Extended Range and Narrowband Color

Many people are happy to stick to the familiar and conventional, but color offers tremendous scope for creatively different approaches to displaying astronomical data. Extended-range color, for example, allows you to show the different stellar populations in a galaxy by placing infrared-, green-, and ultraviolet-filtered images into the familiar RGB color channels. Because the spectral range is wider, stars with different temperatures will be more strongly differentiated than they would be with standard red, green, and blue filters.

Variations on the extended-range theme include IRG, IGB, and GBU—but there is really no limit. Images from the Spitzer Space Telescope combine spectral bands in the deep infrared part of the spectrum, and astronomers using the Chandra X-Ray Observatory make color images from different x-ray wavelengths. In both cases, the color encodes astrophysically significant information and helps astronomers to visualize what's going on.

Narrowband color is another technique that places interestingly different images into the standard RGB color matrix. As we saw at the beginning of this chapter, the spectra of emission nebulae are rich with lines from different elements.

The relative strengths of these lines vary depending on the temperature, pressure, and abundance of the elements that produce them. An image made in the light of three different elements highlights physical features of the nebula that an HaRGB image, dominated by hydrogen, does not. One such combination is Ha, OIII, and SII placed in the R, G, and B channels.

Another possibility is to replace color intensity images with color ratio images. In this technique, the red channel is replaced with an image created by dividing the red image by the green image, and the blue channel is replaced with an image created by dividing the blue image by the green image. Images made in this way enhance very small color differences, and allow professional astronomers to map differing element abundances and stellar types in distant galaxies.

• Tip: AIP4Wiris color tools should be regarded as just that: software tools to create new, interesting, and exciting types of color images. You can generate color images that combine the output of rank processing and histogram shaping with a linear scaling to explore the morphology of comets you're limited only by your imagination!

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