The beginnings of decameter radio astronomy: pioneering works of Semen Ya. Braude and his followers in Ukraine

S.Ya. Braude (1911–2003) was the well‐known radio astronomer, one of the founders of low‐frequency astronomical research in the world, in particular in the former Soviet Union. He began to work in this field of science in 1957, in Kharkiv city (Ukraine), from the design and manufacturing small decameter interferometer ID‐1 and ID‐2. Since that time Braude and his team have developed more sophisticated radio decameter telescopes as UTR‐1 and UTR‐2 (the largest in the world till now) as well as the first decameter VLBI network URAN. They have obtained some important pioneering results about low‐frequency radio emission of objects in our Solar system, Galaxy and Metagalaxy by means of these telescopes. In this paper the key events of early history of decameter radio astronomy research in the former USSR are mentioned with emphasizing the role of S. Braude. For the period of 1957–1962, the quotations of Braude's Personal Diary (2003) are first laying open to the public. The most important results obtained by S.Ya. Braude and his followers as well as perspectives of decameter radio astronomy in Ukraine and in the world are highlighted briefly. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

利用VLBI数据确定"探测一号"卫星的轨道

双星计划的“探测一号”是中国首颗真正严格意义上的科学实验卫星,其运行轨道为中国迄今所发射的卫星中距地球最远,远地点地心距达7．8万公里．采用射电天文的VLBI技术可以对“探测一号”以及更远的深空目标,如探月飞行器实现跟踪．为了验证VLBI技术在我国探月计划中的作用,上海天文台组织了国内目前仅有的上海、乌鲁木齐和昆明3个台站对“探测一号”进行试跟踪,利用对“探测一号”约两天的VLBI观测数据,确定“探测一号”卫星的轨道,对VLBI的定轨能力做初步的探讨．按照测控部门提供的初轨 (其精度仅保证跟踪)推算的轨道与VLBI时延的拟合误差平均约2 km,时延率的拟合误差平均约15 cm／s．而利用VLBI数据定轨后的拟合程度相对于初轨有了很大的改善,结果表明,单独利用VLBI时延定轨,时延的拟合精度约5．5 m,作为外部检核的VLBI时延率的拟合精度在2 cm／s左右．单独利用VLBI时延率定轨,时延率的拟合精度约为1．3 cm／s,作为外部检核的VLBI时延的拟合精度约为29 m．而若将时延和时延率数据联合定轨,采用其内符精度加权,VLBI时延和时延率的残差分别为5．5 m和 2 cm／s．为了合理地评估VLBI定轨的真实精度,利用模拟数据进行误差协方差分析,结果表明VLBI定轨精度受动力学模型误差的影响较大,由于"探测一号”卫星的动力学模型难以精确确定,所以利用两天弧段的VLBI数据确定“探测一号”卫星轨道的位置误差为km量级,而速度误差可达cm／s量级．模拟计算还表明, VLBI和USB数据联合定轨可以大大提高定轨精度．     

Phaseless VLBI mapping of compact extragalactic radio sources

The problem of phaseless aperture synthesis is of current interest in phase-unstable VLBI with a small number of elements when either the use of closure phases is not possible (a two-element interferometer) or their quality and number are not enough for acceptable image reconstruction by standard adaptive calibration methods. Therefore, we discuss the problem of unique image reconstruction only from the spectrum magnitude of a source. We suggest an efficient method for phaseless VLBI mapping of compact extragalactic radio sources. This method is based on the reconstruction of the spectrum magnitude for a source on the entire UV plane from the measured visibility magnitude on a limited set of points and the reconstruction of the sought-for image of the source by Fienup's method from the spectrum magnitude reconstructed at the first stage. We present the results of our mapping of the extragalactic radio source 2200+420 using astrometric and geodetic observations on a global VLBI array. Particular attention is given to studying the capabilities of a two-element interferometer in connection with the putting into operation of a Russian-made radio interferometer based on Quasar RT-32 radio telescopes.     

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．     

星载氢钟VLBI观测实验及分析

中国计划于2025年左右建立月球轨道VLBI (Very Long Baseline Interferometer)测站,将会搭载被动型星载氢钟作为时间频率标准.由于是首次在VLBI观测中使用星载氢钟,需要研究和验证其可行性.因此,利用星载氢钟作为频率基准开展了VLBI观测.实验时,分别使用主动型地面氢钟和被动型星载氢钟作为频率基准,利用上海天文台佘山25 m射电望远镜和其他测站对我国火星探测器天问一号进行了交替VLBI观测.数据处理分析结果表明,基于地面氢钟与星载氢钟的VLBI残余群时延标准差均在0.5 ns以内,表明星载氢钟可满足深空探测VLBI测定轨的精度要求,验证了其作为月球VLBI测站频率基准的可行性.     

