Pete light pollution and gradients are the bane of wide field imagers. Especially with fast optics.
The best invention ever is DynamicBackgroundExtraction in PixInsight.
I used PI DBE on your large image. Here is the result.800k
http://d1355990.i49.quadrahosting.co...07/PeteSMC.jpg
This was done at eight bits and stretched so not ideal.
DBE works much better on 16bit and unstretched data. If you want give me the data and I will use DBE on it for you.
Bert