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.     

An Active Flexure Compensation Method for LAMOST spectrograph based on BP-Neural Network

The Large sky Area Multi-Object fiber Spectroscopic Telescope(LAMOST) is a Chinese national scientific research facility operated by National Astronomical Observatories, Chinese Academy of Sciences(NAOC). The LAMOST survey for the Milky Way Galaxy and extra-galactic objects has been carried out for several years. The accuracies in measuring radial velocity are expected to be 5 km s-1 for the low resolution spectroscopic survey(R = 1800), and 1 km s-1 for the medium resolution mode. The stability of spectrograph is the main factor affecting the accuracies in measuring radial velocity, so an Active Flexure Compensation Method(AFCM) based on Back Propagation Neural Network(BPNN) is proposed in this paper. It utilizes a deep BP(4-layer, 5-layer etc.) model of thermal-induced flexure to periodically predict and apply flexure corrections by commanding the corresponding tilt and tip motions to the camera. The spectrograph camera system is adjusted so that the positions of these spots match those in a reference image. The simulated calibration of this compensation method analytically illustrates its performance on LAMOST spectrograph.     

An adjustment method for active reflector of large high-frequency antennas considering gain and boresight

The design of the Qitai 110 m Radio Telescope(QTT) with large aperture and very high working frequency(115 GHz) was investigated in Xinjiang, China. The results lead to a main reflector with high surface precision and high pointing precision. In this paper, the properties of active surface adjustment in a deformed parabolic reflector antenna are analyzed. To assure the performance of large reflector antennas such as gain and boresight, which can be obtained by utilizing an electromechanical coupling model, and satisfy them simultaneously, research on active surface adjustment applied to a new parabolic reflector as target surface has been done. Based on the initial position of actuators and the relationship between adjustment points and target points, a novel mathematical model and a program thatdirectly calculates the movements of actuators have been developed for guiding the active surface adjustment of large reflector antennas. This adjustment method is applied to an 8 m reflector antenna,in which we only consider gravity deformation. The results show that this method is more efficient in adjusting the surface and improving the working performance.     

Algorithm Analysis of Autonomous Navigation of Spacecraft Based on X-ray Pulsars

A pulsar has the very stable rotation and can be used as the time standard. The astrometric parameters and astrophysical parameters of many pulsars, such as the spatial position, proper motion, distance, rotation period and its derivative, etc., can be all accurately determined. Since the pulsar can provide the time signal and the coordinates of its spatial position simultaneously, the pulsar navigation system installed on a spacecraft enables the autonomous navigation of the spacecraft to be realized. Firstly, the position of the spacecraft is predicted based on the equation of orbit dynamics of the spacecraft and then the Kalman filtering is applied to calculating the estimation error of the spacecraft position through the difference between the pulse arrival time observed on the spacecraft and the predicted pulse arrival time, thereby modifying the position of the spacecraft. Finally, the effects of the initial error, measuring accuracy of the pulse arrival time and number of pulsars on the navigation accuracy are analyzed.     

ALFA: Adaptive Optics for the Calar Alto Observatory Optics, Control Systems, and Performance

The adaptive optics system ALFA differs in some aspects from systems like ADONIS and PUEO which have delivered scientific results since years. Interchangeable lenslet arrays with different numbers of subapertures and a deformable mirror with many more actuators than the number of corrected modesresult in some peculiarities in the calibration of the system and the reconstruction of incident wavefronts.We describe the design of ALFA's optics and its modal control architecture with a focus on a comparative study of the performance of different mode sets used to correct the wavefront aberrations. An outlook on our plans to improve and simplify the use of ALFA is given.The last section is dedicated to issues related to observing with ALFA in its present state. Expected Strehl ratios for different seeing conditions and guide star magnitudes are summarized in a table. AO observations in general, direct imaging and doing spectroscopywith ALFA in particular are discussed.     

Dynamics and Control for a Small Telescope

Our main goal is to determine the dynamic equations of a certain complex system, as is the case of the mechanical system for a small aperture telescope. Causes of this complexity are: the lack of documents about the operation of the elements belonging to the system, and the variation of dynamics with respect to the time and the position of the telescope. To check that we have obtained a valid set of dynamic equations, we will design a computer control system that will implement a self-guide system for the telescope. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dynamics and control of a solar collector system for near Earth object deflection

A solar collector system is a possible method using solar energy to deflect Earth-threatening near-Earth objects.We investigate the dynamics and control of a solar collector system including a main collector (MC) and secondary collector (SC).The MC is used to collect the sunlight to its focal point,where the SC is placed and directs the collected light to an asteroid.Both the relative position and attitude of the two collectors should be accurately controlled to achieve the desired optical path.First,the dy...     

