I like this area of the sky very much because it includes very different objects not easy to image. I combined LRGB and Halpha data for this image.
Image ProcessingAll astronomical processing was done in Siril. I previously tried to do amplify the Halpha signal by using a formula like this: (H*r-R*h)/(r-h). However it turned out that this basically reduced the R-Channel wrt Halpha creating more noise in the image. The results can be seen in the first two revisions of this project.
For the final image I thus used the standard LRGB compositing of Siril. Vicent Hourdin of Siril has created several very useful tutorials, one of them about the RGB composition (4).
I basically used this method treating RGB and Halpha as color channels plus the luminosity data. So there was no special treatment of the Halpha data.
To make the faint cirrus come out as well, a good background subtraction is important. Otherwise, due to the faintness of the cirrus, small differences in the background subtraction between the channels may lead to quite dominant false colors.
In May the sky never was completelly dark. There always was a gradient towards the direction of the Sun. There were other gradients of unknown origin which I am still investigating. Therefore some of the images had to be discarded. For others I used the new RBF algorithm of Siril and placed the background indicators manually. Still some of the background and cirrus colors may not be real.
The final delicate processing of contrast, noise and colors etc. was done in RawTherapee, which as always proved to be an excellent tool for post-processing astronomical images.
Astronomical Objects The two big galaxies M81 & M81 are about 12 million light years away and 150,000 light years apart. M82 is undergoing a starburst and the outflow can be seen very well in the Halpha line. Also the HII regions of M81 can be seen nicely.
Very close to the right of M81 is a dwarf galaxy called Holmberg IX or UGC 5336 (3). Another faint dwarf galaxy is located in the upper center of the image. It is called Holmberg I or UGC 5139. Both dwarfs are considered to be part of the M81 group, have a blueish glow together with a few HII regions barely visible, indicating active star formation. The Holmberg galaxies were already discovered by swedish astronomer Erik Holmberg between 1950 and 1969.
There is yet another faint isolated dwarf galaxy close to M81 and M82. It is called GHOSTS 1 and was only discovered by the Hubble Space Telescope in 2012 (1). This galaxy is
not visible in this image because it is too faint. It is often - also on Astrobin - confused with Holmberg IX.
Then there is the cold and faint galactic cirrus or "integrated flux nebula" visible everywhere. IFN was discovered and named by the amateur astronomer Steve Mandel. It comprises of dust, HII, CO and other molecules at higher galactic latitude. It is illuminated "by the energy of the integrated flux of all the stars of the milkyway". It should be the same stuff observed in the far infrared by the IRAS and COBE satellites (2)
Finally, on the lower left side of the image to the right of NGC2787, there is an area weakly shining red. It is visible in the Halpha image alone too. So to me this looks like a real very weak region of ionized HII gas.
(1) https://iopscience.iop.org/article/10.1088/0004-637X/780/2/179(2) https://www.cosmotography.com/images/galactic_cirrus.html
(3) https://en.wikipedia.org/wiki/Holmberg_IX(4) https://siril.org/tutorials/rgb_composition/
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