Supernova remnant in Cassiopeia.
This target is most difficult to capture and process.
I never realise how dim this really is. "Cartes Du Ciel" gives surface brightness
for this object as low as Mag 37 !
Fast optics, Tokina 300mm @ f2.8 helps to capture those few photons.
There is some internal structure details visible in S-II channel,
they can be seen in Hubble palette image as red.
More information in Wikipedia: http://en.wikipedia.org/wiki/Cassiopeia_A
There is two version of this image here, first image is in "natural" color composition
from narrowband channels, H-alpha 100% + 35%S-II = RED, O-III = GREEN and
O-III 100% + 30%H-alpha = BLUE.
Second version is Hubble palette composition. S-II =RED, H-alpha = GREEN and O-III = BLUE
Imaging data: Camera, QHY8 - Filters, Baader 7nm H-alpha, Baader 8,5nm O-III and Baader 8nm S-II - Optics, Tokina AT-X 300mm @ f2.8 - Exposures, 12X 1200s H-alpha, 1 X 1200 O-III and 6X1200s S-II + flats and bias - Guiding, LX200 GPS 12" + PHD-guiding and Lodestar