In order to identify the source of the radio waves, radio astronomers need to determine an accurate radio source position so that they can compare these with the optical photograph of the same part of the sky.
Positional accuracies are usually measured in arcseeonds. To appreciate the smallness of an arcsecond imagine looking at your fingernail from two miles away. The fraction of your panorama filled by the fingernail is then about one arcsecond. The largest radio telescope system, the Very Large Array in New Mexico, can discern detail down to about a tenth of an arcsecond.
Note: Angles are measured in degrees minutes ('), and seconds (") of arc. A full circle contains 360 degrees. Each degree consists of 60 arcminutes, and each arcminute consists of 60 arcseconds. Our fingernail would be about 20 miles away for it to cover an angle of one tenth of an arcsecond.
The ability to discern detailed structure depends on the beamwidth. or the resolution. of the radio telescope. The resolution can be calculated by dividing the diameter of the reflector by the observing wavelength. Thus the 100-m (330-ft) diameter Robert C. Byrd Green Bank Telescope of the National Radio Astronomy
Observatory (NRAO) operating at 20-cm wavelength has a 9-arcminute beam, which means it can 'see'' details down to 9 arcminutes across. Anything smaller will be blurred by the resolution of the dish and will appear to be 9 arcminutes in diameter. For comparison, the human eye cannot distinguish anything smaller than 20 arcseconds across. This limit is determined by the wavelength of light (about 5 x 10"5 cm) divided by the diameter of the pupil (about half a centimeter). In practice the lens is not perfect and sets a limit on our capacity to see details to about 1 arcminute.
Aperture synthesis is the technique of combining signals from an array of radio telescopes spread over large areas of countryside, or even across half the earth, so as to obtain the resolving power of a single dish whose diameter would have to be hundreds or even thousands of miles, an utter impossibility. The largest aperture synthesis telescope is the Very Long Baseline Array referred to in Chapter 15, which can achieve a resolution measured in millionths of an arcsecond.
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