  The Sydney University Stellar Interferometer (SUSI) now incorporates a new
beam combiner, called the Microarcsecond University of Sydney Companion
Astrometry instrument (MUSCA), for the purpose of high precision differential
astrometry of bright binary stars. Operating in the visible wavelength regime
where photon-counting and post-processing fringe tracking is possible, MUSCA
will be used in tandem with SUSI's primary beam combiner, Precision
Astronomical Visible Observations (PAVO), to record high spatial resolution
fringes and thereby measure the separation of fringe packets of binary stars.
In its current phase of development, the dual beam combiner configuration has
successfully demonstrated for the first time a dual-star phase-referencing
operation in visible wavelengths. This paper describes the beam combiner optics
and hardware, the network of metrology systems employed to measure every
non-common path between the two beam combiners and also reports on a recent
narrow-angle astrometric observation of $\delta$ Orionis A (HR 1852) as the
project enters its on-sky testing phase.
