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Hey Matt, not a bad image considering the conditions!
"onion rings" are a symptom that will show up in any camera under the right conditions - that happen when the camera doesn't have enough dynamic range to represent the data and a lot of pixels all end up on the same value.
It's most prevalent on 8 bit data, cause you only have 256 values available from black to white, and when some of your image is very dim (ie the limb of Jupiter) then a lot of the pixels will end up getting the same brightness value, and after you stack them you get areas where the pixels are exactly the same value instead of slightly different.
Monitors can show the same problem, take a look at any dim image on an lcd monitor, you'll find a viewing angle where the image looks awful.
Best ways to avoid it is to increase the brightness of the subject (eg longer shutter) or switch to a camera that can capture in 12 bit, pretty much any of the more expensive cameras like the dragongly2 or SkyNyx.
Hope that makes sense :-)
cheers, Bird
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