Martyk22 So I believe I should actually use a 7mm spacer to bring the total backfocus to 56.
Yes, you are correct. And I do not see why you cannot use the OAG you are proposing as long as the thread sizes work out.
If your reducer requires a 55mm back focus, and you are adding a 3mm thick filter, the filter will add an additional 1mm to the optical path. So, the mechanical distance (metal flange to metal flange) would need to be increased by 1mm (one third the thickness of glass if you assume that the refractive index of glass is about 1.5).
See page 8 of this thesis:
https://www.researchgate.net/publication/260399858_Useful_Estimations_and_Rules_of_Thumb_for_Optomechanics
(I recommend downloading the entire 3 MB pdf document instead of just reading page 8, since it is a gold mine of information.)
Be aware though that not all all filters are 3mm thick. The IDAS 48mm and 52mm filters are 3mm thick; so is the 48mm and 52mm Radian Quad band filter. But many other manufacturers use thinner glass (especially for smaller filters). Just divide the glass thickness by three and you will be close enough.
Most of ZWO's diagrams do not take the filter glass into account (or they are ignorant of it), and cannot be trusted. When you use the ZWO diagrams that includes a filter wheel or filter drawer, you need to add an additional 1mm spacer (assuming 3mm thick glass filter) to move the camera an extra mm away from the telescope metal back if you plan on inserting a filter.
As a final test, just look at the corners of the plates to fine tune the spacer since there is a small tolerance in the flange to sensor distance in ZWO's camera (most obvious at the corners of the images -- also a good way to detect sensor tilt):
Chen
