I recently processed (RGB) data from this nebula (https://www.astrobin.com/coo844/), and in spite of my efforts with color calibration using SPCC in Pixinsight, the nebula has a definite blue cast rather than the expected red of such a strong Ha area. I've looked at other images on AstroBin, and many that show red in this nebula have incorporated Ha, which explains why the nebula is now red. My question is, why doesn't the red show in RGB images that are not supplemented with Ha? And are there any special techniques to bring out the red in an RGB image? Note that the Ha areas of the nebula on the left side of the image are red.
Ha areas of the Tarantula Nebula are not red... why?
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The simple answer is that the Tarantula isn't mostly Ha, but a mixture with a lot of OIII and Hbeta emissions together with Ha, but your color calibration seems quite off even with that in mind. The image below is a pure RGB with equal exposures in all channels:
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Thanks for the advice. I went back and ran the linear fit again for R, G, and B (against R). Then created, the RGB and, eventually, the LRGB. SPCC seemed to work fine, but it removed most of the red from the Tarantula. I then tried ColorCalibration… same but to a lesser degree. So I think I will forgo color calibration and see what I end up with. Thanks again.
George Hatfield:
Thanks for the advice. I went back and ran the linear fit again for R, G, and B (against R). Then created, the RGB and, eventually, the LRGB. SPCC seemed to work fine, but it removed most of the red from the Tarantula. I then tried ColorCalibration... same but to a lesser degree. So I think I will forgo color calibration and see what I end up with. Thanks again.
Did you CC at the RGB stage or at the LRGB, can't understand which.
I ran SPCC (after solving) on the linear RGB. Then on to LRGB.
RGB
Noise X
IS
SPCC
etc
RGB
Noise X
IS
SPCC
etc
George Hatfield:
I ran SPCC (after solving) on the linear RGB. Then on to LRGB.
RGB
Noise X
IS
SPCC
etc
Seems in the right order other than running SPCC (use CC instead). I don't get what IS is...
IS is the script ImageSolver which is needed to re-plate solve the image for SPCC since the R, G and B were cropped early in processing.
George Hatfield:
IS is the script ImageSolver which is needed to re-plate solve the image for SPCC since the R, G and B were cropped early in processing.
OK, got it.
The concensus of astrobin images is probably a poor guide to setting the 'right' colour balance of an RGB image since many images are NB, have NB mixed in and in any case have colours adjusted for maximum drama and contrast
I suspect that SPCC gets it right and that having the red look more prominent is more fashion than reality?
Tim
I suspect that SPCC gets it right and that having the red look more prominent is more fashion than reality?
Tim
Tim… I agree. In my experience, SPCC does a good job. I just wanted more red without adding Ha. If you look at the other Ha areas in this image, they are very red with SPCC calibration (image 1).
George Hatfield:
Tim... I agree. In my experience, SPCC does a good job. I just wanted more red without adding Ha. If you look at the other Ha areas in this image, they are very red with SPCC calibration (image 1)
Indeed. I like your image but I think that the first is the better in fact. There must be shed loads of blue light - from all those newly formed stars as well as some reflection nebulosity as well as OIII which - as in planetary and some Wolf Rayet nebula - starts to dominate where stars are hotter and the electron temperature up above 40K and more.
Like Andrea said, the nebula has strong OIII and Hb emission. So no matter how you do the color calibration, as long as the calibration is sort of correct, you won't get very red nebulas. The same goes for many other nebulas in LMC and SMC. This is basically a consequence of less dust absorption in LMC and SMC, compared to Milky Way. If you want red nebulas, you will have to either add Ha, or shift the overall color balance to red, or rely on special techniques to enhance the red in the nebulas without affecting the overall color balance.
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