The Cocoon Nebula and Dark Nebula Barnard 168
July 28th, 2011
Well that was a long wait for a clear night, what an unproductive three months for astrophotography but at least the front garden looks nice in it’s new sandstone paving! Sunday night finally delivered the required combination of clear skies, less than half a moon and a following morning I didn’t need to get up to early so I set up and finally got to have a go at a widefield image of the Cocoon nebula and the dark nebula which seems to stream away from it.
The Cocoon itself is a combination or emission and reflection nebulosity, the dark nebula that stretchs out from it is a cloud of dense dust blocking out the light from the stars behind it.
Imaging Equipment
- William Optics ZS66SD
- William Optics MkII Field Flattener
- Canon EOS 1000D (modified)
- Astronomic CLS CCD Filter
Guiding and Mount
- Orion Optics 250mm F4.7 Reflector
- Opticstar PL-131C Camera
- IR/UV Cut Filter
- Celestron CGEM Mount
- PHD Guiding
Exposures
- 44 x 5 minutes
- 15 x bias
- 15 x dark
- 15 x flat
The images were calibrated, aligned, stacked and processed in Pixinsight.
With the processing of this image I took the opportunity to try some new processing techniques within Pixinsight with the aim of improving noise reduction and getting better colour from the image. You can see some of my other attempts from the NGC and Other Catalogs gallery page. The first image is from my standard technique which ended up with a poor colour balance, the second was better in that respect but suffered from noise issues especially in the regions of the dark nebula.
In the version shown on this page I used the following techniques to try and get a better result, the links will take you to the Pixinsight forum post which discusses them :
Wavelet-based noise reduction with the ATrousWaveletTransform tool
I have always used the ACDNR tool in Pixinsight before to manage noise reduction post histogram stretch / sharpening when the image is in a non-linear state. Noise reduction from this alternative newer method however can be applied to a linear image and works as many tools do in Pixinsight by working different structure size layers. This means your noise reduction can be tailored to match the level of noise you have, hopefully that way avoiding blotchy backgrounds where larger areas of noise have been let through as structure.
I was very happy with the result once I had spent some time on trial and error to achieve the best result for my image.
RGB Re-Combination using Linear Fit
Technically I am pretty sure this is not for us single shot colour DSLR imagers but I cannot complain at the result. The technique here is to do a rough stretch of your Luminance data (I used an extracted lightness image) that looks about right, then match that stretch on your RGB images before applying Linear Fit tool to the RGB to match them against a reference image. I still had to use the SCNR tool to remove excess green from the final image but I have to call myself happy with the end result. Colour seems easier to manage, and the red of the Cocoon is closer to red than my normal orange. It is not scientific and probably not right for my data but as an experiment into alternative processing I like the result.
Hopefully the wait between new images can be a bit shorted this time!
