Accurate measurement of the position of the reference fiber is the key to ensure the positioning accuracy of the fiber, but due to the large aperture and large field of view of the LAMOST telescope, the diameter of its focal plate reaches 1.75m. The reference fiber is installed in the unit hole of the detection system as a reference. The reference fiber, which is the reference of the detection system, is installed in the unit hole of the focal plate, and the accurate detection of its position can only rely on the high-precision spatial measurement performance of the laser tracker. However, the systematic error of the laser tracker itself, as well as the temperature change and air flow in the environment have a large impact on the measurement results. This paper proposes a measurement error compensation device based on the relative position of spatial standard length rods, which consists of two or three indium steel rods with extremely small thermal expansion coefficients and accurately calibrated lengths as a reference, and the measurement deviations of the rod lengths and relative positions of the rods and the rods at the time of operation are related to the measurement errors of the reference fiber. According to the relevant algorithm, the position error of the reference fiber on the focal plane can be corrected in real time. Finally, experiments show that this device can effectively reduce the error of the laser tracker in the measurement of the position of the reference fiber.
The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) is undergoing final system debugging for the closed-loop fiber position system. The accuracy of the benchmark position of the fiducial fibers is a key factor affecting the closed-loop fiber position system; The more accurate the benchmark position of the fiducial fibers, the higher the accuracy of the photogrammetric system, and the accuracy of fiber position will also be improved. We have measured the position of the fiducial fibers for two consecutive years. Two laser trackers installed at different sites were used to simultaneously measure the position of the fiducial fibers on the focal plane, and the measurement result were compensated for errors using high-precision indium steel standard rods and control points. Afterwards, we conducted fusion on two sets of the laser tracker data, ultimately obtaining the position of the fiducial fibers. Compared the position of the fiducial fibers obtained from two years of measurement with the theoretical hole position of the fiducial fibers, the results showed that the average measurement accuracy of the fiducial fibers was within 200 μm. When comparing the position of the fiducial fibers horizontally, the stability difference between the two years was around 100 μm. After applying the benchmark position to the closed-loop fiber position system, the Root Mean Square Error (RMSE) value of fiber position is 16.57 μm, and the position accuracy of 97.45% fibers is within 40 μm.
In the closed-loop fiber positioning control mode of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), stringent requirement is needed for time efficiency. Due to the high-resolution image required for fiber positioning and the impact of image transmission, there is still room for improvement in the current closed-loop control's time efficiency. The total time required for all fiber positioner to complete a positioning must be less than 5 minutes. To address this problem, this paper proposes an improved fiber position high-precision detection method based on FPGA (Field-Programmable Gate Array), which can fully utilize the computational resources of the edge hardware platform for image processing and significantly save the time required for computing high-resolution images. This paper compares the impact of several threshold algorithms on the centroid algorithm and uses Vivado HLS to port the algorithm to the FPGA. By labeling the spots, the centroid coordinates of the spots can be obtained in a single scan image. The results show that the FPGA-based centroid algorithm can effectively reduce the image processing time, and the improved centroid algorithm is more suitable for running on the FPGA. The algorithm has been experimentally verified and has been preliminarily applied to the closed-loop detection system of LAMOST. In the future, it can be further optimized and applied to the closed-loop detection system for multiple accumulations to improve detection accuracy, as well as to the next-generation multi-object astronomical telescope detection system with tens of thousands of fiber position detections.
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