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'Yak, these colors are really weird !!!'

Color Presentation of Astronomical Images

Till Credner
Max-Planck-Institut für Aeronomie, 1999

  • Gray Scale and Monochrome
  • Natural Colors
  • Preserved Colors
  • False Colors
  • Mapped Colors
  • Three Color Composites
  • N Color Composites

  • There are thousands of astronomical images, all different in their colors, even if it is one and the same object. For a better understanding of the shown astronomical objects the different forms of image representations are explained. A brief introduction to Color Composites is given and for more advanced readers an enhanced technique of "preserved" colors is introduced. I have kept the text as brief as possible and tried to avoid software topics and related stuff like intensity scaling or color balancing. Many links are courtesy of P. K. Kaiser, York University.

    Definition of "Color"

    First of all, what does the term "color" really mean?
  • In the physical and astronomical sense, a color is the ratio of two intensities measured in two different wavelength regions of the electromagnetic spectrum, also called wavelength bands. For example the ratio of an X-ray intensity to an optical intensity defines a color. I.e. this kind of color definition, also called "color index", always refers to the two used wavelength bands!
  • In the physiological sense, i.e. the human eye and brain, the different wavelength regions are given by the sensitivity curves of the eyes three different color receptors on the retina, also called cones. Our brain creates corresponding impressions similar to red, green, and blue. The ratio of these channels gives our natural color impression (see the CIE Chromaticity Diagram).

    Image Representations

    Every original astronomical image is taken through a distinct wavelength band and thus just shows different intensities and no colors. The way these intensities are shown in an image can be chosen with a color look-up table, i.e. the different original intensities are encoded to certain colors and intensities. Color bars next to the following images show this encoding with increasing intensity.

    Gray Scale and Monochrome

    The most common way to show different intensities is encoding in monochrome color palettes. A monochrome color bar has different intensities, but shows the same or at least similar colors. Normally gray scales are taken (often called black and white, B/W), but also very popular is the "heat" palette from black, dark orange, bright yellow to white. Literally these methods are false color images, nevertheless this term is conventionally used for the kind of images in the next section.

    M 42, Orion Nebula

    M 31, Andromeda Galaxy
    More monochrome images

    False Colors

    (Sometimes mistakingly called Pseudo Colors)
    As the name suggests, false colors have nothing to do with the real intrinsic colors of the shown object. Many different and arbitrary selected colors can be chosen in one color bar as shown in the examples below. If chosen right, this enhances the visible range of intensities in the image. Small differences in intensity are easily visible if they are presented with different colors.
    A non continuous color bar is visualizing isophotes, i.e. lines of equal intensities.

    Comet Hyakutake

    Ring Nebula M 57
    More false color images

    Three Color Composites


    Additive RGB Mixing
    A computer screen and as an approximation also our eyes use the additive mixing of the three primary colors red, green, and blue, called the RGB Color Model. Additive means that the light itself (!) is mixed, for example red and green light is added to yellow. Equal parts of red, green, and blue gives you a neutral color, i.e. gray or white, depending on the intensities.
    For constructing a color image, three images have to be taken in different wavelength regions. They are shown with red, green, and blue color bars in the same chromatic order. The example of the galaxy NGC 4449 shows exposures with the astronomical filters (I + H-alpha), V, and B.
    Different filter exposures shown with RGB color bars

    RGB Three Color Composite of the Irregular Galaxy NGC 4449
    The additive combination of these images results in a natural appearing image with all available colors of the RGB Model. But "appearing natural" might be quite different to a real true color image what is explained in the following:

    Natural Colors

    or True Colors

    Mapped Colors

    or Representative Colors
    If the three different wavelength bands do represent the sensitivity curves of the eyes three different color receptors, we get a "true" or "natural" color image. The astronomical object is shown in a way it would be seen by the eye (if it would be extremely sensitive). Astronomical science of course is not restricted to the wavelengths of the three human sensitivity curves. The electromagnetic spectrum reaches far beyond and often it is very useful to observe in narrow wavelength bands. To visualize these observations, the images are "mapped" from the original wavelengths to the red, green, and blue color bars.

    Constellation Orion

    A conventional color photography has three different layers. The sensitivity curves of these layers approximate those of the human eyes three color receptors and so show natural colors. (But beware of the usual color shifts of emulsions at long exposures.)

