Astronomy Principles and Practice Fourth Edition.pdf

338 Modern telescopes and other optical systems
Figure 20.5. 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 20.6.
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.
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.