Ivo,
Quote:
Yes, so for your luminance data you are working with 1channelx3minutesx3exposures, while for your RGB data you are working with 3channelsx3minutesx3exposures. This is why the luminance data that can be derived from your RGB data is approx. 3x better (not taking into account filter response - the real figure is probably a bit worse). 
We could probably create better aggregate luminance data by making a weighted average of 0.75x the luminance data from your RGB set and 0.25x the luminance data from the luminance-only frame... 
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Yes Ivo,
I see what you mean now.
There wasn't enough data for a top pic & it was from
an outer suburb of Melbourne with light pollution.
The transmission curves for the filters is here:
http://i262.photobucket.com/albums/i...RGBfilters.jpg
You can see a large hole between red & green which helps
to knock out the 2 sodium lines from street lamps.
The luminance filter is picking up all the light pollution which doesn't help.
Your sceond attempt looks a lot better in the center of the nebula
but really needed a vector mask so that it didn't sharpen the less brighter areas.
cheers
Allan