The output beam index (i.e. the spectrometer index) needs to be specified for visibility amplitudes, delay jitter values, etc. Note that the 3-way beam combiner of NPOI has just one baseline per spectrometer, whereas the 6-way combiner will have up to four baselines per spectrometer. In this case, the baseline selection needs to be made in addition to the spectrometer selection. 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, the first one the E-W baseline, the second one the C-W baseline, and the third one the C-E baseline.