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Notes:
Spectra and Astronomical Images
Before continuing I’ll just add a note of warning! Most astronomical images do not show their subjects in their natural colour. For example the Eagle Nebula is comprised of Hydrogen gas which when heated glows red (as seen in the David Malin pic), so to help improve the colour contrast in the HST image which allows us to see the nebula’s strutuce more clearly we enhance the colours by taking three images of the nebula each through a filter which only lets light of a specific wavelength pass through. In the case of the HST image of the Eagle nebula, a filter for Near-IR, Red, and Green (centre of visible spectrum) was chosen with the resulting image from each filter then being coloured Red, Green, or Blue depending upon their respective positions in the spectrum (cooler light Red, and hotter light Blue). These three RGB images are then combined to produce the pretty image that you see on the 6 o’clock news.
The HST can see light over a range slightly greater than the human eye, from Near-Infra-red to Near-UV. Other observatories can see light of different wavelengths, and use similar techniques to represent their images in visible colours with a good colour contrast.