Analysis of LAMOST optical fiber positioning units closed-loop

LAMOST, as the astronomical telescope with the highest spectrum acquisition efficiency in the world, adopts the parallel and controllable optical fiber positioning technology. LAMOST requires high positioning accuracy, and the maximum allowable positioning error is only 40 um. Due to various aberrations, the general photogrammetry system cannot meet the requirements of detecting high-precision optical fiber positioning errors. In this study, we proposed a double telecentric measurement system to detect accurately the actual position of fiber positioner by taking advantage of ultra-low distortion and ultra-wide depth of field of telecentric lens. In this paper, the main sources of telecentric lens distortion were analyzed, and the calibration method of polynomial calibration model and dot matrix calibration target was adopted, and the validity of the five parameters obtained by calibration is verified by experiments. The experiment showed that the calibrated double telecentric measurement system met the requirements of detecting the positioning error of optical fiber unit. Based on the calibrated double telecentric measurement system, the precision verification experiment of the closed-loop positioning scheme was built with three positioners. The experimental results showed that after ignoring the lens positioning error, all optical fibers reached the target after two steps of approximation, and the errors met the positioning requirements which verified the insensitivity of the closed-loop positioning scheme to the initial parameters of the unit and the robustness of the scheme.