Home

Astrophotos

Flathead from Space

What's New

Photographs contain information that digital image processing can isolate and/or enhance easily and quickly. Beginning with a 30-second exposure on Fujicolor 1600 color negative film using a 200mm lens, I scanned the film and made a digital positive. The image is pleasing and shows both the ionized gas trail (left, bluish) and the dust trail (right, pinkish white). The faint tails extend well across the night sky.

Next, I converted the image from a continuous tone image to a discrete tone image using the posterize function in Adobe Photoshop 4.0. By creating discrete regions of luminosity, posterizing exaggerates the gradient of brightness of the comet's tails and the gases surrounding its head.

I then used Adobe Photoshop 4.0 to look at the images formed by different wavelengths of light. The additive primary colors comprising all images are red, green, and blue (the subtractive, or complementary, primaries used in printing are magenta, yellow, and cyan). Color film records a scene's color by recording, on different layers in the film, images of the scene made by red, green, and blue light. A scanner records a film's color by recording the image through red, green, and blue filters. Thus, the digital color image comprises red, green, and blue channels (there are other color models, but RGB is the most useful for this project). Here is what the red, green, and blue channels for the image above look like in grayscale:

Notice how much more detail is in the blue channel. Comets tend to be bluish; thus recording the image in blue light provides more information than recording it in red or green. Recording it in ultraviolet or infrared or other nonvisible wavelenths would produce different images. In true tricolor photography or digital imaging, three grayscale images of an object are made using red, green, and blue filters. The grayscale images are composited, either in the darkroom or digitally, to create a color image. Classic tricolor photography is far more useful for science than my synthesizing technique because the bandpass of each tricolor filter is known.

Finally, a grayscale astrophoto can be inverted to produce a negative image showing the bright objects as dark spots against a clear background (which looks white on a light box). Astronomers find negative images easier to work with. The image below is the inverted blue channel of the posterized image above. The extent of the tails is much more visible. (The dark spots are stars. The gray spots are imaging artifacts.)