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What sort of lens?
Fast lenses are very likely to have significant coma, like Sigma Art F1.4 lenses except for the 40mm. Wide angle lenses are also tough to get to work, like a 14mm. The longer the focal length of the lens the easier it should be.
It sounds a bit like the same issue with quad TCC and an AP refractor with these IMX455 cameras. The smaller pixels are showing up optical aberrations larger pixelled cameras don't show up. Roland Christen did a post recently on the AP users group explaining this.
Short answer, I would experiment with spacing just like you dial in a flattener or a reducer and the resulting ideal point may be a surprisingly large spacing difference.
My Quad TCC seems best at 113mm yet manufacturers recommendation is about 104 including the filters difference.
Speaking of that did you allow to add the difference for the filters and cover glass and chamber glass to the backfocus?
Greg.
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