WST Multi-Object Spectrograph Fiber Positioners:Development of a 32,000-Unit Precision Robotic System

Abstract: The Wide-field Spectroscopic Telescope Multi-Object Spectrograph requires an unprecedented fiber positioning system comprising 30'000 low-resolution and 2'000 high-resolution positioners across a 3.1 deg squared field of view. Each robotic positioner must achieve 5um RMS positioning accuracy in a densely packed focal plane, representing a more than sixfold scale increase over current instruments like 4MOST and DESI. To mitigate risks associated with industrial-scale production of 32'000 precision mechanisms, WST is pursuing a multi-concept development strategy. Four distinct positioner architectures are being prototyped and tested by an international collaboration (EPFL, AIP, UKATC, AAO) using 6.2 mm pitch triangular modules of 63 units each or a new inline modular concept. Performance metrics including positioning accuracy, repeatability, reconfiguration speed, collision avoidance, and manufacturability are being systematically evaluated. Down-selection to one or two concepts is planned for 2026-2027 during the HORIZON Europe-funded conceptual study phase. Current prototype testing demonstrates feasibility of meeting specifications, supporting WST's path toward first light in the early 2040s as ESO's next major spectroscopic facility.