Saturday, July 19, 2014

Narrowband Images of M27

After several weeks of waiting I finally have a working filter wheel for my SBIG camera.  I was hoping to buy another SBIG filter wheel but unfortunately they no longer make the CW10 filter wheel for my camera.  So I opted to go with a third party filter wheel from Xagyl Astronomy.  Initially there was a problem with the clearance of the filters and I had to send it back.  However after waiting a few weeks it arrived and worked perfectly!  So despite the short nights and relatively poor weather I was anxious to try it out.  Below are the first images of M27.

 Narrowband Imaging

15 min HA filter
If you`re not familar with the concept behind narrowband imaging its fairly straightforward.  All elements have a characteristic set of colours they emit that depends on their electron configurations.  What that means is that certain gases will glow in a very specific colour band.  By using a filter that is tuned to that colour astronomers avoid any stray light.  In my case, since my observatory is in a suburban environment, that is a huge benefit.  Once you have the images of several colours (which corresponds to particular elements) the images are combine in photoshop or some other processing software.

15 min OIII filter
There are some drawbacks however; because several different images are combine together you need to take images through three or four different filters.  This can increase the exposure time.  However, I recently read an article that said even though you increase the exposure time the SNR (signal to noise ratio) of each individual image is actually better than it would be for a single, colour image.  I haven`t tested that myself yet but so far I like the images I can get from light polluted skies!

15 min SII filter
Each of the images represent one of the monochrome Sulfur (SII), Oxygen (OIII), and Hydrogen (HA) images.  Each image was stacked from roughly 15 minutes of images.  In total I only got five 3 minute images before I had to hit the sack.  When the weather clears up I plan on imaging M27 again with longer exposures (and better tracking) to bring out more detail and less noise.

 Once the images are gathered I processed each on individually by stretching the histograms so they were all more or less equal.  Nebulosity has several useful tools for aligning and stacking each image. After processing I combine the images using the Hubble Colour Palette.

This maps the Sulfur image into the Red Colour Channel, the Hydrogen Alpha image in the Green Colour channel and the Oxygen III image in the Blue Colour Channel. The result is a nice colour image with the characteristic teal and gold colours of Hubble images.
Hubble Colour Palette

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