The largest fully-steerable parabolic telescope is the 100-m radio dish near Bonn, West Germany, At a wavelength of 6cm it has a resolution of about 2 arc min. To achieve a resolution at radio wavelengths comparable with that set by seeing limitations at optical wavelengths would require a telescope at least fifty times larger. The impossibility of building such a dish, 5km in diameter. does not mean that high resolution radio telescopes cannot be
built.

To see how this is done, consider what happens when we combine the radio signals from two small telescopes. Suppose the two telescopes are observing a point source (very small angular size) of radio waves. The radiation incident at each telescope is collected by the dishes separately and then combined together electronically to give, an output signal. Notice, however, that the radio waves to one telescope have to travel an extra distance, or extra path length, I, before being picked up by the detector at the focus of the dish. The radio source moves across the sky as the Earth rotates on its axis so that the path-length difference changes continually. At some time the two electromagnetic waves will be in-phase and will give a maximum in the output signal. A little later the path difference will have changed so that the waves will be out of phase: they will then cancel giving zero output signal. Then the signals will come back into phase again. The output signal after a time will look similar. When the object is above the centre of the interferometer baseline, the output signal will go from maximum to minimum when the path difference changes by half a wavelength (? /2). This will happen once the object has moved through an angle ? = ? /2D where D denotes the separation of the two telescopes. The amplitude of the output signal then goes from maximum to minimum and back to a maxi¬mum every time the object moves through an angle ? /D across the sky. These amplitude variations are called INTERFERENCE FRINGES, and they are separated by an angle equal to the resolution of a single, large dish of diameter D. By means of these interference fringes we can locate the position of an object with the accuracy and resolution of a telescope of diameter D, which is now the separation of the two small telescopes and not their individual diameters. This does not mean, however, that INTERFEROMETERS (the name given to this arrangement of small telescopes) can be used simply to replace large single dishes. There are a number of substantial problems associated with their use.

In general, we do not know the exact location of the object we wish to observe. If it is giving a maximum in its output signal at one moment, we know that the path-length difference is a whole number of wavelengths but we do not know how many wave-lengths. This means it is not possible to tell at which maximum of the interferometer fringe pattern our object is. We can overcome this problem by observing with a number of different interferometers with different baseline sizes. The various interferometers then give a maximum output signal simultaneously when the object is exactly above the midpoint of all the interferometer base-lines. The other main problem arises when the interferometer is used to observe sources of large angular extent. For a sufficiently large source of radiation, the signals from some parts of the object will be in phase and adding to the total output, while other signals from another part of the same object will be out of phase and there¬fore subtracting from the total output signal. On average, the output signal will vary by only a small amount, if indeed at all, and we say that the source of the radio signals has been RESOLVED by the interferometer. If we want information about the large-scale structure of a radio source we have to use an interferometer with a smaller baseline, and a correspondingly larger fringe spacing on the sky. Astronomers have found that by a series of baselines from the smallest to the largest possible, they are able to deduce the detailed structure of radio sources of nearly all apparent sizes, as described in the next section.

Much of the interferometry done today is SYNTHESIS INTERFEROMETRY in which a substantial number of different baselines are used. However, the largest baselines are those between telescopes situated on different continents (VERY LONG BASELINE INTERFEROMETRY or VLBI). In this arrangement it is impossible to observe with baselines of intermediate size; astronomers have to accept the ambiguities, mentioned above, that are associated with the simple interferometer. Although the positions of objects are then uncertain in most cases, the enormous resolution (thousandths of a second of arc) possible with intercontinental baselines of many thousands of kilometres has yielded valuable data on the angular sizes of some of the most distant and luminous radio galaxies and quasars in the Universe.

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