The accuracy of fiber positioning is crucial for the observation of multi-target fiber spectral telescopes such as LAMOST (Large Sky Area Multi-Object Fiber Spectroscopy Telescope). Most of the methods used for fiber positioning are openloop control or semi-closed-loop control, the efficiency cannot meet the requirements of next-generation spectral telescopes. Considering that, this paper proposes a fiber positioning method that can achieve a completely closed loop without blind spots based on SMART (Special-shaped Micro-lens Aimer for Real-time Targeting) and a dual-rotary positioning mechanism. The entire correction process can be described as follows: first, the light intensity obtained by the 6-channel photodetector is stored in a buffer. Then the pulses required for correction are calculated based on the data in the buffer and the correction algorithm by the computer. The running command is then sent to the stepper motor controller using serial communication to drive the stepper motor. After the movement, the program will run again to verify if the correction is done. After selecting a position in the first quadrant, 8 directions were selected to conduct automatic correction experiments after the optical fiber position offset. The experimental results show that the average farthest distance that the method could correct is 600μm, and 75.9% positioning accuracy under our closed control method can reach 10μm, 94.8% positioning accuracy can reach 20μm, 100% positioning accuracy can reach 30μm. For corrections within the 500μm offset distance, 97.5 % of the correction time is within one minute.
Fiber spectroscopy technology is important in many areas of astronomical surveys. The fiber is used to transfer light from the telescope to the spectrograph. On the detector of the spectrograph, the image of fiber ends after dispersing can be obtained. In multi-mode fiber, multiple modes propagating in the fiber form a granular speckle pattern on the fiber end. In high-resolution spectral measurement, the speckle disturbs the energy distribution of the spot and reduces spectral resolution. The influence of fibers with different parameters on the centroid shift, signal-noise ratio, and radial power spectrum under artificial and mechanical disturbance is explored in this paper. The experimental results show that when the number of modes propagating in the fiber is higher, the precision of the centroid offset of the speckle is higher. Under the same disturbance condition, the speckle suppression effect is better with more mode numbers. This will be a reference for the parameter selection of optical fiber in the new instrument.
Optical fiber spectroscopy technology is widely used in astronomical surveys. Due to the flexibility and long-distance transmission characteristics of the fiber, astronomical observation can gain larger scale and higher precision spectral data. Nowadays, a lot of representative technologies have been presented to enhance spectral resolution, including fiber integral field spectroscopy, fiber positioning technology in the sky survey, adaptive optics, and photonic lantern technology. Fiber spectroscopy technology plays a crucial role in astronomy. The long-distance transmission characteristics of fibers separate the telescope from the spectrograph. The intrinsic flexibility of fibers lends itself readily to reconfigurable sampling of the field. The method to improve the spectral resolution has been gradually proposed. Fiber integral field spectroscopy is one of the most typical techniques to enhance the spectrum resolution. The flexible combination of fiber bundle and microlens is used to improve the sampling rate of target stars and fitting factor. In the observation of target stars by a single fiber, the alignment accuracy between the fiber and the star image determines the spectrum resolution. In the multi-object telescope, the position of a large number of multi-mode fibers needs to be detected. As a kind of optical waveguide device with multi-mode and single-mode conversion, photonic lantern can convert the energy collected by multi-mode fiber into the output of single-mode fiber. This review introduces optical fiber technology on astronomical observation.
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