Jodrell Bank - The Quest for High Resolution

The Lovell-Mark II Interferometer.

The 5 km Ryle Telescope operated by Cambridge University where Martin Ryle and colleagues developed the technique of Earth Rotation Aperture Synthesis.

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The Quest for High Resolution

A problem of wavelength and size

The Lovell Telescope has a large collecting area and thus provides high sensitivity but it is not even as good at discerning fine detail as the human eye! This is because radio wavelengths are typically 100,000 times longer than those of visible light and so, to give the same theoretical resolution, a radio telescope needs to be 100,000 times larger than an optical one. This would require telescopes many tens of kilometres across, clearly not a practical possibility. To solve this problem, radio astronomers use the technique of interferometry in which two telescopes are linked together electronically to imitate a telescope whose diameter is equivalent to the distance between them. A simple interferometer can allow the precise position of a radio source to be found and it is also possible to learn something about its structure.

Arrays

The technique can be extended to link together a number of telescopes to form an array. At a given instant of time each pair of telescopes in the array gathers some information about the structure of the observed radio source. As the Earth rotates, the orientation and effective baselines of the telescope pairs change allowing them to gather further information. After 12 or more hours of observation, sufficient data are acquired to enable a radio image of the source to be made. The technique is called Earth Rotation Aperture Synthesis.

Linking the telescopes

The signals from the telescopes can be brought together in several different ways. Where the telescopes are on one site, cables or waveguides are used. Where greater distances separate the telescopes, as in MERLIN, microwave radio links are used at present, but these may be replaced by fibre-optic links for greater bandwidths and sensitivity. When the telescopes in the array are spaced across a continent, it is not currently feasible to combine their signals as they are received. Instead, the data are stored on large reels of 1" wide magnetic tape with each block of data precisely "time stamped" to better than a microsecond. The tapes from each observatory are then brought to a processing site where they are played back in precise synchronisation before being combined together in a computer to form radio images.

What can be achieved?

The largest array located on one site is the Very Large Array (VLA) in New Mexico, USA where the telescopes can be up to 36 kilometres apart. This provides a typical resolution of 1/3 of a second of arc, similar to that of optical telescopes operating on the Earth's surface. The 217 km MERLIN array, shown on the inside cover, can typically provide a resolution of 1/20 of a second of arc, comparable to images produced by the Hubble Space telescope, whilst the European VLBI Network (EVN) gives a further factor of ten improvement.

The 5 km Ryle Telescope operated by Cambridge University where Martin Ryle and colleagues developed the technique of Earth Rotation Aperture Synthesis.