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How telescopes work

Distant stars and galaxies are too far away for us to reach. We cannot go to them to study them. So everything we know about distant stars and galaxies comes from analysing the radiation they produce.

Telescopes are devices used to observe the universe. There are many different types and some are even sited in space.

Optical telescopes

Optical telescopes observe visible light from space. Small ones allow amateurs to view the night sky relatively cheaply but there are very large optical telescopes sited around the world for professional astronomers to use.

Optical telescopes on the ground have some disadvantages:

  • they can only be used at night
  • they cannot be used if the weather is poor or cloudy.

Other telescopes

Radio telescopes detect radio waves coming from space. Although they are usually very large and expensive, these telescopes have an advantage over optical telescopes. They can be used in bad weather because the radio waves are not blocked by clouds as they pass through the atmosphere. Radio telescopes can also be used in the daytime as well as at night.

X-rays are partly blocked by the Earth's atmosphere and so X-ray telescopes need to be at high altitude or flown in balloons.

Space telescopes


The Hubble telescope is an optical telescope in space.

You may wish to view this BBC News report (2005) about the Hubble Space Telescope.

Objects in the universe emit other electromagnetic radiation such as infrared, X-rays and gamma rays. These are all blocked by the Earth's atmosphere, but can be detected by telescopes placed in orbit round the Earth.

Telescopes in space can observe the whole sky and they can operate both night and day. However, they are difficult and expensive to launch and maintain. If anything goes wrong, only astronauts can fix them.

© NASA, ESA and H. Richer (UBC)

White dwarf stars captured by the Hubble space telescope

Refracting telescopes

A refracting telescope works bending light through a lens so that it forms an image. There are a few problems with refracting telescopes:

  • some of the light reflects off the lens so the image is very faint
  • large lenses are needed to improve the magnification – this can be difficult to do perfectly.
Refracting telescope


A refracting telescope

Reflecting telescopes

In a reflecting telescope the image is formed by reflection from a curved mirror. It is then magnified by a secondary mirror.

Reflecting telescope


A reflecting telescope


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