Optics: Spider and Secondary Mount

The spider and secondary mount began straight from the stellafane site but evolved as I chatted with my expert advisors. The mirror is mounted to a length of dowel. A bolt runs up the dowel and out and through a hub that is a milled cube of aluminum. The corners were milled out of the cube; so that from above it’s shaped like a plus sign. At the intersection of the plus sign a hole was drilled slightly larger in diameter than the bolt. Lengthwise along the arms of the hub, a slot was cut into which the legs of the spider could be inserted. A hole was then drilled and tapped into each arm of the hub to hold a set screw that would hold the spider leg in place. The legs of the spider are four halves of two hacksaw blades. At their far end each leg is attached to a hanger bolt. I drilled holes in the tube out through which the hanger bold comes. Because the length of the spider leg and hanger bolt is a little too long and because I wanted to better distribute the pressure on the tube caused by a tightening of the spider legs, I used thick fender washers on the outside of the tube. For collimation purposes, there is an expansion spring on the central bolt between the diagonal mounting dowel and the aluminum hub. Where the bolt emerges through the hub, there is a wing nut. The spring actually puts pressure on a thin fender washer on the dowel side and a thick fender washer on the hub side. Four holes were tapped in the thick fender washer that correspond to the cut out spaces between the arms of the hub. Long socket-head cap screws run through these taped holes and rest against the thin fender washer, which protects the wooden dowel. Adjustment of these screws allows for collimation. (The fourth screw is not necessary, but I have found it useful to adjust all four at various points.)

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