Measurements at high angular resolution: Speckle interferometry
 
When a very large telescope is used with high magnification to observe a star, the image may be seen to have a rapidly changing grainy structure or exhibit a speckle pattern. Such effects are produced by the turbulence in the atmosphere above the telescope. Any bright portion in the pattern results from

Figure 1. The rapid fluctuations of intensity of a star as it is occulted by the Moon. (A reflection of this curve is obtained at the reappearance of the star.)
interference of essentially plane portions of the beam which, at any instant, happen to be in phase but originate from widely separated areas of the primary mirror. An instantaneous speckle pattern, therefore, contains high spatial resolution information down to the diffraction limit of the telescope. Sketches of an instantaneous pattern of a single star and one for a close double star are depicted in figure 2.
The term speckle interferometry has been coined to cover the technique whereby a series of speckle patterns are recorded by photography, video camera or CCD, combined and then spatially analysed to investigate any difference from what would have been obtained from a point source. In the first place, the speckle patterns must be obtained using a large plate scale and with a short exposure. In order to detect whether a star has a noticeable diameter, a second star needs to be in the field of view at an angular distance such that the seeing affecting both objects is essentially coherent. The speckle pattern of the second star is then used as a reference to detect any difference that the target star’s diameter has had on its recorded speckles. The technique has been most successful in determining the separations of double stars simply by noting the pairing of elements within the recorded speckle patterns and measuring their separations and angles of the lines joining the pairs.

Figure 2. The production of speckle patterns for (a) a single and (b) a close double star.
The first successful speckle interferometric work was done with the 200-in (5·08 m) Mt Palomar telescope. Using a microscope objective at the Cassegrain focus, the focal ratio was converted to be about f/500 giving a plate scale of 1 arc sec per 25 mm and short exposures of the order of 1/100th of a second were made possible by employing image intensifiers. The disks of Betelgeuse, Antares and Aldebaran have been resolved and, for the first time, Capella was resolved as being a double star (see the result from COAST described earlier).
The techniques of speckle interferometry are continuously under development and, no doubt, there will be improvements, particularly in reducing the time between recording the speckles and determining the spatial content of the stellar disc.