Update:
Well due to a very busy schedule, I had not got much time to put into this until this weekend.
This weekend I pretty much finished everything. I had to go back to basics a bit as the original circuit I used just did not provide enough torque to drive/hold the focusser. However that is not all resolved and today I finally installed the prototype unit on my focusser and it seems to work beautifully. Just need a chance to test it under the stars now!
I learned a lot while doing this, primarily that an ungeared stepper will never provide enough torque to drive a loaded focusser. I used some surplus gears to provide a 3:1 reduction (and thus same increase in torque). Connecting this to the 1:10 fine focus dial on my GSO type focusser provides ample torque (since the entire assembly thus provides a 30:1 increase). If you were to attach this to the non geared focus dial you would have to provide a similar gear box (33:1 gearboxes are readily available and should work fine.)
The new and inproved circuit is a bit sweet even if I do say so myself. I included a trimmer (variable resistor) so that the user can adjust the torque to provide just enough torque to drive the focuser but not draw too much current unnecessarily. In my test and without any load the focuser draws 400mA in holding and slewing mode. Not a lot really.
The circuit is now based around bipolar stepper motor. The bipolar motor provides better torque characteristics with less current draw. Bipolar motors are cheap and only have four wires here's the motor I used;
www.rapidonline.com/Electrical-Power/Fan...12V/73192/kw/37-0507
The ASCOM driver package is now complete and tested with Maxim DL and Focusmax. I just need to provide a small application so that the functions of the focusser (connect, slew in and slew out, and positioning functions) can be accessed without the need for 3rd party software. I've tested it inside Maxim DL and FocusMax and it seems to work well reliably. Writing the ASCOM driver was also a big (but useful) learning curve). Not quite as simple as I thought it would be. But thats all done now!
The last thing is that I used a PCB for the prototype. It might be a bit tricky to make up on stripboard. I will be able to provide artwork and simple instructions so you can make the PCB with a maplins PCB kit. However I suspect I'm going to have to provide a PCB service to most people who want to build one of these.
The last thing I need to to (after testing under the stars) is provide detailed documentation for the operation of the unit. and also write up the article on building the thing yourself.
The tricky bit for anyone building this will; be how to mecvhanically attach the thing to your particular focusser. That will be a problem that you may have to solve yourself (of course I will be able to offer advice) but with the range of focusers out there it will, of course, be impossible to cover every angle. You will need a decent focusser to use this to its full potential of course. This type of focusser I built is known as an 'Absolute Positioning' focusser. This means it has the capability to return the focus to a very precise location repeatedly. To acheive this the focusser must have some sort of reference. I acheived this by using a microswitch installed so that it is pressed when the focusser reaches its inner limit. With this known the focusser can then return to any point since it now knows exactly where it is. Of course with cheaper plastic focusers this might not be possible since the accuracy of the focusser might not be good enough. Certainly with my GSO (crayford focusser) the unit repositions to an accuracy better than my 0.05 resolution digital calipers can measure.
This of course presents problems for SCT users since there is no mechanical means to place a microswitch so that it is pressed when the focus reaches a predetermined point. This requires some thought and any ideas here will be most welcome.
Well thats the latest news on this. I expect to have something on the website about this next week.
  