Rosette Nebula - a "semi" BiColor Image

February 1, 2017

The Rosette Nebula - Semi-BiColor

 Altair Astro Imaging Newtonian Telescope 200/800 f4 on EQ6-PRO GOTO Mount and Autoguiding with Orion StarShoot Autoguider. Camera: modified Canon EOS 700D DSLR Camera with Astronomik CLS Light Pollution filter. 35x5min, ISO 800 - semi-OIII-Ha-BiColor image technique


The Rosette Nebula


The Rosette Nebula is a large, spherical H II region located near one end of a giant molecular cloud in the Monoceros region of the Milky Way Galaxy. The open cluster NGC 2244 (big bright stars in the center of the image) is closely associated with the nebulosity, the stars of the cluster having been formed from the nebula's matter. The cluster and nebula lie at a distance of some 5,000 light-years from Earth and measure roughly 50 light years in diameter. The mass of the nebula is estimated to be around 10,000 solar masses. The nebula consists of extensive doubly ionized oxygen (also known as O III) regions around the NGC 2244 cluster. 


About the image

As I have already mentioned in my previous article, the new moon at the end of January this year was very productive. I could shot the Great Orion Nebula and the Bode´s Galaxy Group too. My last planned project for this period was the Rosette Nebula, one of the most beautiful, brightest and biggest nebula of the winter-time Milky Way. For the focal length of my Telescope (800mm) it is a perfect object to shoot. Unfortunately just like in case of my Orion series this part of the southern sky is only limited available from our backyard for astrophotography. Our house stands on the eastern side and a bigger tree limits the view from west. The celestial objects in the gap between them are available for photography only about for 3 hours. I still prefer to do single-night projects and the Rosette Nebula can already show beautiful colours and details with my equipment in a 3-hours-exposure project. 

The weather was first only on the 28th of January clear enough to perform the planned image acquisition. The only problem was that on this weekend I was on call. My plan was to setup the telescope, start the shooting and if I have to go back to work, the system will perform the entire process without any intervention. Fortunately the duty was uneventful -at least for me- so I could acquire all of the raw images without interruption. The present gear was enough stable to achieve a 100% success rate during shooting, therefore at the end of this session I had exactly 3 hours of exposures.


Because of a busy Sunday duty I could start with the digital post-processing first only on Monday. The initial version of the image was already pretty satisfactory: 


The Rosette Nebula - standard RGB

 Altair Astro Imaging Newtonian Telescope 200/800 f4 on EQ6-PRO GOTO Mount and Autoguiding with Orion StarShoot Autoguider. Camera: modified Canon EOS 700D DSLR Camera with Astronomik CLS Light Pollution filter. 35x5min, ISO 800


The natural color-range of this image due to the applied CLS light pollution filter and the relatively moderate exposure time shows mainly some shades of red. As I mentioned above, this object consists of mainly H II clouds but around the NGC 2244 star cluster also O III regions. In a more professional setup due to the application of monochrome CCD cameras and narrowband photographic filters, letting through only this portion of visible light, it is possible to create beautiful, so-called color-mapped images. During post-processing it is necessary to colorise these separate images, usually the H II image in red and the O III image in blue. If we mix these images and create an artificial green channel from the existing red and blue ones the final result will be a synthetic RGB image. This technique is the so called "BiColor H II - O III". 


If we look at the spectral characteristic of my Astronomik CLS Filter one could easily recognise that the filter is nearly a combination of a H II (656.3 nm) and a O III filter (500.7 nm). The UHC Filters are designed to achieve exactly this but the CLS Filters have broader characteristic.


 source:, modified


During the post-processing I tried to create an artificial H II monochrome image from the red channel of the original RGB image and an artificial O III image from the original blue channel. After that the processing was the same as it was described above in case of BiColor H II - O III technique. The result was of course only partly usable therefore I mixed this BiColor version with my original RGB image. In the final "semi-BiColor" picture it was already possible to recognise the location of the extensive H II and O III regions, of course within the mentioned technical limits.











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