Why does my background look like this?

[Nerf_Caching]

Hi all. I tried photographing the Bode's and Cigar galaxy last night under Bortle 9 skies, but my final stacked image had a really mottled background after gradient subtraction in siril. Even before gradient removal, my background still looked quite horrible. Can anybody offer a possible explanation for this? My 30-second subs were quite washed out in the beginning but got a little bit better as the target rose above the skyglow of my area. Any help would be appreciated. Thanks!

[Miguel1983]

Hi, 

did you use dither ? 

[Nerf_Caching]

1 hour ago, Miguel1983 said:

Hi, 

did you use dither ? 

No I did not dither as I used a star adventurer.

[jimjam11]

The stripes are walking noise most likely caused by not dithering as Miguel said.

The inconsistent (not flat) background can have a number of causes, especially if your setup has light leaks or reflections which makes background subtraction much more difficult.

I don't know anything about siril,  but if I got a background like this I would he looking for samples placed too close to stars in the dark areas which are then fooling the background modelling prior to subtraction.

APP can do a decent job but the nuclear option is something called a synthetic flat in PI. This will flatten the background although I don't think it will help with the walking noise:

http://trappedphotons.com/blog/?p=756

[lock042]

Hello. In Siril you can remove the background on each light.

It is really better than removing after stacking.

[lock042]

Oh and looks like you use an old version of Siril.