Yay!
After much diagnosing I think I may have finally cracked this nut (See attached files).
The first image is of 4 clean images (i.e. minimal trailing) from the night. These were taken early on in the piece, circa 12am - all fine, no smears.
The next 3 images are clean and were taken later in the evening, circa 1.45am - again fine, no smears.
The third image is what happens when I stack all 7 together - notice the smearing starting to appear in the background! Note - During the session i actually took 25 subs, however I've only used the best 7 to minimise any other potential issues.
The fourth image is a GIF of the field rotation that's occurring. Normally this wouldn't bother the stacking, however when you look closely at the gif, you can see the background slowly beginning to smear.
1)
Now... I only took 7 darks during the night. I didn't start taking the darks until 4.40am. Somehow I think this is a major contributing factor? (each sub was 300sec at ISO 800)
2)
Something that's also bothering me is the field rotation. It appears to be happening around the SCP or something else out towards the upper left frame... not the DSO. So, my question is this - does field rotation (if poorly polar aligned) occur around the DSO, or will it cause the movement I'm seeing in my image?
As always, thanks in advance.
PS. Hoping to get to the bottom of this as it may also assist other beginners in the future
Cheers, Evan