Measurements at high angular resolution: The intensity interferometer
 
Again, following a concept developed for radio astronomy by Hanbury Brown and Twiss, the principle of the intensity interferometer was considered in the 1950s for the optical region and an experimental arrangement was established at Narrabri in Australia. Rather than utilizing the classical concept of measuring the interference of waves based on their amplitudes and phases, the method involves the measurement of low-frequency beat signals that are generated when incoherent intensity signals from the two collectors are combined. In the Narrabri arrangement, the two light collectors (light buckets!) were large compound mirrors (6·5 m in diameter) but with little effort being made to obtain images of the quality that could theoretically be achieved by a telescope of this diameter.
The current arrangement at Narrabri is shown in figure 1. The experiment involves an investigation of the way in which two collectors receive the radiation from the star according to their separation. The fluctuations of the arrival of energy are compared or correlated. A determination of the behaviour of the degree of correlation according to the separation of the two collectors allows the

Figure 1. An aerial view of the current stellar interferometer, Narrabri, Australia. (By courtesy of the University of Sydney.)
diameter of the star to be calculated. As the strength of the correlation signal is very weak, some tens of hours of integration are required for each star.
Over the years, the Narrabri experiments have provided diameter measurements for a representative sample of stars across the whole spectral range.