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I have expressed my opinion about "true color" in astrophotos (there isn't any), yet acknowledge that much of the visual impact of astrophotographs is due to their color! Modern digital imaging systems incorporate color management to maintain the colors between the original scene and its digital reproduction. How can we utilize these tools to reproduce astrophotos? Since we are at liberty to make rather arbitrary choices in this matter, here is a color interpretation we can apply to astrophotos that has some basis in the physics of stars and nebulas. It is well known (though perhaps not widely known), that full-color reproductions can be made using only three primary colorants. Your CRT or LCD monitor uses three colored light sources to render full-color imagery. The exact colors of these primaries depends on the technology involved: CRTs are limited to available phosphor colors, LCDs use absorption filters, film uses three dye layers etc. The colors that can be created using three colorants include those three primary colors, plus all of their in-between colors. A convenient way to show those colors is on a chromaticity diagram, which is a 2D plot of their (conceptual) "hues". The figure below shows the chromaticities of two sets of primaries. One is the set of primaries from the popular Trinitron display. The triangle that is formed represents the range of colors that the display can make; this is the gamut of the display.
The other triangle is a set of primaries for a fictitious display. This display, if it could be built, would be ideal for displaying deep-space scenery: it uses the hydrogen-alpha emission line as the red primary, the oxygen-3 line for making green, and a filtered high-temperature, black-body radiator (to be described) for blue. These are the colors generated by emission nebulas, emitted by hot stars, and reflected by reflection nebula! I'll refer to this set as the "HOT" (Hydrogen-Oxygen-Thermal) primaries. Even though we do not have real-world displays that use these "cosmic" colors, we can simulate what it would look like using color management tools (in Photoshop for example). The simulation is limited to the capabilities of our real displays and printers, but it will provide another interpretation of the colors in an astrophoto, one which may communicate better the content of the scene. For more information about this rather specialized color space, continue to the following topics, or use the previous/next links at the top of the page:
Some general and clarifying topics: |
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