机器视觉高斯拟合法自动导星定心系统设计

丽江2.4m光学望远镜自动导星系统升级改造需要计算CCD图像中实际星像中心的位置,在现有导星定心算法的基础上,利用机器视觉Canny边缘检测和椭圆拟合的方法识别星像轮廓,然后对星像直接二维高斯拟合,计算星像中心位置.研究了导星算法中涉及的天光背景参数、星像轮廓识别所需阈值、星像拟合边界参数的整定方法,总结出有效的自动导星定心方法,并开发了基于Linux平台的高速自动导星定心系统软件.软件对星像的高斯拟合结果与IRAF软件高斯拟合结果一致;同一天区相邻时间图像中星像中心偏移量计算结果也表明算法的有效性.     

ePSF拟合法与Gaussian拟合法的比较

为了解决哈勃空间望远镜的图像欠采样问题,美国学者Anderson和King提出了精确测量星像位置和通量的有效点扩散函数(ePSF—effective point-spread function)拟合法。然而,他们却不加比较地将其应用于地面CCD图像中星像的位置高精度测量。因此,我们试图将ePSF拟合法与经典的高斯函数(Gaussian)拟合法作比较研究。调用CFITSIO库生成模拟的背景图像,并应用不同参数条件下的星像轮廓模型产生非欠采样的星像。最后,分别采用ePSF拟合法与Gaussian拟合法对这些星像进行拟合,并对它们的拟合精度进行比较。实验结果表明,在非欠采样的图像中这两种算法对星像位置的测量几乎是等精度的。     

近地天体望远镜的PSF分布研究

盱眙近地天体望远镜CCD视场为1.94°×1.94°,作为大视场望远镜由于存在场曲现象,因此根据单颗星像调焦会导致PSF(Point Spread Function)分布不均匀.研究所拍摄图像的PSF分布特征,并通过拟合半高全宽和离焦量的函数关系模拟出弯曲焦面,从而确定出最佳成像平面.制定出根据PSF的半高全宽直方图来判断最佳成像平面的调焦方法.     

基于几何扭曲模型的CCD图像扭曲校正残差及分析

在已知图像几何扭曲模型的基础上,对2013年2月5日使用云南天文台2.4 m望远镜观测近地小行星(99942)Apophis资料中的58幅CCD图像进行了几何扭曲校正,并对校正后的图像测出的星像位置进行了几何扭曲残差分析。结果发现,星像位置X、Y两个方向的几何扭曲残差约在±0.02 pixel内。为了探究影响几何扭曲残差大小的因素,设计了仿真实验。将模拟星像函数经过设定的扭曲函数模型添加几何扭曲,得到受扭曲的星像函数,并对扭曲前后的星像进行几何扭曲残差分析。结果显示,星像的半峰全宽(FWHM)越大,几何扭曲模型的扭曲变化越快,几何扭曲残差越大。     

基于BATC 1999-2008十年观测值班记录的兴隆天气情况统计

基于BATC巡天组近十年的观测值班记录,统计了兴隆观测站可观测时间随月份的变化,结果表明平均每年大约有195个可观测夜(约1986小时),其中大约有87个观测夜(约918小时)用来巡天,约有18个观测夜用来测光.另外还统计了兴隆观测站施密特望远镜十年来北极天区的观测图像中星像的半高全宽(FWHM)随月份的变化,结果表明一年内平均星像的半高全宽约为2.3像元(老CCD)或2.2像元(新CCD).一般来说,夏季的星像要好于冬季,这与刘颖(Liu)等人2003年[1]的结果基本一致.     

