It looks like a clone stamp tool was ran in a squiggly line.
My guess would be an issue with your flats?
Oscar:
I need to know some more things and might be able to help
1. I'd like to know whether this artifact and the other you mention that are in all of the RGB frames, are in very different places, or the same/similar places (if you were to combine RGB, how would the artifacts look then?)
2. I'd like to see your rejection frames
3. I'd like to see your flats
the more info you can give, the better
I hope I can help solve this
Daniele Borsari:
Mayebe a slowly moving asteroid that was partially rejected?
Die Launische Diva:
I agree with @Daniele Borsari, this is something slow-moving which was partially rejected, like an asteroid or a comet. My best guess is that we have a moving comet here. I have seen such artefacts when I integrate light frames which include a comet and its head. Blink your registered frames and see if there is something appear moving at that specific region.
Riccardo Civati:
the Answer of Daniele Borsari is correct. is a small objects that moves. you can eliminate this in the image integration parameter
Rodrigo Roesch:
It looks like there is a slow moving object that the center has been removed with a rejection algorithm during stacking such as sigma clipping. I have seen that during comet stacking if those algorithms are usedi
Oskari Nikkinen:
Try stacking without rejection to see what the smudge looks like. If its a comet or an asteroid it should turn into a line instead of this weird looking smudge. (definitely looks like a rejection remnant of something moving).
You could also try stacking the dataset in split groups, for example stack first 50 subs into an image, next 50 into another etc. You might see the object move between these partial stacks. If it is an object that is, could be an insect or a boring piece of dust that is moving.
