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The black ring is vignetting, and flats should correct it if you can get your flats right. I have 5d MKII and an 8" newt. I notice that with the full frame you can't get the coma corrected across the entire chip, or at least I couldn't. If you extend the mPcc out it gets worse, it's like it needs to be closer to the chip than you can get.
If you can find a solution I'd be all ears.
Thanks
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