The very high astrometric accuracies possible with long baseline interferometer are possible only if two or more stars are observed simultaneously. This is in contrast to all other stellar interferometer built todate. The basic idea behind a dual star interferometer is quite simple and is shown below.

Each telescope at the ends of the interferometer uses a field splitter. Shown here is a mirror with a pinhole in the coating. Light from one star goes through the hole while light from the other star is reflected by the coating. Two separate beams of starlight are directed to two separate beam combiners. A laser metrology system "ties" the two interferometers together, so that active optics can be used to compensate for vibration and thermal drift of the equipment that are different between the two optical trains.

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