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Magenta hue

Processing Color Calibration Deep Sky NGC7023 Nebula
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Arun H avatar
Arun H avatar
In processing my most recent image taken with Chroma RGB filters, I noticed a magenta hue in my image (see rev 0):

The Iris Nebula (Arun H) - AstroBin

Now all three masters were background subtracted using Multiscale Gradient Correction with Flux calibration prior, combined, background subtracted with Spectrophotometric Color calibration. The color calibration graphs were all properly fitted, so I am not sure why there is the hue.

I was able to get rid of it in the final version, but why is there a hue in the first place? Why doesn't spectrophotometric color calibration get rid of it?
andrea tasselli avatar
andrea tasselli avatar
SPCC is far from perfect, as so are the curves fed into it. Try CC, you'll be surprised.
John Hayes avatar
John Hayes avatar
HI Arun,
I run into this kind of issue every once in a while and it's pretty extreme in this case.  There are a number of things that can throw SPCC off.  First, you have to be careful about the stars that are being selected for the sample.  If they are flat topped in one of the channels, you'll have big problems.   You also want to make sure that the algorithm isn't picking the very faintest stars so look carefully at the star selection parameters.  Even if all that stuff is properly configured, you may just be running into a problem with extinction, which will shift the white balance.   Remember that extinction scatters short wavelengths (blue light) so that the star colors will be biased toward warmer colors.  In this case, you want to move the red-ish stars more toward white, which will make blue appear more prominent.   With that idea in mind, you might try using SPCC with class M type stars for the white reference.  I don't know if class M is going to work right out of the box so you'll have to experiment a bit.   I had to really fiddle with the white balance when I processed my Horse Head data for similar reasons and I wrote quite a lot about it in that image so you might want to check it out.

Good luck with it!

John
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andrea tasselli avatar
andrea tasselli avatar
AFAIK SPCC does compensate for atmospheric relative extinction although I never put it to a real test.
John Hayes avatar
John Hayes avatar
andrea tasselli:
AFAIK SPCC does compensate for atmospheric relative extinction although I never put it to a real test.

I was referring to Mie and/or Rayleigh scattering within the nebula itself.
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Arun H avatar
Arun H avatar
Thanks for responding John and Andrea.

I did try straight color calibration, but some version of the hue remained. 

I think I am going to try John's suggestion. In SPCC, I tried G2V star (my usual default) and Average Spiral Galaxy without good results.

The odd thing is that it seems specific to this region. My image of IC405 was fine straight out of the box with acceptable color, so John's extinction theory probably has something to do with it, given the dust in the region.

The color of the dust in the Iris Nebula seems to have huge variation in the images I see on AB, and I'd rather choose something that's reasonably color accurate.
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andrea tasselli avatar
andrea tasselli avatar
Can you exclude human error? I my experience reflection nebulae calibrate rather nicely straight out of the box as long as you got good SNR. You seem to prefer husky tones with more red thrown into the mix than what I would otherwise do.
John Hayes avatar
John Hayes avatar
I think that it's important to point out that while SPCC generally does a good job of color calibration, as with any color calibration method, the white balance is up to you.  The average spiral galaxy option uses the average of trillions of stars to define a workable white balance for most situations–particularly for galaxies.  Once you get into regions with high extinction, the ASG white balance isn't always the best choice–and this isn't due to any kind of error in the method.  It's due to the way colors are defined and measured–and by what we want to appear as "white".

John
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Brian Puhl avatar
Brian Puhl avatar
As I've generally moved away from SPCC,  I think basic background neutralization + color calibration works far better in 90% of my imagery.     There are occurrences where I can't find a good neutral background sample however, leading me to use SPCC.
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Dan Kearl avatar
Dan Kearl avatar
I took your jpeg into Photoshop and Capture one to adjust WB.
The image was very oversaturated, so the right color balance is difficult to get from a jpeg but
this is what I would do with your image using basic WB tools outside of the Astro world.