Also, I wonder if the backfocus is exact? When my Esprit 120 is set up, the focal length as solved from actual images works out at ~860mm, not the spec'd 840mm. This is for the total scope/flattener setup, of course, but I've never had issues to bother with figuring out which was to blame.
It's a bit out of the box, but maybe you want to dial it in by spending an evening dialing in a v-curve by shooting 1mm increments in back focus and seeing which one gives the best results?
Markus
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