Circular Pattern in Night Sky Image

I do not think this artifact is caused by reflections in the optical path. I think this is due to software correction of light falloff as a function of distance from image center (i.e., vignetting compensation). Normally such correction is not very noticeable because the signal-to-noise ratio (SNR) is high enough. But in this case, the image is so dark, that lifting the the image by a couple stops towards the corners greatly magnifies the noise in those regions as well.

Notice that the noise at the top, above the outer ring, and in the bottom center and lower right, is substantially more pronounced ("grainier") than in the center. There is nothing optically that would cause radially-biased digital noise. That has to be a result of software increasing the signal level (i.e., digital multiplication). The upper-right and upper-left corners outside of the ring especially show a lot more noise. Optically, these regions should be darkest due to vignetting, so their SNR is lowest. After digitally boosting the corners for light falloff, the only signal there is just boosted noise.