The complicated way: interferometry

To overcome this problem, the technique that is now used by radio astronomy (and the Westerbork Synthesis Radio Telescope is one example) is interferometry. The idea is that it is not really necessary to have a "filled" parabola as long as the signals coming from the "separate" parts of the telescope can be combined. In other words, a big telescope, like Arecibo, does not have to be made with a single large "filled dish", but it would work almost as well even if we take some parts of it away! We would still be able to produce an image of the celestial object (although the "collecting area" would, of course be smaller and therefore the telescope less sensitive). Radio interferometry works using this idea.

A number of separated radio telescopes are used like pieces of a giant single dish antenna: this technique is called "aperture synthesis". In the case of the WSRT, the 14 dishes (each only 25 m) are used to "create" a huge radio telescope of 2.7 km diameter, i.e. the distance between the two most distant antennae.

In Westerbork the telescopes are distributed along an east-west line. Thanks to the rotation of the earth, they "simulate" a 2.7 km big radio telescope. Observations last usually for 12 h. During this time the earth makes half a rotation around its axis. Thus, as seen from the radio source in the sky, the position of the WSRT antenna changes continuously and at the end of the 12 h the 14 WSRT antennae have described a full circle and have filled part of an "imaginary" huge single-dish telescope. The more antennae available, the more the "imaginary" dish is "filled" and better images can be made.

In order to make aperture synthesis possible, the signal coming from each telescope has to be combined in a coherent way. This means that the signals should be combined as they would be at the focus of a single dish. Thus, the same part of the wavefronts has to be added from the different telescopes. However, as illustrated in the figure (in the case of two antennae used as an interferometer) the path the wave has to travel to reach one telescope is longer than that to reach the other and therefore there is a "delay" in the arrival of the wavefront at one telescope. This means that at a certain time, the trough of the wave could be arriving at one telescope while the crest is arriving at another. If these signals would be "blindly" combined they would (in the worst case) cancel out! Thus, because of a delay in reaching the different telescopes. a correction has to be introduced electronically to compensate for this. The signals arriving from all the telescopes are then combined by a powerful computer after this correction is made.

Images made using WSRT data

Because the array of dishes is one-dimensional, at any instant of time they produce a response which is "sharp" in one direction only. The instrument then uses the rotation of the earth to view the source from different orientations, and in this way a telescope beam can be built up which is sharp in all directions.

Click on picture to see a movie showing how the image builds up during a 12 hour observation, first with a long, narrow beam almost horizontal, but then rotating in a clockwise direction as more data are accumulated, until at the end of the observation (after 12 h) the image is complete. The prominent rings around bright, compact sources are part of the beam produced by a telescope like the WSRT. They result from the fact that we have collected information like a set of concentric rings rather than from an entire dish surface.

Go further: Radio telescopes in the world