Conceptual drawing of the FB. Only four fibers (science and references) go all the way from the input ferrule to the output ferrule. The other fibers are for calibration or AIT, or simply dummy fibers filling the geometrical patterns in the ferrules. All calibration fibers enter or exit the bundle at the level of the junction between the SPHERE and telescope sections. The SPHERE and CRIRES sections have a smaller diameter (4.8 mm) and are more flexible than the central telescope section (9.2 mm), which is reinforced. All fibers are identical Nufern 1310M-HP SMF, except for the AO MMF guide fiber that is multimode with a 50 μm core.