Our skies are filled with nebulae. The nebulae of interest in this tutorial are huge regions of gas – gas clouds. The nebulae I’ll discuss here are called emission nebulae. We image emission nebulae by means of the light from the surrounding gas cloud which is ionized by radiation from one or more nearby stars.
The most common emission nebulae are of the hydrogen emission type. They emit a very narrow band of red light, and with the appropriate filter (Hydrogen alpha, or H-a), the camera sees essentially only the nebula light, and moon light and terrestrial light pollution is nearly completely eliminated.
Imaging with H-a levels the imaging playing field. No matter where you are imaging from, with an H-a filter you have the equivalent of magnitude 8 super dark skies! Honest!
Pros of H-a Imaging
• Eliminates light pollution
• Background is now inky black – backgrounds have a zero or nearly zero black point
• Stars are much dimmer, minimizing star bloat, chromatic aberration and “fat” stars.
Cons of H-a Imaging
• Very little signal is passed by the filter, so imaging times have to be fairly long – five minute subs are possible if your camera has very low readout noise
• Autoguiding is probably required
• DLSRs without mods don’t have much sensitivity to the H-a light
Here’s an example of 2 hours and 15 minutes in nine 15-minute subframes
http://home.att.net/~dpersyk/new.htm
The target is NGC 281, a/k/a the “Pac Man” nebula and Sharpless 2-184. I used a Takahashi astrograph, f/3.3 and Starlight Xpress SXV-H9 camera. The filter was a 6 nm FWHM H-a. Most processing was done with ImagesPlus. The false color was added in Photoshop with Noel Carboni’s actions. The Hubble employes false color, so I don’t feel like I cheated.
I recommend H-a imaging to everyone who is plagued by sky pollution. No longer will you have to journey to a dark site to get good images.
Clear skies,
Dennis Persyk
Igloo (Now Roll-Off) Observatory Home Page http://dpersyk.home.att.net
Hampshire, IL
The most common emission nebulae are of the hydrogen emission type. They emit a very narrow band of red light, and with the appropriate filter (Hydrogen alpha, or H-a), the camera sees essentially only the nebula light, and moon light and terrestrial light pollution is nearly completely eliminated.
Imaging with H-a levels the imaging playing field. No matter where you are imaging from, with an H-a filter you have the equivalent of magnitude 8 super dark skies! Honest!
Pros of H-a Imaging
• Eliminates light pollution
• Background is now inky black – backgrounds have a zero or nearly zero black point
• Stars are much dimmer, minimizing star bloat, chromatic aberration and “fat” stars.
Cons of H-a Imaging
• Very little signal is passed by the filter, so imaging times have to be fairly long – five minute subs are possible if your camera has very low readout noise
• Autoguiding is probably required
• DLSRs without mods don’t have much sensitivity to the H-a light
Here’s an example of 2 hours and 15 minutes in nine 15-minute subframes
http://home.att.net/~dpersyk/new.htm
The target is NGC 281, a/k/a the “Pac Man” nebula and Sharpless 2-184. I used a Takahashi astrograph, f/3.3 and Starlight Xpress SXV-H9 camera. The filter was a 6 nm FWHM H-a. Most processing was done with ImagesPlus. The false color was added in Photoshop with Noel Carboni’s actions. The Hubble employes false color, so I don’t feel like I cheated.
I recommend H-a imaging to everyone who is plagued by sky pollution. No longer will you have to journey to a dark site to get good images.
Clear skies,
Dennis Persyk
Igloo (Now Roll-Off) Observatory Home Page http://dpersyk.home.att.net
Hampshire, IL
