Here's a pic of the original wooden tracker with my DSLR attached:

It uses a stepper motor driven at a fixed 32-steps-per-second rate, courtesy of a 32kHz quartz crystal clock and frequency divider. The motor drives a nut along a curved, threaded rod, which opens the two halves at the required 15degrees-per-hour rate. The down-side is that the curved rod tends to waggle (the thread isn't very accurate) so very long exposure images are a little blurry. Overall accuracy is also limited by poor dimensional tolerance when drilling the holes for the bearings - the whole thing has to be large to minimise the tolerance effects.

Nevertheless, here's a pic I managed to take by stacking dozens of short(ish) exposures:

This is the benchmark to try and beat!