This is a curved-bolt (AKA curved-rod) variation of a barn-door tracker that I made in 1998 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. Power for the motor is provided by two 9-volt batteries in series through an LM317 adjustable regulator. The regulator is adjusted to achieve a precise 1 rpm motor rate under load. If I were building this today I would probably use a stepper motor instead of a DC motor, but the circuitry would be a lot more complex. Note that the two hinges I used contain nylon bushings and are totally free of any play, which is important for consistent tracking (by maintaining polar alignment). The tracker performs decently with lenses up to 135mm, allowing tracked exposures up to 10 minutes, which is the longest exposure time I tried and which is unnecessarily long now with digital cameras.
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