Technically, there is no backfocus requirement for the Redcat 51 so I'm not sure what you mean by achieving the "56 mm back focus."
Thus, you don't need to have a fixed spacing between the Redcat 51 and the camera's sensor, whatever spacing that brings the scope to focus with an adjustment to the helical focuser is just fine.
That said, if after adding the OAG and filter wheel you can't reach infinity focus (not enough inward travel on the helical focuser) then that means you have too much spacing between the Redcat and the camera (i.e. there is a limited range of spacing that can exist between the end of the Redcat and the point of infinity focus which varies upon the position of the helical focuser). So, it's likely that you need to find something that can be removed to shorten that spacing.
However, I'd be certain that you are testing the focus correctly before you begin any changes. Are you doing this testing during the day on a VERY distant object? Sometimes it can be hard to find focus when you have nothing but stars to go by.
What you probably want is a spacing that allows you to reach focus when the helical is near to its mid-point, or (better?) at a point where it is nearly fully retracted. Of coarse, if you have very little spacing between the Redcat and the camera then there is nothing particularly wrong with having to extend the helical to its full outward travel.
In any case, the Redcat offers anywhere from 59.7 to 77mm of available spacing between itself and the point of focus depending upon whether you are using its M48 or M56 threads. So, worse case (and if you can't remove anything else) then perhaps you can find a shorter M56 to M42 adapter to sit between the Redcat and your OAG. However, I'm not certain where the helical was positioned when WO determined those 59.7 to 77mm of available spacing (or it you want to call it so, the "backfocus").
Edited by james7ca, 20 March 2023 - 06:41 PM.