共查询到20条相似文献，搜索用时 15 毫秒
1.
This paper presents an analysis of the phase‐calibration problem encountered in astronomy when mapping incoherent sources with aperture‐synthesis devices. More precisely, this analysis concerns the phase‐calibration operation involved in the self‐calibration procedures of phase‐closure imaging. The paper revisits and completes a previous analysis presented by Lannes in the Journal of the Optical Society of America A in 2005. It also benefits from some recent developments made for solving similar problems encountered in global navigation satellite systems. In radio‐astronomy, the related optimization problems have been stated and solved hitherto at the phasor level. We present here an analysis conducted at the phase level, from which we derive a method for diagnosing and solving the difficulties of the phasor approach. In the most general case, the techniques tobe implemented appeal to the algebraic graph theory and the algebraic number theory. The minima of the objective functionals to be minimized are identified by raising phase‐closure integer ambiguities. We also show that in some configurations, to benefit from all the available information, closure phases of order greater than three are to be introduced. In summary, this study leads to a better understanding of the difficulties related to the very principle of phase‐closure imaging. To circumvent these difficulties, we propose a strategy both simple and robust (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
2.
Robert I. Reid 《Monthly notices of the Royal Astronomical Society》2006,367(4):1766-1780
A new technique is presented for producing images from interferometric data. The method, 'smear fitting', makes the constraints necessary for interferometric imaging double as a model, with uncertainties, of the sky brightness distribution. It does this by modelling the sky with a set of functions and then convolving each component with its own elliptical Gaussian to account for the uncertainty in its shape and location that arises from noise. This yields much sharper resolution than clean for significantly detected features, without sacrificing any sensitivity. Using appropriate functional forms for the components provides both a scientifically interesting model and imaging constraints that tend to be better than those used by traditional deconvolution methods. This allows it to avoid the most serious problems that limit the imaging quality of those methods. Comparisons of smear fitting to clean and maximum entropy are given, using both real and simulated observations. It is also shown that the famous Rayleigh criterion (resolution = wavelength/baseline) is inappropriate for interferometers as it does not consider the reliability of the measurements. 相似文献
3.
Results concerning the dependence of photometric and radial velocity amplitudes on metallicity are presented based on about 200 Galactic classical Cepheids pulsating in the fundamental mode. The Galactic distribution of the [Fe/H] value of Cepheids is also studied. We show that the photometric amplitude ratio AI/AV is independent of metallicity. The observed dependence of this ratio on the pulsation period does not correspond to the theoretical predictions. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
4.
M. Vannier R. G. Petrov B. Lopez F. Millour 《Monthly notices of the Royal Astronomical Society》2006,367(2):825-837
We present the high angular resolution technique of colour-differential interferometry for direct detection of extrasolar giant planets (EGPs). The measurement of differential phase with long-baseline ground-based interferometers in the near-infrared could allow the observation of several hot giant extrasolar planets in tight orbit around the nearby stars, and thus yield their low- or mid-resolution spectroscopy, complete orbital data set and mass. Estimates of potentially achievable signal-to-noise ratios are presented for a number of planets already discovered by indirect methods. The limits from the instrumental and atmospheric instability are discussed, and a subsequent observational strategy is proposed. 相似文献
5.
6.
We present a study of the dynamic range limitations in images produced with the proposed Square Kilometre Array (SKA) using
the Cotton-Schwab CLEAN algorithm for data processing. The study is limited to the case of a small field of view and a snap-shot observation. A new
modification of the Cotton-Schwab algorithm involving optimization of the position of clean components is suggested. This
algorithm can reach a dynamic range as high as 106 even if the point source lies between image grid points, in contrast to about 103 for existing CLEAN-based algorithms in the same circumstances. It is shown that the positional accuracy of clean components, floating point
precision and the w-term are extremely important at high dynamic range. The influence of these factors can be reduced if the variance of the
gradient of the point spread function is minimized during the array design. 相似文献
7.
J.H. Telting G. Avila L. Buchhave S. Frandsen D. Gandolfi B. Lindberg H.C. Stempels the NOT staff S. Prins 《Astronomische Nachrichten》2014,335(1):41-45
FIES is a cross‐dispersed high‐resolution echelle spectrograph at the 2.56 m Nordic Optical Telescope (NOT), and was optimised for throughput and stability in 2006. The major 2006 upgrade involved the relocation of FIES to a stable environment and development of a fiber bundle that offers 3 different resolution modes, and made FIES an attractive tool for the user community of the NOT. Radial‐velocity stability is achieved through double‐chamber active temperature control. A dedicated data reduction tool, FIEStool, was developed. As a result of these upgrades, FIES is now one of the work‐horse instruments at the NOT. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
8.
