"Iris" NGC 7023

Defocus?

It looks like a tilt of the matrix relative to the optics.

Well, I like this better. I don't know if the ICQ interface shows this or not, but there, when you hover over an object in the atlas, you position a red square, and when the camera aligns, it matches with a blue square. For me, one corner of the squares' frames matches, but the other three are out of sync, just like in geometry when two planes don't lie flush against each other. I'll attach a link now, I took a screenshot.

This frame doesn't say anything. It's just the program displaying line overlays. Use Siril or pixel analysis of the images and check the tilt of the sensor.

Understood, thank you.

I'm talking about them.

Here is the link to the raw FITS files and a random selection of individual frames — they are better visible there.

Dunno, maybe I overdid the saturation, I'm still not used to processing in Siril, also the stretched image looks more overstretched in the interface than it actually is, there's probably something somewhere that needs to be adjusted