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I'm new to this stack. I have a question about astrophotography. A while ago I took pictures of the crab nebula with R, G and B filters with a 120cm telescope. These pictures have already been postprocessed with the flat fields and the biases. The only thing left to do is to combine these in GIMP. We did this in class (like a workshop), but I don't remember how it was done, because the tutorial was quite fast. Moreover, the final result has a transparent green gradient.

I'm looking for someone with experience in this kind of postprocessing, who knows

  1. how to combine R, G and B pictures in GIMP to create a full colour image; and
  2. how to get rid of such a gradient if that is at all possible.

The pictures are .fits files. I've tried to play with the channel mixer under Colors > Components, but it doesn't get much better.

For those interested, I put the fits files and the GIMP file on Github.

Crab nebula

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2 Answers 2

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The green gradient on the side is actually present in all three channels, but a bit more in green and red:

enter image description here

(this histogram also shows that your blue channel is somewhat undexposed)

So the problem has nothing to do with your methodology, it is part of your data (parasitic light?).

To restore the image from the individual channels, I used:

  • Open the first channel image in Gimp.
  • Convert the image to RGB (Image > Mode > RGB)
  • Start the bucket fill tool
  • Set the foreground color to the intended color of the channel
  • Set the bucket-fill tool to Multiply mode

enter image description here

  • Bucket fill the whole layer. That should set all the white to the channel color (so if you started with red, all your stars will turn red)

For the other two channels:

  • Add the channel with File > Open as layers
  • If necessary adjust the layer position
  • Bucket fill in Multiply mode with the required color as above
  • Set the layer to Addition mode.

enter image description here

If you wan to go to the next level:

  • For each color you make a group with:

    • the stars image at the bottom
    • a layer filled with the channel color, set to "Multiply" mode
  • You set the top two groups to Addition mode

This way you can do fake colors by re-filling the color layers with other colors, and if necessary play with the opacity of the two top groups (if you want to play with the opacity of the bottom thrid group, add a black background layer outside of the three groups).

enter image description here

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    \$\begingroup\$ It could be moon light, maybe? The moon was almost full that night. The blue filter also shows a weird waffle pattern, some kind of interference. Thank you! \$\endgroup\$ Commented Feb 15 at 12:31
  • \$\begingroup\$ Yes, but I wonder why that pattern has a vertical spread when the R/G parasitic light is horizontal. So it could be completely unrelated, and be some sensor noise that is masked on R/G channels because there is more signal (and more noise)(the "waffle" concerns a very narrow band of values). \$\endgroup\$
    – xenoid
    Commented Feb 15 at 12:57
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This green tint is usual in astrophotography and it is because sensors (even from black/white sensors) are usually more sensitive to green light than in other wavelengths and this is so, because the sensor manufacturers ultimately model the eye‘s sensitivity, which is best in green (and those sensors are only by chance used in astrophotography). Just have a look at spectral sensitivities as published for ZWO ASI, QHY or similar cameras.

So how do you correct for this and what is the „right“ color for an astrophoto? Well, it depends: Rene Heller published „Colors by Temperature“, which provides colors with-out influence of the atmosphere (as Hubble would see it) and takes into account the different sprectral photon fluxes by temperature and composition of the stars, I.e. it is calculated from the spectrum of a star and taking into acount the eye‘s sensitivity to different wavelengths.

„Best“ methods implemented as color calibration in sophisticated software like PixInsight uses this information to adjust the color balance of a picture, see Spectophotometric Color Calibration (SPCC): A list of stars is extracted from a picture and then their spectral classes are used to determine the right color balance. This may take into account, which spectral filters are used for taking the pictures.

Sorry, I can‘t help you with Gimp (as I am using PixInsight), but hope that this background infos sheds some light on the problem (pun intended).

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