We wanted to produce a Gabriela Mistral in SHO with RGB stars. We took about 22 hours of SHO, and 1 hour total of RGB.
We noticed that in the RGB image, when stretched enough for the stars to be brighter and fatter than the SHO stars, there was a lot of nebulosity, with Gabriela's face still very recognizable, but in the "wrong" colours for SHO.
If we stretched less, then when they were combined, there was still a conspicuous annular ring from the very highly stretched SHO stars. What to do?
I used multiple linear regression to statistically remove ("correct for") the emission nebulosity in the RGB image, given the SHO image.
If I used the whole image, it just didn't work.
But if I ignored the brightest purple nebulosity close around the bright star under Gabriela's chin (the one with the purple lightning bolts coming out of it), it worked beyond splendidly: 99.99% of the variance in the background of the RGB image was accounted for by SHO emission nebulosity.
If you scroll through the five attached images, you will see:
(a) The straight SHO image, with big purple stars
(b) The straight RGB image, with Gabriela's face clearly visible (
big one here)
(c) RGB given SHO (shorthand: RGB|SHO): the RGB image "corrected for" the SHO image. (
big one here). There should be less than 0.01% of the SHO emission nebulosity left in the image. Any nebulosity that is still there is not SHO, but something else. This is the important result.
(d) A starless, deconvolved, wavelet sharpened SHO image.
(e) Just for fun, the SHO image with the "corrected" RGB stars dropped in (result = brighter of RGB|SHO and starless SHO.
The final image is here.
So what is the remaining nebulosity in the RGB|SHO image? What is the nature of those purple "whiskers" or lightning bolts?
The most obvious answer is reflection nebulosity. Gas well behind the big bright star, with the ropy whiskers nothing to do with the star, but to do with shock fronts in the distant gas.
Another answer is that it is something exotic. One possibility is another emission line, for example N2. But one would expect N2 to be statistically highly correlated with one or more of SII, H-alpha, and OIII. There are other weird kinds of radiation, such as synchrotron radiation (the blue glow in the Crab nebula) due to accelerating charged particles in a magnetic field, but the blue glow in the crab nebula is uniform, not ropy.
Conclusion: the bright excess nebulosity close to the bright star under the chin, after exclusion of SII, H-alpha, and OIII, would most prosaically be reflection nebulosity from gas and dust well behind the star. It could just possibly be something more exotic. A spectrometer could distinguish between continuum light (such as reflected starlight) and exotic narrowband emission.
Best,
Mike