The input beam index corresponds to any station based index, e.g. siderostats, delay lines, paths, etc. Input and output beams refer to stellar beams on the input and output side of the beam combiner. An input beam has the light of a single siderostat in it, whereas an output beam has light of more than one. Currently, with the 3-way combiner, there are three output beams which are each connected to a spectrometer. Each one of these output beams or spectrometers has light from one baseline in it.