Big Bertha Is Still Too Heavy

Big Bertha Is Still Too Heavy

I mentioned back in June that I should clearly have used a Serrurier truss, instead of my own clever (?) design. While adding turnbuckles and wires gave Big Bertha sufficient stiffness to hold collimation, it is still too heavy for the Celestron CI-700 mount. The CI-700 had a nominal weight capacity of 60 pounds, and Big Bertha is right at 60 pounds--but that's part of why this is a nominal weight capacity.

When this became apparent a few months ago, I thought of selling the CI-700 mount, and buying a Losmandy Titan, which has a nominal capacity of 100 pounds, and should be more than sufficient. But a Losmandy Titan new costs about $6000. That was a sufficiently breathtaking amount of money that I didn't just run right out and buy one--and then my job at HP evaporated. So I went back to asking, "How can I knock some more weight off of Big Bertha?" Something closer to 50 pounds.

In the meantime, carbon fiber composite materials have become easier to find, and cheaper (at least for off the shelf components). Dragonplate, for example, sells square carbon fiber composite tubes from which I could construct a Serrurier truss. But the more that I looked at this, the more that I liked the idea of buying off the shelf parts for this. Moonlite Telescope Accessories sells connectors and poles for exactly this purpose. I did the math, and concluded that I could replace my current Frankenstein collection of parts with a total of six poles--keeping at least that part of the aluminum channel that bolts the telescope to the mounting plate (which also adds stiffness to that part of the telescope with the most deflection problem).

I'm not quite sure how to calculate the stiffness of a Serrurier truss, but I am quite sure that because of the diagonals, it is stiffer (probably substantially stiffer) than the same tubes parallel to the optical axis, as I have now. Even parallel to the optical axis, the six 1" aluminum tubes would give maximum deflection of 0.00251" for the heavy (mirror) end of the telescope--and the total weight would be about 52 pounds (which was about my original goal for Big Bertha).

Most attractive of using these off the shelf parts is that they are designed for quick assembly and disassembly. I could turn six bolts at the top of the scope, and six bolts at the bottom, and end up with two fairly short assemblies that could be put in the trunk of almost any passenger car. The six poles are five feet long, but can be put into almost any front seat without problem. It also simplifies putting Big Bertha onto a mount. The lower end will weigh about 35 pounds, and is small enough to pick up and handle by myself. Once located on the mount, I would bolt the poles in place, install the upper assembly (which should weigh less than ten pounds), and tighten down the bolts.

I'll scratch my head about this for a while, then look to see if I can find carbon fiber composite tubes that would be lighter than the aluminum tubes--even knocking 2-3 pounds off the total weight would be a win.