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1.
卫星双向时间传递(Two-Way Satellite Time and Frequency Transfer, TWSTFT)是目前精度最高的时间传递方法之一,同时也是参与国际原子时计算的守时实验室之间比较原子时尺度的一种主要方法.提高TWSTFT链路的短期稳定度,降低周日效应对链路时间传递结果的影响,对优化TAI (International Atomic Time)的性能具有现实意义.提出了一种基于条件平差的TWSTFT链路性能优化方法,先依据TWSTFT链路测量噪声水平与谱分析结果建立TWSTFT链路性能优化网络(简称优化网络),再根据优化网络中各链路测量噪声分析结果设置权系数阵,建立条件平差模型.选取亚太地区的中国计量科学研究院(National Institute of Metrology, NIM)-中国科学院国家授时中心(National Time Service Center, NTSC)卫星双向时间传递链路作为待优化链路,以NTSC、NIM以及德国联邦物理技术研究所(Physikalisch-Technische Bundesanstalt,PTB)之间的TWSTFT链路组成优化网络,对优化网络的组网方法和条件平差模型进行实验验证.结果表明,平差后待优化链路短期稳定度得到了改善,同时其受周日效应的影响降低了约24.6%.使用该方法,能够有效提高待优化链路的时间传递性能.  相似文献   

2.
闫凯  郭晶 《天文学报》2022,63(5):55
偏振定标单元(Polarization Calibration Unit, PCU)对于定标由偏振系统和天文望远镜产生的仪器偏振至关重要, 然而偏振定标单元 中偏振元件光轴的方位角误差是限制定标精度的主要因素之一. 为解决该问题, 提出了一种基于约束非线性最 小化优化的方位角误差定标方法, 该方法具有定标精度高、定标速度快的优点. 首先将偏振定标单元中的线性 偏振片和四分之一波片的光轴方位角误差设置为两个待优化的自由变量, 然后利用产生和测量的Stokes参数 以及偏振定标获得的响应矩阵定义优化目标函数, 最终使用约束非线性最小化优化方法来确定 两个偏振元件的方位角误差. 分别从理论模拟和实际测量两个方面对优化方法进行了验证, 实验结果表明, 该 优化方法能够成功获得上述两个方位角误差, 精度分别优于2.79$''$和2.72$''$. 此外, 从理论上 计算分析了不同方位角误差对各Stokes分量的影响情况. 该优化方法有望应用到我国太阳望远镜中偏振定标 装置的误差定标及研制之中.  相似文献   

3.
Two-Way Satellite Time and Frequency Transfer (TWSTFT) is one of the time transfer methods with the highest accuracy at present, and it is also the main method to compare the atomic time scale among the time-keeping laboratories participated in the international atomic time calculation. Improving the short-term stability of TWSTFT links and reducing the influence of Diurnal on the link time transfer result have practical significance for optimizing the performance of TAI (International Atomic Time). In this paper, a method of TWSTFT link performance optimization based on conditional adjustment is proposed. Firstly, the optimized network of TWSTFT link performance (hereinafter briefly called as optimized network) is established according to the noise level and spectrum analysis result of TWSTFT link measurements, and then the weight coefficient matrix is set according to the analysis result of measurement noises of all the links in the optimized network to establish the conditional adjustment model. The Two-Way Satellite Time and Frequency Transfer link between the National Institute of Metrology (NIM) and the National Time Service Center (NTSC) of Chinese Academy of Sciences in the Asia-Pacific region is selected as the link to be optimized, and the TWSTFT links among NTSC, NIM, and Physikalisch-Technische Bundesanstalt (PTB) are used to compose the optimized network. The networking method and conditional adjustment model are verified by experiments. The results show that the short-term stability of the link to be optimized after adjustment is improved, and the influence of Diurnal is reduced about 24.6%. Using this method, the time transfer performance of the link to be optimized can be improved effectively.  相似文献   

4.
In the second paper of the series we continue the investigation of open cluster fundamental parameters using a robust global optimization method to fit model isochrones to photometric data. We present optical UBVRI CCD photometry (Johnsons-Cousins system) observations for 24 neglected open clusters, of which 14 have high quality data in the visible obtained for the first time, as a part of our ongoing survey being carried out in the 0.6 m telescope of the Pico dos Dias Observatory in Brazil. All objects were then analyzed with a global optimization tool developed by our group which estimates the membership likelihood of the observed stars and fits an isochrone from which a distance, age, reddening, total to selective extinction ratio RV (included in this work as a new free parameter) and metallicity are estimated. Based on those estimates and their associated errors we analyzed the status of each object as real clusters or not, finding that two are likely to be asterisms. We also identify important discrepancies between our results and previous ones obtained in the literature which were determined using 2MASS photometry.  相似文献   

