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I've got an older C8 which I use with a f6.3 reducer corrector.
I don't think it's a collimation issue, I think its backfocus.
When the backfocus is incorrect, I see the same type of out of focus donuts as in your image. If you focus you'll see coma increasing towards the edge of field.
I'd check the backfocus measurements and remember that filters affect backfocus distance, so you'll need to correct for that. Try increasing backfocus and see if the donuts change.
That's my best guess.
Chris
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