then further secured it with a screw at each end, in from the back side.
To attach the secondary mirror to the spider would require more help from the blacksmith. I had him attach a short plate bored through with a 3/8ths channel that would allow for some adjustment of secondary position.
The Secondary Holder
I had originally planned on buying a secondary holder from well known supplier. As the project went on, I began to feel that I could make my own and save the expense and that is what I did.
I assembled the entire holder from hardware store items and a piece of wood that I turned on my brother's wood lathe (a first for me by the way). In the end this was a really satisfying part of the project. I am proud to say that it works well and I saved about $40 doing it.
To attach the secondary to the holder I used Silicone adhesive. Using a technique learned from the web, I placed 3 dabs of silicone on the holder and positioned 2 nails laid flat across the face of the holder between the dabs. The process is to place the mirror and let the mirror settle to it's own level, finally resting on the nails. Once the silicone is set, simply slide out the nails. This process assures that no unnatural stresses are introduced to the mirror and silicone.
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