MOSAIC at ELT: the instrument core subsystem (ICOS) design status

MOSAIC, the wide-field multi-object spectrograph for the Extremely Large Telescope (ELT), is advancing through Phase B2, where subsystem architectures mature toward the Preliminary Design Review. Central to the Front-End channel is the Instrument Core Subsystem (ICOS), which provides the mechanical support, alignment stability, and controlled rotation required by the focal-plane assembly and the GLAO, ORSS, and CALEMOS subsystems. ICOS comprises a steel Static Structure Module interfaced to the Nasmyth platform, a motorized Rotating Structure Module that delivers field -rotation compensation with accuracy better than 0.095 mrad , and a Cable Wrap Module enabling continuous ±180 deg motion. ICOS must include an alignment strategy capable of restoring the optical axis to the sub-100µm level required for injection efficiency and pupil stability. With refined interfaces and a mass current estimated mass of 12.97 t, within the 13.3 t allocation, ICOS is converging toward a robust preliminary design for Phase C.