ASI 2600mm with EFW and OAG

Biggy

Hello, I read the connection guide for the ASI 2600 mm to have the 55 mm back focus.
I have an open telescope at f/5.6 (ASKAR FRA600), the connection with the new EFW 36 mm and M68 OAG works very well, because visibly it's for a telescope up to f/4.2. If I put the focal reducer, I'm f/3.9 and the connection no longer works.
To keep the same configuration, do I have to switch with 2 inches EFW wheel to avoid vignetting?

I'd like to keep the same configuration when I switch from one setup to another

Excuse me for my English. I'm French
Thanks

Biggy

Tech@ZWO

The solution we provide is for the situation without the reducer. Let's see if others have better ideas about this.

Biggy

I understand, but if i pass with 2 inches EFW, does it work at f/3.9 ?

Tech@ZWO

Biggy
It does work at F3.9 with 2" filter. But the thing is with reducer, there is no need to refer to 55mm back focus requirement in the first place. So we are not sure if it still works with current solution.

jrrjrr

Biggy

The online calculator at this link should help you find the right answer:

https://astronomy.tools/calculators/ccd_filter_size