Hi astrophotographers.
I'm really happy to be able to share this image that I've been working on for a long time, considering the total integration time and hours of postprocessing.
It all started from the most detailed web search for the Abell 21 there was and I stopped on the HOO version photographed by the Mayall telescope (Kitt Peak) with a 4 meter primary mirror:
https://upload.wikimedia.org/wikipedia/commons/d/d3/Abell21_-_Noaoann09008a.jpgThis planetary nebula is made up of a very bright shell in SII-Ha-OIII and very faint outer OIII and Halpha shells, which is why I decided to integrate as many hours as possible with equally long exposures (1200s).
During the shooting of OIII and SII the seeing was really bad and I had to keep the shots that did not go beyond 5 FWHM. Fortunately, during the shooting of Halpha the seeing was phenomenal (1.5/2) and therefore the detail given by Halpha removed the FWHM problems on OIII and SII.
BlurXTerminator was applied with a low percentage so as not to lose the true nuances of the nebula
The image has a total integration of approximately 100 hours in SHO + RGB for the stars
The brighter shell is composed of overlapping layers of OIII Halpha and SII so in order to best bring out the OIII layer and the nuances between Halpha-SII I had to use a "Super Dynamic Narrowband" with a HO multiplication factor.
The result is a dynamic HSO-SHO.
This formula was designed together with my friend Daniele Borsari
The stretch phase was performed with HT, the last step instead I performed a blend between 2 images with different stretch curves (to maintain the faint nebulosity and at the same time maintain the details on the bright regions)
In the center of the shell, the White Dwarf (blue) responsible for all this is clearly visible
In revision B you can see some really weak Halpha waves along the background, probably the gases that were expelled first.
Hope you like it
Riccardo
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