I got a Starizona 0.65 Reducer for my 100 ED. The nominal backfocus distance should be 58 mm. I tested a lot, but I got always elongated stars in the corners, which indicates, the camera (APS-C-IMX571) is too close. I am now at 59.5 mm and still get elongated stars. Has anyone found a working distance with this combo?

Clear skies,