Amateur telescope making by Stephen F Tonkin

By Stephen F Tonkin

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Sample text

The extra 13 lb on only one side of the optical tube assembly of Skinflint created the instability. Counterweighting the side opposite the focuser board with lead shot in PVC tubing eliminated the problem. The weight hangs by a wire hook on the open end of the mirror box. Assembly and Use Before an observing session, to get a head start on equilibration, I leave the mirror on the front porch in its pizza box carrying case. Order of assembly is no different from that for any other truss system. I install the mirror only after the focuser board is in place.

In those earlier projects, the optical tube assembly was completed in advance before figuring. However, a bad case of laziness prevented me from making the optical tube assembly for this project in advance. Astigmatism is something that I have avoided in the past with other projects, but I had never attempted a thin, large mirror until this project. Astigmatism is avoidable in thin mirrors and I will redouble my efforts to eliminate it in the future. In hindsight, I believe the astigmatism was detected in the Ross Null test used in the Ronchi mode.

The bottom of the rocker and front board are 1/2 in (12 mm) birch plywood. The top of each side board has a curved cut conforming to the curve of the side bearings. The pine board frame was deep enough to prevent cutting into the foam panel. Two pieces of Teflon per each curved section allow the side bearings to pivot smoothly in altitude. A piece of FRP covers the bottom of the rocker assembly. This in turn sits on the ground board, held by a through-bolt. 5 in (38 mm) thick foam surrounded by a pine frame of equal height.

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