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\item when a new beam train is installed or configured, the AAS must allow for measurement of the "piston" term due to the optical path between the beam combiners and the telescopes, in order to allow for rapid acquisition of stellar fringes;  \item at the beginning of the night, the AAS must allow the pathlength differences between the fringe tracker and the science instrument to be measured so that these can be equalized.  \end{enumerate}  \section{Outline of the existing scheme}