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Here's a couple of shots and my understanding of what's going on as I eventually fixed it.
1_ obviously field rotation. Everything rotates in reference to the guide star in this case in the keyhole vicinity. (10min exposures). Crisp arc lines. No coma, just field rotation. Combination of field rotation + coma makes stars on the edges look like squares. Still digging out an old tarantula shot. (that's why I said it's a bad choice to test)
2_ Over-correction. MPCC is too far. Stars are bright from the edge blurring inwards (inverted comet). T shape on an ED80 is more pronounced the further out you are.
3_ Collimation. Your field is like a horse saddle. Some of it is perfect some of it is worse. No uniformity. Primary/secondary/focuser axis offset.
4_ Coma (under correction). Stars are bright from the center blurring outwards (comet shape)
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