deletions | additions       diff --git a/section_Outline_of_the_existing__.tex b/section_Outline_of_the_existing__.tex  index 388c29d..7694592 100644  --- a/section_Outline_of_the_existing__.tex  +++ b/section_Outline_of_the_existing__.tex 
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The existing design of the AAS is shown schematically in Fig.~\ref{fig:classic}. A beam of light is injected in the BCA by the Magical Optical Box (MOB), with one beam going "upstream" towards the telescopes and one going "downstream" towards the beam combiners (Fringe Tracker FT and Science Instrument SI). The injected beam (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.   At the Nasmyth table of the telescope, the upstream beam is split by the FTT dichroic so that some is transmitted to M3 of the telescope and the rest is reflected to a corner cube on the table, which in turn reflects light into the FTT. A deployable shear measurement camera on the Nasmyth table allows the shear (transverse displacement) of the primary fiducial to be compared with the shear of the telescope pupil.  The initial coarse alignment of the system involves using pop-up quad cells at M5 and M4, which allow the shear of the primary fiducial to be measured at these locations, and manual adjustments can then be used to center the beams.  Space does not permit a full description of the start-of-night tilt and shear re-alignment procedure, but it can be split into two phases:  \begin{enumerate}  \item The tilt of the upstream primary fiducial is measured by the FTT and the shear is measured by the UTM shear measurement camera. These measurements are used to (a) align the primary fiducial with the delay-line mechanical axis by adjusting the MOB and (b) to align the primary fiducial with the UTM inner axis by adjusting M4 and M5 (this is the "tilt-and-shear algorithm").  \item The tilt and shear measurements from the in-built tilt and shear system in the fringe tracker are used to align the primary fiducial with the fringe tracker, making use of M11 and M12, the switchyard mirrors.  \end{enumerate}  The combination of these phases is designed to ensure that a light beam can travel from the telescope to the fringe tracker so that the errors in the tilt and shear are within the error budget at the start of the night. To keep within this error budget for the duration of the night, the BEASST system uses a fraction of the starlight when observing bright stars to measure any changes in the tilt and shear of the beams and applies appropriate corrections.  The piston term is also measured using the primary fiducial: the upstream beam is retroreflected off M3 and travels back to the fringe tracker. The delay line positions are adjusted until fringes are seen and the relative position of the delay lines gives the piston term.  The downstream beam is used to equalise the pathlength differences between the fringe tracker and science instrument by adjusting the switchyards in both combiners until fringes are seen.