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) 相似文献
9.
S. F. Snchez 《Astronomische Nachrichten》2006,327(9):850-861
This paper describes the general characteristics of raw data from fiber‐fed spectrographs in general and fiber‐fed IFUs in particular. The different steps of the data reduction are presented, and the techniques used to address the unusual characteristics of these data are described in detail. These techniques have been implemented in a specialized software package, R3D, developed to reduce fiber‐based integral field spectroscopy (IFS) data. The package comprises a set of command‐line routines adapted for each of these steps, suitable for creating pipelines. The routines have been tested against simulations, and against real data from various integral field spectrographs (PMAS, PPAK, GMOS, VIMOS and INTEGRAL). Particular attention is paid to the treatment of cross‐talk. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
10.
M. D. Albrow K. Horne D. M. Bramich P. Fouqué V. R. Miller J.-P. Beaulieu C. Coutures J. Menzies A. Williams V. Batista D. P. Bennett S. Brillant A. Cassan S. Dieters D. Dominis Prester J. Donatowicz J. Greenhill N. Kains S. R. Kane D. Kubas J. B. Marquette K. R. Pollard K. C. Sahu Y. Tsapras J. Wambsganss M. Zub 《Monthly notices of the Royal Astronomical Society》2009,397(4):2099-2105
The numerical kernel approach to difference imaging has been implemented and applied to gravitational microlensing events observed by the PLANET collaboration. The effect of an error in the source-star coordinates is explored and a new algorithm is presented for determining the precise coordinates of the microlens in blended events, essential for accurate photometry of difference images. It is shown how the photometric reference flux need not be measured directly from the reference image but can be obtained from measurements of the difference images combined with the knowledge of the statistical flux uncertainties. The improved performance of the new algorithm, relative to isis2 , is demonstrated. 相似文献
11.
Flux Calibration of the BATC Survey and Its Application for Checking the Consistency of the Oke—Gunn Standards 总被引：1，自引：0，他引：1
1 INTRODUCTIONThe B eij lug- Ahs ona~ TaiP ei- C onnect lout Color S urvey of t he S by (Hereaft er B AT C ) ut driesthe 15 intermediate band filters to make CCD forage photometric ohs~ion. The BATCphotometric system ties its maghtude zero P~Oint to the spectro-photometric AB maghtudesystem. The AB system is a monochro~ic fi system fort introduced by Oke in 1969 with aprovisional calibration designated AB69.The AB system selects F subdwarfs around visual magnitude 9 as standa… 相似文献
12.
超宽带接收机面临众多技术挑战,而关键技术难点之一是超宽带低噪声放大器.采用以砷化镓材料为基底的70 nm栅长改性高电子迁移率晶体管和双电源偏置4级放大电路结构,设计了一款4–40 GHz超宽带低噪声单片微波集成放大器,完整覆盖C、X、Ku、K、Ka共5个波段.设计仿真结果表明,该放大器增益为(40±2.5) dB,常温下噪声温度平均95 K, 4–12.5 GHz噪声温度全频带低于83 K,直流功耗130.5 mW.整个频带内输入反射系数典型值-10 d B,输出反射系数典型值-15 d B,全频带范围内稳定,无自激振荡现象.该器件可做为前置放大器,应用于超宽带接收机和大规模多波束接收机中,可有效提高射电望远镜观测效率. 相似文献
13.
Radio interferometry significantly improves the resolution of observed images, and the final result also relies heavily on data recovery. The Cotton-Schwab CLEAN(CS-Clean) deconvolution approach is a widely used reconstruction algorithm in the field of radio synthesis imaging. However, parameter tuning for this algorithm has always been a difficult task. Here, its performance is improved by considering some internal characteristics of the data. From a mathematical point of view, a peak signal-to-noise-based(PSNRbased) method was introduced to optimize the step length of the steepest descent method in the recovery process. We also found that the loop gain curve in the new algorithm is a good indicator of parameter tuning.Tests show that the new algorithm can effectively solve the problem of oscillation for a large fixed loop gain and provides a more robust recovery. 相似文献
14.
讨论有关天体测量中的测光问题，介绍一些地面天体测量仪器和空间天体测量望远镜的测光研究。给出一种测光资料的处理方法，并建议利用DCMT开展天文测光课题研究。 相似文献
15.
