There are two ways to use radio telescopes:
- a (conceptually) "easy" way: a single dish
- a complicated way: interferometry

The (conceptually) "easy" way: a single dish

A single-dish radio telescope is usually an antenna shaped as a parabola that collects the radio waves from a celestial object and focuses them onto a receiver. The signal received is then electronically processed so that it can be stored and analysed by a computer.

The radiation from a certain direction in the sky reaches the parabolic surface of the telescope and is reflected into the "focus".
In the antenna the radiation induces a certain electric current that goes in to the receiver.
This receiver amplifies the signal many thousands of times. The signal goes through a cable to the control building where it is amplified again and converted to a more understandable format, recorded in a computer and from this an image is made.

The telescope has to have two important characteristics to be really effective and allow one to detect faint and distant celestial objects:
- it should have a large collecting area (large telescopes can collect more electromagnetic waves and therefore detect fainter objects)
- the resolution (how close things can be together and still be distinguished as separate) should be as good as possible.

A big telescope can better separate objects that are close together. However, size in meters is not the whole story: the diameter of the telescope should also be many times (as many as possible!) greater than the wavelength of the radiation it detects. In other words, the longer the wavelength of the radiation that we radio astronomers want to receive, the larger the telescope has to be in order to have the same resolution.

An optical telescope with a diameter of 10 cm already gives the best resolution obtainable from the ground due to the effect of the atmosphere: such a telescope gives a resolution of about 1 arcsec which means it will be able to separate two people 1 meter apart at the distance from Groningen to Amsterdam (about 200 km). On the other hand, a parabola of 30 metre (quite typical for a single-dish radio telescope) observing at 1 cm wavelength will separate the two people only if they are 60 metre apart. So to get the same resolution as obtained from the optical telescope we need a parabola of more than one kilometre: something that is impossible to build with the current technology!

This has been a big limitation for single-dish radio telescopes and they are at most the size of 100 m (with the exception of the 300 m antenna in Arecibo, that, however, being built in a valley cannot move and can track objects only over a small part of the sky). For a long time, at the beginning of radio astronomy, this was the main limitation: it was not possible to obtain information about celestial objects comparable to what the optical telescopes were getting. In particular, it was very difficult to identify which object in the sky seen by optical telescopes was actually emitting radio emission, because the resolution of the radio telescopes was so poor.

Go further: The complicated way: interferometry