Telescopes

When Galileo Galilei first pointed a telescope at the sky in 1609, a run of discoveries began: first mountains on the moon, a few weeks later, in January 1610, four moons around Jupiter, and countless stars in the Milky Way that nobody had seen individually before. His instrument was worse than any toy telescope today. Since then telescopes have become humanity's most powerful eyes, and their most important job is not the one most people assume.

Gathering light instead of magnifying

The most common beginner's mistake: choosing a telescope by its magnification. But the real problem in the night sky is not that objects are too small, it is that they are too faint. A telescope is above all a funnel for light: the larger its aperture, the more light it catches, and the amount of light gathered grows with the square of the diameter. Your dark adapted pupil opens to about 7 mm. Even a small telescope with a 70 mm aperture collects a hundred times more light and thus shows you star clusters and nebulae that stay invisible to the naked eye.

Lens or mirror

There are two basic designs. The refractor focuses light with a lens at the front of the tube, like Galileo's telescope. The reflector gathers it with a curved mirror at the back end, a design invented by Isaac Newton in 1668. Lenses have a catch: they bend different colours by different amounts, which creates colour fringes, and large lenses sag under their own weight because they can only be held at the edge. A mirror reflects all colours equally and can be supported from behind. That is why all the world's giant telescopes are reflectors.

Resolution: making out detail

The second strength of a large aperture is resolution, the ability to separate fine details and stars that stand close together. It is limited by the wave nature of light: the larger the aperture, the finer the angles a telescope can distinguish. Such angles are given in arcseconds, one degree holds 3600 of them, and one arcsecond is about how wide a one euro coin looks from 5 km away. Magnification, by contrast, is easy to change: it is the telescope's focal length divided by the eyepiece's focal length, so a different eyepiece is all it takes. That way you can magnify an image almost without limit, but beyond a certain point it just gets bigger and mushier, because no new detail is added. It is like a digital photo: the aperture sets how fine the grid of image information is, and blowing it up creates no new pixels.

Onto mountains and into space

In practice it is usually not the telescope that limits sharpness but the shimmering air above it, the same air that makes stars twinkle. That is why the great observatories stand on high mountains like Mauna Kea in Hawaii at around 4200 m or in Chile's Atacama Desert: there a large part of the restless, moist air lies below the telescope, and the nights are dark and clear. The most radical step leads out of the atmosphere altogether. The James Webb telescope works with its 6.5 m mirror around 1.5 million km from Earth and observes in the infrared. This lets it look right through dust clouds and receive the light of the very first galaxies, stretched into the infrared by the expansion of the universe during its journey of billions of years.