Choose a mission from below to learn more:

• ALMA
• Herschel
• JWST
• Planck

Why we use space telescopes

Although there are very many powerful telescopes operating on the surface of the Earth, often in the mountainous regions of the world (eg.Vista which will be built in the mountains of the Atacama Desert), there are many advantages to sending telescopes into space. The major advantage is that telescopes in space are not hindered by the Earth's atmosphere. This is vital for detailed infrared astronomy.

The image to the right (click to enlarge) shows a graph which demonstrates which wavelengths of light can pass through our atmosphere. Atmospheric opacity is the factor to which light is blocked by the atmosphere, thus 100% represents a total block.

As shown, the majority of the infrared region of the spectrum is absorbed by atmospheric gases. Only a small range of near infrared light (with wavelengths just longer than visible red) is observable from Earth.

Therefore for astronomers to make use of the entire infrared range it is necessary to use space telescopes which are above the Earth's atmosphere and thus are not affected by the gases in the Earth's atmosphere which absorb light.

Survey telescopes

Iras and Vista are examples of infrared survey telescopes. Survey telescopes have a much wider field of view than normal telescopes or observatories than thus can take images of a greater portion of the sky. However, they lack the capacity to look at objects in depth. So while a survey telescope can see entire galaxies, they cannot focus in on one particular group of stars.

Survey telescopes are often used to make sky-maps by sweeping their wide-field view across all of the celestial sphere and thus making a map of all of the universe that is visible from earth. These maps are then used to identify and catalogue new infrared sources and to locate more areas of scientific interest for more detailed study by observatories.

Infrared observatories

When scientists have found areas of interest such as regions where stars are being formed in stellar nurseries using the sky maps, more detailed study is then performed using infrared observatories, such as ISO, Herschel and JWST.

Observational satellites have much greater imaging and resolution capabilities than survey telescopes, which have a much narrower field of view allowing for in-depth analysis of specific galaxies and star clusters. Observatories often also include non-imaging instruments such as spectrometers.

Interferometers

Beyond the infrared, at wavelengths longer than the sub-millimetre ranges, it becomes much harder to make clear images.

Resolution power is a measurement of the minimum angle between two observed objects at which they appear clear and separate. The relationship between resolution and wavelength is such that to gain a suitable level of resolution for longer wavelengths, the size of the telescope used must be much larger. Thus radio telescope are often parabolic dishes several metres in diameter (eg. the dish of the Lovell telescope, shown left, is 76.2 m in diameter).

However, even the largest telescopes do not provide the resolution needed for detailed study of distant galaxies on their own.

Interferometry is a technique where several radio telescopes are used together in an array to view the same object at the same wavelength. This, in effect, combines the telescopes into the equivalents of a single telescope with a diameter equal to the distance between the outermost telescopes. This dramatically improves the resolution of the instruments and has been so successful that larger arrays are being constructed; ALMA will have an effective diameter of 10 km.