丽江2.4米望远镜检偏器控制系统设计

偏振测量是天文四大测量(光谱,光度,偏振,图像)手段之一,而检偏器是进行天体偏振测量的有力工具之一。通过TCP/IP协议控制步进电机驱动检偏器旋转一定的角度值,通过RS485串行通讯实时读取光电绝对编码器的读数作为检偏器绝对位置的反馈量,从而构成一个闭环控制系统。最终实现了转动误差0.42°,步进角度22.5°,转动范围0°～360°的偏振闭环控制系统,完全满足天体偏振测量的基本要求。     

The GREGOR telescope control system

This article describes the architecture of the new GREGOR telescope and its instrument control system. A short summary is given on the communication structure between instruments and other devices during observation. Because of its importance to all instruments the main functions of the telescope control system are described in detail (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Adapted Fuzzy Controller for Astronomical Telescope Tracking

This paper presents a novel application of fuzzy logic (FL) controller driven by an adaptive fuzzy set (AFS) for position tracking of the telescope driven by electric motor. Also, the proposed FL controller, driven by AFS, is compared with a classical FL control, driven by a static fuzzy set (SFS). Both FL controllers algorithm use the position error and its rate of change as an input vector. The mathematical model of the telescope driven by electric motor is highly nonlinear differential equations. Therefore the use of the artificial intelligent controller, such as FL is much better than the conventional controller, to cover a wide range of operating conditions. So, the output of FL control is utilized to force the electric drives, of the telescope, to satisfy a perfect matching of the predefined desired position of the telescope arms. Both of FL controllers, using AFS and SFS, are simulated and tested when the system is subjected to a step change in reference value. In addition, these simulation results are compared with the conventional Proportional-Derivative (PD) controller, driven by fixed gain. The proposed FL, using an adaptive fuzzy set, improve the dynamic response of the overall system by improving the damping coefficient and decreasing the rise time and settling time compared with other two controllers.     

Non-uniform Solar Temperature Field on Large Aperture,Fully-Steerable Telescope Structure

In this study, a 110-m fully steerable radio telescope was used as an analysis platform and the integral parametric finite element model of the antenna structure was built in the ANSYS thermal analysis module. The boundary conditions of periodic air temperature, solar radiation, long-wave radiation shadows of the surrounding environment, etc. were computed at 30 min intervals under a cloudless sky on a summer day, i.e., worstcase climate conditions. The transient structural temperatures were then analyzed under a period of several days of sunshine with a rational initial structural temperature distribution until the whole set of structural temperatures converged to the results obtained the day before. The non-uniform temperature field distribution of the entire structure and the main reflector surface RMS were acquired according to changes in pitch and azimuth angle over the observation period. Variations in the solar cooker effect over time and spatial distributions in the secondary reflector were observed to elucidate the mechanism of the effect. The results presented here not only provide valuable realtime data for the design, construction, sensor arrangement and thermal deformation control of actuators but also provide a troubleshooting reference for existing actuators.     

Modern photographic catalog work: Aspects of observing techniques and reduction methods

The consequent application of present optical technology for the construction of substantially improved instrumentation in photographic astrometry and the application of rigorous reduction techniques provide all means for the construction of new, global position catalogs with a precision of a few 0.01 arcsec. At the same time the limiting magnitude of those catalogs can be extended to about 17 without decreasing positional accuracy. High speed measuring machines combined with real time reductions and storage on optical disks can handle the huge amount of information on a single plate. The throughout application of block adjustment reduction of global and hemisphere covering catalogs can provide a rigid stellar net which can be adjusted for the first time to a quasar based future reference frame directly. Instrumental requirements and optimal reduction techniques for achieving this goal are discussed.     

A CCD‐based guiding and control system for solar telescopes

We describe a system for guiding and control of motion of solar telescopes based on a determination of the position of the full solar disk on a CCD chip. The system computes the position of the disk center and, according to the difference between the instantaneous and required positions, it controls the motion of the telescope to compensate the deviation. The instantaneous position of the disk center is the basic input for the telescope's guiding and coordinate system. Brightness inhomogeneities of the solar limb caused by clouds are checked in real time. The function of the system can be suspended automatically if the clouds degrade the accuracy of guiding. The system also makes possible to check the correct focus and to evaluate the image quality.     

Research on actuator distribution and panels for a radio telescope

Trying to achieve the best surface accuracy control with the fewest actuators, this article mainly studies the distribution of actuators and the method of panel design. The influence of the number of faulty actuators on the accuracy of the surface shape is demonstrated. In addition, the method incorporating a triangular panel, node index and the fitting solution method of a single panel is also given. This method provides a reference for the design and realization of an active surface or a deformable sub-reflector for high performance large aperture radio telescopes.     

