With the FSQ-85, with any sensor size larger than APS-C, you need to purchase the special Takahashi 1.01x flattener to achieve full frame flat field. Otherwise, edge stars will be slightly elongated in all corners.
Under APS-C size, the field is DEAD FLAT.
The flattener can be tricky to get (I do have one, but don't use it seeing I am under APS-C, only slightly
but have no plans selling it) - best to order it and wait if you intend to use a large sensor camera