(Edited to reflect points in the discussion)
Short version: I derived a physically-based formula to synthesize RGB "true color" images from narrowband Hydrogen-alpha (Ha), OIII, and SII filters:
R = 0.5*Ha + 0.5*SII
G = 0.5*OIII + 0.094*Ha
B = 0.295*OIII + 0.1*Ha
And, I like how it looks. Here's "Frank's Special Blend" on 4 hours of the Eagle Nebula (M16), side by side with a more typical HOO->RGB mapping. Both were processed identically apart from the PixelMath equation used in PixInsight:
Here's my full image on Astrobin.
Long version:
So, here's how I arrived at these numbers. Please check my assumptions and math; I'm still pretty new to astro-imaging and I am definitely not an astrophysicist, so if someone else already arrived at these same numbers or if I'm doing something stupid, just tell me.
Looking at spectra of emission nebula yields a couple of insights:
- Hydrogen-beta emissions are generally about 20% of Hydrogen-alpha emissions. I think it makes sense that this ratio remains fairly constant; I mean, it's the same element just excited a couple of different ways. But these are very different wavelengths to our eyes. That's nothing new; I've seen others use this as justification for blending some Hydrogen-alpha into your blue channel (credit to Richard Wright for enlightening me on the physical basis of this, though.)
- Similarly, OIII emissions are actually (mostly) on two emission lines, with the secondary one being about 39% of the primary in strength.
So, we can synthesize Hydrogen-beta emissions if we have Hydrogen-alpha, and we can also get a couple of different colors from OIII using a similar thought process.
The wavelengths we have to work with are:
Hydrogen-alpha: 656 nm
OIII: 501 nm (primary) and 496 nm (secondary) - although these numbers are very close, they're quite distinct colors to our eyes.
SII: 672 nm
Hydrogen-beta: 486 nm
Now, we can convert those wavelengths to RGB values.
Hydrogen-alpha and SII are both straight up red to our eyes, or an RGB color of (1.0, 0.0, 0.0)
The primary OIII line converts to the RGB values (0, 1.0, 0.53), and the secondary is (0, 1.0, 0.75).
Hydrogen-beta converts to (0, 0.94, 1.0)
So we end up with:
R = Ha + SII (both Ha and SII emissions are pure red to our eyes)
G = 0.72*OIII + 0.28*OIII + 0.2*0.94*Ha (this is the green contribution from the primary and secondary lines of OIII, weighted by their relative strength as they are both captured by an OIII filter, and the contribution of Hydrogen-beta, which has a G of 0.94 and further is 20% of Hydrogen-alpha)
B = 0.72*0.53*OIII + 0.28*0.75*OIII + 0.2*Ha (again, this is the blue values from OIII, weighted by their contribution to the total OIII signal, and adding in the synthesized Hydrogen-beta data as 20% of Hydrogen-alpha.)
Simplified, this is the same as:
R = Ha + SII
G = OIII + 0.188*Ha
B = 0.592*OIII + 0.2*Ha
But, if some stars are near saturation in the linear images you're combining, this could result in them getting clipped. We need to scale everything down 50% to prevent that:
R = 0.5*Ha + 0.5*SII
G = 0.5*OIII + 0.094*Ha
B = 0.295*OIII + 0.1*Ha
In theory this only works on emission nebula, and furthermore on emission nebula that are excited mostly in Hydrogen, Oxygen, and Sulfur. Some nebulae have strong Nitrogen components too and I didn't factor that in.
The stars with this look tight and still have some color to them, which is good. But I think I'll take some real RGB data just for the stars on this image later on - I don't think you can really capture most stars' true color with NB filters.
So, how real is it? Well, we can compare it to a RGB image from the ESO's La Silla Observatory:
Image credit: ESO
So, reality seems to lie somewhere in between my formula and HOO. But maybe it's just the impact of light pollution on my raw data... I took my image at the start of this post from a "red zone" on the night before a full moon! That probably did not-so-great things to the quality of my OIII data. It will be interesting to see how it works under better conditions.
However, I think it's pretty darn exciting that a guy in a suburban driveway with a 7-inch telescope can produce an image that holds its own against a 2.2-meter telescope in the Atacama desert using narrowband techniques.
Hope this proves useful to others, or launches you on your own exploration for the perfect color mapping.
-Frank Kane
Edited by BoldlyGoing, 21 May 2019 - 07:11 AM.