General Summary

In this paper Albert Abraham Michelson describes the use of interference methods in measuring the angular size and the one-dimensional brightness distribution of sources that are too small to be resolved by a single telescope. This fundamental technique has found widespread application both in optical and in radio astronomy. At optical wavelengths, radiation from a source is received by two mirrors and combined on the focal plane of a telescope. If the separation of the mirrors, D, is not too great, the source is unresolved and the coherent light will produce interference fringes of alternating light and dark bands. As the two mirrors are gradually separated, the fringes will disappear when the source is resolved. In this case, the angular diameter of a source is 1.22 λ/D, where λ is the wavelength of the observation. The first successful measurement of the angular diameter of a star was made by Michelson and Francis G. Pease on December 13, 1920, by using two mirrors separated by 20 ft and placed at the end of the open tube of the 100-in Hooker telescope. They found that the angular diameter of the supergiant star α Orionis (Betelgeuse) was sec of arc. Although the angular diameters of six giant stars were measured with this instrument, subsequent efforts to extend the measurements to the smaller main-sequence stars failed. This was due partly to the effects of atmospheric scattering and partly to the practical difficulty of constructing large mirror separations. These problems were finally overcome with the development of the intensity interferometer described in our next selection.When extraterrestrial radiation was found at radio wavelengths, Michelson interferometers were constructed to measure the angular sizes and the brightness distributions of the sources. In this case, radio frequency signals received at two telescopes were transmitted to some central location for correlation, and coherence was maintained by the transmission of a common local oscillator signal to the two telescopes. In this way, the angular sizes of several bright radio sources were found to be a few minutes of arc; later detailed maps showed that many of the radio objects consisted of two components. The Michelson type of interferometer has been since used to give angular resolutions up to 0.1 sec of arc at a radio wavelength of 6 cm; and the very long baseline (V.L.B.I.) adaptation using signals recorded simultaneously at two independent radio telescopes has given angular resolutions up to 0.001 sec of arc.R. Hanbury Brown and his colleagues suggested that mutually coherent local oscillators were not necessary at the two telescopes, and showed that post-detection correlation of signals recorded at twodifferent telescopes could be used to measure the angular sizes of radio sources (see selection 2). This method has vastly extended the interferometer baselines at both optical and radio wavelengths.