宽波段傅里叶变换太阳光谱仪等光程差采样系统设计

太阳成像光谱探测是诊断太阳大气磁场和热力学参数的主要手段. 傅里叶变换太阳光谱仪(Fourier Transform Solar Spectrometer, FTSS)具有宽波段的优势, 是当前中红外高分辨率太阳光谱探测的最佳选择. FTSS通过采集目标辐射等光程差干涉图, 反演获得光谱图, 等光程差采样的间隔决定了反演光谱波长范围. 因此从FTSS宽波段光谱观测对不同等光程差采样间隔需求出发, 基于现场可编程门阵列(Field Programmable Gate Array, FPGA)技术, 采用全数字分频、倍频方案, 设计了一套宽波段FTSS等光程差采样系统. 采用分布式余数补偿方法, 有效解决了在参考激光干涉信号倍频过程中, 输出采样信号在输出信号周期间误差累积问题, 并降低了输出采样信号的误差及非均匀性; 经功能仿真及实验测试, 系统在200Hz--50kHz频率范围内, 频率误差delta $<$ 0.04%, 可有效满足FTSS的300nm--25μm宽波段的光谱观测数据采集需求, 为后续可见和红外波段FTSS的研制奠定了技术基础.     

光学综合孔径望远镜光程探测方法研究

光干涉技术需要对光程差进行精确的补偿和控制,因此光程差的精确探测尤为重要。主要针对色散条纹法在光程差探测中的应用进行讨论,通过理论分析、仿真计算和实验,验证了在一个波长范围内的光程差与色散干涉条纹纵向光强峰值偏移量之间存在线性关系。分析实验获得的白光色散干涉条纹数据发现,理论计算预测值有一定的偏差,通过对实验系统中各主要误差源进行分析发现,分光棱镜在运动过程中引入的俯仰起伏是导致实验误差的主要原因。     

一种基于傅里叶变换和图像二值化阈值遍历的干涉条纹角度计算方法

消除综合孔径望远镜子孔径之间的相对光程差是实现高分辨率干涉成像的前提条件,条纹检测法是一种检测相对光程差的有效办法。因子孔径的空间位置排布使干涉条纹具有一定的方向性,若不能精确获知干涉条纹的角度,则无法沿条纹的法线方向行采样,进而无法根据对比度变化曲线的最大值获得子孔径之间的最小光程差位置。提出了一种基于傅里叶变换和图像二值化阈值遍历的获得干涉条纹角度的方法,首先介绍了算法的基本原理,其次通过对条纹角度为43°的仿真数据进行算法验证得到的角度为43.007 8°,与理论值的误差为0.018%,证实了方法的可行性。最后对比了未旋转相机和旋转相机两种情况下的条纹对比度变化曲线,可知通过旋转相机使条纹转至相机靶面纵轴方向再进行采样的办法,更有利于精确得到相对光程差的最小位置。     

环焦天线口径面相位分析

环焦天线具有特殊的电磁特性和应用领域.对环焦天线的口径面相位误差进行了理论和仿真分析,推导了馈源和副面位置偏差引起的相位误差、主副面之间的补偿关系以及全息测量中天线转动引起的光程差.研究结果将对环焦天线的精确面形测量和补偿提供理论依据和参考.     

基于小波分析和神经网络的卫星钟差预报性能分析

为了有效地进行卫星钟差预报和更好地反映卫星钟差特性,提出了一种基于小波分析和神经网络的4阶段混合模型来实现卫星钟差的预报,并给出了基于小波分析和径向基函数(Radial Basis Function,RBF)神经网络进行卫星钟差预报的基本思想、预报模型和实施步骤.采用"滑动窗"划分数据,利用神经网络预测小波分解和去噪后的钟差序列各层系数,更精确地把握钟差序列复杂细致的变化规律,从而更好地逼近钟差序列.为验证该混合预报模型的可行性和有效性,利用GPS卫星钟差数据进行钟差预报精度分析,并将其与灰色系统模型和神经网络模型进行比较分析.仿真结果显示,该模型具有较好的预报精度,可为实时GPS动态精密单点定位提供较高精度的卫星钟差.     

