After using the focuser for a time I found the small stepper motor had insufficient torque to turn the threaded shafts on the coldest nights.
I have been planning the filter wheel for a while and the focuser would have to cope with the added weight too.
The backlash in the gearbox was akward to manually compensate for by always moving the focuser in the same final direction.
A dc motor and encoder to provide feedback was chosen.

ST7 ccd camera / filter wheel mounted on the focuser which is attached to a f 6.3 focal reducer.
The encoder is the small black part under the camera head.

Focus control application.

The Mechanics'
I have changed the reduction of the gearbox to 250:1 and replaced the stepper motor with a dc motor.

The Electronics'
A miniture rotary encoder provides 500 pulses per rev.
An Atmel 80c2051 micro-controller is instructed via a rs232 link to a pc running a simple focus application.
Focus can be changed by either clicking on the in/out buttons or by typing in a position in the blank window next to the target position.
The control buttons set the amount of focus change for each mouse click.
The bottom blank window shows the raw rs232 comms to / from the focuser. Helped debugging only.
 
 
Focusing with the modified head.
Focusing is much the same, only slightly faster, and my desk is less cluttered having removed the hand controller.
The encoder gives 0.0025mm resolution and is directly mounted on the shaft so that the gearbox backlash is automatically removed. This has been a major improvement.
When imaging at f6.3 on most nights I can detect correct focus to with in about +- 30 counts which equates to +-0.07mm.
Further changes to the focus application will be to add RGB focus offset buttons for each filter and also to emulate one of the auto-focuser protocols on the market to allow standard software to provide auto-focus ability with my hardware. Maybe I get around to adding temperature compensation too.