Top: location of the interferometer between the secondary mirror and the focal plane inside the cryostat (top); ray-tracing with the interferometer excluded (bottom). Bottom left: general ray-tracing in the interferometer. Bottom right: ray-tracing of a single field in one FTS. Radiation coming from the telescope is reflected by the 45° folding mirror towards the wedge mirror. A first collimator (Coll I) reflects it towards the input Wire Grid (WG in) that splits the polarizations towards the two inteferometers. In each FTS radiation from this first WG proceeds towards a second collimator (Coll II) that focuses it on the beam splitter (WG BS). Both the reflected and transmitted components are collimated by collimator mirrors (Coll RM) on the Roof Mirrors (RM) of the delay lines. The roof mirrors rotate the polarization by 90° so that the beams proceed towards the output section, where Coll III collimates the beam on the output wire grid (WG Out). Radiation is finally collimated by Coll Out and reflected by the rear face of the 45° folding mirror so that the exit is aligned with the optical axis of the telescope. A HDPE lens adapts its divergency to the initial one.