全天相机在多设备巡天中的应用

天文观测站夜天空星像星等信息和天区分布信息可用于指导多设备巡天观测.建立全天相机监测系统(Monitoring all-sky system)对本地天区夜天空实时监测,获取的监测图像需要有效的方法进行处理以提取全天图像星像信息.由于全天图像视场大和高阶扭曲的影响,采用天顶等距投影与多项式函数组合的方法计算图像的底片常数.天文定位的均方根残差约为0.15个像素.通过对图像中亮星部分测光得到的星等差,改正大气消光误差.最后使用HEALPix (Hierarchical Equal Area isoLatitude Pixelation)方法实现天区划分和每个天区可观测极限星等值的存储.     

基于系列斑点图像的视宁度估算精度方法研究

半高宽法和像运动法是估算大气视宁度的常用方法,两种方法的估算精度都受到望远镜跟踪误差、风或者机械振动等因素的影响,提高视宁度估算精度在高分辨成像、选址、台站视宁度监测等方面有重要意义。对于半高宽法,用二维高斯函数拟合长曝光点源单星星像并选取合适方向的半高宽估算视宁度,可以有效改善跟踪误差、风或者机械振动等因素的影响。对于像运动法,用主成分分析法找到一系列斑点图重心位置在合适方向上投影的方差,以此估算视宁度,与谱比法对照可以发现,两种方法均能有效改善跟踪误差、风或者机械振动等因素的影响。     

基于能量集中度的广角望远镜自动调焦快速清晰度评价算法

地基广角相机阵由36个广角相机拼接组成,主要观测目标为光学瞬变源,具有超大视场、高时间采样率、实时数据处理的技术特点。实时自动调焦对于系统自动化观测与提高科学产出至关重要。自动调焦的核心目标为提高图像的能量集中度,目前常用评价指标如星像50%能量半径、点扩散函数的半高全宽均存在计算量过大的问题。基于地基广角相机阵调焦过程中星像中心区域能量变化大、星像总流量及分布范围基本不变这一点,提出了通过统计中心区域流量占星像总流量百分比的快速清晰度评价算法,并对两个流量孔径展开详细研究。进一步对算法中流量计算方法进行简化,细化了背景取值、选星标准、选星数目等对算法有影响的参数,最终确立了一套适用于地基广角相机阵系统实时自动调焦的清晰度评价算法。算法结果准确,单图运算时间在0.1 s左右,满足系统快速自动调焦的需求。     

VLBI相位参考中的残余对流层延迟修正

VLBI (very long baseline interferometry)相位参考在高频波段(约大于10 GHz)的主要误差源是对流层延迟模型误差,减小该项误差的有效途径是采用实测资料对其精确拟合。常用的修正方法有两种:一种是在相位参考观测中穿插测地观测,用以拟合剩余大气延迟;另一种是使用GPS并置站的对流层产品改正。两种改正方法的精度约为2 cm。从残余相位中提取出了残余大气信息,提出了传统方法与残余相位拟合大气参数相结合的改正方法,并通过实测数据对该方法进行了验证。     

CCD图像的一维定心方法

在天体测量学中，数字星象的高精度定位方法日益重要。本文利用边缘分布的方法将二维星像转换到一维，比较了高斯拟合法、修正矩法、中值法和寻导法四种一维定心算法。从精度上看，高斯拟合法最佳，修正矩次之，而寻导法最低。但从计算速度上看，高斯拟合法太慢了。因此作为一个满足高精度和高效率的定心方法，修正矩是最佳选择。     

山东大学威海天文台视宁度研究

山东大学威海天文台拥有口径1 m的赤道式反射光学望远镜,于2007年6月建成并投入使用。对天文台2008年、2009年的所有观测数据用编写的IRAF自动处理程序进行处理得到了大气视宁度值,对得到的大气视宁度进行了分析研究,并与天文台气象站获得的气象数据一起进行了分析。经分析得到了山东大学威海天文台的大气视宁度状况,同时得到了一些大气视宁度随气象因素变化的规律。     

