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We see the Universe as it looks in visible light, the kind of radiation our eyes can detect. Optical telescopes collect more light than our eyes can pick up on their own, and so help astronomers see fainter objects. The bigger a telescopes the more light it can collect and the fainter the things it can see. Photographs and electronic light detectors also help to get the best results from a telescope. But today's astronomers are not only collecting visible light. Stars and galaxies, and the material between them, give out radiation across the whole of the electromagnetic spectrum: radio, and millimetre waves, and infrared with wavelengths longer than visible light; ultraviolet light, X-rays and gamma-rays with shorter wavelengths. There are telescopes to collect all these different kinds of radiation. Only visible light and radio waves pass easily through the Earth's atmosphere. Though some places are much more suitable than other, optical and radio telescopes can work almost anywhere on the Earth. Radio waves even pass through clouds. On high mountain sites, where the air is very clear and dry, it is possible to have infrared and millimetre-wave telescopes. However, infrared observations are best done from telescopes orbiting the Earth in space, well above the atmosphere. Ultraviolet light, X-rays and gamma0rays cannot get through the atmosphere at all and have to be studies from orbiting observatories. Even for optical telescopes, the atmosphere creates a problem. Star images constantly twinkle and look blurred rather than as sharp points, and dust and clouds can make them look dimmer. In April 1990 the Hubble Space Telescope was put in orbit by the Space Shuttle. It is a reflecting telescope with a mirror 2-4 m across. After it was launched, astronomers discovered that a mistake in shaping the mirror meant that images could not be focused as sharply as they should have been. Nevertheless, the Space Telescope has already produced some remarkable pictures, much better than anything taken from the ground, and it should be possible to correct the fault on a future space mission.
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