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Hi Peter,
Definitely check for light leaks as previously suggested by others. Bias frames will still be black due to their short exposure so if you do have light leaks, they won’t show up on the bias frames.
I had a light leak issue and found the sources by looping 1 second exposures and shining a torch around the OTA. For my Newtonian, the weak spots were the rear end and the focuser. Once I fixed these, I never got those weird patterns again and can stretch my images so much more with no trace of leaks.
Also for flats, is your camera orientation exactly the same? I learnt the hard way that it must be the same.
I hope you fix your issues soon.
Regards,
Stéphane
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