FIRST instrument principle: the redundant pupil of the telescope is divided into several subapertures (A). The selected subapertures, circled in red, are injected into single-mode fibers (B) and rearranged into a nonredundant pattern (C). The signal of each subaperture is dispersed using anamorphic optics and a prism (D). Coherent recombination produces the interferometric signal on the camera, delivering Optical Path Difference (OPD) information as a function of wavelength (E). RP = Redundant Pupil, µL = micro-lenses, FB = Fiber Bundle, SMF = Single Mode Fibers, VG = V-groove, Cyl = Cylindrical lenses, P = Prism.