Stuart you are quite correct. There is a light pollution gradient from scatter due to particles and gases in the atmosphere. There are two contributions to the scattered light the Moon and streetlights etc. This LP gradient gets worse as you move awayfrom the zenith. If you sample this gradient with RGB and for a full night you will get lovely colours. The FOV is 3x2 degrees so this problem is much worse than with longr focal lengths.
I managed to get a new data set of O3 last night. Here is a quick repro using the new data 3MB
http://d1355990.i49.quadrahosting.co.../2070nbnew.jpg
I will recollect the HA even though it is not as badly compromised by LP as the O3. See the third image of 8min O3 data where the gradient is obvious
This 150mm f/4.25 achro is very fast so as well as getting the faint stuff it is also a very good LP detector even in narrow band. The image below shows how big this achro is. That is a 90mm mak on top used as the guidescope.
Bert