i have 6 narrowband frames of the rosette nebula (O3) and the top left corners are inconsistently vignetted. when i do a BPP to preprocess them, it turns out weird and as if the flat i took didnt really do anything. i'm using darks and biases as well.
here are the 'raw' frames:
here are the registered/calibrated frames from BPP:
here's the master flat for my O3 filter:
and when they are aligned and stacked, they produce a severely vignetted image:
any advice?
