As Greg suggested, would be interesting to see some calibration frames. Also some lights much shorter than 30 mins. Was that sub taken with a filter? What sort? Maybe your light pollution was bad enough to completely saturate the frame in 30 mins or perhaps there is a light leak somewhere? How are you converting the FITS file? Might be better to view the original FITS directly.
I wouldn't jump to any conclusions on the basis of a single 30 minute sub...
Cheers,
Rick.
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