Hi Niall,
you'll find this is a controversial topic depending on where it gets discussed. Try asking about this in the autoguiding Y! group
- you have a smaller field of view and because you're looking at the edges of your field of view you can have more distorted star shapes than a guide scope would present. I ran into this with my C11.
- Getting both the imaging and guide cam to focus at the same time can be challenging depending on the OAG model.
- longer FL numbers mean the field of view of your guide cam is small and less light per star hits it. So you are restricted in two ways:
- fewer stars to choose from
- dimmer stars
both these restrictions mean you may have to rotate your camera to make sure you get a bright enough star on your guide cam. The guide star dictates image framing in that case. Not desirable.
Image train gets more complicated and you may run out of backfocus space. Or the spacing between flattener and sensor is compromised.
Anyway, it all depends on needs and wants. I can get away without an OAG right now. If I have to I would use an OAG but not for fun, only because it's the only available option to get the image quality I desire.