1969

Astronomy Goes Digital

Willard Boyle (b. 1924), George Smith (b. 1930)

For millennia, astronomy was a visual craft, with practitioners relying on their keen eyesight and excellent night vision to find and characterize celestial objects. Even two hundred years after the invention of the telescope, the only “detector” available to astronomers was still the human eye. The introduction of astrophotography in 1839 provided astronomers with more-sensitive light-detection devices—first silvered glass plates and then sensitized photographic films. Although it was a huge advance in data archiving and observational repeatability, photography still provided only a modest improvement in the ability to record light from fainter astronomical sources.

A gigantic advance in astronomical detector sensitivity was made possible by the development of radar and navigation electronics for aircraft and weapons during World War II. These advances led to the invention of the first electronic switches—diodes—around 1939, and the first electronic amplifiers—transistors—in 1947. These devices relied on the properties of certain elements, such as silicon or germanium, that are not quite conductors (as metals are) and not quite insulators. Such semiconductors are materials that generally don’t conduct electricity but that can be coerced, by applying the right kind of voltage or, in some cases, just by shining light on them, to conduct electricity.

The critical semiconductor advance for astronomers (and, ultimately, for digital camera and cell phone users) came in 1969, when the American physicists Willard Boyle and George Smith at AT&T Bell Labs created an array of semiconductors that converted incoming light—photons—into analog voltage signals that could then be stored, amplified, and converted into digital numbers. The invention was called a charge-coupled device (CCD) because the incident light on the array is coupled to the electrical charge generated.

Astronomers instantly fell in love with CCDs as they started becoming available in the 1970s and 1980s, partly because their output signal is linearly proportional to the brightness of objects shining on them, and partly because they are one hundred times more sensitive than photographic film. CCD cameras are now standard equipment for astronomical telescopes as well as space missions.

Example of a modern charge-coupled device (CCD) semiconductor detector used for astronomical as well as consumer electronics imaging.