Measurements at high angular resolution: Aperture synthesis
 

The larger the telescope, the greater is the potential angular resolving power to investigate finer detail in an image. Unfortunately, the recorded sharpness is degraded by a large factor as a result of atmospheric seeing.
One approach to redress the situation is by the technique of aperture synthesis. If the telescope aperture is fitted with a mask so that only a few small patches of the primary mirror are used, the light is still brought to a focus but the image will be in the form of an interference pattern. In the experimental arrangement, the sizes of the smaller apertures should be no larger than the Fried parameter. By altering the structure of the mask, so that the smaller apertures are redistributed over the main mirror, the resultant image will again be an interference pattern but with a different form. From a combination of the various recorded patterns, a resultant image can be constructed as though achieved by the complete large aperture. Thus, the resolution potentially available from the large aperture telescope is synthesized from the smaller element combinations used to produce the ‘component’ images. To make such a technique more powerful and to allow the synthesizing of an effective telescope aperture of some tens of metres, i.e. much larger than the currently available optical telescopes, it is possible to use an array of independent small telescopes. This notion was first developed by radio astronomers but the technological requirements for the optical region are more complicated because of the much smaller wavelengths in the radiation relative to the physical sizes of the engineered apparatus. The approach of a multi-telescope array is incorporated in COASTW 20.5 (Cambridge Optical Aperture Synthesis Telescope). The instrument comprises five 40 cm telescopes arranged in a ‘Y’ configuration, the maximum separation of the elements being 22 m. Rather than by moving the individual elements to synthesize an aperture of this order, a range of effective configurations is achieved by taking records at different times during the night, the Earth’s rotation changing the orientation of the array relative to the observed star.
A notable success of the system was the very clear imaging of the close binary star, α Aur (Capella), with a separation∼0·05. Records taken weeks apart revealed movements of the components as they orbit their common centre of gravity with a period of 104 days. The COAST group are now planning an array with a baseline of 400 m, with the potential of 0·25 milli-arc sec angular resolution.