基于傅里叶变换色散条纹法的实验研究

光干涉要求实现各光路共相位叠加,对于宽波带的白光干涉要求更加苛刻。因此,要实现稳定的白光光干涉需要对光程进行实时探测和控制。对基于傅里叶变换的色散条纹法进行了理论推导、算法设计,并利用实验室建立的Fizeau型干涉实验装置开展开环、闭环实验研究。在开环实验中,采集不同光程差的多幅色散条纹图像研究了图像频谱次峰偏移量和光程差的关系,结果显示,偏移量与光程差之间呈现良好的线性关系。在闭环实验中,利用光程差的实时计算结果和光程补偿机构对光程差进行闭环控制,结果显示,在外加扰动的情况下,系统能够始终保持在初始的干涉状态。     

差分VLBI技术在采样返回任务中的应用

针对采样返回任务中多探测器精密短弧定轨问题,研究了甚长基线干涉测量(Very Long Baseline Interferometry,VLBI)技术在两探测器间的交替观测模式、2π模糊度解算方法和数据差分处理方法,给出了星载信标的设计原则和方案。利用日本SELENE探月卫星的两个小卫星R-star和V-star的同波束VLBI相关相位生成了交替VLBI相位观测量,对其进行了差分处理求解差分时延,然后利用差分时延和测速测距数据进行定轨计算。对差分时延的分析表明,交替VLBI差分群时延RMS值为46 mm,测量精度与同波束VLBI差分群时延相当;交替VLBI差分相时延RMS值为1.6 mm,测量精度与同波束VLBI差分相时延相当。定轨结果表明,交替VLBI在进行多探测器的短弧定轨时能达到同波束VLBI相当的精度。     

地月DRO星载光学测量近地小行星轨道确定

针对地基光学监测系统对近地小行星在近太阳方向的监测存在盲区的问题,提出了远距离逆行轨道(Distant Retrograde Orbit,DRO)天基光学平台对近地小行星进行跟踪定轨的方法.通过可视性分析,筛选仿真观测数据,利用美国宇航局喷气推进实验室(Jet Propulsion Laboratory,JPL)公布的小行星初始轨道信息对不同轨道类型的目标天体进行轨道确定,将计算结果与参考轨道对比分析.仿真结果表明:在测量精度2角秒,定轨弧长3年的情况下,DRO平台对仿真算例中所选择的近地小行星的定轨精度可以达到几十公里量级,其中Atira型轨道精度可达10公里以内.由此可见,DRO天基平台对近地小行星具有较好的监测能力,定轨精度能实现对目标小行星的精确跟踪,并对其进行轨道预报.     

基于FPGA的空间太阳磁像仪自校正稳像系统研制

太阳磁像仪是开展太阳磁场观测研究的核心仪器,其中的稳像系统是空间太阳磁像仪的关键技术之一,针对深空探测卫星系统对载荷重量、尺寸限制严苛的要求,设计了基于图像自校正方法的稳像观测系统.介绍了一套基于现场可编程门阵列(Field-Programmable Gate Array, FPGA)和数字信号处理器(Digital Signal Processor, DSP),通过基于自相关算法的高精度稳像方法设计,并结合精确偏振调制、准确交替采样控制等系统软硬件设计,克服由于卫星平台抖动、指向误差等因素造成的图像模糊,实现实时相关、校正、深积分的稳像观测系统.针对像素尺寸为1 K×1 K、帧频为20 fps的CMOS (Complementary Metal Oxide Semiconductor)探测器,实现了1像元以内的实时稳像观测精度.在完成实验室测试后, 2021年6月18日在国家天文台怀柔太阳观测基地35 cm太阳磁场望远镜上开展了实测验证,结果表明该系统能够有效地完成太阳磁像仪自校正稳像观测,获得了更高分辨率的太阳磁场数据.稳像系统的成功研制不仅可以为深空太阳磁像仪的研制提供轻量化、高...     

利用国内VLBI网跟踪大椭圆轨道卫星

2004年7月,昆明VLBI站经过改造,由上海、乌鲁木齐和昆明站组成的中国VLBI网(CVN)采用统一的MARK4格式编制器和CVN硬盘记录系统,对大椭圆轨道卫星“探测1号”的2圈轨道的共同可视弧段进行了跟踪观测．软件相关处理程序已成功地用于检测卫星遥测信号的干涉条纹和数据相关处理．采用基于条纹幅度的加权最小二乘条纹拟合方法,获得了卫星VLBI观测量及其精度估计,完成了卫星VLBI观测量的3基线闭合误差检验．应用河外射电源校准方法和多频点相位校正信号提取方法,进行了台站钟差和仪器延迟等系统误差改正．经系统差改正后的卫星VLBI观测量序列已用于“探测1号”卫星的轨道确定．     

