My main camera setup is ZWO ASI1600MM-pro, ZWO EFW-7, 12.97 mm spacer, Orion TOAG, AstroTech FR/FL which requires 55 mm backfocus. The light path with this setup is 56 mm and the stars with the main camera are round and sharp. So the main light path is correct.
I am using a ZWO ASI290mm for a guide camera. I am not sure what the back focus is for the ASI290mm camera. I have seen 12.5 mm or 8 mm of back focus for the guide camera.
The light path from the AstroTech FR/FL is 9.5mm to the prism and the light path from the prism is 31 mm from the prism to the base of the guide camera platform. This looks similar to the ZWO OAG. SO this gives a light path of approximately 41.5 mm. SO I would need 13-14 mm to achieve 55 mm back focus for the FR/FL or do I need to take in account the required back focus also of the ASI290mm guide camera?
The guide camera came with a 25mm (1.25") extender if I use it to compensate for the back focus of ASI290mm (assuming 12.5mm) plus the 13-14 mm additional required for the FR/FL; I can not achieve focus of the ASI290mm
The Orin TOAG is similar in measurement to the ZWO OAG.
What have other users done to achieve focus on the guide port with a ASI290mm
I would appreciate any help - Thanks