基于冗余递减匹配的星图识别方法

空间目标光学观测过程中,当望远镜的指向误差与跟踪误差显著时,很难使用邻域匹配的方法完成背景恒星的配准.基于星对角距冗余递减匹配的思想,实现了一种背景恒星的高效识别方法.利用多种条件下的仿真星图以及实测图像进行了试验,结果表明,方法提高了识别成功率,且耗时少,可以准确、快速地完成星图匹配工作.     

小视场CCD图像的几何定标

针对长焦距望远镜CCD观测天然卫星的情况,导出了用两颗亮星作为定标时,CCD视场几何参数(比例尺和指向)测量的误差公式,进而设计了多定标星定标时正确的归算方案。新方案不仅对天然卫星的位置测量,也对同类型的小视场CCD位置测量具有指导意义。     

STEP天体测量误差源分析

系外类地行星空间探测计划(Search for Terrestrial Exo-Planets,以下简称"STEP")采用天体测量法和微像素级焦平面定标测量技术,设计望远镜焦平面检测精度达到1μas.在假定焦平面设计能达到检测精度的前提条件下,系统分析了恒星自行、视差、卫星速度和位置、光学系统的光心等关键因素对检测系外行星的影响.有别于传统的窄视场照相底片常数法,提出了一种恒星相对角距测量方法,以检测由于可能存在的系外行星而引起星对角距变化的非线性项,消除了传统窄视场天体测量中参考星位置和自行精度对检测系外行星的直接影响.针对同一天区内的8颗参考星和1颗具有行星系统的待测星,分别模拟出5 yr内的观测数据,利用最小二乘法进行处理,发现基于STEP自身1μas的观测精度,在这种情况下是可以观测到类地行星的.     

小视场CCD图像的几何定标

针对长焦距望远镜CCD观测天然卫星的情况，导出了用两颗亮星作为定标时，CCD视场几何参数(比例尺和指向)测量的误差公式，进而设计了多定标星定标时正确的归算方案。新方案不仅对天然卫星的位置测量，也对同类型的小视场CCD位置测量具有指导意义。     

望远镜几何扭曲实测方法仿真研究

精确求解望远镜几何扭曲效应,有利于提高望远镜的天体测量定位精度,这对天文学的诸多学科具有十分重要的意义.为此,前人发展了一种针对密集星场抖动观测并针对观测底片迭代求解几何扭曲的自校准方法,取得了较好的效果.但是,先前的工作并未对星场的密集程度或抖动方式做进一步要求,而是经验地选择较为密集的星场和较多的抖动次数进行观测.这些经验的观测方式固然能够较好地给出几何扭曲,但有时会占用较多的望远镜观测时间导致效率较低.首先介绍了上述求解望远镜几何扭曲的一般方法;通过仿真模拟,对上述方法的有效性做评估,同时对该方法要求的星场密度和抖动次数等条件做进一步优化;最后,针对实际应用中几何扭曲改正后的定位精度与视场参考星数量的关系也做了进一步的仿真和分析.     

土星主要卫星天体测量定标试验

在地面望远镜观测土星主要卫星的精确定位中,目前普遍采用CCD成像技术。为了测量视场中暗卫星的位置,常采用4颗主要卫星(土卫三-土卫六)进行定标。鉴于技术的进步,目前CCD视场正在逐步增加,如何精确定标图像值得研究。利用2010年1月20日在云南天文台1 m望远镜上观测的不同取向的105幅CCD图像进行了试验研究。采用了美国喷气推进实验室土星主要卫星历表和美国海军天文台的UCAC4恒星星表进行计算,相对于不同的参考对象进行定标。结果表明,采用4颗主要卫星进行定标并测量离卫星较近的观测对象时具有较好的测量准确度和内部精度。测量离定标星较远的卫星时,这种定标方法具有较大的不确定性。采用6颗主要卫星(土卫三-土卫八)进行定标,所有卫星的测量结果具有更好的外部和内部符合。即便如此,采用四常数模型和六常数模型定标的结果都不是令人满意,这揭示CCD视场存在明显的扭曲效应。如果采用视场中的恒星定标,卫星的外部符合和内部精度明显变差,这说明UCAC4恒星参考星表也不是理想的定标用参考星表。     

基于直接成像法的多星系统观测

系外行星直接成像探测能够获取系外行星更全面的物理信息,是未来搜寻系外生命的关键技术之一.针对近期地基望远镜高对比度成像观测数据,对新发现的多星候选体进行系统展示.前期,结合地基系外行星高对比度成像设备观测能力,从已发表文献整理的Gaia星表恒星数据中筛选,得到约1000个观测目标.这些目标分布于不同的年轻星团中.近期,使用Palomar天文台Hale望远镜对上述观测目标中的42个目标在K波段开展了高对比度成像观测.这些目标恒星在可见光波段为7.5-14.2019年经过两轮观测,发现了6个多星系统候选体,这些目标在Gaia Data Release 2星表和Gaia early Data Release 3星表中难以确认是单星还是多星系统.     

