FWIW....There was an article on how to build a 'Automated Stepped Focuser Unit' for just such a procedure, back in Sept. '98 in US Sky & Telescope, by Mr. Ernie Piini. He used a simple base board to hold the camera & the drive assy., with limit switches at either end to stop & reverse it, etc etc etc. (Pic. explains more). The electronics controlled the stepper motor, time interval & how much the focus changed in each step.... Was very good set-up. Bit rough around the edges but gave very nice results.
See attached for few details...
Another interesting little project for those so inclined.....
L.