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Stretched, I can see massive differences in sub alignment (as you say "overlapped rectangular frames at various angles"). You have some serious movement in your image train between subs, almost as if the camera is loose and flopping around (rotation and position), but only between subs, not during exposure. Are you bumping the scope or moving stuff between exposures?.
And, what was the ISO?, 15mins is a long exposure for a DSLR, yes low noise, but the bright stars look saturated.
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