Why do comets look green in photos?

21 April 2015

Question

Just been watching the BBC's monthly 'Sky at Night' programme on iPlayer and they were talking about a comet that is now visible with a low-powered telescope or even binoculars. One of the show's presenters helpfully gave us some tips on how to take photographic images of the comet, but also stated that the photos showed the comet's trailing gasses to have a green tint, whereas the naked eye looking through a telescope can't pick that up.

Answer

We put Tad's question to astronomer Carolin Crawford...

Carolin - Again, this is such a good question. You look at these wonderful pictures in coffee table astronomy books and you have these multi-coloured swirls, nebulae, comets. They all look fantastically coloured. But when you look at them through the telescope with your naked eye, it's often disappointing - they're sort of grey and whitish. The problem is not with the comet or the nebula. It's with your eye, because the part of the eye that detects faint diffuse substances, these are the rods. They're around the outside of your eye. They're not colour-sensitive. It's the cones at the centre of your eye that are colour-sensitive. And so, if you can see something faint and fuzzy through a telescope, you won't be picking up the colour. And that's why the camera of course doesn't have any of these problems, it picks up all the colours that there are. So it's to do with your eye rather than anything to do with the comet.

Kat - Great question.

Chris - I have noticed that myself when you look at the night sky. If you look out of the corner of your eye the stars are much brighter, and then you look at them and they seem to almost disappear. Especially if it's a star that's a little bit dim in the first place. You can see the star and you look at it and then it's gone. That's why, because you're then focusing it on the centre of your retina where you've got these cones which are pretty good at seeing text on a bright screen, but they're not very good at seeing dark things.

Carolin - Yeah, you're exactly right.

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