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Multiwavelength Milky Way: Making an Image

mosaic image of portions of separate Milky Way images

Return to Maps of the Sky
I. Telescopes and detectors - how they gather light
II. Reconstructing the image
III. False colors - how colors are used to "see" an invisible image

Telescopes and detectors
See "Different telescopes for different wavelengths".

Reconstructing the Image
If data can be stored from a detector digitally, the information can be quickly transferred from the instrument to a receiving computer. The data is first converted into binary numbers (numbers created using only 1s and 0s). After transmission to a receiving computer, the data is reconverted to the original digital information, which can then be used to construct an image. Above we said that, for a CCD, the number of photons hitting a pixel is assigned a single number representing the total brightness for that pixel. Each pixel on the CCD has a specific location. For example, there is only one pixel on row20, column300. When reconstructing an image, the data is laid out on a grid that matches the CCD. Then each brightness level can be assigned a color (see below), and an image appears.

example of CCD image
An example of what a CCD image
might look like
example of CCD image grid
A grid with numbers corresponding to
brightnesses in the CCD-like image to the left

False Colors
A true-color image is the kind of photo we're used to. If we substitute orange for purple, blue for yellow, then we have a false-color image.

cartoon tree
true-color tree 		cartoon tree-false color
false-color tree

Colors can be associated with the intensity of light from an image, or with temperature.

Remember the "paint by numbers" kit, where each number corresponded to a specific color? If you followed the directions, a picture would appear. Astronomers do a similiar thing when viewing and interpreting electromagnetic radiation (light) from distant objects. Other than visible light, electromagnetic radiation can't be seen with the eye. So, when light (such as X-ray light) is detected, astronomers assign different colors to different brightness levels. That way, they can visualize the object being studied.

Usually, the brighter levels are color-coded in red colors and the fainter levels are coded in blue/purple colors. For example, in the CCD-image example above, a false color image might look like the following:

example of CCD image in color
Noteworthy:
In connection with COBE and the TARP (Technology And Research Partnership) project, two high school students have written a short discussion on how DIRBE scanned the sky.

Resources:
Getting the Picture from Hubble Space Telescope - how images are made

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This service is provided by the Astrophysics Science Division (ASD) at the NASA Goddard Space Flight Center (GSFC).
Web Curator:
Karen Smale

Responsible NASA official:
Dr. David Leisawitz: David.T.Leisawitz@nasa.gov

Version 2.0, September 2001