Last week I bought one more specimen on ebay, for $75.. This one is branded Rokinon, DX variant.
First star tests indicated it is better than my older one (Opteka DG), it seems it doesn't have so prominent coma in frame corners..
So I decided it will pay off to improve the focusing mechanism, by adding stepper motor.
The control circuit was developed with
Nico13 some 12~13 years ago, I built couple of them, and I am still using it :-)