VLBI and VLA observations of intraday polarization variability in 0917+624 and 0954+658

Total intensity and polarization λ =6 cm Very Large Array (VLA) and global very long baseline interferometry (VLBI) images of the quasar 0917+624 and the BL Lacertae object 0954+658 (both at epoch 1991.43) are analysed. Integrated measurements using the VLA during the VLBI observations indicated that, although there were no substantial total intensity variations, there were significant polarization variations for both sources during the 24-h VLBI experiment. The VLBI data were divided into 2–3 h segments in order to try to identify corresponding rapid variability in the VLBI structure. This analysis revealed intraday variability (IDV) in the VLBI core of 0917+624: both the polarized flux and the polarization position angle varied substantially on time-scales of ∼5–10 h. There is evidence that the VLA polarization variations for 0954+658 occurred in an inner VLBI jet component, where the polarized flux varied by ∼30–40 per cent on time-scales of ∼2 h. 0917+624 and 0954+658 were observed together with 0716+714, an object that also displayed IDV in the polarized flux density measured during our experiment (analysed in a separate paper). These three sources were targeted for the VLBI observations since they had been previously identified as intraday variables, but we had no way of knowing whether they would vary during our observations. The fact that all three exhibited IDV in polarization (but not in total intensity) during our experiment suggests that polarization IDV occurs frequently in at least some IDV sources.     

The radio remnant of SN 1993J: an instrumental explanation for the evolving complex structure

We present simulated images of Supernova 1993J at 8.4 GHz using Very Long Baseline Interferometry (VLBI) techniques. A spherically symmetric source model is convolved with realistic u v -plane distributions, together with standard imaging procedures, to assess the extent of instrumental effects on the recovered brightness distribution. In order to facilitate direct comparisons between the simulations and published VLBI images of SN 1993J, the observed u v -coverage is determined from actual VLBI observations made in the years following its discovery.
The underlying source model only exhibits radial variation in its density profile, with no azimuthal dependence and, even though this model is morphologically simple, the simulated VLBI observations qualitatively reproduce many of the azimuthal features of the reported VLBI observations, such as appearance and evolution of complex azimuthal structure and apparent rotation of the shell. We demonstrate that such features are inexorably coupled to the u v -plane sampling.
The brightness contrast between the peaks and the surrounding shell material are not as prominent in the simulations (which of course assume no antenna- or baseline-based amplitude or phase errors, meaning no self-calibration procedures will have incorporated any such features in models). It is conclusive that incomplete u v -plane sampling has a drastic effect on the final images for observations of this nature. Difference imaging reveals residual emission up to the 8σ level. Extreme care should be taken when using interferometric observations to directly infer the structure of objects such as supernovae.     

An Approach of Tropospheric Correction for VLBI Phase-Referencing using GPS Data

The dominant source of error in VLBI phase-referencing is the troposphere at observing frequencies above 5 GHz. We compare the tropospheric zenith delays derived from VLBI and GPS data at VLBA stations collocated with GPS antennas. The systematic biases and standard deviations both are at the level of sub-centimeter. Based on this agreement, we suggest a new method of tropospheric correction in phase-referencing using combined VLBI and GPS data.     

New concept for testing General Relativity: the Laser Astrometric Test Of Relativity (LATOR) mission

This paper discusses a Fundamental physics experiment that will test relativistic gravity at the accuracy better than the effects of the second order in the gravitational field strength, ∝ G². The Laser Astrometric Test Of Relativity (LATOR) mission uses laser interferometry between two micro‐spacecraft whose lines of sight pass close by the Sun to accurately measure deflection of light in the solar gravity. The key element of the experimental design is a redundant geometry optical truss provided by a long‐baseline (100 m) multi‐channel stellar optical interferometer placed on the International Space Station (ISS). The spatial interferometer is used for measuring the angles between the two spacecraft and for orbit determination purposes. In Euclidean geometry, determination of a triangle's three sides determines any angle therein; with gravity changing the optical lengths of sides passing close by the Sun and deflecting the light, the Euclidean relationships are overthrown. The geometric redundancy enables LATOR to measure the departure from Euclidean geometry caused by the solar gravity field to a very high accuracy. LATOR will not only improve the value of the parameterized post‐Newtonian (PPN) γ to unprecedented levels of accuracy of 1 part in 10⁸, it will also reach ability to measure effects of the next post‐Newtonian order (c⁻⁴) of light deflection resulting from gravity's intrinsic non‐linearity. The solar quadrupole moment parameter, J₂, will be measured with high precision, as well as a variety of other relativistic effects including Lense‐Thirring precession. LATOR will lead to very robust advances in the tests of Fundamental physics: this mission could discover a violation or extension of general relativity, or reveal the presence of an additional long range interaction in the physical law. There are no analogs to the LATOR experiment; it is unique and is a natural culmination of solar system gravity experiments. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

