Sky on a Chip: The Fabulous CCD

TELESCOPES today are no bigger than those in use two generations ago. Yet, curiously, astronomers can now study objects that are hundreds or even thousands of times fainter th;3.ll those accessible in the 1930's. • The explanation is simple: the last few decades have brought an unprecedented improvement in the sensitivity of light detectors. The changes in our own era are as significant as those caused by the development of the telescope in the 17th century or the photographic plate in the 19th century. Of all the new astronomical tools, one in particular stands out because it is an almost perfect radiation detector. Known as a charge-coupled device, or CCD, it has the ability to register almost every photon (particle of light) that strikes it over wavelengths ranging from the near infrared to X-rays. In contrast, the most sensitive photographic emulsion used by astronomers records two or three percent of the light striking it, and over a much narrower range of wavelengths. The CCD revolution came about indirectly. It began in the late 1960's when two researchers at Bell Telephone Laboratories, Willard S. Boyle and George E. Smith, sought to invent a new type of computer memory circuit. In particular, they wanted to construct an electronic analog to magnetic bubbles. While Boyle and Smith conceived the CCD as a memory element, it rapidly became apparent that the tiny chip of semiconducting silicon they first demonstrated in 1970 had many other applications, including signal processing and imaging (the latter because silicon responds to visible light). In recent years the CCD's initial promise as a memory element has vanished. It has, however, established itself as the premier imaging device for scientific applications, especially astronomical ones. Indeed, CCD's are at the forefront of an ongoing revolution in all types of electronic photography. They are already common in lightweight video systems and auto-focus cameras. This picture of Uranus is thought to be the first astronomical image made with a charge-coupled device, or CCD. It was obtained in 1975 by scientists from the Jet Propulsion Laboratory and the University of Arizona, using the 61-inch telescope in the Santa Catalina Mountains near Tucson. Recorded at a wavelength of 8900 angstroms in the near infrared, it shows a region of enhanced methane absorption (dark area) near Uranus' south pole.