Fiber positioning technology is widely used in spectroscopic telescopes, and the accurate identification of the fiber position on the focal plane directly affects the efficiency of the astronomical spectrum. At present, fiber positioning usually uses the “back-illuminate” technique to illuminate one end of the fiber. The other end of the fiber is used for detection. The fiber could be stressed or twisted during locator motion, resulting in a difference between the detected fiber position and the actual fiber core. However, the fiber-optic back-illuminated device in the spectrometer system increases the complexity of the system and the time loss of fiber positioning. This paper attempts to use a new method combining image processing with deep learning to identify the fiber ferrule by the front-illuminated method. We built an experimental platform in the lab and experimented with a CMOS camera and telecentric lens. We tested the repeated errors and displacement measurement errors of the two methods. A series of comparative experimental results show that the final detection accuracy of this method can meet the needs of optical fiber positioning in the laboratory, although it has not yet reached the accuracy of the back-illuminated approach. In the future, if the light source and fiber ferrule were specifically designed for the front-illuminated method, its accuracy could be further improved.
The entire system of the LAMOST ((Large Sky Area Multi-Object Fiber Spectroscopic Telescope) requires high positioning accuracy of the fiber positioning unit. In order to acquire accurately target celestial objects, fiber view metrology system for positioners can efficiently and accurately detect thousands of fiber spots simultaneously in a large scale is required. The traditional method mainly used the "back-illumination method" for detection. With the advent of 8k*6k high-resolution CMOS cameras, fiber position detection based on the "front-illumination method" becomes feasible. This paper mainly studies the fiber position detection based on the "front-end illumination image processing method". The image is preprocessed first, and then the edge detection of a large number of fiber target points in the image is performed. Considering the constant radius of the white ceramic head where the fiber is located, the article proposes a "front-illuminated" image algorithm based on radius-based Hough space conversion and optimal radius error center search. This algorithm improves the speed and accuracy of fiber pixel coordinate detection. At the same time, it can be coordinated and compared with the "back-illuminated method" to further optimize and improve the detection accuracy of the fiber position.
During the LAMOST observation, to accurately align a large number of fibers with the target star positions, we used a closed-loop feedback system based on visual measurement in fiber positioner operation mode. The fiber was illuminated at the end of the spectrometer and the fiber light spots on the other end of the focal plane could be captured by the metrology system for positioning. The system can have a larger field of view and a single measurement can cover thousands of fibers. The metrology accuracy which is based on camera accurate calibration, is critical in the fiber positioning system. In general, calibration of a standard camera requires a reference surface with a known precise position marker and covering the camera's field of view. Theoretically, it is necessary to design a standard target surface that covers the camera's field of view to calibrate the camera's error. However, it is not realistic to manufacture and install a large standard target that meets the accuracy requirement. To ensure that the camera calibration error is within the limited range, and the fiber positioner can obtain higher positioning accuracy, we use the focal plane unit hole to insert a dedicated reference unit to serve as its calibration reference. In this paper, a reference fiber unit structure was designed according to the requirements of closedloop positioning. Through the test experiment on the reference fibers, it was finally verified that the reference fiber unit meets the accuracy requirements of closed-loop control.
LAMOST, as the astronomical telescope with the highest spectrum acquisition efficiency in the world, requires high positioning accuracy, the maximum allowable positioning error is only 40μm. 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 experimental results showed that all optical fibers reached the target after two steps of approximation, and the errors met the positioning requirements.
The fiber positioner is the core component of the multi-object spectral survey telescope. Its precise scanning plays a key role in the observation of the astrology. The eccentric bracket is an important part of the fiber positioner in LAMOST. To ensure the installed accuracy of the fiber positioner, a visual detecting instrument is designed for quick and accurate measurement of critical dimensions. Using two orthogonally placed cameras, the distances between the spatial intersecting shafts of the complex eccentric bracket are precisely detected. The position of the circular hole in the image is extracted by Hough Transform. However, the accuracy of this method is not high after experiments. An active luminous target was designed, manufactured and installed on the eccentric bracket. Finally, according to the experimental data, it is shown that the measurement of the instrument can meet the installed accuracy of the fiber positioner and improve the measuring efficiency.
