charge-coupled device (CCD)

For a century astronomers caught starlight on photographic plates — sheets of glass coated in light-sensitive chemicals. Then in the 1970s a small silicon chip changed everything. The charge-coupled device, or CCD, is the same kind of electronic light sensor that later went into digital cameras, and it turned astronomy from a craft of darkrooms into a science of precise, countable numbers.

A CCD is a grid of millions of tiny light-sensitive cells (pixels) etched in silicon. When a photon strikes a pixel, it knocks loose an electron, and the pixel stores that electron as a packet of charge. During an exposure each pixel quietly piles up charge in proportion to how much light fell on it. Afterwards the chip 'couples' the charge from pixel to pixel, shuffling it row by row to an output where it is counted and turned into a number for each pixel — building a digital image whose brightness values can be measured exactly.

CCDs revolutionised astronomy because they are vastly more efficient than film. A good photographic plate detects only a few percent of the photons that hit it; a CCD can detect 80 to 90 percent or more, so the same telescope reaches far fainter objects in far less time. CCDs are also linear — twice the light gives twice the signal — which makes precise photometry possible. They have rivals now in CMOS sensors, but the CCD remains the workhorse detector behind most of modern observational astronomy.