When making an observation of an astronomical radio source with a parabolic dish radio telescope it is often advantageous to use as large a dish as possible. A larger dish makes it possible to observe weaker sources (increasing the sensitivity), as well as allowing more detail to be seen (increasing the resolution). Unfortunately, the maximum size of a parabolic dish radio telescope is limited by enormous cost associated with building extremely large dishes.
In order to obtain higher resolution than is possible with the largest single dish radio telescope, astronomers have devised a method of combining the signals from an array of several smaller radio telescopes. The distance between any two radio telescopes in such an array is called a baseline, and the resolution that can be achieved is comparable to that obtained with a single dish radio telescope, whose diameter equals the largest baseline of the array.
This technique of combining the signals from an array of radio telescopes is called "radio interferometry". When the radio telescopes that comprise an array are very far apart (possibly on the order of thousands of kilometres), then the technique is called VLBI, or Very Long Baseline Interferometry. Global arrays of radio telescopes now routinely make observations with a resolution equivalent to that which could be achieved by a radio telescope as large as the Earth. To achieve an even higher resolution, a radio telescope was launched into space in 1997.
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