Of all these diverse imaging accomplishments stretching across the century, perhaps the greatest revolution has been in telescopes. A telescope's light-gathering power is determined by the size of its aperture: the wider its diameter, the more light a telescope can gather and, therefore, the dimmer the celestial objects it can detect. Before 1900 the best telescopes were refractors, gathering and focusing light through a series of lenses arranged in a long tube. But refractors can get only so big before the weight of their lenses, which must be supported just at their edges, becomes too great; the practical limit turned out to be 40 inches. Reflecting telescopes, on the other hand, use a mirror to gather and focus light, and the mirror can be supported under its entire area. Improvements in mirror-making techniques after the turn of the century opened the door for the telescope revolution. Under the direction of American astronomer George Ellery Hale, engineers built a series of increasingly larger reflecting telescopes: a 60-inch version in 1908, followed by a 100-inch giant in 1918, and then a 200-inch behemoth, named after Hale and completed in 1947, 9 years after his death. This trio stood at the pinnacle for many years and unlocked a host of cosmic secrets, including the existence of galaxies and the fact that the universe is expanding. Today, the largest reflectors, using sophisticated techniques that link the light-gathering power of multiple mirrors, have effective apertures of up to 400 inches. And dramatic advances in light-sensing equipment, including photodiodes that can detect a single photon, have added to the wonders revealed.
The year after the Hale telescope was completed, a radio engineer named Karl Jansky made a discovery that initiated yet another telescopic revolution. He determined that a constant background static being picked up by sensitive radio antennas was actually coming from space. It was ultimately identified as residual radiation from the Big Bang that gave birth to the universe. Within a few years, astronomers were training radio dishes on the heavens and learning to see with a whole new set of eyes. Radio telescopes were even easier to link together; using a technique called interferometry, engineers could create radio telescopic arrays made up of dozens of individual dishes, with a combined aperture that was not inches, but miles, across.
Telescopes now also orbit Earth on satellites, the most famous being the Hubble Space Telescope, which includes several different imaging devices—optical among others—and has produced cosmic views of astounding clarity. Crucial to that clarity are detectors called charge-coupled devices (CCDs), electronic components that convert light into electrical signals that can be interpreted and manipulated by computer. The most refined CCDs consist of hundreds of millions of individual picture elements, or pixels, each capable of distinguishing tens of thousands of shades of brightness. CCDs have become essential components not only in optical telescopes but also in digital cameras, achieving resolutions that rival the best of older photographic techniques.
Hubble is not alone out there. Its spectacular optical images are only part of what we can see of space. Orbiting x-ray observatories give us a ringside view of the most violent cosmic events, from the birth of stars to the gravitational collapses that form black holes. Gamma-ray detectors tell stories of other cataclysmic events, some still so mysterious as to defy explanation. And infrared instruments, picking up dim signals from the deepest reaches of space, reveal details about the whole history of the universe, back to its very beginning. With this new breed of imaging devices the eyepiece is long gone, but the view is still riveting.