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I get increasing sky fog as exposure time increases. Depends upon a lot of factors: If it is cold; If target is at Zenith; If target has high declination (ie:+20 degrees); and ISO settings & exposure time.
For example. I find I can image up to zenith and past for around 1.5hrs for for lets say Helix Nebula (given it's declination) around a month or so ago. But after this it is travelling west towards city lights...so I have to stop as the sky fog increases dramatically.
See attached raw Helix image subs (no processing), both 300s...which one has more fog? As you can see I reached my limit, IMHO.
Of course I can process a lot of this fog out but there is a limit to what is possible. Same with my camera, better to try to get the best data I can. My limit is 5 mins unless it is very cold or dark or ..conditions right. You may get more than me..depends on a lot as mentioned.
So all in all..take lots of subs...try different sub times but try to keep them as short as practical.
I did have once see a web site that had users publish their sub exposure data and times on quite a few a few DSOs...but I cannot find that page...Damn!
Your image appeared as if you clipped out some data? Just a thought.
edit:
the images are slightly not exactly the same frame as I was manually dithering...what a pain that is. But Dithering is well worth it.
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