No need to change a thing! Very good as is.
Our philosophy on colour using three narrowband filters:
(1) Our primary goal is to convey meaningful astrophysical information. If Mike wants to be
primarily artistic, he gets out the clay or the oils.
(2) We make the picture as beautiful as we can consistent with the primary goal.
(3) The kinds of astrophysical information we want to show:
- Hydrogen is very easy to ionize, so H-alpha tends to show where the bulk stuff is. It also shows shock fronts nicely.
- Oxygen requires high energy to ionize. OIII can be seen near the extremely hot white dwarf in a planetary nebula. Otherwise, it tends to indicate extreme violence. It is mostly seen near hot young OB stars that will soon go supernova, but can be excited by extremely violent shock fronts, e.g. in an SNR.
- Sulphur is found only where there is dredged-up material from the guts of a star where nucleosynthesis has progressed a long way. Sulphur is rare rather than ubiquitous. It also requires high energy to ionize, so SII emission is rare. The [SII] to H-alpha ratio is unusually high in supernova remnants. You can also get [SII] in the atmosphere of a PN or WR nebula.
(4) Now for the nitty gritty:
- Because OIII is less common than H-alpha, it requires more exposure. Since SII is dead rare, it requires long exposure.
- We don't try to show the relative abundances of H-alpha, OIII, and SII. That would be like a map of the mineral wealth of Australia trying to show the relative abundances of sand, coal, and gold. All you'd see was sand.
- Instead, we're trying to show action and structure: to show the locations of what little OIII and SII there might be, as these are special places.
- Since our primary goal is to convey true and genuine astrophysical information, we use the Hubble palette, because everyone immediately understands what they are looking at. If you translate a beautiful poem into let's say ancient Hittite or Martian, only very old Hittites and Uncle Martin will understand. Hence we map SII, H-alpha, and OIII to red, green, and blue respectively.
- We remove gradients from moonlight or light pollution, and set the black point very carefully in narrowband images. Otherwise one can falsely fill the background with OIII and SII, when all you've really got is moonlight and air glow and the floodlights at the Council car park. Makes a beautiful picture, but 'tis a lie.
- As regards the stars, they are incidental to the story, and their colour is of little or no interest. Consequently, we divide the image into two components: the stars and the nebulosity, and we process them separately.
- To avoid having the image flooded by H-alpha (with OIII getting lost and SII undetectable) we colour balance the nebulosity so that the image as a whole is colour neutral. That is to say the
mean values for red, green, and blue for the whole image should be the same.
- At this point, it is easy to use reasonably strong wavelet sharpening on the nebulosity-only image. (It is very difficult if the stars are still in).
- To avoid distracting magenta haloes around the stars, and to avoid implying that the colours of the stars have significance, we suppress the OIII and SII channels in the star image, keep only H-alpha, and map the H-alpha to white.
- We then recombine the stars with the nebulosity to produce the final image.
Having said all that once again, your image, exactly as it is, is very pleasing. The goal of the diatribe is to save you from the Dark Side. We've tried to write the above description in a non-commercial way, concentrating on the rationale and goals, rather than saying "We use Bob's WobbleBobble action from his GobbleBobble package", when we have no real idea of what those actions do.
Very best,
Mike (in charge of maths) and Trish (colour and aesthetics)