Multiobject spectroscopy is applied in numerous modern astronomical facilities conducting observations of a large number of targets per pointing. Assigning the maximum number of targets to these instruments requires efficient algorithms. We present a simple and effective algorithm, the averaging (Aver) algorithm, to maximize the number of assigned targets for the first few visits of a given field. In comparison to the draining (Dra) algorithm, our algorithm increases the target completeness by 1% to 2% by employing Poisson distributed and real catalogs from the Large Sky Area Multiobject Fiber Spectroscopic Telescope survey. Moreover, our algorithm performs ∼375 times faster than the conventionally applied simulated annealing algorithm and yields a slightly higher completeness. We further optimize the Aver and Dra algorithms by combining the genetic algorithm (GA) and the differential evolution method. The Aver is slightly optimized by this method, whereas the Dra algorithm is improved by 0.9% to 1.6%, suggesting that our proposed Aver algorithm approaches maximum completeness. Furthermore, we find that the GA can optimize the rotation angle with a specially designed fitness function in the case of focal-plane rotation that is expected to be realized in the future, achieving a 1.8% increase in the number of the targets observed. In particular, our Aver algorithm assigns the maximum number of targets within the first few visits.
This paper gives a scheme of optical fiber positioner structure of a miniature, by use of the DC servo motor with the diameter of 3mm driver, the distance can designed to 8.5mm, and can arrange more than 12000 fibers in the focal plane with the diameter of 1 meters, it is especially suitable for telescope with small dimension focal plane and has high density fiber positioning requirements. Based on the principle of double rotary fiber positioning principle, It consists of a hollow shaft revolving mechanism, and eccentric axis revolving mechanism relative to hollow shaft. The hollow shaft turns round at the range of -180 degrees to +180 degrees and the eccentric axis turns round at the range of -90 degrees to +90 degrees at the half of radius driving by each control motor. When positioning, the optical fiber end moves on the focal plate throughout, and can never deviate from focal plane. optical fiber is fixed in the mounting hole of fiber support which installed on the eccentric rotary shaft (fiber support’s hole axis is parallel to the axis of the hollow shaft), and fiber will lead to pass through the inner hole of the hollow shaft and focal plate then connected to the spectrometer. positioner center shaft adopts planetary gear driving principle, with small module motor’s gear and the fixed ring gear can driving motor and positioner planetary rotate, the eccentric shaft by DC servo motor with the diameter of 3mm drived coaxial optical fiber on the eccentric shaft, the center and the eccentric shafts adopts micro rolling bearing support; in order to prevent the positioner’s center and eccentric shaft to rotate out of bounds, both limiting devices have designed to ensure the safety of fiber positioning; both center and eccentric shaft are designed with a spring structure to eliminate the influence of gear clearance; because positioner size is very small, the positioner driving wire is embedded in the slot of the hollow shaft sleeve wall. This will not affect the fiber go through the center shaft’s holes and pass through the focal plane; positioner sample test results show that the closed-loop positioning can reached accuracy of 0.01mm unit, and can meet with the demand of optical fiber positioning.
