23 October 2019
Remember a couple of weeks ago I posted my narrowband Helix photo? Well a while later, ASV member Geoff Healey posted a visible light photo of the same object. It's labelled "LRGB". Compare the two photos here.
My photo (HOO) is an image captured using two filters that only pick up a pair of highly specific colours. One colour, a pinkish "Hydrogen" is displayed as red on the computer screen, and a teal "Oxygen" is displayed on both the blue and green pixels on the screen.
If you look closely, they're quite different. Geoff's images is "true colour" and mine is "false colour". But there are a couple of other differences.
You'll note that Geoff's image has lots more stars in it than mine. Geoff's camera accepts the whole spectrum, so it allows a lot more light in, including light near infra-red and ultra-violet. My filters accept light from the nebula but shut out a lot of the rest - to the point where my camera just didn't detect most of the dimmer ones.
Look closely at the fineness of the detail in the nebula. Geoff's image is streets ahead, mainly due to the sheer time of exposure - it took him two nights to take the photo (that's dedication).
In processing, I've made the background a bit darker for aesthetic reasons (both to make a stronger contrast between background and foreground, and also to hide a bit of the near-black noise). It's actually well established that this is poor practice, as this tends to hide dark background objects.
Have a look at the next group of my images. They're all actually the same one, just with the colours swapped around.
Geoff's visible light image shows that the outside of the Helix is red and the inside is blue. My image shows that the red bit is mostly Hydrogen and the blue bit is Oxygen. I made Hydrogen red and Oxygen blue and green, but I needn't have. In the third and fourth images I've swapped them round, just to show the differences, as well as how arbitrary "false colour" can be.