A while ago I imaged Corona Australis with my Canon 5D Mark III on a Orion 8" telescope. (All CCDs and modded Cameras were busy shooting at the time).
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I was curious to see what the superb sensor would do on the fast Orion 8” Astrograph – I imaged 12x 300sec @ ISO800 of this target. Due to the full frame sensor I was concerned about vignetting. My husband, Cory Schmitz's method of shooting flats post-imaging worked well, and corrected the uneven illumination extremely effectively, some coma left in the left edge but tolerable.
Check out the blog he wrote for a step-by-step guide how to shoot effective flat frames! It really helped my data!
Shooting flats seems difficult, but once you figure it out it’s easy and very reproducible. Flat frames fix vignetting and brightness imperfections and can really save your astro photos.