F for flat frames

On a rare clear night, I went out to try and shoot Orion widefield with my X-T3 and my newly acquired 85mm F1.4 on my tripod. I used 2 second shutter speed to prevent star trail, ISO 500 and F1.4 (of course). Shot 430 light frames. Initially I did not process the images with flat frames and there is a green tinge in the middle of the image after stretching.

I searched online and discovered that some people on cloudynights forum were reporting the same problem. But luckily the solution seems to be pretty easy: use flat frames when stacking. So I took my lens, attached the lens hood and just pasted it to my monitor that is displaying an uniformly grey background. Set my camera to aperture priority with 0EV exposure compensation at F1.4 and snapped a few shots. The vignetting/light fall off in the corners are super obvious just from the picture previews. I added the flat frames and proceeded to re-stack the images.

When stretching the final image, I no longer experience the green tinge and I can recover more details as well! Sadly I lost the raw files and cannot do a before/after comparison, but at least we all learnt something new. Flat frames are actually really important :).

In the image you can see the Flame nebula, Horse Head nebula, Running Man nebula and the Orion nebula (which looks really pretty imo)- from left to right.

The dark blue circles around bright stars is due to my lens not being apochromatic. This can be solved by stepping down from F1.4 to slower F stops, but I would be losing precious photons (since my shutter speed is kind of fixed) so I opted not to do that.