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Creating Colorful Comets

by Noah Goldman
U. Maryland, College Park Scholars

"I see skies of blue, and clouds of white ... and I think to myself 'What a wonderful world.'"
-Louis Armstrong, "What a Wonderful World"

The universe is indeed a colorful place, but what does it look like? For decades astronomers have taken pictures of the heavens, and have produced some spectacularly colorful images. Astronomers have allowed mankind to view the heavens in all its colorful glory, and shown the universe in a rich palette of colors. But where do these color pictures come from? How can a camera tell the difference between one color and another, and how can a black and white image become color?

In the days before digital photography, astronomers recorded images from their telescopes onto photographic films and plates. Photographic film is made from a light sensitive (or photosensitive) material, which changes color when exposed to light. The change in color varies for different wavelengths (or colors) of light, and records the image onto the film. Once the image is recorded, a series of chemical processes are used to transfer the image and colors from the film onto paper, thus producing a color photograph.

Digital photography, on the other hand, uses revolutionary techniques to create an image, which introduce their own advantages and disadvantages. Digital photography uses a device called a Charge-Coupled Device, or CCD, to record an image. The CCD measures and records how much light strikes its wafer thin surface, and stores the pattern using computer memory (for more info on CCDs check out Swinburne University's PowerPoint presentation). A CCD records light with no regard for its color. As a result it can only record black and white images. How, then, does a digital camera record a color image? The color must be added in at a later time.

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In 1985 Harold Reitsema coined the term falsi colori (Italian slang for "false color") for describing the color display of digital images. At the time Reitsema was working in Europe to develop a method for broadcasting Halley Multicolor Camera (HMC) images, of the comet Halley encounter, on the world TV network that looked just as good on black & white TVs as on color TVs. This way the networks could use a single signal to broadcast their shows, and they would still look good on both color and black and white TV sets. This was not an easy task, but Reitsema devised a way to make it work. In tribute to his work, some astronomers still use Reitsema's terminology today when creating digital color images, Falsi Colori.

Scientists have developed two different methods for creating false color based on the idea of adding the color in later; true color composition and color tables. In the color table method a number of color schemes are created that assign color to an image based on brightness. When a color table is applied to an image, the color of each pixel is chosen based on its brightness relative to all of the other pixels. Every range of intensities is then assigned a specific color, and the appropriate color is added wherever the image brightness falls within that range.

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“When we see comets up in the sky they're really spectacular. But unless you get close to a comet, you can't really figure out what's going on.”

-Joe Veverka