Rosette Nebula - Narrowband Imaging

One of the modern day challenges of astrophotography is how to deal with light pollution.  Lets say you go out and spend $10k-$20k on all the right equipment to get yourself to the same level as all the best amateurs in the hobby and night after night all your looking at is dull skies awash in light pollution. For you imagers within 5-10 miles of a major metro area, you know how painful this can be. I actually live within 5 miles of Manchester to the east and Nashua to the south, so I generally need to photograph away from those directions, directly overhead, west and north are good areas of the sky for me, however, every once in a while I want to shoot something that lies in that area of the sky that’s awash in light pollution.  Narrowband imaging to the rescue!  Fortunately, sky pollution is very predictable and so is the light from DSOs (deep sky objects) that I want to gather.  The subject photo in this blog entry is the rosette nebula which was located in the south west in some heavy light pollution when I photographed it. 

The problem of course with light pollution is that your camera can’t tell the difference between incoming light, be it from the DSO or from your local 7-11 lighting up the sky. With this particular image, I shot the rosette using two different filters, the first being a Baader-planetarium Hydrogen-alpha (HA) filter.  The HA filter isolates the glowing red of the nebula and blocks all other incoming light.  The second part of the photo was taken with a Baader-Planetarium Oxygen-3 (OIII) filter, which records the teal color of triply ionized oxygen. These two exposures were both about two hours each and allowed me to create grayscale images of these two dominant spectral lines of the rosette nebula.  Using photoshop, I used the OIII image for blue, the HA image for red and combined the two to create a synthetic green channel, these three “channels” (to use photoshop-speak) make up the RGB (red green blue) image attached.  Comparing this image to a rosette nebula shot at a very dark, light pollution free sight, there is very little difference other than the dominance in the image of the OIII (bluish-purple). This image is absolutely a false color image, but done so in a way to try and as realistically reproduce what is seen from a dark sky. 

This second image is of the same rosette nebula from a dark sky sight, notice the dominance of the red (hydrogen-alpha) which overwhelms the picture. It’s a matter of aesthetics as to which one would like more, but to me, the mere fact that I can continue to shoot from my backyard despite the light pollution makes the argument between false color and true color moot.

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Skyglow reduces the contrast between stars and galaxies in the sky and the sky itself, making it more difficult to detect fainter objects. Some astronomers use narrow-band "nebula filters" which only allow specific wavelengths of light commonly seen in nebulae, or broad-band "light pollution filters" which are designed to reduce the effects of light pollution by filtering out spectral lines commonly emitted by sodium- and mercury-vapor lamps, thus enhancing contrast and improving the view of dim objects such as galaxies and nebulae.