A CCD (Charge Coupled Device) is an electronic sensor that can detect and record patterns of light in a way similar to photographic film. But for astronomy, CCDs have several advantages over film, the chief being that they are much more sensitive than film and therefore can be used to photograph celestial objects too faint to record on film. (An example of this is the famous Hubble Deep Field image, which captured thousands of extremely distant galaxies too faint to be recorded with film.)

Another advantage of CCDs is that while film has to be processed and developed chemically, CCD sensors can be read directly by a computer and the image transmitted by wire or radio. That capability has made possible remotely operated and spaceborne telescopes such as the Hubble Space Telescope. It would have been impractical for the Hubble to shoot rolls of film that then had to be developed (and sent back to Earth); instead its CCD sensors are read by its onboard computers and the images transmitted electronically to Earth as soon as they are taken.

For these reasons and a number of others, CCD sensors are the basis of all modern optical astronomy. At the focal point of every modern telescope there is a CCD camera recording the images and passing the data to a computer for recording and analysis. If that seems like exotic technology, consider that you probably own or use CCDs yourself: All modern camcorders use CCD sensors, as do all digital cameras, image scanners, and many bar code readers.

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