A multi-axis plane mirror interferometer uses shared measurement and
reference beams that respectively reflect from measurement and reference
reflectors before that shared beams are split into individual beams
corresponding to the measurement axes of the interferometer. An N-axis
interferometer thus requires only N+1 measurement beam paths, one for the
shared measurement beam and N for individual measurement beams, to
provide for each measurement axis the two reflections that cancel angular
misalignment between the measurement and reference reflectors.