Although it was proposed by Albert Michelson almost a hundred years ago, stellar interferometry is today even more important as a means of researching the dynamics of binary stars as it was then. Michelson's idea led to the construction of an interferometer for the 100-inch reflector on Mount Wilson in the 1920s, consisting of a 20-foot structure with flat mirrors at each end mounted at the top end of the telescope tube.
This instrument uses the interference of light to determine whether a bright single star is either extended i.e. its diameter is resolvable at the Earth, or a close double. By combining the light from each of the two small mirrors and adjusting the separation of the mirrors until the fringes thus formed combined in such a way that they cancelled each other out then the separation of the two components could be found from the separation of the mirrors and the position angle from the orientation of the fringes. With so little light available only bright stars could be measured.
In 1925 Frederick Pease5 first resolved Mizar A using this equipment. It was also used for observations of extended sources, so that, for instance, the diameters of supergiant stars such as Betelgeuse could be determined. Other stars measured included the binary system Capella which turned out to have a separation of between 0.03'' and 0.05'' and a period of 104 days.
In the 1970s double-star observation underwent a revolution with the invention of speckle interferometry (see Chapter 17). This technique effectively removes the effect of the atmosphere and allows telescopes to operate to the diffraction limit. In the case of the 4-metre reflectors on which it was used, this corresponded to about 0''.025 or about four times closer that Burnham or Aitken could measure. In addition the accuracy of this method was much greater than visual measures and since then it has proved its worth by discovering new very close and rapid binaries and improving the older visual orbits.
The launch of the Hipparcos satellite in 1989 also heralded a new era of double-star discovery. Operating high above the atmosphere its slit detectors found some 15,000 new pairs, most of which are difficult objects for small telescopes but a number have already been picked up in very small apertures.
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