5.
This paper describes the Wide Field Spectrograph (WiFeS) under construction at the Research School of Astronomy and Astrophysics (RSAA) of the Australian National University (ANU) for the ANU 2.3 m telescope at the Siding Spring Observatory. WiFeS is a powerful integral field, double-beam, concentric, image-slicing spectrograph designed to deliver excellent throughput, wavelength stability, spectrophotometric performance and superb image quality along with wide spectral coverage throughout the 320–950 nm wavelength region. It provides a 25×38 arcsec field with 0.5 arcsec sampling along each of twenty five 38×1 arcsec slitlets. The output format is optimized to match the 4096×4096 pixel CCD detectors in each of two cameras individually optimized for the blue and the red ends of the spectrum, respectively. A process of “interleaved nod-and-shuffle” will be applied to permit quantum noise-limited sky subtraction. Using VPH gratings, spectral resolutions of 3000 and 7000 are provided. The full spectral range is covered in a single exposure at R=3000, and in two exposures in the R=7000 mode. The use of transmissive coated optics, VPH gratings and optimized mirror coatings ensures a throughput (including telescope atmosphere and detector) >30% over a wide spectral range. The concentric image-slicer design ensures an excellent and uniform image quality across the full field. To maximize scientific return, the whole instrument is configured for remote observing, pipeline data reduction, and the accumulation of calibration image libraries.  相似文献   

6.
The French soft -ray coded mask imaging telescope on board Soviet spacecraft GRANAT has experienced some unexpected gain drifts during its operation in the space. This gain instability seriously affected its data analysis and scientific interpretation of the results. After a detailed study of its background spectra and the long term performance of the detector, we have identified several background features which enable us to perform the in flight energy calibrations and gain corrections for the observations. The precision of such an energy calibration is estimated to be in the order of 0.5%, which is accurate enough for all types of data analysis such as spectrum de-convolution, and summation of observation spectra. More importantly it makes the search of astrophysical line features in the spectrum possible.On leave fromUniversity of Southampton, U.K.  相似文献   

7.
A spectrum extraction method based on a two-dimensional (2-D) exponential polynomial model is proposed in this paper. It is presented for extracting the flux of the fiber spectrum from a two-dimensional image observed by Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). Due to the effects of the environments and the instruments, centers of the fiber spectra deviate from the ideal, and the energy is expanded to the neighbors in the form of a point-spread function (PSF) in two dimensions. Thus we use a 2-D model to fit the PSF. In spatial orientation and wavelength orientation, the flat-field images and the calibration lamp images are respectively used to evaluate the parameters of the 2-D exponential polynomial model. With the 2-D exponential polynomial model, the spectrum flux is extracted based on the least square theory. The experiments are processed through simulated profiles to demonstrate the effectiveness of the proposed algorithm. Especially, its superiority of restraining noise and fitting asymmetric fiber profiles is proved.  相似文献   

8.
On 7 February 2008, the SOLAR payload was placed onboard the International Space Station. It is composed of three instruments, two spectrometers and a radiometer. The two spectrometers allow us to cover the 16?–?2900 nm spectral range. In this article, we first briefly present the instrumentation, its calibration and its performance in orbit. Second, the solar spectrum measured during the transition between Solar Cycles 23 to 24 at the time of the minimum is shown and compared with other data sets. Its accuracy is estimated as a function of wavelength and the solar atmosphere brightness-temperature is calculated and compared with those derived from two theoretical models.  相似文献   

9.
We have constructed a new high resolution solar reference spectrum in the spectral range between 250 and 550 nm. The primary use of this spectrum is for the calibration of the Dutch – Finnish Ozone Monitoring Instrument (OMI), but other applications are mentioned. The incentive for deriving a new high resolution solar reference spectrum is that available spectra do not meet our requirements on radiometric accuracy or spectral resolution. In this paper we explain the steps involved in constructing the new spectrum, based on available low and high resolution spectra and discuss the main sources of uncertainty. We compare the result with solar measurements obtained with the OMI as well as with other UV-VIS space-borne spectrometers with a similar spectral resolution. We obtain excellent agreement with the OMI measurements, which indicates that both the newly derived solar reference spectrum and our characterization of the OMI instrument are well understood. We also find good agreement with previously published low resolution spectra. The absolute intensity scale, wavelength calibration and representation of the strength of the Fraunhofer lines have been investigated and optimized to obtain the resulting high resolution solar reference spectrum.  相似文献   