Research on the Application of Fast-steering Mirror in Stellar Interferometers

In stellar interferometers, the fast-steering mirror (FSM) is widely utilized to correct the wavefront tilt caused by the atmospheric turbulence and internal instrumental vibration, because of its high resolution and fast response frequency. In this study, the non-coplanar error between the FSM and the actuator deflection axis introduced by the manufacturing, assembly, and adjustment is analyzed systematically. Via a numerical method, the additional optical path difference (OPD) caused by the above factors is studied, and its effect on the fringe tracking accuracy of a stellar interferometer is also discussed. On the other hand, the starlight parallelism between the beams of two arms is one of the main factors for the loss of fringe visibility. By analyzing the influence of wavefront tilt caused by the atmospheric turbulence on fringe visibility, a simple and efficient real-time correction scheme of starlight parallelism is proposed based on a single array detector. The feasibility of this scheme is demonstrated by a laboratory experiment. The results show that after the correction of fast-steering mirror, the starlight parallelism meets preliminarily the requirement of a stellar interferometer on the wavefront tilt.     

Improvements for group delay fringe tracking

Group delay fringe tracking using spectrally dispersed fringes is suitable for stabilizing the optical path difference in ground-based astronomical optical interferometers in low light level situations. We discuss the performance of group delay tracking algorithms when the effects of atmospheric dispersion, high-frequency atmospheric temporal phase variations, non-ideal path modulation, non-ideal spectral sampling, and the detection artifacts introduced by electron-multiplying CCDs are taken into account, and we present ways in which the tracking capability can be optimized in the presence of these effects.     

A novel Doppler frequency measurement method based on the closed-loop signal correlation for deep space exploration

In deep space exploration,it is necessary to improve the accuracy of frequency measurement to meet the requirements of precise orbit determination and various scientific studies.A phase detector is one of the key modules that restricts the tracking performance of phase-locked loop(PLL).Based on the phase relationship between adjacent signals in the time domain,a novel phase detector is presented to replace the arctangent phase detector.The new PLL,which is a closed loop signal correlation algorithm,shows good performance in tracking signals with high precision and the tracking accuracy of frequency of1 second integration is close to Cramer-Rao lower bound(CRLB) when setting proper parameters.Actual data processing results further illustrate the excellent performance of the novel PLL.     

KAGRA引力波探测器中蓝宝石测试镜光学性质研究

低温制冷技术是下一代激光干涉仪引力波探测器的核心技术之一. 日本引力波探测器KAGRA (Kamioka Gravitational Wave Detector)作为该技术的前沿开拓者, 将运行在20K的超低温环境中, 并使用在低温下热噪声较低的单晶蓝宝石晶体作为测试镜. 然而, 高质量大尺寸低吸收率的蓝宝石晶体极难制备. 此外, 由于蓝宝石晶体存在晶格结构不均匀, 很容易导致不必要的双折射效应, 从而影响探测器的目标灵敏度. 基于上述问题, 开发了两套大尺寸光学测量系统, 首次系统研究了KAGRA低温蓝宝石测试镜的光学特性. 首先, 根据探测器对测试镜热噪声的要求, 开发了一套基于光热共光路干涉技术的光学测量系统, 该系统可对测试镜以及测试镜表面涂层的光学吸收进行有效的表征. 其次, 基于光学吸收测量系统, 开发了一套双折射效应测量系统, 该系统可以有效表征测试镜中双折射的均匀性. 目前两套测量系统的搭建与调试已完成, 对蓝宝石测试镜光学吸收的测量灵敏度达到了1.5ppm/cm, 双折射测量系统的空间分辨率小于0.3mm times 0.3mm. 该工作对降低大尺寸低温测试镜双折射效应及提高探测器灵敏度具有重要意义.     

High time-resolution spectroscopic imaging using intensified CCD detectors

The micro‐channel plate intensified CCD photon counting detector developed at University College London has been upgraded to allow time-resolved spectroscopic optical data to be acquired on periodical sources such as pulsars. First observing trials have been carried out, acquiring spectroscopic data on the Crab pulsar. The detector was phase locked to the pulsar period and a temporal resolution of 41.4 μs employed. The phase locking allowed the coaddition of time slices over a large number of pulsar periods to build up quantifiable spectroscopic data when observing in a flux-limited regime.     