上海天文台MK5A终端系统的安装与测试

简要阐述了上海天文台VLBI终端系统的发展历史与性能；介绍了在没有MK4解码器、互联网又不能满足需要的特殊情况下，利用数据质量分析仪(DQA)完成MK5A终端系统安装与测试的过程和方法。目前，MK5A终端系统已经成功地应用于上海天文台所有的VLBI实验。     

Space VLBI Geodesy: background of an experiment proposal

Space VLBI is a new observing technique which will be available in the second half of this decade by radio telescopes as interferometer elements in orbit. Besides the main astronomical interests, this new development has some very interesting potential applications in satellite dynamics, geodesy and geodynamical research which qualify space VLBI a potential new technique in space geodesy. A geodesy demostration experiment (GEDEX) is being proposed for the Japanese space VLBI satellite-VSOP, with the main objectives of reference frame interconnections and orbit accuracy improvement of the space radio telescope. The paper gives a review of the special characteristics of space VLBI and a general background of the GEDEX proposal.     

Mark5B格式器及10G网络传输系统在VLBI中的应用

由于遥远的天体发出的无线电信号极其微弱,信噪比极低,VLBI系统要想得到较高的测量精度,必须尽量加大测量带宽和提高采样位数,但是这样会导致VLBI终端(比如数字基带转换器)产生的观测数据激增。传统的VLBI终端数据传输系统采用VSI接口,数据最高传输速率限制在2 Gbps,且数据记录设备必须采用定制的Mark5B设备,极其不灵活,因此已经不能适应现在VLBI系统的观测需求。为了提高数据传输速率,增加数据记录设备的灵活性,上海天文台新研制的基于多相滤波器组和快速傅里叶变换方式的数字基带转换器的数据传输系统采用了高速灵活的10 G网络接口。10 G网络系统中数据传输采用报文交换方式,因此数据到达接收端的时间不是精确可靠的,这要求数据在进入10 G网络接口之前必须已经具有标准的VLBI数据格式,所以在10 G网络前端设计了Mark5B格式器。详细介绍了基于现场可编程门阵列的Mark5B格式器及10 G网络传输系统的设计原理,并在文章的最后通过三组实验验证了其功能的正确性和性能的稳定性。     

Peculiarities of processing VLBI observations of pulsars

We determined the features of pulsars that were disregarded in standard amplitude-calibration procedures for VLBI observations. We suggest additional amplitude-calibration methods. These methods were used to process the VLBI observations of the pulsar PSR B0329+54 carried out with the HALCA ground—space interferometer. Data from the space radio telescope are corrected for a nonuniform reception band. We estimated the diameter of the scattering disk for this pulsar at a frequency of 1600 MHz: \( < 1\mathop .\limits^{''} 8 \times 10^{ - 3}\).     

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.     

HERMES at Mercator,competitive high‐resolution spectroscopy with a small telescope

HERMES, a fibre‐fed high‐resolution (R = 85000) échelle spectrograph with good stability and excellent throughput, is the work‐horse instrument of the 1.2‐m Mercator telescope on La Palma. HERMES targets building up time series of high‐quality data of variable stellar phenomena, mainly for asteroseismology and binary‐evolution research. In this paper we present the HERMES project and discuss the instrument design, performance, and a future upgrade. We also present some results of the first four years of HERMES observations. We illustrate the value of small telescopes, equipped with efficient instrumentation, for high‐resolution spectroscopy. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of stellar loop structures using VLBI

In this paper we discuss a set of three consecutive VLBI observations of the binary system UX Arietis. The most interesting result is the variation with time of the source structure. The usual interpretation in terms of gyrosynchrotron emission from relativistic electrons trapped in a magnetic loop and undergoing collisional and radiative losses is not able, alone, to explain the observed variations. By using optical, radio and X-ray information we have produced a model of two giant loops anchored on a rotating star. As the star rotates, the loops change their relative position and orientation with respect to the line of sight, causing the observed variation of the source structure. The qualitative agreement found is consistent with our hypothesis and makes these observations a sort of a pilot experiment for a new way of using VLBI to observe radio-stars. In order to quantitatively test our model of evolving electrons confined in loops anchored on a rotating star, we plan in the near future a set of several phase-reference VLBI observations fully covering the 6.4 day rotational period.     

