Radio telescopes observe the Universe at very long wavelengths (up to several metres) in the same general part of the spectrum as an ordinary radio. Just as streetlights can block out the light from stars, so mobile phones and satellite transmissions can prevent radio telescopes observing faint radio sources such as distant galaxies.

The larger the telescope, the more detail you can see. To see the same amount of detail as the human eye you would need a radio telescope over 500m in diameter! As this is impractical, astronomers use interferometers where several telescopes are spread out over a large area and the signals from each receiver are combined in a computer called a correlator.

Famous radio observatories include MERLIN in the UK, the EVN in Europe, the VLA in the USA, and ATNF in Australia.