Telescopes

Most people think a telescope is for making things look bigger. Its main job is something else: to catch more light.

The opening in your eye is only a few millimetres wide, so it catches very little. A telescope is like a much bigger eye. It gathers light with a wide lens or mirror and squeezes it all into yours. Faint things become bright enough to see.

So the wider a telescope is, the more it can show. The first ones, four hundred years ago, were about as wide as a coin. The biggest today have mirrors wider than a house.

Some telescopes are sent into space, above the air. The air makes stars twinkle, which is pretty but blurs the view. Above it, everything is perfectly sharp.

Two jobs. A telescope does two things, and both depend on the width of its main lens or mirror, called its aperture.

Collecting light. The light gathered goes with the area of the opening. A wide-open eye has a pupil about 7 mm across. A modest telescope with a 200 mm mirror collects about 800 times as much, and so shows stars 800 times fainter.

Seeing detail. Light is a wave, and waves spread a little as they pass through any opening. The wider the opening, the less they spread and the finer the detail. Doubling the width doubles the sharpness.

Magnifying is the easy part: any telescope can be made to magnify as much as you like by changing the eyepiece. Past a certain point it only makes a bigger blur.

Lenses and mirrors. Galileo’s telescopes of 1609 used lenses. A lens can only be held round its edge and sags under its own weight if it is large, and the biggest ever used for astronomy is about a metre across. In 1668 Isaac Newton built a telescope that used a curved mirror instead. A mirror can be supported from behind, so it can be made far bigger, and every large telescope since has used one.

Telescope	Mirror width	Where
Galileo’s, 1609	A lens of about 4 cm	Italy
Hubble, 1990	2.4 m	In orbit, about 500 km up
Webb, 2021	6.5 m, in 18 gold-coated segments	1.5 million km from the Earth
Rubin, first pictures 2025	8.4 m	Chile
Gran Telescopio Canarias, 2009	10.4 m	La Palma
Extremely Large Telescope, being built	39 m	Chile

The trouble with air. Air is always moving, and it bends starlight this way and that many times a second. That is why stars twinkle, and it smears their images to several times the size a big telescope could otherwise achieve. There are three answers. Build on high mountains in dry places, above much of the air. Put the telescope in space, as with Hubble. Or measure the blurring as it happens and bend a thin mirror hundreds of times a second to cancel it, which is how the largest telescopes on the ground now match or beat Hubble’s sharpness.

Other kinds of light. Radio waves are far longer than light waves, so to see the same detail a radio telescope must be vastly wider. The largest single dish, in China, is 500 m across. Better still, the signals from dishes far apart can be combined so that they act like parts of one dish as wide as the gap between them. In 2019 a team linking radio dishes on four continents, in effect a telescope the size of the Earth, published the first picture of the shadow of a black hole, in a galaxy about 55 million light years away.

Looking out is looking back. Light takes time to arrive, so a telescope shows distant things as they were when the light left. The galaxies at the limit of Webb’s sight appear as they were more than 13 billion years ago, a few hundred million years after the universe began.

Not only light. Since 2015 there have been detectors for gravitational waves, ripples in space itself, and there are others deep under ice and rock for neutrinos. They are the first ways of studying the universe that do not use light at all.