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Pixelmath for HaRGB blending, objects with bright areas

Astrophotography Software
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#1 balykos

balykos

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Posted 01 October 2022 - 06:35 PM

Hello,

 

I've made my first attempt to collect Ha + LRGB data using M31 as my target. My equipment is EQ6-R, Canon 400mm lens, ZWO ASI 1600MM with filters. I also used NINA, Siril & Gimp for software.

 

I collected the following subs under a Bortle 4 sky:

37x180" and 16x60" L, (76 gain, 15 offset)

16x120" R,G,B, (76 gain, 15 offset)

12x300" Ha 7nm ZWO filter (139gain, 21 offset) - I plan to add 2-3 hours of Ha to clean up the signal

 

so far I've done an LRGB composition in Siril using the 180"L & 120" RGB data and the results seem fine to me.

when I try to use Pixelmath to add Ha data to the galaxy, I always seem to blow out the core and add noise to the R background. At this point I'm thinking of blending the R and the R+Ha image using masks in GIMP.

 

formula: iif ((R<m), R, R + l*((Ha -k*R) - m*(Ha-k*R)))

k = scaling factor, l=boost factor, m=median value of Ha-k*R image.

 

it seems that the formulas circulating on websites and social media seem to work for smaller galaxies where there are not a lot of bright spots such as the core of M31. how do you typically handle blending Ha with R for M31 or perhaps the Horsehead with Altinak in the frame?

Assuming of course you would be willing to share the recipe to your secret sauce!




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