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 . 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.

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.

Figure 2

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.

Using a color table is often quite useful in enhancing certain features of an image for analysis. As an example, the image in figure 2 uses the false color scheme entitled Hardcandy. This color selection has sharpened the difference between the flash of the impact and the ejecta, making the shape of the ejecta easier to define. Unfortunately, such color schemes do not correspond in any way to the true colors of the object under observation, and require a color bar (such as the one in figure 1) in order to interpret them. Furthermore, images shown using color tables, while informative, often appear cartoonish and unrealistic, and can obscure some of the more spectacular features of an image. For this reason the method of "true color" composition was created.

Figure 3

In true color composition a few pictures are taken of the same scene using different color filters. A filter will block all frequencies of light except for a certain color (or colors), and give a complete image of the scene in that color. In the case of astronomy, astronomers usually take pictures with three primary filters, red, blue, and green, and, using a computer, paint each image in its respective color. The three images are then combined together and the colors mix together. As figure 3 shows, when two or more of the primary colors overlap in any part of the image colors other than the primary three are produced. These composite images are then released and described as "true color."

True color images offer a glimpse of how the object might appear if it could be viewed with the naked eye. They provide stunning pictures of planets, moons, and comets that give them realistic faces. Such images fuel the imagination. They are, quite literally, the stuff that dreams are made of. Yet, true color images are not all sugar and spice, either. The resulting picture can vary greatly depending on the filters used, and the colors used to paint each filter image. These choices are still somewhat arbitrary, and depend heavily on the available filters, as well as the personal preferences of the astronomer making the image. This can result in images that while pretty and awe inspiring, still do not necessarily reflect the true colors of the astral body.

Color images are often some of the most dramatic and informative images in astronomy. In truth, however, the color is false, and not completely scaled to our eyes' response. The colors can vary from image to image and astronomer to astronomer with few bounds. Ultimately, they are nothing more than falsi colori.

References

Delamere, Alan. "More Falsi Colori" E-mail to Alan Delamere. Aug. 23, 2005.

Wilson, T., Nice, K. and Gurevich, G. "How Digital Cameras Work," HowStuffWorks.com, Aug. 26, 2005

Figures

Figure 1: An example color bar for the Hardcandy IDL color table. This color bar shows that the lowest values are displayed in dark blue and green and the highest values are shown in orange and forest green with this color table. This image was created by Noah Goldman for Deep Impact.

Figure 2: Impact of comet Tempel 1 shown using the Hardcandy IDL color table. The nucleus of the comet is visible in blue and maroon and the impact flash is ringed in multiple colors, reflecting the decrease in flash intensity further from the site of impact. This image was created by Alan Delamere for Deep Impact.

Figure 3: The three primary filter colors, red, green, and blue, mix together to create other colors. Red and green make yellow, green and blue make cyan, blue and red make magenta, and blue, green, and red make white. This is how astronomers create colors in true color composition. This image was created by the Maryland Institute College of Art. Reused with permission from www.mica.edu

Noah Goldman first started working with Deep Impact as a student intern from the College Park Scholars program, a freshman-sophomore living-learning community at the University of Maryland. Noah has continued to work with the project working mostly on analysis but also writing articles for the website.