Charge-Coupled Device

Astronomers began using charge-coupled devices in their work in the early 1980s, when the increasing power and clock speed of semiconductors, and the computers needed to drive the hardware and analyze the data became both fast and affordable. Almost every field of astronomy was directly impacted by CCDs: for observations of asteroids, galaxies, stars, and planets, whether by direct imaging or the recording of spectra, the CCD rapidly became the detector of choice.

CCDs are also useful to astronomers because an average, CCDs are about ten times more light-sensitive than film. Astronomers are notorious for finding desperately faint objects to observe, so the CCD gave them the ability not only to see fainter objects than they could before, but to reduce the amount of time spent tracking and observing a given object. A CCD camera can record in a 15 minute exposure the same information that would take a standard camera loaded with film two hours or more. While film typically records only 2–3% of the light that strikes it, charge-coupled device cameras can record between 50–80% of the light they detect. Furthermore, CCDs can capture light outside the visible spectrum, which film cannot do. The devices operate without darkrooms or chemicals, and the results can be reconstructed as soon as the information is loaded into an image processing program.

However, CCD cameras do have some drawbacks. The small size of the most affordable arrays results in a much smaller field of view. Large celestial bodies such as the moon, which are easily photographed with a 35mm camera, become very difficult to reproduce as a single image with a CCD camera. Although larger arrays are coming to the market, they remain pricy and beyond the resources of the amateur astronomer. They require complicated systems to operate, any many of them have to be cooled to typical temperatures of -112°F (-80°C) to reduce their background electronic noise to an acceptable level. Finally, color images for astronomical CCD cameras (unlike commercially-available video and digital still cameras) require three separate exposures for each filter used. The final image has to be created by combining the data from each exposure within the computer.