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Field flatness and illuminated imaging area would be the two key points.
Field flatness can often be achieved through external adapters but illuniated imaging area is something you cant correct without changing your telescope parts.
Take a standard visual 8" Newt, most have a small secondary of around 50mm, whereas an imaging Newt will have a secondary of around 70mm or greater, creating a larger illumunated area for your CCD to minimise vignetting.
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