Wow! That was the exact part I was looking for Scott so thankyou. I may give it a go yet but am trying a different strategy at the moment. The roller-coupled-to-encoder idea was a backup plan for me anyway. Putting long, unwieldly encoder arms on it are a last resort for me, but possible. BTW, I am useless with tools too but still built it - thanks to Dave Kriege's
The Dobsonian Telescope book and the internet.
Bojan how accurate is your system? It looks a bit more heavily engineered than what I had in mind. Also the ALT roller would need to ride on the ebony star laminate of the ALT bearings - which is very slippery.
Glenhuon, the timing belt and toothed pulley approach would not slip but is not possible on the ALT bearing at least.
A DSC manufacturer informed me that they tried very hard to engineer a friction roller-coupled-to-encoder approach for dobs but that it always ended up with slippage errors.
Next time I design a scope, I'll factor in Goto from the beginning.
Tim