Fiber spectroscopic telescopes are important tools for astronomy research. In spectroscopic telescopes, the positioning of thousands of fibers on the focal plane is a big issue, which is usually solved by some compact structured devices with small-size motors installed. Stepper motors are normal choice for fiber positioner, however, stepper motors’ low efficiency leads to serious heating, so brushless DC motor becomes a more possible option when the fiber positioner is required to be less heating. Moreover, the size of brushless motor is much smaller than stepper motor in same situation. Brushless DC motor are synchronous motors powered by DC electricity via an inverter. Brushless DC motors complete commutating by switching power supply with an inverter, instead of with the help of carbon brush. Because of the absence of Mechanical Structures like carbon brush and slip ring, BLDC Motors can prevent problems like friction that caused by carbon brush. Both brushless DC motors and Steppers are DC synchronous motors, they generally share the same mechanical structures, but they have many different features. Brushless DC motors perform well in acceleration and they are less likely to generate remarkable amount of noise or heat. Steppers is made for position control, but they sometimes step out, what’s more, they generate a lot of heat and noise while running. Brushless DC motors are usually used when high speed and small size are required, while Steppers are used for positioner. If we can use brushless DC motors for position control, in one way, we can solve the stepping out problem of Steppers and managed to achieve higher accuracy and performance control of position, in the other way, circumstances that need small size and low heat and position control, which may have bothered us a lot, may have an convenient economic solution by using BLDC Motors. Although BLDC Motors are not made for position control, but the Mechanical Structure similarity with Steppers make it possible to realize some kind of position control theoretically. By now, there many ways (with sensor and without sensor) to detect the position of the brushless DC motors’ rotor’s position, which make the idea of using BLDC Motors for position control possible in practice. Generally, ways with sensors detect the rotor position by placing a sensor in the motor, and those without sensors usually calculate the rotor position by detect the voltage and current of the motor. This paper presents our efforts in applying sensorless rotor positioning technology in driving the brushless DC motors and in using the method of sensorless positioning to realize position control. During the process of rotor position detection, a three-terminal resistance network of star arrangement and a simple RC filter is used to get the zero-crossing point of the back EMF. The resistance network is used to extract the neutral voltage and the filter can filter high-frequency background and direct current component. The filtered signal’s zero-crossing point is a shift of the back EMF’s zero-crossing point and almost can be directly used as a reference of the rotor position. Pulse-width-modulation is used to make it possible for the driver to adapt to different brushless motors and different torque. For the purpose of inspecting the performance of the technique when used in position control, the brushless DC motor is mounted to an optical fiber positioner to avoid no-load running and to verify the possibility of applying it for small positioner. The detailed control scheme is introduced, the circuit is analyzed and experimental results obtained form position control experiment are shown to estimate the accuracy of the motor. The result of this paper may help to simplify the control of brushless DC motor and improve the performance of brushless DC motor.
Measuring the position of the end of 4000 optical fibers on the spherical focal plate for the LAMOST (Large Sky Area Multi-Object Fiber Spectroscopy Telescope) optical fibers positioning system is one of the key problems for LAMOST. The accuracy of optical fibers positioning system is guaranteed by feedback from measuring the position of the end of optical fiber. The position of the end of optical fiber is measured by photogrammetry with precision calibration. However, given the complexities in the optical fiber focal plane and the fiber positioner, the accurate standard point is considerably difficult to obtain, which results in insufficient calibration accuracy. To solve this problem, a convenient calibration method based on the Flexible Planar Target (FPT) is proposed. In this method, each fiber positioning unit positions the fiber to 16 designed locations, which are relatively accurate. These points form a high-precision 2D point array that can be used as the planar target. In this manner, each fiber positioning unit can be regarded as a small high-precision planar target. All small high-precision planar targets are assembled to form the Flexible Planar Target (FPT), which is used for calibration. Experimental results indicate that this improved method can reach a higher precision than that of previous method.
The LAMOST telescope has been officially observed for the past seven years since 2009, and many parts of the telescope are currently being upgraded. The fiber positioning unit of the focal plane instrument is also planned to be upgraded again. In order to ensure a higher positioning accuracy of the fiber positioning unit, the newly developed fiber positioning system adopts a closed-loop camera to photograph the unit fiber position in real time, and feeds back to the control system to implement multiple positioning to improve the positioning accuracy. This article focuses on an improved optical center of gravity algorithm for optical fiber location based on the optical center of gravity algorithm. The factors affecting the position measurement of the optical fiber spot are optimized, and the recognition accuracy of the spot position under different conditions is improved.