10.
A new composite model eliminates discrepancies between synthetic spectra for the L2000 model of β Lyrae and spectrophotometric scans and charge-coupled device (CCD) spectra of that binary star. The source of the discrepancies is the use of synthetic spectra from standard model atmospheres to represent the radiation characteristics of the accretion disc rim. Substitution of a composite synthetic spectrum to represent the accretion disc rim eliminates the discrepancies and produces accordance with both scan data, including their temporal variation, and CCD spectra.  相似文献   

11.
The last solar minimum activity period, and the consequent minimum modulation conditions for cosmic rays, was unusual. The highest levels of galactic protons were recorded at Earth in late 2009 in contrast to expectations. Proton spectra observed for 2006 to 2009 from the PAMELA cosmic ray detector on-board the Resurs-DK1 satellite are presented together with the solutions of a comprehensive numerical model for the solar modulation of cosmic rays. The model is used to determine what mechanisms were mainly responsible for the modulation of protons during this period, and why the observed spectrum for 2009 was the highest ever recorded. From mid-2006 until December 2009 we find that the spectra became significantly softer because increasingly more low energy protons had reached Earth. To simulate this effect, the rigidity dependence of the diffusion coefficients had to decrease significantly below ～?3 GeV. The modulation minimum period of 2009 can thus be described as relatively more ‘diffusion dominated’ than previous solar minima. However, we illustrate that drifts still had played a significant role but that the observable modulation effects were not as well correlated with the waviness of the heliospheric current sheet as before. Protons still experienced global gradient and curvature drifts as the heliospheric magnetic field had decreased significantly until the end of 2009, in contrast to the moderate decreases observed during previous minimum periods. We conclude that all modulation processes contributed to the observed increases in the proton spectra for this period, exhibiting an intriguing interplay of these major mechanisms.  相似文献   

12.
The Atmospheric Imaging Assembly (AIA) instrument onboard the Solar Dynamics Observatory (SDO) is an array of four normal-incidence reflecting telescopes that image the Sun in ten EUV and UV wavelength channels. We present the initial photometric calibration of AIA, based on preflight measurements of the response of the telescope components. The estimated accuracy is of order 25%, which is consistent with the results of comparisons with full-disk irradiance measurements and spectral models. We also describe the characterization of the instrument performance, including image resolution, alignment, camera-system gain, flat-fielding, and data compression.  相似文献   

13.
The effectiveness of equal power spectrum filtering in image restoration is analyzed via digital simulation. We start with a gray-scale map of concentric rings with different grey values. To obtain a dirty image, the original image is first transformed via a linear space-invariant system. Then, white noise is mixed into the image. The equal power spectrum filter is then used for the image restoration. The signal to noise ratio (SNR) and a defined steepness of the steps, together with the visual impression, are accepted to characterize the performance of the image restoration. It is concluded that a satisfactory restoration can be realized when the SNR > 15. When SNR < 5, the image is completely un-restorable. The effect of system's transfer function on image restoration is also discussed. The effectiveness of the presented model of equal power spectrum filter is confirmed by applying it to the restoration of an actual astronomical image. Finally, a typical application of the equal power spectrum filter is given.  相似文献   

14.
We show how traditional instrument quality control trending can be augmented by the use of a physical instrument model. The ESO VLT archive contains a detailed record of instrument diagnostics and calibration parameters while ESO quality control monitors changes in critical parameters. The physical model allows changes in positions, orientations and other physical properties of a spectrograph to be determined from standard wavelength calibration exposures via an optimisation process that seeks the physical model parameters that best reproduce the calibration features in the data. We introduce physical model parameters to the quality control monitoring. When applying this technique to archived calibration exposures, we find that the results are sensitive to the combination of parameters open to the optimisation process. Therefore we determine the most favourable set of physical parameters to optimise for each arm. We then show correlations between several physical parameters and instrument temperature sensor readings and epoch. In addition we find clear discontinuities in some physical parameter values that correspond to known maintenance events.  相似文献   

15.
We describe a method for deriving the position and flux of point and compact sources observed by a scanning survey mission. Results from data simulated to test our method are presented, which demonstrate that at least a 10-fold improvement is achievable over that of extracting the image parameters, position and flux, from the equivalent data in the form of pixel maps. Our method achieves this improvement by analysing the original scan data and performing a combined, iterative solution for the image parameters. This approach allows for a full and detailed account of the point-spread function (PSF), or beam profile, of the instrument. Additionally, the positional information from different frequency channels may be combined to provide the flux-detection accuracy at each frequency for the same sky position. Ultimately, a final check and correction of the geometric calibration of the instrument may also be included. The Planck mission was used as the basis for our simulations, but our method will be beneficial for most scanning satellite missions, especially those with non-circularly symmetric PSFs.  相似文献   

