Shades of opinion

Colour in astronomy is a difficult and often controversial topic. Rather than just lump all the photons together, we like to use separate filters which allow a standard set of wavelengths through; that means that when I talk about a ‘blue’ image, then I will certainly be talking about the same ‘blue’ as anyone else. Each colour encodes information; blue represents young stars, red old stars or emission from dust. This is all very well, but the problems start when we try to produce colour images. Which filters do we use? No camera has the same response as the human eye, so this can be a controversial question. The Galaxy Zoo images are based on three of the Sloan filters – blue is blue, but red is green and infrared is red. That’s why the Voorwerp appeared as a blue blob even though it’s really green.

Yet it’s worth the effort; colour images are a natural way for us to process information. So much so that I’ve been jumping ahead of the careful work that Bill and Anna have been doing in processing the images and producing rough and ready colour versions of a few. Here’s a standard field; the colours are blue=blue and red=very near infrared, with green being an artificial combination of the two.


Of course, the power of this approach is in looking in detail at our overlapping galaxies. Remember this system? From the monochrome image it’s hard to tell which galaxy is on top of which, but in (exaggerated) colour it’s obvious.


You can see the blue spiral arms crossing in front of the edge-on galaxy, whose dust lanes are highlighted in red. One of the arms is clearly thicker than its tenuous neighbour; quantifying the difference between the two is one of the results we’re after.

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6 responses to “Shades of opinion”

  1. garrett_cw says :

    “No camera has the same response as the human eye, …”

    The human eye does not have the same response as the human eye. I noticed as a child that my two eyes see colors slightly differently. Best seen when looking at a saturated blue and blinking from one eye to the other. I suspect that I am not the only one.

  2. oino says :

    “You can see the blue spiral arms crossing in front of the edge-on galaxy,”

    No, I don’t. I see it as just the opposite. The spiral’s arms disappear behind the edge-on. Otherwise the red dust lane would be cut in half, and it’s not.

    Maybe I’m not cut out for this business; I don’t seem to see the way others do.

  3. oino says :

    While we’re on this subject, what is that bright round object in the upper right? According to the merger voting instructions, we’re supposed to be ignoring such a result because it will spoil what you want to look at. Yet you seem to be fine with having such a bright star-like object in the frame.

    I’m confused.

  4. Chris says :


    The bright object in the top right is a star. Different projects; this is follow up on overlapping galaxies, rather than mergers. As for the arms, I’ll try and make a higher res image available, but look closer to the nucleus – there’s an arm there that clearly goes over the red galaxy.

  5. JayT says :

    Do you mean green is blue, not blue is blue?

    That is what I remember reading in the SDSS
    stuff on image composition.

  6. Adam Primus says :

    Well it seems that the human eye sees differently not only from a camera, but from other people’s eyes… like oino, when I look at the image it appears that the edge-on galaxy is in front… curious…

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