This is Sh2-135, another obscure nebula from the Sharpless catalog.  It’s almost entirely red because it’s almost entirely made up of hydrogen – and even the H-alpha signal is pretty weak.  The brightest part of it (near the center) also contains a bit of Sulfur, which I have assigned to blue-green, so it is leaning very slightly toward magenta, but still looks pretty much red.  In summary, the hydrogen was weak, the sulfur was extremely weak, and the oxygen was non-existent!  I integrated about 11 hours of exposure time here, including the O-III, but it would look pretty much the […]