基于光学辅助指向测量的太赫兹望远镜指向误差模型研究

针对太赫兹波段天文点源目标较少, 指向测量相对困难的特点, 研究了利用与太赫兹天线共轴的小型光学望远镜来辅助太赫兹望远镜指向测量以及建立指向误差修正模型的方法. 依托紫金山天文台1.2 m斜轴式太赫兹天线开展了光学辅助指向测量的实验研究, 利用一台安装在天线背架上的100mm口径折射式光学望远镜获得了优于2$''$的指向测量精度. 此外, 通过对斜轴天线的结构分析以及大气折射和本地恒星时(Local Sidereal Time, LST)偏差等误差来源的分析, 建立了包含23个误差项的斜轴式光学指向修正模型, 实现了约3$''$的拟合精度. 最后, 借助高精度数字摄影测量对光电轴一致性进行了标定, 并针对其对指向模型精度的影响进行了讨论. 研究成果将为南极5 m太赫兹望远镜(The 5m Dome A Terahertz Explorer, DATE5)及其他太赫兹望远镜提供指向测量和指向修正模型方面的技术参考.     

掩星驿站

月掩星是一种很有趣味的天文观测项目,使用非常简单的设备就可以很容易进行观测。用口径60毫米的折射望远镜就可以观测到不少的月掩星,使用更大口径的望远镜可以看到更多的月掩星。因为较其他恒星、行星等天体而言,月球运行速度较快,所以在观测月掩星时,我们能明显觉察到月球的运动。     

一米R-CC望远镜定位误差及相对弯程检验

本文用20颗星的两次观测资料,计算了一米望远镜的定位误差,主光路与导星镜的相对弯程以及相对弯程与地理纬度φ,恒星的赤纬δ和时角t之间的关系。     

Radiation transfer for an inhomogeneous half-space with an internal source

The emergent flux for radiation transfer in an inhomogeneous half-space with internal source is obtained in terms of the source-function of the diffuse reflection problem. Modified Eddington method is used to calculate the diffuse problem and, hence, the emergent flux. Numerical result are given and compared with previous work.On leave of absence from the Nuclear Research Centre, Cairo, Egypt.     

Eruption of an Active-Region Filament Driven by an Emerging Bipole

1 INTRODUCTION Filaments are cool, dense material suspended in the hot, tenuous corona. It is widely accepted that the global magnetic field surrounding the filaments plays a key role in their formation, structure and stability (Tandberg-Hanssen1995). Fil…     

Optimal low-thrust takeoff from an orbit about an oblate planet

Future space missions to the outer planets may depend upon the use of low-thrust propulsion systems. As these planets are decidedly oblate, the question of the effect of that oblateness on a low-thrust trajectory is of some interest. In this paper the problem of optimal energy increase is attacked under the assumption that the coefficients for the second zonal harmonic, i.e.,J ₂, and the nondimensional thrust acceleration are the same order of magnitude. Using a two variable asymptotic expansion technique, a near optimal control program is generated and the first order uniformly valid approximation for the corresponding trajectory is obtained. Tangential thrust is shown to be a good near-optimal thrust program even in the presence of oblateness effects. The optimal control program is found to be oscillatory and quite similar to the optimal control for energy increase in an inverse square gravitational field.This research was supported by the National Aeronautics and Space Administration under Grant NGR-23-005-329.     

Response of an isothermal bounded atmosphere to an applied random body-force

The response of a one dimensional isothermal atmosphere with rigid lower boundary and bounded upward by a very hot isothermal atmosphere to an applied random source is studied. It is shown that a concentrated white noise source merely excites the damped eigen-modes of the model and that the enhanced response at the cut-off frequency found in semi-infinite isothermal model is removed by the reflections. We believe that this rules out the attempts to relate the photospheric and chromospheric oscillations to the singular response of an excited semi-infinite isothermal atmosphere.     