三孔较差视宁度监视仪及试观测报告

本文介绍了云南天文台研制的三孔较差视宁度监视仪，统计了１９９４年３月至７月期间的试观测情况，及仪器定标和数据处理结果。     

恒星自转的引力昏暗效应对谱线的影响

对于快速自转的恒星,谱线的等值宽度（ＥＷ）受引力昏暗效应的影响,即使自转速度达到临界也是很小的;而谱线半宽（ＦＷＨＭ）受引力昏暗效应影响相对较大,在（ω≈０．９９）时,相对变化达到８％．对于光谱型在Ｂ０～Ｂ９范围内的主序星,谱线半宽的相对变化在自转速度较小时（ω＜０．８）与光谱型无关;在自转速度较大时（ω≈０．９９）,谱线半宽的相对变化随光谱次型的增加而下降．     

Simultaneous seeing measurement through the Subaru Telescope in the visible and near-infrared bands for the wavelength dependence evaluation

Stellar images have been obtained under natural seeing at visible and near-infrared wavelengths simultaneously through the Subaru Telescope at Mauna Kea. The image quality is evaluated by the full-width at the half-maximum (FWHM) of the stellar images. The observed ratio of FWHM in the V-band to the K-band is 1.54 ± 0.17 on average. The ratio shows tendency to decrease toward bad seeing as expected from the outer scale influence, though the number of the samples is still limited. The ratio is important for simulations to evaluate the performance of a ground-layer adaptive optics system at near-infrared wavelengths based on optical seeing statistics. The observed optical seeing is also compared with outside seeing to estimate the dome seeing of the Subaru Telescope.     

Contribution of chromospheric features to UV irradiance variability from spatially-resolved Ca II K spectroheliograms

We have digitized the Ca ii K spectroheliograms, observed at the National Solar Observatory at Sacramento Peak, for the period 1980 (maximum of solar cycle 21), 1985 (minimum of solar cycle 21), 1987 (beginning of the ascending phase of solar cycle 22), 1988 and 1989 (ascending phase and maximum of solar cycle 22), and 1992 (declining phase of solar cycle 22). A new method for analyzing the K spectroheliograms has been developed and applied to the K images for the time interval of 1992. Using histograms of intensity, we have segregated and measured the cumulative intensity and area of various chromospheric features like the plages, magnetic network and intranetwork elements. Also, the full width at half maximum (FWHM) derived from the histograms has been introduced as a new index for describing the chromospheric activity in the K-line. The full-disk intensity (spatial K index) has been derived from spatially-resolved K images and compared to the spectral K index derived from the line profiles for the full disk. Both the spatial K index and FWHM have been compared to the UV irradiance measured in the Mg ii h and k lines by the NOAA9 satellite and found that they are highly correlated with the Mg ii h and k c/w ratio.NRC Resident Research Associate, on leave from Indian Institute of Astrophysics, Bangalore 560034, India.     

Improved astrometry of space debris with image restoration

In order to implement an observing strategy,image degradation that occurs during optical observation of space debris is ineluctable and has distinct characteristics. Image restoration is presented as a way to remove the influence of degradation in CCD images of space debris,based on assumed PSF models with the same FWHM as images of the object. In the process of image restoration,the maximum entropy method is adopted. The results of reduction using observed raw CCD images indicate that the precision in estimating positions of objects is improved and the effects of degradation are reduced. Improving the astrometry of space debris using image restoration is effective and feasible.     

Saturn's vertical and latitudinal cloud structure 1991-2004 from HST imaging in 30 filters

