Please check the included gif-anim
The images are part of 180x360 panorama. I have total 32 columns of 16 images. They all overlap at Polaris. The bight star rotating is Polaris. As you see all images have grading, darkest at the bottom, no matter what way camera is poiting.
Here is the same gifanim, processed the same way but with the flats turned 180 degrees:
I’m sure your eyes are more trained than mine to see what is going on here, but for me it looks like the grading is gone and that the sky is little darker at oposite side of polaris. The darker area seems to rotate with camera. Location was very dark place in mountains, very cold winter night with great visibility. No light pollution, but the sky can be a little brighter looking north due to aurora borealis.
Maybe the flats are not upside down as it seems to me, but there may be some other source of confusion? Maybe my flats are just bad somehow? But I’m getting the same results with 2 sets of flats. One set made inside house, white walls, white ceiling, camera in white lightbox, lens covered white piece of plastic. The other flat set is made outside a foggy day, with lens covered by white tshirt.
Any ideas?