A motion planning algorithm for the feed support system of FAST

The paper relates to a motion planning algorithm for the feed support system of the Five-hundred-meter Aperture Spherical radio Telescope(FAST).To enhance the stability of the feed support system,the start/termination planning segments are adopted with an acceleration and deceleration section.The source switching planning adopts a combination of a line segment and focal segment to realize stable control of the feed support system.Besides,during the observation trajectory,a transition segment which is not used for observation data is planned with a required time.Through an example simulation,a smooth change is realized via the motion planning algorithm and presented in this paper.     

On the cable car feed support configuration for FAST

This paper introduces the cable car feed support configuration for China's FAST project. Recent advances on the mechanics of the proposed supporting structure and the control of the Stewart platform for secondary feed stabilization are presented. Difficulties associated with the configuration are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Position and attitude determination by integrated GPS/SINS/TS for feed support system of FAST

This paper presents a new measurement system based on integration method that can provide all-weather dependability and higher precision for the measurement of FAST's feed support system. The measurement system consists of three types of measuring equipments, and a processing software with the core data fusion algorithm. The Strapdown Inertial Navigation System(SINS) can autonomously measure the position, speed and attitude of the carrier. Its own shortcoming is that the measurement data diverge rapidly over time. SINS must combine the Global Positioning System(GPS) and the Total Station(TS)to obtain high-precision measurement data. The Kalman filtering algorithm is adopted for the integration measurement system, which is an optimal algorithm to estimate the measurement errors. A series of tests are carried out to evaluate the performance. For the feed cabin, the maximum RMS of the position is 14.56 mm, the maximum RMS of the attitude is 0.095, these values are less than 15 mm and 0.1° as the precision for measuring the feed cabin. For the Stewart manipulator, the maximum RMS of the position is 2.99 mm, the maximum RMS of the attitude is 0.093°, these values are less than 3 mm and 0.1° as the precision for measuring the Stewart manipulator. As a result, the new measurement meets the requirement of measurement precision for FAST's feed support system.     

Design and optimization of cross bow-tie dipole feed with cavity for FAST

A cross bow-tie dipole feed with cavity and symmetrical E and H plane pattern is presented for the Five hundred-meter Aperture Spherical radio Telescope(FAST).In this paper we describe the design,optimization and simulation results of a wide-band cross dipole feed with cavity covering the frequency range from 300 MHz to 600 MHz for FAST.The main goals of our design are to ensure that,(1)we cover the octave bandwidth,(2)the feed has symmetrical E and H plane pattern,and(3)the physical dimension is suitable for mounting it in the reserved position of the FAST feed cabin.The initial results indicate that we have met most of our design goals.This kind of feed had been equipped with the multi-beam receiver to carry out observation on the platform of FAST cabin.     

Study of central control system for FAST

FAST(The Five-Hundred-Meter Aperture Spherical Radio Telescope) is an under-building radio telescope which will be the largest single dish in the world. Through the study of the central control system, in accordance with the actual operation of the telescope and observation process, this article introduces the physical models for engineers and observers, the central control system architecture design, basic support modules and the necessary interfaces. We simulated observation control process and telescopes monitoring and control process, and took Active Reflector System as a subsystem example to complete the control system design and implementation using EPICS (Experimental Physics and Industrial Control System). The Central control system, with active reflector systems, feed supporting system has been taken to an integration test at Miyun model. In the case of the normal operation of the various sub-systems of the Miyun model by the central control system, a coordinated control is achieved.     

Simulation of FAST EM performance for both the axial and lateral feed defocusing

Five-hundred-meter Aperture Spherical radio Telescope(FAST)is the world’s largest single dish radio telescope,which is located in Guizhou Province,in southwest China.The FAST feed cabin is supported and positioned by six steel cables.The deviation of the feed position and orientation would lead to loss in the telescope efficiency.In this paper,a series of electromagnetic(EM)simulations of the FAST facility with varying feed positions and orientation offsets was performed.The maximum gain of FAST is about 82.3 dBi and the sibelobe is–32 dB with respect to the main beam at 3 GHz.The simulation results have demonstrated that the telescope efficiency loss is more sensitive to the lateral feed deviation compared with the axial deviation.The telescope efficiency would decrease by 8.2%due to the FAST feed position deviation of 10 mm rms when the observing frequency is 3 GHz.The FAST feed deviation basically has no effect on the sidelobes and cross polarization characteristic according to the simulations.     

