You are correct that Back focus is your issue, that is entirely dependent upon your telescope specs and where the scopes back focus point is, it has very little to do with the OAG, or cameras as they don’t determine focus.
so most importantly, what scope do you have?
you first set you back focus between the back of the scope and the main camera sensor, ignore the OAG. You need to achieve perfect main camera backfocus so you image is not abnormally shaped and then you need to achieve main camera focus via the telescopes focus mechanism. Don’t mix or combine setting backfocus and focus. They are two very different things, but yes it is confusing when starting out. I highly reccomend doing some reading up on it or looking through other forums like cloudynights where this is discussed at length. Only after all of that do you then make minor changes to the OAG’s back focus and then focus points to bring everything into focus. So in total you have 4 focuses to achiece in this order:
- scope to main camera back focus
- scope to main camera focus
- OAG back focus
- OAG focus
This will take time to get right, so be patient and there is a lot to learn, you likely don’t have the right spacing either and need to buy some additional stuff. It took me many many hours to learn, understand and get it right so it might take a few nights out and some adjustments along the way
