The existing design of the AAS is shown schematically in Fig. \ref{fig:classic}. A beam of light is injected from the alignment table in the BCA (the BCA beam injection system is also known as the Magical Optical Box or MOB), with one beam going “upstream” towards the telescopes and one going “downstream” towards the beam combiners (Fringe Tracker FT and Science Instrument SI). This pair of injected beams (known as the “primary fiducial”) can consist of either laser light at a visible wavelength or broadband light which has components at visible and infrared wavelengths. The design of the MOB ensures that the upstream and downstream beams are anti-parallel to one another.