Direct detection and characterization of extrasolar planets: The Mariotti space interferometer

Space infrared nulling interferometry has been identified as one of the most promising techniques for direct detection of Earthlike extrasolar planets and spectroscopic analysis of their atmospheres in the near future. After a review of various nulling interferometer schemes, we introduce the concept of internal modulation. As an illustration, we describe a two-dimensional array of telescopes that provides full internal modulation capabilities: the Mariotti space interferometer. It consists of six free-flying telescopes positioned on the sides of an equilateral triangle and grouped into three nulling interferometers. Their nulled outputs are suitably phase-shifted with respect to each other, coherently recombined, and detected. The phase shifts applied between the nullers are periodically changed, providing signal modulation at a frequency that can be selected to minimize instrumental and background noise. The frequency upper limit is set by the read-out noise of the detectors, and turns out to be ¹⁰⁻¹-¹⁰⁻² Hz for currently available Si:As BIB devices. This “fast” signal modulation allows much better monitoring of the background and detector drifts than when one relies solely on the external modulation provided by the slow rotation of the whole interferometer (at typical frequencies of 3×¹⁰⁻⁴-3×¹⁰⁻⁵ Hz). Mariotti internal modulation, also known as “phase chopping,” thus appears as a major step toward the feasibility of the Darwin and TPF space missions.     

Achromatic interfero-coronagraph with variable rotation shearing for studying extrasolar planets

Direct observation of exoplanets will make it possible to clarify many principal questions connected both with extrasolar planets and planetary systems and to measure atmospheric spectra of the planets. Obtaining an exoplanet image not distorted by the light from a star is at the cutting edge of present-day optical technologies owing to the combination of tremendous brightness contrasts and small angular distances between the planet and star. To observe the exo-Earth, it is necessary to weaken the brightness of the parent star image by 9–10 orders of magnitude (in the optical and near-IR ranges). To compensate the influence of the atmosphere, ground-based (e.g., 8–10 m) telescopes intended for observing exoplanets are equipped with adaptive optics systems, the spatial and temporal resolutions of which are not yet sufficient. A meter-class space telescope equipped with a star coronagraph will make it possible to observe the nearest exoplanets. In this paper, an improved tool for star coronagraphy is considered, namely, the achromatic interferometric coronagraph with a variable rotational shear. It is fabricated according to the optical scheme of the common path interferometer for studying extrasolar planets by direct observations. Theoretical and experimental estimations for the main characteristics of the scheme were carried out. Laboratory experimental measurements were carried out on a coronagraph model.     

Diffraction losses in ground-based optical interferometers

We present a numerical analysis of free-space propagation of the beams inside a long-baseline optical/infrared interferometer. Unlike the models of beam propagation used in most previous studies, our analysis incorporates the effects of atmospheric seeing on the wavefronts entering the interferometer. We derive results for the changes in throughput, coherence loss and fringe-detection signal-to-noise ratio arising from diffraction along the propagation path. Our results for conditions of moderate seeing show that although the flux throughput decreases with propagation distance for a given beam diameter, the fringe contrast increases at the same time. In this case it becomes possible for diffraction to increase the signal-to-noise ratio of the fringe measurements. Previous studies have only considered an arrangement where all the apertures in the beam-propagation system have the same diameter. If the light at the end of the propagation path is collected with a fixed size aperture, we find that in many cases the signal-to-noise ratio for fringe detection is maximized when the initial beam diameter is approximately 30 per cent smaller than the final collector diameter. We discuss the implications of our results in the context of future interferometer designs.     

基于SLR精密轨道的天文定位精度分析

利用全球卫星激光测距服务系统(ILRS,International Laser Ranging Service)标准点资料对Ajisai卫星进行精密定轨,残差均方根(RMS)优于3 cm,得到该星的精密轨道.进而对长春站40 cm空间碎片光电望远镜获得的Ajisai卫星的天文定位资料进行精度分析,外符合精度约3″左右.单独利用天文定位数据进行轨道改进,内符合精度优于3″.改进轨道的x、y、z坐标3分量在观测数据覆盖范围内的精度在100 m之内.同样地对Jason-1卫星作数据分析,结果和Ajisai卫星精度相当.分析各个弧段的精度变化,发现定标星个数减少,会导致天文定位精度下降.据此提出可以把最少定标星比例作为评定数据质量的参考指标之一.