The Sloan Digital Sky Survey monitor telescope pipeline

The photometric calibration of the Sloan Digital Sky Survey (SDSS) is a multi‐step process which involves data from three different telescopes: the 1.0‐m telescope at the US Naval Observatory (USNO), Flagstaff Station, Arizona (which was used to establish the SDSS standard star network); the SDSS 0.5‐m Photometric Telescope (PT) at the Apache Point Observatory (APO), NewMexico (which calculates nightly extinctions and calibrates secondary patch transfer fields); and the SDSS 2.5‐m telescope at APO (which obtains the imaging data for the SDSS proper). In this paper, we describe the Monitor Telescope Pipeline, MTPIPE, the software pipeline used in processing the data from the single‐CCD telescopes used in the photometric calibration of the SDSS (i.e., the USNO 1.0‐m and the PT). We also describe transformation equations that convert photometry on the USNO‐1.0m u ′g ′r ′i ′z ′ system to photometry the SDSS 2.5m ugriz system and the results of various validation tests of the MTPIPE software. Further, we discuss the semi‐automated PT factory, which runs MTPIPE in the day‐to‐day standard SDSS operations at Fermilab. Finally, we discuss the use of MTPIPE in current SDSS‐related projects, including the Southern u ′g ′r ′i ′z ′ Standard Star project, the u ′g ′r ′i ′z ′ Open Star Clusters project, and the SDSS extension (SDSS‐II). (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

激变变星Ⅴ1159 Ori 和BZ UMa 的测光研究

Ⅴ1159 Ori 是SU UMa 型中 ER UMa 亚型激变变星,BZ UMa 介于 U Gem 型和 WZ Sge 型之间,但又具有 SU UMa 的周期特征,存在争议.在2008年2月24日和 25日,用云南天文台 1 米 RCC(Ritchey-Chretien-Coude)望远镜对两者的测光观测显示: Ⅴ1159 Ori 在正常爆发的下降阶段存在 superhump,这为 superhump 现象普遍存在于 ER UMa 型星中提供了观测证据;BZ UMa 观测时处于爆发极大,并未观测到确凿的 superhump 周期,而 AAVSO(American Association of Variable Star Observers)近年的 BZ UMa 观测亦从未发现明确的 superhump;两者均表明 BZ UMa 可能并非 SU UMa 型星.基于星等变化幅度考虑,BZ UMa 较 WZ Sge 更为接近.     

Speckle observations with PISCO in Merate: VIII. Astrometric measurements of visual binaries in 2007 and new orbits of the multiple system Zeta Aqr

We present relative astrometric measurements of visual binaries made during the second semester of 2007, with the speckle camera PISCO at the 102 cm Zeiss telescope of Brera Astronomical Observatory, in Merate. Our sample contains orbital couples as well as binaries whose motion is still uncertain. We obtained 283 new measurements of 279 objects, with angular separations in the range 0″.17–4″.4, and an average accuracy of 0″.014. The mean error on the position angles is 0°.6. Most of the position angles were determined without the usual 180° ambiguity with the application of triple‐correlation techniques and/or by inspection of the long integration files. We also present the new orbit we have computed for Zeta Aqr AB (ADS 15971), for which our measurements lead to large residuals with the previously computed orbit. We were also able to compute the elements of the perturbation orbit Bb‐P caused by an invisible companion, whose mass is estimated at 0.7 M_⊙ (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An Approach of Tropospheric Correction for VLBI Phase-Referencing using GPS Data

The dominant source of error in VLBI phase-referencing is the troposphere at observing frequencies above 5 GHz. We compare the tropospheric zenith delays derived from VLBI and GPS data at VLBA stations collocated with GPS antennas. The systematic biases and standard deviations both are at the level of sub-centimeter. Based on this agreement, we suggest a new method of tropospheric correction in phase-referencing using combined VLBI and GPS data.