Metrology Camera System (MCS) was designed to make a closed-loop control of the optical fiber position in Fiber Positioning System (FPS) on the focal plate of the LAMOST. The stability of the metrology platform is the key factor to the quality of camera shooting. A precise adjustable mechanism was designed in this paper to achieve the platform’s pitching and horizontal rotation adjustment. And also a vibration isolation system using Magnetic Negative Stiffness (MNS) and positive spring in parallel was designed to decrease the effect of vibration, which was caused by the multiple complex vibration loads existing in the working environment, on the platform. Furthermore, an air conditioning system using the semiconductor refrigerator and resistance heater was designed to ensure working temperature of the camera and lens in extreme temperature environments. The simulation results showed that these designs were effective to improve the stability of the metrology system
Since the large scale use of paralleled controllable fiber positioner in LAMOST, the newly designed spectral survey telescope project generally uses the fiber position unit which similar to LAMOST to obtain the target spectrum. The positioning accuracy of the fiber positioner is directly related to the performance of the telescope. In order to further improve the positioning accuracy of positioners system, it is an important way to improve the accuracy by measuring the position of the optical fiber end on the positioners by using the visual metrology system. This paper mainly introduces the research design of LAMOST closed-loop metrology system, and the closed-loop system was established in different positions within the telescope to acquire best results. The metrology system will improve the fiber positioner system operation accuracy and reliability after the completion of the entire system in the future.
In this paper, a compact optical fiber positioner is proposed, which is especially suitable for small scale and high density optical fiber positioning. Based on the positioning principle of double rotation, positioner’s center shaft depends on planetary gear drive principle, meshing with the fixed annular gear central motor gear driving device to rotate, and the eccentric shaft rotated driving by a coaxial eccentric motor, both center and the eccentric shaft are supported by a rolling bearings; center and eccentric shaft are both designed with electrical zero as a reference point, and both of them have position-limiting capability to ensure the safety of fiber positioning; both eccentric and center shaft are designed to eliminating clearance with spring structure, and can eliminate the influence of gear gap; both eccentric and center motor and their driving circuit can be installed in the positioner’s body, and a favorable heat sink have designed, the heat bring by positioning operation can be effectively transmit to design a focal plane unit through the aluminum component, on sleeve cooling spiral airway have designed, when positioning, the cooling air flow is inlet into install hole on the focal plate, the cooling air flow can effectively take away the positioning’s heat, to eliminate the impact of the focus seeing. By measuring position device’s sample results show that: the unit accuracy reached 0.01mm, can meet the needs of fiber positioning.
The surface accuracy of astronomical telescope focal plate is a key indicator to precision stellar observation. Combined with the six DOF parallel focal plane attitude measurement instrument that had been already designed, space attitude error compensation of the attitude measurement instrument for the focal plane was studied in order to measure the deformation and surface shape of the focal plane in different space attitude accurately.
In the telescope observation, the position of fiber will highly influence the spectra efficient input in the fiber to the spectrograph. When the fibers were back illuminated on the spectra end, they would export light on the positioner end, so the CCD cameras could capture the photo of fiber tip position covered the focal plane, calculates the precise position information by light centroid method and feeds back to control system. A set of fiber back illuminated system was developed which combined to the low revolution spectro instruments in LAMOST. It could provide uniform light output to the fibers, meet the requirements for the CCD camera measurement. The paper was introduced the back illuminated system design and different test for the light resource. After optimization, the effect illuminated system could compare with the integrating sphere, meet the conditions of fiber position measurement.Using parallel controlled fiber positioner as the spectroscopic receiver is an efficiency observation system for spectra survey, has been used in LAMOST recently, and will be proposed in CFHT and rebuilt telescope Mayall. In the telescope observation, the position of fiber will highly influence the spectra efficient input in the fiber to the spectrograph. When the fibers were back illuminated on the spectra end, they would export light on the positioner end, so the CCD cameras could capture the photo of fiber tip position covered the focal plane, calculates the precise position information by light centroid method and feeds back to control system. After many years on these research, the back illuminated fiber measurement was the best method to acquire the precision position of fibers. In LAMOST, a set of fiber back illuminated system was developed which combined to the low revolution spectro instruments in LAMOST. It could provide uniform light output to the fibers, meet the requirements for the CCD camera measurement and was controlled by high-level observation system which could shut down during the telescope observation. The paper was introduced the back illuminated system design and different test for the light resource. After optimization, the effect illuminated system could compare the integrating sphere, meet the conditions of fiber position measurement.
