Referring to the FS60CB system chart and use of the flattener.
Having followed the diagram and all combinations of adapters / elements shown, stars in all corners are eggy. More like long grain rice?
Thinking this through. Given that there is no way of attaching the flattener backwards, it seems that the distance between the flattener and camera rotator / focus tube and / or flattener and camera is important - though, the SD ring is optional, which leaves distance to the camera.
Worse, could the flattener elements have moved? Is that likely? I do not recall dropping the flattener and there are no rattles.
Looking around, this question has been asked before, one way or another. And, I'm not sure why the documented setup doesn't work for me. Takahashi specs cite 56mm back focus. Now back focus appears to be as indicated in the diagram. However, another source suggests BF is measured from the back of the flattener. I'll measure it all up and see what gives.
Edit: 'Metal back focus' is the distance from the union of the flattener and camera adapter to the photographic plane.
Where are all the TAK experts on this? Certainly sounds like spacing, at least on any other scope it might be. You could try increasing the spacing a bit at a time. Most reducer correctors seem to have a tolerance range of at least +-2mm. A variable spacing ring is a nice tool for this sort of fiddling.
I may have asked too soon. Added metal back focus table which includes a definition. It's just the way the information is presented. It doesn't matter whether a spacer is used ahead of the flattener. Providing there is sufficient focus travel, which gives me an idea to minimize camera tilt.