The concept of a 3 lines of sight telescope for space astrometry is evaluated in terms of implementation feasibility and some science applications. The beam combination by pupil splitting exploits the Hipparcos concept, extending it to true large bidimensional angle. The PSF symmetry provides equivalent astrometric performance and immediate field identification on the common detector. Operation aspects are investigated in terms of field identification and observing strategy, in particular pointing constraints and sky density of reference bright stars. The approach, based on flat mirrors mounted on a pyramidal structure, provides a simple geometric framework, with immediate mechanical materialization and clear interface to metrology. The geometric arrangements considered place the lines of sight either along the (x, y, z) Cartesian axes, or in a planar layout for a Gaia-like astrometric mission. The application to a single head, multiple boresight star tracker is also outlined.
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