Formal integrals for an autonomous Hamiltonian system near an equilibrium point

In this paper we give a new method to construct formal integrals for an autonomous Hamiltonian system near an equilibrium point. Our construction is reminiscent of the algorithms introduced by Hori and Deprit; we show however how it presents itself quite naturally if one follows the line of attack of Whittaker, Cherry, and Contopoulos.     

MHD free-convection flow of an elasto-viscous fluid past an infinite vertical plate

A theoretical model of shock wave propagation in a self-gravitating radiative magneto-hydrodynamic medium has been studied. The effects of the magnetic field, radiation, and gravitation have been discussed separately. The results discussed depend upon the numerical variations of flow variables behind the shock.     

Hydromagnetic free-convection laminar flow of an elasto-viscous fluid past an infinite plate

The effect of a transverse magnetic field on unsteady free-convection flow of an electrically conducting elasto-viscous fluid (Walters liquidB') past an impulsively started infinite vertical plate is discussed. The Laplace transform technique is used to obtain the expressions for velocity and shearing stress. The influence of the various parameters, entering in the problem, on the velocity field and shearing stress is extensively discussed.     

Asymmetric signatures in simple craters as an indicator for an oblique impact direction

Abstract— In oblique impacts with an impact angle under 45°, the bilateral shape of the distal ejecta blanket is used as the strongest indicator for an impact vector. This bilateral symmetry is attenuated and is superimposed by radial symmetry towards the crater rim, which remains circular for impact angles down to 10–15°. The possibility that remnants of bilateral symmetry might still be present in the most proximal ejecta, the overturned flap and the crater rim was explored with the intention of deducing an impact vector. A model is presented that postulates bilateral patterns using proximal ejecta trajectories and predicts these patterns in the orientation of bedding planes in the crater rim. This model was successfully correlated to patterns described by radial grooves in the proximal ejecta blanket of the oblique Tooting crater on Mars. A new method was developed to detect structural asymmetries by converting bedding data into values that express the deviation from concentric strike orientation in the crater rim relative to the crater center, termed “concentric deviation.” The method was applied to field data from Wolfe Creek crater, Western Australia. Bedding in the overturned flap implies an impactor striking from the east, which refines earlier publications, while bedding from the inner rim shows a correlation with the crater rim morphology.     

Theory of an experiment in an orbiting space laboratory to determine the gravitational constant

The paper analyzes an experiment in an orbiting laboratory to determine the gravitational constantG. A massive sphere, according to a suggestion of L. S. Wilk, is to have three tunnels drilled through it along mutually perpendicular diameters. The sphere either floats in the orbiting laboratory, with its center held fixed by means of external jets issuing from the spacecraft, or is tethered to the spacecraft. In either case it is free to rotate; in the second case this freedom would be achieved by a system of gimbals.Each tunnel contains a small test object, which is held on the tunnel's axis by means of a suspension system, perhaps electrostatic, and held at rest relative to the sphere by slowly rotating the latter by means of inertia reaction wheels, governed by a servomechanism. Fundamentally, one balances the gravitational forces on the test objects by centrifugal force, determines the latter by measuring the components of angular velocity, and calculatesG from the resulting balance. It is better to use three tunnels than one because their use minimizes the effects of the Earth's gravity-gradient.Many other measurements and corrections are required. The latter arise from Earth gravity-gradient, aerodynamic drag (with the tethered sphere), gravitational forces produced by the spacecraft itself, and the force reductions produced by the empty space in all three tunnels. After the consideration of these effects there is a presentation and discussion of the equations required to reduce the observations to obtainG. There then follow the extra equations, not needed in the reduction, that are required for a computer simulation to investigate the possible extraction of a test object and to aid in designing the servomechanisms.In Appendix B, I have devised another version of the experiment, in which the sphere is kept intact, but has short thin hollow vestigial tunnels attached to the outside of the sphere, along perpendicular diameters. These external tunnels would contain the test objects and the suspension systems. The servomechanisms would then have to prevent collision of a test object with the sphere, as well as extraction. This second method could allow for some inhomogeneities in the sphere, would require no accurate drilling, and would make the suspension systems more accessible for construction and adjustment.This paper was prepared under the sponsorship of the National Aeronautics and Space Administration through NASA Contract NAS 9-8328.