Yonggang Gu’s research while affiliated with University of Science and Technology of China and other places

The positioning accuracy of the robotic fiber positioners (RFPs) is a crucial issue because incorrect fiber position can significantly affect the spectral quality. The fiber viewing camera is implemented to guide the fiber positioning. However, the current camera calibration methods are inconvenient for the narrow space of the telescope. To solve this problem, a flexible separated calibration method is proposed in this study. Firstly, the initial calibration is completed by traditional Zhang’s method. Secondly, multiple small planar targets (SPTs) are placed on the focal plane to form a spliced large target (SLT) to calibrate the lens aberrations. Then, the home position of the RFPs is used as a variable control point to calibrate the extrinsic parameters. Furthermore, an improved weighted multiple harmonic (WMH) method is proposed to obtain accurate centroids of the deformed light spot. A prototype fiber position detection system is constructed in the laboratory. Physical experiments verify the effectiveness and flexibility of the proposed method, which has practical engineering significance.

Fiber positioning technology is widely used in spectroscopic telescopes. The Z-axis position of the optical fiber tip must be strictly constrained on the focal plane, otherwise the skylight of the celestial body cannot be focused on the optical fiber tip, thus affecting the observation efficiency of the telescope. Taking the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) as an example, 4,000 optical fibers are densely distributed on the focal panel, and the distance between adjacent ones is very small, which means that contact-type instruments cannot be used. Therefore, this article proposes a fiber Z-axis position measurement method based on depth from focus (DFF). First, a new sharpness function based on the maximum edge gradient is proposed according to the edge diffusion distribution of spots in a backlight environment, which can accurately calculate the sharpness value of light spot images at different focus levels. Secondly, in order to improve the measurement speed of DFF, a focus search algorithm based on the blurred spot radius is proposed, which establishes the relationship between the defocus radius of the spot and the focus position. The displacement stage only moves 4 steps to obtain an initial focus position. Finally, a micro-step search is performed on both sides of the initial focus position to obtain an accurate focus evaluation curve, and Gaussian fitting is performed on the focus curve to obtain an accurate focus position. In the laboratory, an experimental platform was built to confirm the efficiency and accuracy of this method, results show measurement accuracy is about 10 micrometers.

The positioning accuracy of the robotic fiber positioners (RFPs) is a crucial issue because incorrect fiber placement can significantly affect the spectral quality. The fiber viewing camera (FVC) is implemented to guide the fiber positioning. However, the current camera calibration methods are inconvenient for the narrow space of the telescope. To solve this problem, an innovative calibration method is proposed in this study. Multiple small planar targets are placed on the focal plane to form a spliced large target (SLT) to calibrate the lens aberrations. Then, the home position of the RFPs is used as a variable control point (VCP) to calibrate the extrinsic parameters. Furthermore, a weighted multiple harmonic (WMH) method is proposed to obtain accurate centroids of the deformed light spot. A fiber position detection system is constructed in the laboratory. Physical experiments verify the effectiveness and flexibility of the proposed method, which has practical engineering significance.

The perspective camera model has difficulty handling refracted light in the underwater environment. To achieve accurate and convenient calibration in large underwater scenes, we propose a method based on the underwater refractive camera model in this paper. First, the initial values of the refraction parameters are solved using refraction coplanarity constraints. Then the initial values are optimized nonlinearly using co-point constraints, which simplifies the optimization process of existing methods. In the field of view of ${200}\;{\rm mm} \times {200}\;{\rm mm}$ 200 m m × 200 m m , the experiment results show that the reconstruction accuracy of the proposed method can reach below 0.02 mm, and it is equally effective in the case of sparse calibration.

The multiobject fiber-fed spectrograph is the core scientific instrument for large-scale spectroscopic surveys. For closed-loop control of fiber positioning, fiber metrology systems are implemented in numerous fiber-fed spectrographs. The position accuracy of the fiducial fiber in the focal plate directly affects the performance of the fiber metrology system. However, there are currently no suitable methods and devices for measuring the fiducial fibers with high accuracy in the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). To solve this problem, this study proposed a novel online scanning measurement method for fiducial fibers in which a scanning camera was set up in front of the focal plate, and the rotation and translation movements of the focal plate were combined to set a polar coordinate measurement system. First, the pole and polar axis of the polar coordinate frame were determined, and the compensation values of the polar radius and angle of the polar coordinate of the fiducial fibers were solved in the field of view of the scanning camera. A prototype measurement platform was set up to verify the feasibility and scientific validity of the method. Experiment results show that the polar radius accuracy of the proposed method met the measurement requirements. The X – Y measurement accuracy can be further improved when a higher-precision rotary stage is adopted. Thus, the difficulties in online accurate measurement for fiducial fibers can be tackled by the proposed method with good operability in LAMOST.

Traditional dental implant navigation systems (DINS) based on binocular stereo vision (BSV) have limitations, for example, weak anti-occlusion abilities, as well as problems with feature point mismatching. These shortcomings limit the operators’ operation scope, and the instruments may even cause damage to the adjacent important blood vessels, nerves, and other anatomical structures. Trinocular stereo vision (TSV) is introduced to DINS to improve the accuracy and safety of dental implants in this study. High positioning accuracy is provided by adding cameras. When one of the cameras is blocked, spatial positioning can still be achieved, and doctors can adjust to system tips; thus, the continuity and safety of the surgery is significantly improved. Some key technologies of DINS have also been updated. A bipolar line constraint algorithm based on TSV is proposed to eliminate the feature point mismatching problem. A reference template with active optical markers attached to the jaw measures head movement. A T-type template with active optical markers is used to obtain the position and direction of surgery instruments. The calibration algorithms of endpoint, axis, and drill are proposed for 3D display of the surgical instrument in real time. With the preoperative path planning of implant navigation software, implant surgery can be carried out. Phantom experiments are carried out based on the system to assess the feasibility and accuracy. The results show that the mean entry deviation, exit deviation, and angle deviation are 0.55 mm, 0.88 mm, and 2.23 degrees, respectively.