Thanks for the detailed explanation, lot's of post processing going on there 
And now I noticed the wavelength filters: Hα - OIII - Hβ (must read better next time 
)
So 3 wavelengths and the rest of the light is blocked out.
Indeed, inferential narrow band filter was used; light pass for 7~8nm around the frequency, all is block out.
Plus the 5D2 used is also modified, the internal IR-cut filter is not the original one; replaced by a full spectrum Astrodon filter (400~700nm):
This is better if you want to use special filters because the original Canon IR-cut block more or less some wavelength, so you loose a lot of _interesting_ informations.
Sometimes I make time lapses of the night sky, with maximum exposure times of 15 seconds (I don't have a tracking mount...)
I live nearby a big city, so lot's of sodium street lights over here.
Do you think I can benefit from a clip in (clicks in DSLR in front of the mirror) type of this filter ?
http://www.astronomik.com/en/visual-filters/uhc-e-filter.html
Of course I don't expect a filter like that too give results like the photo you posted (For that I need a tracking mount and lot's of knowledge in post processing astro pictures)
EOS clip filters are very interesting; you can use them with a lot of optics/lens (specific astronomical filters are design to be used only with astronomical optics because of the angular light flux issue). But I never used EOS XL clip filter for full frames camera yet (I see you use a 6D). It should be done next months. It looks tricky, because of the mirror (block in up position?).
The UHC-E filter is adapted if you have a very polluted sky, it gives very good results. All sodium (high and low pressure lamps) emissions are blocked.
But if the sky is moderately polluted, IDAS LPS gives better results because this filter preserve a kind of spectrum continuum (for RVB imaging in one shot) but some sodium emissions are not blocked.
In images:
Left to right: no filter, LPS, UHC
Did try to capture the summer milky way a few months ago:
https://drive.google.com/folderview?id=0B1BxGc3dfMDaSkNlZ0ZtaHN4Ym8&usp=sharing
Although not bad, it could use a lot more contrast(and I already added a lot contrast in Lightroom in this example).
Not bad! We can see NGC7000 Nebula, like in the image top of this thread
Your sky doesn't look so polluted in fact. If you are in Europe I know good pollution maps, if you need to have a idea.
You have used Dual ISO on a such a long exposure? It worked?! Can you share a CR2?
Yes, I did.
There's a lot of hot pixels without cooling, and we can't use the cr2hdr hot pixels removing in this case (because of stars) so it's a bit tricky, but it works. With a good cooling it works better, of course (it's better than in regular photography actually).
But the major issue is the spacial resolution loose, important in astrophotography.
Here it was not a problem because it was not for an high end imaging, just for filter tests (we can see horrible reflections in OIII band) and I planned to bin + resample since the beginning, so..
I'll see to PM you CR2 ASAP (I'm not front of my home computer for couple of days).
Beside, the most important stuff in my eyes is the ADTG/DFE Gain + black offset + white clipping definition, drive the Canon sensor like a specialized/scientific/astronomical camera is enormous. I'm thinking to make a ML module with special tweaks for astronomy (and some specialized functions like FWHM map assist in liveview), just need to find time to do a public friendly version.