Tim Naylor 《Monthly notices of the Royal Astronomical Society》1998,296(2):339-346
This paper is primarily an investigation of whether the 'optimal extraction' techniques used in CCD spectroscopy can be applied to imaging photometry. It is found that using such techniques provides a gain of around 10 per cent in signal-to-noise ratio over normal aperture photometry. Formally, it is shown to be equivalent to profile fitting, but offers advantages of robust error estimation, freedom from bias introduced by mis-estimating the point spread function, and convenience. In addition some other techniques are presented, which can be applied to profile fitting, aperture photometry and the 'optimal' photometry. Code implementing these algorithms is available at http://www.astro.keele.ac.uk/~timn/. 相似文献
16.
Coryn A. L. Bailer-Jones Mike Irwin & Ted von Hippel 《Monthly notices of the Royal Astronomical Society》1998,298(4):1061-1068
We describe a method for the extraction of spectra from high-dispersion objective prism plates. Our method is a catalogue-driven plate solution approach, making use of the right ascension and declination coordinates for the target objects. In contrast to existing methods of photographic plate reduction, we digitize the entire plate and extract spectra off-line. This approach has the advantages that it can be applied to CCD objective prism images and spectra can be re-extracted (or additional spectra extracted) without having to re-scan the plate. After a brief initial interactive period, the subsequent reduction procedure is completely automatic, resulting in fully reduced, wavelength-justified spectra. We also discuss a method of removing stellar continua using a combination of non-linear filtering algorithms. The method described is used to extract over 12 000 spectra from a set of 92 objective prism plates. These spectra are used in an associated project to develop automated spectral classifiers based on neural networks. 相似文献
17.
Difference imaging is a technique for obtaining precise relative photometry of variable sources in crowded stellar fields and, as such, constitutes a crucial part of the data reduction pipeline in surveys for microlensing events or transiting extrasolar planets. The Optimal Image Subtraction (OIS) algorithm of Alard & Lupton (1998) permits the accurate differencing of images by determining convolution kernels which, when applied to reference images with particularly good seeing and signal‐to‐noise (S/N), provide excellent matches to the point‐spread functions (PSF) in other images of the time series to be analysed. The convolution kernels are built as linear combinations of a set of basis functions, conventionally bivariate Gaussians modulated by polynomials. The kernel parameters, mainly the widths and maximal degrees of the basis function model, must be supplied by the user. Ideally, the parameters should be matched to the PSF, pixel‐sampling, and S/N of the data set or individual images to be analysed. We have studied the dependence of the reduction outcome as a function of the kernel parameters using our new implementation of OIS within the IDL‐based TRIPP package. From the analysis of noise‐free PSF simulations of both single objects and crowded fields, as well as the test images in the ISIS OIS software package, we derive qualitative and quantitative relations between the kernel parameters and the success of the subtraction as a function of the PSF widths and sampling in reference and data images and compare the results to those of other implementations found in the literature. On the basis of these simulations, we provide recommended parameters for data sets with different S/N and sampling. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
18.
A. W. J. Cousins J. A. R. Caldwell 《Monthly notices of the Royal Astronomical Society》2001,323(2):380-384
Bouguer lines have been used to measure the atmospheric extinction of U − B , and two approaches to correcting U − B for the atmosphere are examined. We find that the conventional photometric reductions may be sufficiently accurate when the range of airmass is small enough (in terms of a maximum hazarded error of 0.04 mag per airmass), but we recommend the use of a colour term in the correction for U − B extinction when this condition is not satisfied. 相似文献
19.
This article describes a novel algorithm for cosmic‐ray rejection in single spectroscopic CCD images. This algorithm is based on a variation of template matching. It focuses on identifying those pixels belong to spectra, while other conventional algorithms tried to locate the cosmic‐ray hits directly. The main principle is applying template matching to find suspicious blocks, which is followed by surface patching to locate the legitimate pixels accurately. Therefore, the rest pixels are the ones corrupted by cosmic‐ray hits. Meanwhile, almost all the parameters are automatically extracted from the images. Examples of its performance are given for both simulated and observed images. It shows an advantage of significantly low false alarm rate with relatively high detection rate (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
20.
针对LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope)2维光谱图像数据,对6种抽谱算法进行了分析与比较.比较的算法包括孔径法、轮廓拟合法、直接反卷积方法、基于Tikhonov正则化的反卷积抽谱算法、基于自适应Landweber迭代的反卷积抽谱算法以及基于Richardson-Lucy迭代的反卷积抽谱算法.通过实验对这些算法在信噪比和分辨率两个方面进行了比较,发现基于Tikhonov正则化的反卷积抽谱算法、基于自适应Landweber迭代的反卷积抽谱算法以及基于RichardsonLucy迭代的反卷积抽谱算法是6种算法中最为可靠的3种抽谱算法.最后,对今后的工作进行了展望. 相似文献