Perhaps you are a competent radio spectroscopist, probably without realising it! Every time you turn the tuning knob on a radio or television receiver you are looking for peaks in the radio-frequency spectrum present at the antenna (or aerial) of your receiver. The peaks are radio and television broadcast stations.

The same technique is the essence of simple RADIO ASTRONOMICAL SPECTROSCOPY. In the radio region of the spectrum there are a number of spectral lines produced by clouds of neutral hydrogen (Ho regions) and dusty H+ regions and molecular clouds. The observing frequency of a radio telescope may be changed gradually while the astronomer watches the strength of the signal on a meter. In this way he may look for peaks and dips in the spectrum of the object. However, this is a relatively inefficient way of using the signal from a faint object since only one small part of the spectrum is actually being used at any one moment. Much more efficient is a technique using a large number of separate receivers each tuned to a slightly different wavelength. Another valuable method is very similar to that used optically in the Fourier spectrograph. In radio astronomy they are known as AUTO-CORRELATION RECEIVERS.

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