We analyzed 134 images of Saturn taken by the Hubble Space Telescope between 1991 and 2004. The images cover wavelengths between 231 and 2370 nm in 30 filters. We combined some 10 million calibrated reflectivity measurements into 18,000 center-to-limb curves. We used the method of principal component analysis to find the main latitudinal and temporal variations in Saturn's atmosphere and their spectral characteristics. The first principal variation is a strong latitudinal variation of the aerosol optical depth in the upper troposphere. This structure shifts with Saturn's seasons, but the structure on small scales of latitude stays constant. The second principal variation is a variable optical depth of stratospheric aerosols. The optical depth is large at the poles and small at mid- and low latitudes with a steep gradient in-between. This structure remains essentially constant in time. The third principal variation is a variation in the tropospheric aerosol size, which has only shallow gradients with latitude, but large seasonal variations. Thus, aerosol sizes and their phase functions inferred at a particular season are not representative of Saturn's atmosphere at other seasons. Aerosols are largest in the summer and smallest in the winter. The fourth principal variation is a feature of the tropospheric aerosols with irregular latitudinal structure and fast variability, on the time scale of months. Spherical aerosols do not display the spectral characteristic of that feature. We suspect that variations in the shape of aerosols may play a role. We found a spectral feature of the imaginary index of aerosols, which darkens them near 400 nm wavelength. While we can describe Saturn's variations quite accurately, our presented model of Saturn's average atmosphere is still uncertain due to possible systematic offsets in methane absorption data and limitations of the knowledge about the shape of aerosols. In order to compare our results with those from comparable investigations, which used less than 30 filters, we fit models to spectral subsets of our data. We found very different best-fitting models, depending on the subset of filters, indicating a high sensitivity of results on the spectral sampling.     

Modeling of Hα Eruptive Events Observed at the Solar Limb

We present spectra and slit-jaw images of limb and on-disk eruptive events observed with a high temporal resolution by the Ond?ejov Observatory optical spectrograph. Analysis of the time series of full width at half-maximum (FWHM) in Hα, Hβ, and radio and soft X-ray (SXR) fluxes indicates two phenomenologically distinct types of observations which differ significantly in the timing of FWHM and SXR/radio fluxes. We investigated one such unusual case of a limb eruptive event in more detail. Synthesis of all observed data supports the interpretation of the Hα broadening in the sense of regular macroscopic plasma motions, contrary to the traditional view (emission from warm dense plasma). The timing and observed characteristics indicate that we may have actually observed the initiation of a prominence eruption. We test this scenario via modeling of the initial phase of the flux rope eruption in a magnetohydrodynamic (MHD) simulation, calculating subsequently – under some simplifying assumptions – the modeled Hα emission and spectrum. The modeled and observed data correspond well. Nevertheless, the following question arises: To what extent is the resulting emission sensitive to the underlying model of plasma dynamics? To address this issue, we have computed a grid of kinematic models with various arbitrary plasma flow patterns and then calculated their resulting emission. Finally, we suggest a diagnostics based on the model and demonstrate that it can be used to estimate the Alfvén velocity and plasma beta in the prominence, which are otherwise hard to obtain.     

A software package for evaluating the performance of a star sensor operation

We have developed a low-cost off-the-shelf component star sensor (StarSense) for use in minisatellites and CubeSats to determine the attitude of a satellite in orbit. StarSense is an imaging camera with a limiting magnitude of 6.5, which extracts information from star patterns it records in the images. The star sensor implements a centroiding algorithm to find centroids of the stars in the image, a Geometric Voting algorithm for star pattern identification, and a QUEST algorithm for attitude quaternion calculation. Here, we describe the software package to evaluate the performance of these algorithms as a star sensor single operating system. We simulate the ideal case where sky background and instrument errors are omitted, and a more realistic case where noise and camera parameters are added to the simulated images. We evaluate such performance parameters of the algorithms as attitude accuracy, calculation time, required memory, star catalog size, sky coverage, etc., and estimate the errors introduced by each algorithm. This software package is written for use in MATLAB. The testing is parametrized for different hardware parameters, such as the focal length of the imaging setup, the field of view (FOV) of the camera, angle measurement accuracy, distortion effects, etc., and therefore, can be applied to evaluate the performance of such algorithms in any star sensor. For its hardware implementation on our StarSense, we are currently porting the codes in form of functions written in C. This is done keeping in view its easy implementation on any star sensor electronics hardware.