New method of orbit determination from two position vectors based on solving Gauss’s equations

The classic problem of finding the orbit of a celestial body from its two position vectors for two instants of time is considered. A solution to the problem free from uncertainties is obtained which can be applied for all three kinds of Keplerian movement. The main part of the computational procedure is reduced to solving one equation with one unknown. Formulas are derived for the initial value of the equation root, which makes the application of the Newton-Raphson method successful. The efficiency and reliability of the suggested algorithm is illustrated by examples of the orbit determination for the asteroids Adeona and Icarus, as well as for Halley’s Comet and Bowell’s Comet.     

Determination of the orbit of a natural satellite with hyperbolic flybys

We show that, when a natural satellite like Titan is invisible (e.g., due to an opaque atmosphere) its planetary orbit and its mass can be determined by tracking a spacecraft in close flybys. This is an important problem in the Cassini mission to the Saturnian system, which will be greatly improved by a good astrometric model for all its main components; in particular, an accuracy of a few hundred meters for the orbit of Titan is necessary to allow a measurement of its moment of inertia. The orbit of the spacecraft is the union of elliptical arcs, joined by short hyperbolic transitions: a problem of singular perturbation theory, whose solution leads to a matching condition between the inner hyperbolic orbit and the elliptical orbital elements. Since the inner elements are given in terms of the relative position and velocity of the spacecraft, accurate Doppler measurements in both regions can provide a satisfactory determination of Titan's position and velocity, hence of its Keplerian elements. The errors in this determination are discussed on the basis of the expected Allan deviation of the Doppler method; it is found that the driving errors are those in the elliptical arcs; the fractional errors in Titan's orbital elements are expected to be 10^–7. It is also possible to measure the mass of the satellite; however, when the eccentricity e of the flybys is large, the mass and a scaling transformation are highly correlated and the fractional error in the mass is expected to be e times worse.     

1米太阳望远镜光谱仪像旋转及消旋控制

光谱仪是1 m太阳望远镜的主要终端设备之一,该望远镜采用地平式的机架结构和修正的格里高利光学系统。在望远镜跟踪太阳时,由于地平式望远镜的自身运动特点和光学系统中平面反射镜的存在,其光谱仪狭缝所在平面上的太阳像随时间绕主光轴旋转,因此光谱仪必须进行消旋才能正常工作。首先深入研究了光谱仪狭缝平面上像的旋转变化,分析其旋转范围、速度和加速度随时角变化的特性,然后根据光谱仪消旋精度并结合像的旋转特性提出伺服系统位置检测和驱动电机的主要性能指标,最后给出光谱仪消旋伺服控制方案。     

基于X射线脉冲星的航天器自主导航算法分析

脉冲星自转非常稳定,可以用作时间标准,许多脉冲星的空间位置、自行、距离、自转周期及其导数等天体测量参数和天体物理参数都能被精确测定.由于脉冲星能够同时提供时间信号和空间位置坐标,安装在航天器上的脉冲星导航系统能够实现航天器的自主导航.首先根据航天器轨道动力学方程预测航天器的位置,再通过航天器上观测的脉冲到达时间和预报的脉冲到达时间之差,应用Kalman滤波计算航天器位置估计的误差,从而对航天器的位置进行修正.最后,分析初始误差、脉冲到达时间测量精度、脉冲星个数对导航精度的影响.     

BPM标准时号放大器的研制

本文简要论述BPM标准时号放大器的主要技术指标、L作原理和设计特点。     

Forthcoming close angular approaches of planets to radio sources and possibilities to use them as GR tests

During close angular approaches of solar system planets to astrometric radio sources, the apparent positions of these sources shift due to relativistic effects and, thus, these events may be used for testing the theory of general relativity; this fact was successfully demonstrated in the experiments on the measurements of radio source position shifts during the approaches of Jupiter carried out in 1988 and 2002. An analysis, performed within the frames of the present work, showed that when a source is observed near a planet’s disk edge, i.e., practically in the case of occultation, the current experimental accuracy makes it possible to measure the relativistic effects for all planets. However, radio occultations are fairly rare events. At the same time, only Jupiter and Saturn provide noticeable relativistic effects approaching the radio sources at angular distances of about a few planet radii. Our analysis resulted in the creation of a catalog of forthcoming occultations and approaches of planets to astrometric radio sources for the time period of 2008–2050, which can be used for planning experiments on testing gravity theories and other purposes. For all events included in the catalog, the main relativistic effects are calculated both for ground-based and space (Earth-Moon) interferometer baselines.