3D measurement method for LAMOST fiber end-faces under front and back illumination

Abstract In the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) project, fiber positioners align the fiber ends to designated positions on the focal plane to capture high-quality spectra of specific celestial objects. Therefore precise measurement of fiber end positions is essential for accurate control in the closed-loop positioning system. Current measurement systems primarily rely on 2D planar methods, which cannot achieve precise positioning of fiber ends on LAMOST’s curved focal plane. However traditional 3D measurement methods often struggle to reconstruct specific targets, leading to excessive data redundancy and reduced accuracy. To address these issues, this paper proposes a novel monocular vision-based 3D measurement method for LAMOST fiber ends under both front and back illumination. In this method, coding points and a baseline ruler are placed on the focal plane; the coding points help determine camera poses, while the baseline ruler recovers the physical scale of the measurement results. Images of the coding points and fiber end-faces are first captured under front and back illumination using a single camera that takes frames with movement, and the enhanced spot-grayscale distribution model algorithm is introduced as an efficient and accurate method for spot center extraction. Subsequently, the decoding method of coding points-clustering and pattern matching method is proposed for automated recognition and decoding of the coding points. Finally, a fusion-coded structure-from-motion based 3D reconstruction method, is employed to measure the fiber end positions in 3D. Results show that the method achieves a root mean square error of 0.049 mm in fiber optic 3D measurement error. The proposed 3D measurement method for fiber ends is vital for closed-loop control in LAMOST and is also applicable to high-precision measurement of the fiber ends in other multi-object spectroscopic telescopes.