I usually don’t use flat frames, but this time there is visible vignetting ruining the photo. If I don’t use flat, the artifact/noise does not appear. It shows only after stacking, regardless of the method. Camera is Nikon Z6, tracked, but unguided with bad alignment (I don’t see north pole from my balcony). Flat is taken by pointing the camera to LCD screen (brightness reduced) with same settings, t-shirt over the lens and only 1/400s exposure (surprisingly, longer exposures cause overcompensation and bright corners, took me several days and >100 flats to figure that out). It happens regardless of whether I use bias frame or not. Noise is present on all 3 channels, especially blue. Siril is configured to use 16 bit FITs and compression.
I’ve read in the past on this forum and elsewhere that it’s the camera induced correlated noise when alignment is near-perfect and to use dithering, but in my case alignment is far from perfect and it shows only when subtracting flat. Flat frames itself look ok, min value is 1072, max 5497, mean and median ~1260 (I can upload if someone wants to look at it) which is somewhat low, however no such pattern is visible. Hmm, now when I think, could it be possible that, when stacking pre-processed light frames, we’re also stacking (now negative) flats which are always at the same position and thus correlated and that dithering would need to be applied while subtracting flats? Perhaps I need to stack many more flats, currently I have 5-10 of them.