Getting the colours
You might remember me talking in the past about the different ways you can represent the light you capture in your camera. You can display red light as red, but more interestingly, you can display the light that comes from different elements as different colours on your computer display.
This is particularly good for separating light from two gases, say, Hydrogen and Sulphur, which are both reds. You can display one as red and one as green.
When I take photos, I use filters to capture different colours. For foreground stars, I typically capture using broad “red”, “green” and “blue” filters, and for background nebulas I use Hydrogen, Oxygen and Sulphur “narrowband” filters.
Presenting the colours
Later on, in my imaging process, I recombine these channels in any way I find aesthetic.
A couple of weeks ago, a new version of my image processing software (Astro PixelProcessor) was released. One of the new features is that I can change combinations at the touch of a button.
For example, I can display the light I record coming from Hydrogen in a nebula to red on the computer, the light coming from Sulphur to green, and that coming from Oxygen to blue (we call this HSO). Then I can rearrange that to SHO by clicking a button. The difference is arbitrary because the human eye can't really see much light coming from those elements.
Last year imaged the Eagle Nebula using all these filters, so I thought I’d present some variants. They're named in a conventional manner., with simply “RGB”, using the broad spectrum images I captured, or “HSO”, which doesn’t use the broadband data at all, but maps Hydrogen to red, Sulphur to blue and Oxygen to green. It also gets complicated, such as “RGBHOO”, mixing broad and some narrowband data. Red data and Hydrogen both display as red, green and Oxygen display as green, and blue and the same Oxygen display as blue.
Very Andy Warhol!
And here they all are...
I've put them all (well, nearly all) below. Remember, they're all the same photo, just with different colour information presented on the red, green and blue pixels of your computer monitor.
My preferred one? SHO (known as the Hubble palette) with the green turned down a medium amount. I put that one right at the start of this article. My wife disagrees with my choice. What do you think?
RGB - visible colour
This one is plain RGB - red, green, blue. It's pretty much what your eyes see.
These ones are the same, with the addition of some Hydrogen alpha, which gets added to the red channel.
SHO - Hubble palette
These next ones are variants of the Hubble palette, where Sulphur goes to the red channel, Hydrogen goes to green and Oxygen goes to blue. For the first version I've included a few levels of green reduction. Hydrogen tends to come through very strongly, and it's nice to turn it down a bit.
The last in this group include another version with a lighter channel mixture (courtesy of Astro Pixel Processor) and then an "RGBSHO", which includes the broadband information. I think it gives the stars a better colour.
The next group are "HSO", where Hydrogen goes to red, Sulphur goes to green and Oxygen goes to blue. I'm not fond of it - it's purely aesthetic, but I don't like the magenta tones.
This final group are "HOO", where we don't use Sulphur at all. Hydrogen goes to red, and Oxygen goes to both green and blue. You can actually get a filter that you can put in front of a colour camera to get this effect.
My thoughts about this is that if you've got the Sulphur information available, you may as well use it!