Canon 5D3 + 200mm f/2.8 lens 2015 Nov 6
Red = hydrogen (H-alpha) Green/blue = oxygen (OIII)
This is a composite of two images. Hydrogen is shown by the H-alpha filter. Another narrowband filter for a specific wavelength in the green part of the spectrum detects light from doubly ionised oxygen.
We are seeing a shock front resulting from the explosion of a star a few tens of thousands of years ago. Gas is being compressed in a ring that is expanding from the explosion and parts of it are being ionised by radiation from stars near it. That makes it light up as electrons recombine with atoms.
This nebula is difficult to photograph in normal light even from a dark site, so I was very pleased to be able to do this from suburbia with narrowband filters.