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I've also started getting similar rings since switching 2" nosepieces from a Baader to a Moonlite nosepiece. In my case, I think it's internal reflections off the Moonlite nosepiece as it's quite shiny (the Baader is much duller). It completely disappears with flat frames, so I'm not terribly worried about it.
Flat frames are easy to take using the "white T-shirt method". A couple of hours after dawn / before dusk, point the scope at a patch of clear sky in the opposite direction to the sun, place a white T-shirt or towel over the scope, and adjust your exposure duration until you get a middle-histogram exposure. Take a reasonable number of exposures (say 30+). It'd be a good idea to take "dark flats" too - same as normal darks, but with equal duration to the flat frames.
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