Got the tracking correct, finally.

What was it?.. a miscalculation.. no.

mis-specced/counted motor/worm.. no.

straight lack of understanding.. no.

pure and simple foul up.. that'll be it.

The software has a sin lookup table for generating the microstepping, it used to use a 128 sample 1/4 sine ramp to generate it, but I found that a 150 sample 1/4 sine ramp improved theoretical performance. I had changed the lookup table without error, then forgot to change the software that turns that 1/4 wave ramp into sine and cosine waves:BangHead:

end result was a 'choppy sinewave' running 150/128 too fast.

At least it was a simple fix. So tried some pics and got Jupiter, M1 and the trapesium. Tracking still dragging in fits and spurts, found the worm was slipping against the shaft coupler, so need a new and better one... but at least the images are starting to work.