Guitar Machine Head Spider Tensioners

Spider Tensioners
I decided on a wire spider when there was a discussion about them on the Amateur Telescope Makers' list, about the time I was finding my tube and finishing up my mirror.  Several people had discussed using guitar strings for the wire (they're handily packaged, reasonably priced, stainless steel, and come in a wide range of precision diameters), and I remember at least one person who wasn't impressed with using guitar tuners for the tensioners.

I, in contrast, rather like them.  These adjustments, arranged in pairs at three radial locations, allow me to adjust the centering, tilt, and even the fore and aft position of the diagonal; only rotation has to be handled by other means (and doesn't need adjustment anything like as often as the regular collimation).  One key point, however, is to use sturdy machine heads designed for steel strings; the lightweight goods made for nylon strings simply won't hold up when you put enough tension on the wire to suspend a diagonal with reasonable rigidity as well as positioning it in the tube with the required precision.

As you can probably tell from the images, the tensioners consist of short lengths of aluminum angle with the machine heads mounted to them with small machine screws and nuts.  These photos don't show the sheet metal screws penetratign through the tube, but they do -- and hold the tensioners firmly in place.  You can see the two holes, 1/4" diameter, that serve to feed through the guitar strings (which themselves are invisible in the right hand photo due to lighting).  Unfortunately, due to adjustments required after the holes were drilled, the holes are rather too close together; next time I have the diagonal out, I plan to use my nibbler to convert the holes into slightly longer slots and free the wires from interference with the tube.

There's one other significant drawback to using any kind of tensioned spider: the tension has a tendency to pull the tube out of round, which makes for a rather esthetically annoying effect which is, fortunately, optically insignificant.

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