Your flattener will have instructions for teaching 55mm of backfocus. In this case, it is the base adjustment implied by the adapter. Your backfocus is consumed in part by the flange distance of your mount (Sony E Mount) and the rest needs to be consumed by an adapter. In this case, the simplest solution would be a T-mount adapter, which extends from an astronomy-standard thread to Sony E Mount and consumes 55mm of backfocus in the process. That flattener will probably have T/M42 threads or maybe Wide T/M48 threads. The later is best for full frame. (Flattener offers M48 threads, I just saw.)
That said, I would not expect great correction from that flattener even with backfocus dialed in ideally. But, since you own it, you can experiment. It is a bit bizarre that it offers such a backfocus range when the flattener itself should define what is relatively ideal. And they don’t define an ideal starting point relative to telescope characteristics, that I noticed, so you will probably need to start at 55mm and adjust if that isn’t suitable (by adding, since you can only subtract with a short E Mount adapter).
If all else fails, this is the flattener paired to your telescope:
(Even with a quality flattener and perfect backfocus there will likely still be some noticeable concessions in star shape and rendering on the outside of the frame, and fairly significantly light falloff which will require proper flats to correct.)
Edited by James Peirce, 30 November 2022 - 04:56 PM.