radio telescope

Visible light is only a sliver of the electromagnetic spectrum. Stretch the waves to centimetres or metres long and you reach radio waves, which the universe pours out in floods — and which pass right through clouds, dust, and even daylight. A radio telescope is an instrument built to gather these long waves and turn them into a picture of the radio sky.

Most radio telescopes look like enormous metal dishes — curved reflectors, often tens of metres across, that focus incoming radio waves onto a receiver at the focus, exactly as a mirror focuses light. Because radio waves are millions of times longer than light waves, a dish must be huge to capture much signal and, by the diffraction limit, to see any fine detail at all. The signal collected is not an image but a faint electrical current, which electronics amplify and computers convert into maps of brightness across the sky and across radio frequencies.

Radio astronomy opened a hidden universe. Radio waves trace cold hydrogen gas through the 21-centimetre line, mapping the spiral structure of our Galaxy; they reveal pulsars, quasars, the synchrotron glow of relativistic electrons, and the faint afterglow of the Big Bang. Because a single dish has poor resolution at these long wavelengths, radio astronomers routinely link many dishes into interferometers, synthesising the sharpness of an instrument as wide as the array — the trick behind facilities like ALMA and the Event Horizon Telescope.