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Hi, Bo,
That's really pretty.
The blue gradient toward the right is subtle. We notice it stops at a sharp edge about 10% from the right hand side. That suggests that it is due to the scope not quite pointing to the same spot on different nights (a very good thing, as it helps with all kinds of other problems), and there being slightly different conditions on the two nights (could be darks, temperature, altitude of the object, moon or sky glow, etc etc).
As mentioned, it's not so very severe, but if it bothered you, the trick would be to normalize the background to be the same, perhaps even down to removing the gradients on a per-sub basis, before stacking. Certainly set the zero point on the histogram very accurately for each channel of each image before stacking.
One again, a lovely image.
Best,
M & T
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