Telescopes: introduction
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. They can be based:
- on Earth
- in space
Telescopes can be built to look at not only the visible wavelengths of electromagnetic radiation, but all the other wavelengths as well. Sometimes these specialised telescopes are called observatories.
Different wavelengths are blocked to varying degrees Visible light is only slightly affected by the Earth's atmosphere, but infrared, gamma rays, X-rays, and much of the ultraviolet, infrared, microwave and radiowave parts of the spectrum are significantly blocked. However all wavelengths can be detected by telescopes placed in orbit round the Earth - though placing telescopes on high mountain tops or on hot-air balloons helps a lot with some wavelengths.
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.
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.
- They cannot be used in areas where there is bright street lighting (“light pollution”).
Radio 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-ray, ultraviolet & infrared telescopes
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Chandra X-ray Observatory
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International Ultraviolet Explorer
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X-rays are partly blocked by the Earth's atmosphere and so X-ray telescopes need to be at high altitude, flown in balloons or be satellites in space. The same applies to infrared telescopes and ultraviolet telescopes.
Some well-known telescopes
• Hubble Space Telescope
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This optical telescope was launched into orbit round the Earth in 1990. Because there is no atmosphere to get in the way, images are not blurred. The telescope has viewed the most distant objects ever. It detects near ultraviolet, visible and near infrared parts of the electromagnetic spectrum. |
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| • Spitzer Space Telescope | This was launched into space in 2003. It detects
infrared energy radiated by objects in space. |
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| • SETI |
The Search for Extraterrestrial Intelligence (SETI) project uses radio telescopes to look for non-natural signals coming from space. It should be possible to detect even alien TV programmes, if they exist! (It is likely such alien civilisations will be far too sophisticated to watch programs like Eastenders or Big Brother.) |
 Part of an array of SETI radio telescopes in California USA |
Space probes
Space probes are unmanned spacecraft that are not designed to return to Earth and that can visit other planets without the need for astronauts. A lot of the information we have about our planets has been provided by probes. Some of the missions undertaken by such craft include the following Mars probes:
| • Viking 1 and Viking 2 | Landed on Mars in the 1970s, took
photographs and analysed soil samples |
 The Spirit roving vehicle |
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| • Mars Global Surveyor | Went into orbit around Mars in 1997 and mapped
the surface in 3D |
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| • Spirit and Opportunity | Twin robot vehicles that landed on Mars in
2004. |
Unmanned spacecraft are cheaper than manned spacecraft, and safer, as they do not carry any astronauts. It is very difficult to repair an unmanned spacecraft in Earth’s orbit, and impossible once it leaves orbit. So the components are designed to be rugged and reliable. But history shows that systems in unmanned spacecraft do fail. This may mean that parts of the mission cannot be completed. Sometimes the whole mission is lost.
Extraterrestrial life
Astronomers have detected planets around some nearby stars.
As there are a lot of stars in the Universe, many scientists think it is likely life exists on other planets. But there is no evidence yet to support this view.
Various kinds of telescope are being used to search for:
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Oxygen-using life forms. The Earth's atmosphere contains a lot of oxygen - the result of photosynthesis by plants. If we find evidence of oxygen in the atmosphere of another planet, it could indicate the presence of life forms which function something like those on Earth. . It is possible to detect oxygen and other gases on other planets by studying the light (from their sun) reflected off them.
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Intelligent
life forms. It is thought possible that alien civilisations
capable of transmitting radio signals may exist. In addition,
some researchers have considered the possibility that alien civilisations
might be using powerful lasers for interstellar communications
at optical wavelengths.
[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science ]