What is false colour?

8 May 2019

What is colour? A strange question maybe...

Recently I took a photo of NGC 6188. The Fighting Dragons of Ara is one of my favourite nebulas. I'd got a test shot last year in black and white, and really wanted more.

I went up to the ASV dark sky site and got over 6 hours of images through my three narrowband filters. Then I roughly processed what I'd got on my computer. I used the free program Deep Sky Stacker to create stacked and aligned images from each filter, then recombined them in Photoshop using the "Hubble palette". Images taken through the Sulphur filter end up as red in the final image, Hydrogen goes to green and Oxygen goes to blue.

This is a "false colour image". If you were there, it wouldn't look like that. If you could see it at all, it'd probably be pink.

I put the photo on the ASV Facebook page, and there was some some discussion about the green colour. It's not that it's wrong, just ... unusual.

One awesome person even pulled the channels of my posted photo apart and rebalanced them, getting the second, and far more conventional image here.

As an aside, the folk at the ASV are really great to know. Everyone should join!

I eventually had a second go at it (the last photo) and used a more complex mapping. What's the difference between these photos? It's a good question.

You see, "false colour" images are just that. False. It's entirely arbitrary how we assign the filters to colours.

Because there are no rules covering how filters get "mapped" to colours in the final image, all ways of doing it are equally valid. How pleasing you find it is entirely up to you, or to put it another way, beauty really is in the eye of the beholder.

Which is your favourite?

Oh, and YOU can take a photo like this yourself. if you want to find more, just ask me!