Richard Shagam:
It will be interesting to see how it works out. I've always been kind of dubious of OAG--how do you get enough, if any stars in the field of view. Would an ASI120mini mm work ok for the guider? My initial trials with the MiniCam were with a vintage orange tube C8, and I hardly got enough stars in that field of view to even do stacking properly. (I know, a C8 is totally inappropriate for a sensor the size of the IM585, but I had to try, given I had just inherited the OTA.)
I've actually done more trial and error than research, so there may be a way to calculate the exact ratio and sizes, but I've learnt a few things the hard way:
1. ASI174MM and ZWO OAG does not work
The sensor of the ASI174MM is too large at 11.3x7.1mm for the ZWO OAG, which has a prism size of 8mmx8mm. The vignetting is too severe to allow effective guiding.
2. ASI17MM and Celestron OAG works
The 12.5mmx12.5mm prism of the Celestron OAG provides enough light (FoV in a sens I guess) for the ASI174mm.
3: Celestron SCT field correction
I have a Celestron C9.25 XLT (without field correction) and the stars are extremely distorted at the edge where the OAG prism picks off the light. PHD2 does an incredible job, but the stars, well, don't look much like stars at all.
Based om my experience, the ASI120MM Mini should work, as it has a tiny sensor at 4.8x3.6mm. It shoudl get enough light. There may be issues with the star distortion depending on your focal length and field correction. One option may be to run the guide camera in bin 2.