16.
K. Serkowski 《Icarus》1976,27(1):13-24
A method of wavelength calibration is proposed which may enable measuring changes in radial velocity of bright solar-type stars to an accuracy of about 5 meters per second. Such accuracy would be sufficient for detecting Jupiter-like planets around these stars. The stellar spectrum is imaged by a slitless echelle spectrograph onto a 100-channel Digicon image tube. Instrumental profiles of Digicon diodes are narrowed down by a Fabry-Perot etalon, making the profiles less dependent on atmospheric seeing. The spectrograph and the etalon act merely as a series of narrow band filters for the individual diodes; effective wavelengths of these “filters” are monitored by a crystal retarder (phase retardation plate) kept at a constant temperature. For artificially linearly polarized stellar light which passes through this retarder and through a quarter-wave plate, the plane of polarization varies rapidly with wavelength. The precisely measured position angle of polarization provides wavelength calibration for every resolution element in the spectrum.  相似文献   

17.
With increasingly large data sets, weak lensing measurements are able to measure cosmological parameters with ever-greater precision. However, this increased accuracy also places greater demands on the statistical tools used to extract the available information. To date, the majority of lensing analyses use the two-point statistics of the cosmic shear field. These can be either studied directly using the two-point correlation function or in Fourier space, using the power spectrum. But analysing weak lensing data inevitably involves the masking out of regions, for example to remove bright stars from the field. Masking out the stars is common practice but the gaps in the data need proper handling. In this paper, we show how an inpainting technique allows us to properly fill in these gaps with only   N log  N   operations, leading to a new image from which we can compute straightforwardly and with a very good accuracy both the power spectrum and the bispectrum. We then propose a new method to compute the bispectrum with a polar fft algorithm, which has the main advantage of avoiding any interpolation in the Fourier domain. Finally, we propose a new method for dark matter mass map reconstruction from shear observations, which integrates this new inpainting concept. A range of examples based on 3D N -body simulations illustrates the results.  相似文献   

18.
By analyzing impact glass, the evolution of the impact melt at the Mistastin Lake impact structure was investigated. Impact glass clasts are present in a range of impactites, including polymict breccias and clast‐rich impact melt rock, and from a variety of settings within the crater. From the glass clasts analyzed, three petrographic subtypes of impact glass were identified based on their clast content, prevalence of schlieren, color, texture, and habit. Several alteration phases were also observed replacing glass and infilling vesicles; however, textural observations and quantified compositional data allowed for the identification of pristine impact glass. Although the various types of glasses show significant overlap in their major oxide composition, several subtle variations in the major oxide chemistry of the glass were observed. To investigate this variation, a least‐squares mixing model was implemented utilizing the composition of the glass and the known target rock chemistry to model the initial melt composition. Additionally, image analysis of the glass clasts was used to investigate whether the compositional variations correlated to textural difference in the lithologies. We propose that the textural and compositional dichotomy observed is a product of the evolution, assimilation, and emplacement of the glass. The dichotomy is reflective of the melt either being ballistically emplaced (group 2 glasses: occurring in melt‐poor polymict breccias at lowermost stratigraphic position outside the transient crater) or the result of late‐stage melt flows (group 1 glasses, occurring in melt‐bearing polymict breccias and impact melt rocks at higher stratigraphic positions outside the transient crater).  相似文献   

19.
G. Kovács 《Solar physics》1983,82(1-2):123-128
The accuracy of frequency determination by a least squares technique for an autoregressive spectral estimator is studied and compared with the Fourier method. Using numerical tests the probability distribution function of the peak location is calculated. The autoregressive filter order is optimized in the sense of minimum variance of the peak location. Simple sinusoidal signals with additive Gaussian noise are considered and the effect of other components is only indicated. Generally, a filter order between 1/3 and 1/2 of the total data number and a not very dense data sampling, gives the most stable spectrum. The results are numerical.  相似文献   

20.
For submicrometer-sized particles of the carbonaceous chondrite Nogoya and, for comparison purposes, also of terrestrial chamosite, the mass absorption coefficients have been derived from laboratory spectra for the wavenumber range 250–1300 cm–1. Using these data the expected spectrum of an infrared source has been calculated under simple model assumptions and compared with the observed flux from the infrared source OH 26.5+0.6. Although certain discrepancies are left in the 20-m region it is concluded that phyllosilicates, especially such occurring in carbonaceous chondrites, are interesting candidates for the material of interstellar grains.  相似文献   

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