Parallel controlled fiber positioner as an efficiency observation system, has been used in LAMOST for four years, and
will be proposed in ngCFHT and rebuilt telescope Mayall. The fiber positioner research group in USTC have designed a
new generation prototype by a close-packed module robotic positioner mechanisms. The prototype includes about 150
groups fiber positioning module plugged in 1 meter diameter honeycombed focal plane. Each module has 37 12mm
diameter fiber positioners. Furthermore the new system promotes the accuracy from 40 um in LAMOST to 10um in MSDESI.
That’s a new challenge for measurement. Close-loop control system are to be used in new system. The CCD camera
captures the photo of fiber tip position covered the focal plane, calculates the precise position information and feeds back
to control system. After the positioner rotated several loops, the accuracy of all positioners will be confined to less than
10um. We report our component development and performance measurement program of new measuring system by using
multi CCD cameras. With the stereo vision and image processing method, we precisely measure the 3-demension position
of fiber tip carried by fiber positioner. Finally we present baseline parameters for the fiber positioner measurement as a
reference of next generation survey telescope design.
Multi-objects survey system because of its high efficiency have been planned to build in many telescope such as
Mayall 4m telescope and have been working well on LAMOST. The telescope could control massively robotic fiber-positioners
carried with fibers on the top, received thousand galaxies and quasi-stellar objects at one time observation.
How to measure every fiber's position accurately is the key techniques for the telescope to improve its performance.
There is a good way to measure the fiber’s position by photogrammetry with no touches measurement. The camera
could capture the position of backside illuminated fibers. In this paper we described the trial measurement for multi
positioners system in different measuring parameters, and compared these conditions which influenced the measuring
accuracy. Finally the test results were presented the baseline parameters for the measurement system to provide a site
measurement option for the positioner location.
Modern multi-spectral sky survey requires the use of greater quantity and smaller size of the fiber positioner. This paper
presents a high-density integrated optical focal plane positioning system, which includes 150 groups fiber positioning
module and a 1 meter diameter honeycomb-shaped focal plane framework in that have about 150 hexagonal hole. Each
module has a pedestal includes 37 holes and 37 fiber positioner of 11.8 mm diameter. 37 fiber positioner integrated can
greatly reduce the difficulty of the design and installation. The modular structure also facilitates maintenance and
replacement in the field of telescope, and greatly reduce the difficulty of the drive system design. Numerical simulation
results show that: the honeycomb-shaped focal plane framework whose thickness is 100mm and who is in a variety of
working positions and load conditions, its maximum deformation is about 0.02mm. This meet the needs of the
general astronomical telescopes. The positioning accuracy of test 12mm diameter fiber positioner is about 0.04 mm,
and it is expected to reach 0.01mm if have the closed-loop control.
Large sky area multi-object fiber spectroscopy telescope (LAMOST) is an innovative reflecting Schmidt telescope. One
of its key technology is 4000 dual rotational fiber robot located in the focal plane. This article analyzes the calibration
requirements of the 4000 fiber robot. And then, proposes a fast calibration method in the complex field environment, and
discribes the specific process how to obtain positioning parameters of the fiber robot rapidly.