    NGC 7635, the Bubble Nebula

    Narrow band filters are used to determine different gases in nebulae. The chemical abundances and temperatures can be reveiled by such images. To visualize these physical circumstances, the images are mapped onto the RGB-color bars and combined to get a natural appearing(!) image. These images are very different from natural colors. You can compare the above image with a natural color image HERE.

    Center of the Globular Cluster Omega Centauri

    Using the astronomical Johnson R, V (visual), and B filters and representing these with red, green, and blue and the right color balance gives a crude approximation of the human eye.

    Open Cluster NGC 2194

    To examine stellar parameters Johnson I, V, and B broadband filters were used. V and B more or less match the human green and blue sensitivities. But Johnson I is a near infrared filter and was mapped to the red color bar to construct this three color composite. But nevertheless such a stellar image with the right chromatic order is very similar to a true color image, because stars do radiate mainly with continuum and not line emission.
    More true three color images The term "false colors" is also used quite often for such representative three color images, whereas I think this is somewhat misleading, because the shown RGB-colors do visualize the physical colors within the used wavelength bands. So the shown colors do have a strong meaning and I propose the more characteristic usage of the above names and having in mind the observed wavelengths.
    More mapped three color images


    N Color Composites

    There is no urgent need to use the three colors red, green, and blue in a composite similar to the human eyes technique. You can use different colors and a different number of colors. The restriction is just that the used colors should be complementary, i.e. the sum of equal parts must result in the neutral color white (more general gray).


    Additive Mixing of the two complementary colors orange and blue
    The most obvious way to construct two color composites on a computer is using the colors orange and a bright blue. In comparison to the RGB-colors as written above, the green color is split into two equal parts and added to red and blue. This is equivalent to constructing the green channel in the RGB-system from the average of the red and blue channel.
    Two images have to be taken in different wavelength regions for constructing a two color composite. They are presented with orange and blueish color bars in the same chromatic order. The example of the galaxy NGC 1961 shows exposures with the astronomical filters I and V.
    Different filter exposures shown with orange and blue color bars

    Two Color Composite of the Peculiar Galaxy NGC 1961
    The combination of these two images results in a natural appearing image. But of course it isn't a natural color image. The resulting colors are restricted to the mixing of the two used primaries. This method is just the best we can do with two available wavelength bands, to show the object as natural as possible.

    Also four and even more primary colors can be merged to a color image with the equal number of different filter exposures. But it is impossible to show these colors independently to the human eye. Our perception is restricted to three primaries. If a color image is shown with four or more primaries, it always will be reduced to a smaller gamut (color sub-space) inside the three color space of our eyes (CIE Chromaticity Diagram). But the point is that we always use media like a monitor screen, color film or a printer that already have restricted gamuts inside the CIE diagram. And using more than three primary colors, like the available four layer films or six color printers, helps maximizing this gamut inside the entire visible color space. But a computer screen is limited to the RGB-colors and a four color image shown as RGB-image would have a much smaller gamut inside the RGB-space. So there is not much sense to do this just for a screen.

    The above described method of mapped colors also applies to the more general n color techniques.


    Color Composites
    with preserved colors

    The usual way of additional combining images to a color image suffers from a loss of color or even false color information. It is common to enhance contrast and faint features in images with nonlinear look-up tables, for example a logarithmic intensity scaling. But doing this to the channels of a color image, changes the ratios of the channels and thus the colors (see definition above). Stars for example usually appear white on color images due to nonlinear scaling and not just because of detector saturation! Realizing this problem, a method of image combination was constructed that preserves the original colors to see the maximum astrophysical content of the image (full article).


    Center of the Globular
    Cluster Omega Centauri

    Open Cluster NGC 1245

    Comet Wirtanen
    More images with preserved colors


    All astronomical images are clickable to get the whole HTML-document with description and larger image size.

    References
  • Foley, J. D., et al., 1996, Computer graphics: principles and practice, Addison-Wesley, 563-604
  • Kaiser, P. K., 1999, The Joy of Visual Perception: A Web Book, York University
  • Kylander, K., Kylander, O. S., GUM, The GIMP User Manual
  • Malin, D., Murdin, P., 1984, Colours of the Stars, Cambridge University Press, 198

    © all photographs taken by Sven Kohle and Till Credner et al.