This is a curved-bolt (AKA curved-rod) variation of a barn-door mount (tracker) that I made in 2002 and used with my 35mm film cameras. It uses a 10-32 threaded rod bent into a curve to increase the separation between the two boards at a constant rate (a straight rod wouldn't keep a constant rate). A gear driven by a 1 rpm 12VDC electric motor through an identical gear turns on the bolt threads, increasing the separation at the required sidereal rate. It performed decently with lenses up to 135mm, allowing well-tracked 10 minute exposures. But I had no such luck when I tried 200mm. Either the job I had done bending the threaded rod to the required radius was not sufficiently accurate or the extra weight of the 200mm lens was too much for the somewhat undersized ball head. In any case, today I do completely successful tracked shots with lenses up to 200mm without a tracker at all. I use Astrotracer, which is built in to my Pentax K-3II DSLR. Ain't technology wonderful?
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