False Colour

I've just seen a neat little video on YouTube showing how a Hubble Space Telescope image of a galaxy is made. The video is created by the Space Telescope Science Institute - the people behind HubbleSite. It shows 10 hours of image processing compressed down into 2 minutes. The final colour image of NGC 3982 was created from 7 grey-scale images taken with Hubble's cameras. It is really interesting to see the work that has to go into image processing to create the beautiful finished products we expect.



Although I try not to read YouTube video comments, one caught my attention. JackyRBKwan asked "so the galaxies are in fake colour??" This issue of true verses false-colour is one that is often brought up in comments online. I've seen criticisms of Chromoscope complaining that "invisible" parts of the spectrum are given 'false' colours with the implication that this is somehow deceiving or misleading. The same sorts of arguments are heard when people find out images from Cassini or the Mars Rovers have had their colours adjusted. The implication is that adjusted colours mean the image is 'doctored' or untrue.

I find arguments over 'true' colours very frustrating. What does true-colour even mean? Our eyes don't see the true colours of reality; they respond in certain ways to blue, red and green parts of the spectrum but there is some variation from person to person and our eyes fail to see all the interesting light emitted at other wavelengths (IR, UV, radio etc). Even if we define 'true' as equivalent to the average human eye, it is almost impossible to recreate this colour in print or on screen. Even on computers the colours of red, green and blue will appear differently depending on the colour space (RGB, CMYK, AdobeRGB, CIELAB etc) and display used. Ultimately, all images are false-colour and we should understand that.

Most multi-colour astronomical images are created by combining images taken through different filters (and sometime different cameras/detectors) which often don't directly correspond to the colour detection of our eyes. That isn't a bad thing. Sometimes, by altering or stretching the colours used it is then possible to see or understand something that we couldn't before. That doesn't make the final result false even if the colour is.

Posted in astro blog by Stuart on Thursday 17th Feb 2011 (22:38 GMT) | Permalink
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