Suggested quasi-Cassegrain system for multi-beam observation of FAST

FAST, the largest single-dish radio telescope in the world, has a 500-meter diameter main reflector and a 300-meter diameter illuminated area. It has a main reflector that can vary its shape, which continuously changes the shape of the illuminated area in reflector into a paraboloid. In this article, we propose a quasiCassegrain system for FAST. The detailed design results are provided. Such a quasi-Cassegrain system only needs to add a 14.6-meter diameter secondary reflector, which is close to the size of the feed cabin. The distance from the secondary reflector to the focus is only 5.08 m, and it has excellent image quality. In this quasi-Cassegrain system, the shape of the illuminated area in the main reflector continuously changes into an optimized hyperboloid. Using this quasi-Cassegrain system from frequency 0.5 GHz to 8 GHz, the multi-beam system can include 7 to 217 feeds. If this system is used in combination with Phased Array Feed(PAF) technology, more multi-beam feeds or a higher working frequency can be used.     

Extending the observable zenith angle of FAST using an offset feed

The five hundred meter aperture spherical radio telescope is will use an active spherical reflector. When the zenith scan angle is changed, the illuminated part of the reflecting surface is made to fit a paraboloid of revolution in real time by active control. The maximum zenith scan angle |ψ_max| of FAST is 30^° under conditions of the geometry selected in order that the feed does not illuminate the ground. The result of this paper shows that the maximum zenith scan angle |ψ_max| can be extended to 69^° by offsetting the feed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Development of wideband orthomode transducers for FAST cryogenic receiver system

This paper describes the design,construction,and performance of the wideband orthomode transducers(OMTs)for the L-(1.2–1.8 GHz),the S-(2–3 GHz)and the P-(0.56–1.12 GHz)band receiver systems of the Five-hundred-meter Aperture Spherical radio Telescope(FAST).These OMTs operate at the cryogenic temperature of 70 K to reduce their thermal noise contribution to the receiver chains.The development on the FAST L-and S-band quad-ridged waveguide(QRWG)OMTs is carried out based on the theoretical mode analysis.In view of the miniaturization of FAST cryogenic receiver system at P-band,a novel wideband compact bowtie dipole OMT is designed with an octave bandwidth as well as a length of only quarter wavelength.The proposed L-,S-and P-band OMTs are designed and optimized by using Ansys High Frequency Structure Simulator(HFSS),and then manufactured,tested at room temperature.Measurement of FAST cryogenic receiver system noise is also performed with the L-,S-and P-band OMTs installed.The measured results fully comply with the design specifications.     

The time and frequency standard system for FAST receivers

This paper reports on the time and frequency standard system for the Five-hundred meter Aperture Spherical radio Telescope(FAST),including the system design,stability measurements and pulsar timing observations.The stability and drift rate of the frequency standard are calculated using 1-year monitoring data.The UTC-NIM Disciplined Oscillator(NIMDO)system improves the system time accuracy and stability to the level of 5 ns.Pulsar timing observations were carried out for several months.The weighted RMS of timing residuals reaches the level of less than 3.0μs.     

The control system of the active main reflector for FAST

The main reflector of FAST consists of about 1800 elementary units. Each unit has three adjustable supports to fix its position, and its position is adjusted by mean of mechanical actuators. According to the radio source position at any given time, all the actuators are continuously adjusted to form a proper paraboloid in real time during the course of the observation. The basic requirements of such a control system are discussed. A field bus control system based on LonWorks technology is suggested to control all 1800actuators. The main advantages of this distributed control system are its reliability, flexibility, efficiency and economics. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structural analysis of FAST reflector supporting system

According to the deformation and movement requirements of the FAST reflector, a multi-purpose analysis, including the load-bearing behavior, deformation, construction costs of the reflector supporting structure and its model, is presented in this paper. The advantages and disadvantages of steel and aluminum alloy structures are also discussed and compared through detailed design calculations under load-bearing capacity and normal working conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Similarity solutions for an axially symmetric explosion model in magnetogasdynamics. I

Similarity solutions, for a point explosion in a spheroid with axially symmetric density distribution obeying power laws in the presence of magnetic field, are obtained. A new technique suggested by Bhowmick (1978) has been utilised to study the character of flow variables behind the shock front in an axisymmetric model. The total energy of the wave is constant.     

An open-loop control algorithm of the active reflector system of FAST

An open-loop control algorithm is put forward for continuous paraboloid deformation of the active reflector system of the Five-hundred-meter Aperture Spherical radio Telescope(FAST).The method is based on a calibration database and interpolation in 2D spatial domain and temperature domain,respectively.It is completely independent of real-time measurement of cable nodes so that it has advantage of working all-weather and no additional electro-magnetic interference(EMI).Furthermore,its control accuracy can be effectively improved via reasonable layout of the calibrated paraboloids and increasing calibration accuracy.Meanwhile deformation safety is considered via calibration as well.Finally its control accuracy is also confirmed via site measurements of paraboloid deformations.     

