  The Planet Formation Imager (PFI) is a future kilometric-baseline infrared
interferometer to image the complex physical processes of planet formation.
Technologies that could be used to transport starlight to a central
beam-combining laboratory in PFI include free-space propagation in air or
vacuum, and optical fibres. This paper addresses the design and cost issues
associated with free-space propagation in vacuum pipes. The signal losses due
to diffraction over long differential paths are evaluated, and conceptual beam
transport designs employing pupil management to ameliorate these losses are
presented and discussed.
