M 51, also known as the Tourbillon galaxy (Whirlpool Galaxy in English) is a couple of galaxies located around 27.4 million light years in the constellation of Hunting dogs.
This famous couple is composed of a regular spiral galaxy massive whose diameter is estimated at 100 000 light years and a small irregular galaxy .
Spring star, it is one of the most famous galaxies of astrophotographers, with very pronounced colors and presenting beautiful nebulae, accentuated here by the joint use of a Halpha Hydrogen filter.
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Project M27: Stéphane Gonzalez (Exaxe), Sébastien Kuenlin, Mathieu Guinot. November 2021
Click to view the full resolution image (4096x4096) on Astrobin!
Some time after the finalization of my M27 image in summer 2021, made with Newton 250mm and ASI2600mm camera in 9:40 pm long exposures through HO and RGB filters, I came across the superb high resolution image produced by Stephane Gonzales (Exaxe ) in lucky imaging (short exposures) with a Newton 300mm and Playerone Neptune color II camera (Halpha: 3600x4s and 5000x2.5s / RGB and IR: 40000x500ms).
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I then decided to contact Stéphane to offer him to mix our two images and thus benefit from its high resolution on the heart of the nebula thanks to its short exposures in Halpha and IR, and from the signal / noise ratio obtained on the extensions of the nebula. thanks to the long exposures on my HORGB image to create a composite image making the most of both images.
Crops of our two images showing the difference in resolution of details at the heart of M27 (click on Stéphane's image to access full resolution on his Astrobin gallery).
Stephane is up for the project and therefore sends me his luminance in Halpha + IR so that I can start tests to embed his image in mine.
Having already tried this type of experiments with other astrophotographers, Stéphane quite rightly anticipates that the result may lack consistency due to the difference in sampling and resolution between our images.
Having for my part also tried this type of mixing with the group Astropotos ( see other collaborations ), with sometimes quite significant sampling differences, I remain confident but it is true that there is no had no images made in short exposures in the mixes made within this collaborative group. However, it is here the important difference in resolution of the details, more than the sampling, which proved to be problematic in the sense where his much more resolute heart contrasted too much with the extensions close to the heart in my picture. For more distant extensions it is less problematic because more diffuse, they do not show chiseled details.
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At the same time, and by pure coincidence, my friend Sébastien Kuenlin contacted me because: he would like to try to mix his image of M27 made a few months earlier with his new RC400mm f / 8 telescope with mine in order to benefit from the details of its image and the signal on my extensions! Sébastien made 200 poses of 300s in Halpha and Oiii at 3200mm focal length with ASI2600mm camera.
The junction then emerges since the image of Sébastien made at 0.5 "/ pixel with an FWHM around 1.8" is slightly more resolved than mine made at 0.8 "/ pixel with an average FWHM of around 2.5" and approaches thus of the image of Stephane carried out at 0.3 "/ pixel with an FWHM close to 1".
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The project then materializes in a new form using 3 images: the heart of Stephane, close extensions of Sébastien, and distant extensions of my image. The coherence then becomes more convincing between the samples with this progressiveness from the center at 0.3 "/ pixel towards the extensions close to 0.5" / pixel then the more distant extensions at 0.8 "/ pixel.
On the left the image of Sébastien Kuenlin (4:40 p.m. exposure with RC400mm f / 8 and ASI2600mm camera) showing finer details in the core than my image, but with extensions to the lower signal-to-noise ratio, especially for the more distant ones. (click to accéde r in full-resolution images on our Astrobin galleries).
- The raw stacks, in linear out of stacking, collected, it follows then the first phase of the work I do by embedding the heart + close extensions of Sébastien's M27 in my image. This step is carried out linearly (before the histogram rise) in order to ensure a correspondence of dynamics so as not to alter the difference in luminosity of the object between the core and the extensions. This mixing is carried out on the Halpha and Oiii layers.
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- Sébastien is in charge of the second phase which consists of processing the composite images obtained Halpha and Oiii (deconvolution, noise processing, histogram rise, accentuations etc ...), their mixing to obtain a color image HOO, and the integration of the color of the stars obtained by the RGB layer of my image.
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- Finally, we collaboratively carry out with Sébastien the last phase which consists in embedding the nonlinear image provided by Stéphane, on which the deconvolution treatments, accentuations etc. have already been carried out by him with his technique resulting from planetary astrophotography which uses the same software and process for processing deep sky images acquired in lucky imaging.
From left to right, Mathieu's Halpha stack and Oiii stack in autostrech to capture the real dynamics of the object, and Stéphane's Halpha-Ir stack
The treatment of the M27 nebula always presents a certain difficulty and constrains to choices due to the large difference in luminosity between the core and the extensions on the one hand, and to the strong presence of the Halpha and Oiii signals on identical zones d 'somewhere else.
These artistic choices were made while respecting as much as possible the scientific coherence of the object, although the latter is not possible at 100%: in fact we obtain a saturated core if we want to take advantage of the signal on the most extensive extensions. distant, or then we do not distinguish these if we want to maintain an unsaturated heart. HDR processing must therefore be used as sparingly as possible, which inevitably alters the dynamics of the object but allows a compromise to enhance the different signals making up the nebula.
Crop on the core of the final image: the structural details and the resolution on the comet nodes show the potential of lucky imaging
Comparison with the image of the Subaru telescope of 8.2 m in diameter
In the end, we are very happy to present the fruit of this work with the objectives fulfilled:
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- enjoy the details of short poses
- take advantage of the signal / noise ratio on long exposure extensions
- ensure consistency between the levels of detail thanks to the intermediate image produced in long exposure to long focal length
- maintain the most "real" possible dynamics of the object and the most "natural" treatment possible
- not perceive any artifact or transition related to the mixing of the three images
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Sébastien, Stéphane and Mathieu, November 2021