Towards the Characterization of Next Generation Robotic Positioners for 3D Mapping of the Universe

Spectroscopic surveys play a central role in observational cosmology by enabling precise measurements of galaxy redshifts, which are used to map the large-scale structure of the Universe and constrain cosmological parameters. These surveys rely on large arrays of robotic fiber positioners, each required to place an optical fiber on the telescope focal plane with micrometer-level precision. In this work, we present a calibration and characterization framework for next-generation robotic fiber positioners designed to operate under realistic observing conditions. Positioners are tested on a telescope mount at three distinct orientations relative to gravity, replicating configurations encountered during on-sky survey operations. Performance characterization is conducted inside a 140 cm long light-tight dark-box, motivated by lessons learned from DESI, where orientation-dependent effects arising from gravity loading were observed after installation on the telescope. The framework includes calibration of the metrology system, modeling of the positioner kinematics, and extraction of key performance metrics such as absolute positioning accuracy, stability, and repeatability. Using the commissioned system, reliable spot detection was achieved across all tested mount orientations, with an average positional uncertainty of 0.31 μm for fiducial bright spots. Sub-micron positioning precision was demonstrated for both the SDSS-V model and the Orbray trillium raft prototype robotic positioners in two of the three mount orientations. For the SDSS-V positioner, repeatability measurements yielded root-mean-square residuals below 1 μm at mount orientations of −89◦ and 0◦, increasing to 2.46 μm at 90◦. Investigation of the degraded performance at 90◦ identified thermal turbulence within the test environment as the dominant source of error. After mitigation, the radial repeatability for the Orbray positioner at 90◦ improved from 2.46 μm to 0.55 μm. Absolute positioning measurements for the SDSS-V positioner yielded an average accuracy of 27 μm. Together, these results demonstrate a robust and practical approach for validating fiber positioner performance and identifying orientation-dependent effects, providing a foundation for future spectroscopic instruments and surveys.