Hi John, thanks for coming back so quickly. Yes indeed an awesome project. Basically I am aiming to build this: http://www.amazon.com/iOptron-3302B-SkyTracker-Camera-Mount/dp/B00EFRN1GE for a fraction of the costs and enjoying the project time with my son whilst building and using it!
I am aiming to drive one of these motors (undecided):
https://www.pololu.com/product/2267 Nema 17, 200 step, 1.7 amp per phase
https://www.sparkfun.com/products/10846 Nema 17, 400 step, 1.7 amp per phase
https://www.sparkfun.com/products/10847 Nema 23, 200 step, 2 amp per phase
using a vertical shaft worm gear with a gear ratio of 30:1, 120:1 180:1 or 360:1 (again to be decided depending on motor power and internal friction....working that)
To get basically to one revolution per astronomical day. The motor will be running somewhere between 4 and 12 minutes per revolution. So micro stepping is important and smooth operation is everything as a jerking motion will spoil the pictures!
As motor driver I am looking at the following options (see hook up schemes attached). The choice will be determined by whether we think we can get this to work. I can also chicken out and not risk it and just buy an arduino, but that is the last resort.
Big_easy_driver_HookupFixed available via spark fun
Pololu DRV 8825
and the EV3 connected up via the dexter break out board. At the moment I am thinking of using the 4.3V line to power the motor board, have a separate 12 V motor supply and I am hoping that I can use the SCL and SDA ports as the 2 interface boards as per the simplified connection schemes attached to this post for step and dir.
So would this all work and would the EV3 see this as a motor or do I need to build a dedicated I/O block.
As you can see I did my research but I am really reaching the limit of my understanding of electronics here....thanks for the effort and help.