Hi Mike, finding guide stars can be a bit of a pain on some objects with the 120 colour, (the ASI 120 mono would be a lot better) sometimes I have to move the object off center to find a good guide star.
The hardest part in setting up the thin oag is getting the right spacers so that both cameras are at focus, but once that is done you don't have to worry about that again. With my DSLR (1100D crop sensor) I found it easier to move the oag prism right in and adjust the guide camera spacing for focus with a thin spacer. With my camera I only needed a 5mm spacer and the thin locking ring from the variable spacer in the kit to get the 120mc to the same focus as the 1100D.
https://www.bintel.com.au/product/bi...v=6cc98ba2045f
Rick