RFI measurements and mitigation for FAST

Radio Frequency Interference(RFI)mitigation is essential for supporting the science output of Five-hundred-meter Aperture Spherical radio Telescope(FAST)due to its high sensitivity.In order to protect FAST from RFI,an Electromagnetic Compatibility(EMC)study has been carried out and the operation of a Radio Quiet Zone(RQZ)is ongoing.RFI measurements of the telescope instruments and monitoring of the active radio services outside the site have revealed the radiation properties of the RFI sources.Based on the measurement results and theoretical analysis,various EMC methods have been implemented for the telescope to decrease the RFIs.Meanwhile,the main RFI sources in the FAST RQZ,such as mobile stations,broadcast stations and navigation instruments,have been identified,and the technical measures have been adopted to protect the quiet radio environment around the site.The early science outputs of FAST have demonstrated the efficiency of RFI mitigation methods.     

Design of the cryogenic system of the wideband phased array feed for QTT

Phased array feeds(PAFs),which illuminate the dish with digitally synthesized beams instead of separated horns,provide the capability for wider and continuous field-of-view surveys.As a promising technology for next generation radio telescopes,PAFs will provide the Qi Tai Telescope(QTT),which will be next world-class fully steerable radio telescope,an opportunity of reaching several cutting-edge science goals.This paper presents a brief introduction of the wideband PAF for QTT,and the detailed design and simulation of the cryogenic system.Based on this design,a scaled prototype of the spherical vacuum window,which is the key part of the cryogenic system,has been built and the performance is verified.     

Prospects for detecting ultra-high-energy particles with FAST

The origin of the highest-energy particles in nature, ultra-high-energy(UHE) cosmic rays, is still unknown. In order to resolve this mystery, very large detectors are required to probe the low flux of these particles — or to detect the as-yet unobserved flux of UHE neutrinos predicted from their interactions. The‘lunar Askaryan technique' is a method to do both. When energetic particles interact in a dense medium,the Askaryan effect produces intense coherent pulses of radiation in the MHz–GHz range. By using radio telescopes to observe the Moon and look for nanosecond pulses, the entire visible lunar surface(20 million km~2) can be used as a UHE particle detector. A large effective area over a broad bandwidth is the primary telescope requirement for lunar observations, which makes large single-aperture instruments such as the Five-hundred-meter Aperture Spherical radio Telescope(FAST) well-suited to the technique. In this contribution, we describe the lunar Askaryan technique and its unique observational requirements. Estimates of the sensitivity of FAST to both the UHE cosmic ray and neutrino flux are given, and we describe the methods by which lunar observations with FAST, particularly if equipped with a broadband phased-array feed, could detect the flux of UHE cosmic rays.     

A 2-D model for the support of a polar-crown solar prominence

We present a 2-D potential-field model for the magnetic structure in the environment of a typical quiescent polar-crown prominence. The field is computed using the general method of Titov (1992) in which a curved current sheet, representing the prominence, is supported in equilibrium by upwardly directed Lorentz forces to balance the prominence weight. The mass density of the prominence sheet is computed in this solution using a simple force balance and observed values of the photospheric and prominence magnetic field. This calculation gives a mass density of the correct order of magnitude. The prominence sheet is surrounded by an inverse-polarity field configuration adjacent to a region of vertical, open polar field in agreement with observations.A perturbation analysis provides a method for studying the evolution of the current sheet as the parameters of the system are varied together with an examination of the splitting of an X-type neutral point into a current sheet.Program Systems Institute of the Russian Academy of Sciences, Pereslavl-Zalessky 152140, Russia.     

EMC design for actuators in the FAST reflector

An active reflector is one of the three main innovations incorporated in the Five-hundredmeter Aperture Spherical radio Telescope(FAST).The deformation of such a huge spherically shaped reflector into different transient parabolic shapes is achieved by using 2225 hydraulic actuators which change the position of the 2225 nodes through the connected down tied cables.For each different tracking process of the telescope,more than 1/3 of these 2225 actuators must be in operation to tune the parabolic aperture accurately and meet the surface error restriction.This means that some of these actuators are inevitably located within the main beam of the receiver,and Electromagnetic Interference(EMI)from the actuators must be mitigated to ensure the scientific output of the telescope.Based on the threshold level of interference detrimental to radio astronomy described in ITU-R Recommendation RA.769 and EMI measurements,the shielding efficiency(SE)requirement for each actuator is set to be 80 d B in the frequency range from 70 MHz to 3 GHz.Therefore,Electromagnetic Compatibility(EMC)was taken into account in the actuator design by measures such as power line filters,optical fibers,shielding enclosures and other structural measures.In 2015,all the actuators had been installed at the FAST site.Till now,no apparent EMI from the actuators has been detected by the receiver,which demonstrates the effectiveness of these EMC measures.