Here’s a “cool” looking image for all the hot summer days we are having right now! A really amazing object in the southern sky in the constellation Canis Major (the Big Dog), is a big gas bubble cataloged as Sharpless 308. It has a popular (but unofficial) name of the Dolphin Nebula. It’s in the Sharpless catalog because of it’s Hydrogen Beta emissions, but it is especially bright in ionized Oxygen III. This image is about two hours of exposure time through a 3nm Chroma OIII narrowband filter. It’s amazing that we can photographically identify elements this way, but the OIII atoms emit a very specific wavelength of light, and this filter let’s only that wavelength through (well, plus or minus 1.5 nanometers!).
The central star is pre-supernova and is responsible for blowing off all this gas, and is about 4,530 light years from Earth.
Artistically, this was a tough image for me. I took it back in February of 2023 at the Winter Star Party and only now (late July) am I satisfied to publish it. I did take about 1/2 hour of RGB data to give the stars color, but this is basically just an Oxygen emission image of this target. I went back and forth on the coloring. OIII is Cyan (blue/green), but of course the light is far too dim to see. What color is something that’s invisible? I settled on this, and finally even decided I liked the smoky background oxygen that is glowing around/behind the main focus of the image.
Gear used:
Sky-Watcher Esprit 150 refractor
Night Crawler Focuser
Player One Poseidon-M Camera w/Phoenix Filter Wheel
Chroma 3nm OIII narrowband filter
Software Bisque Paramount MYT
30 minutes RGB exposure time
2 Hours OIII exposure time