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In astronomical observations at optical wavelengths, a fast image tracking system can be adopted to reduce the effects of the atmospheric seeing and telescopic tracking error, and therefore improve the observing efficiency. Aiming at the need of astronomical observations, totally 5 kinds of algorithms in two categories were selected to make a comparative study on their accuracies and stabilities under different noise conditions by both numerical experiment and laboratory test. The results indicate that the normalized cross-correlation method and barycenter method have not only a higher accuracy but also a better reliability against interferences, they will be applied to the high-resolution spectrograph of the Xinglong 2.16 m telescope and the scienti?c instruments of the SONG (Stellar Observations Network Group) project, respectively. 相似文献
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A. Ziad R. Gredel J. Aceituno J. Borgnino F. Hoyo A. Irbah F. Martin U. Thiele S. Pedraz 《Monthly notices of the Royal Astronomical Society》2005,362(2):455-459
The main atmospheric optical parameters have been measured at the Calar Alto Observatory simultaneously using the Generalized Seeing Monitor (GSM) and a Differential Image Motion Monitor (DIMM) during several nights in 2002 May. The temporal evolution of the seeing, the outer scale, the isoplanatic angle and the coherence time have been analysed. There is excellent agreement between the seeing measurements provided by the two instruments, particularly when the turbulence is slow. Indeed, the GSM measurements are corrected from the exposure time when the DIMM data were recorded for at least 5 ms. From almost three years of DIMM (at 5 m height above ground) data, a seeing of 0.92 arcsec with a standard deviation of 0.31 arcsec has been obtained for this site. The outer scale , the isoplanatic angle θ0 and the coherence time τ0 measured with the GSM are well fitted with log-normal distributions with median values of 22.9 m, 2.27 arcsec and 3.7 ms, respectively. 相似文献
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The degradation of astronomic images by optical inhomogeneities in the earth's atmosphere is generally called seeing. It represents the angular diameter of the stellar images as seen through a turbulent medium. Several techniques can be used to determine this parameter. The knowledge of the optical strength of atmospheric turbulence, namely, the integrated structure coefficient of the atmospheric refractive index Cn
2 allows to predict the atmospheric optical quality in terms of seeing. We tried in this study to assess an astronomical seeing using a model forecast using meteorological data collected in three stations in Latin America from 1958 to 1991. The efficiency of the model is tested by comparison with simultaneous seeing measurements, at Chiliean astronomical sites. 相似文献
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A. Bounhir Z. Benkhaldoun E. Carrasco M. Sarazin 《Monthly notices of the Royal Astronomical Society》2009,398(2):862-872
This paper aims at studying the wind at 200 mbar over the Moroccan observatory Oukaimeden, as high-altitude winds have been adopted as a useful parameter for site characterization in terms of the suitability of a site for the development of some adaptive optics techniques. The data used come from the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data base, which is widely acknowledged as being reliable. Statistical analyses of 200-mbar wind speed since 1983 are performed. Comparison with some of the main observatory sites worldwide qualifies Oukaimeden as one of the best observatory sites in terms of 200-mbar wind statistics. Our analysis of a record of seeing measurements during the years 2003 and 2004 concludes that while 200-mbar wind speed can be used as a parameter for ranking astronomical sites in term of their suitability for adaptive optics, it cannot be used for the whole atmospheric seeing prediction. A comparison of monthly values of the seeing parameter at Oukaimeden, La Silla and Paranal demonstrates the high seeing quality of Oukaimeden, as the seeing values measured were lower than those of La Silla and Paranal for most of the time during the comparison period. Furthermore, a statistical analysis of atmospheric stratified seeing, wavefront coherence time and isoplanatic angle measured with a Multi-Aperture Scintillation Sensor instrument over Paranal from 2004 to 2007 have been performed. We found good correlations between 200-mbar wind velocity and levels 4, 5 and 6 seeing, wavefront coherence time and isoplanatic angle, with corresponding correlation coefficients of 0.74, 0.79, 0.70, 0.97 and 0.78. 相似文献
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Catalogues of non-telescopic sunspot observations from the Orient have been presented by several authors. Since atmospheric dust facilitates naked-eye observation of the Sun we investigate its possible influence by comparing the historical records of sunspot sightings and atmospheric dust storms. A distinction is made between the record up to the end of the Ming dynasty that is based on court documents and the post-Ming reports that are all provincial sightings. The earlier record is found to be significantly anti-correlated with C-14 fluctuations while the latter one is weakly anti-correlated. The provincial sightings contain a much larger signature of atmospheric turbidity, as is also indicated by their poor comparison with telescopic data. 相似文献
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A 48 GHz five-radiometer front end was installed at the Cassegrain focus of the 13.7-m Itapetinga antenna for the observation of solar bursts. The system works with five beam patterns partly overlapping. The five antenna temperatures are recorded with a temporal resolution of 1 millisecond, including time and antenna position. The ratios of the incoming antenna signals are used to determine the centroid of burst emission. Its coordinates are determined from groups of three receivers by using a least-square fit. In favourable observing conditions we obtain an angular accuracy of about 2 arc sec (r.m.s.), with a time resolution of 1 ms and a sensitivity of 0.05 s.f.u. The accuracy of the antenna tracking, the absolute pointing and the quality of radio seeing at Itapetinga are discussed. A preliminary analysis of an impulsive solar burst event is used to illustrate the capabilities of the method described here. 相似文献
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Extensive air showers, induced by high energy cosmic rays impinging on the Earth’s atmosphere, produce radio emission that is measured with the LOFAR radio telescope. As the emission comes from a finite distance of a few kilometers, the incident wavefront is non-planar. A spherical, conical or hyperbolic shape of the wavefront has been proposed, but measurements of individual air showers have been inconclusive so far. For a selected high-quality sample of 161 measured extensive air showers, we have reconstructed the wavefront by measuring pulse arrival times to sub-nanosecond precision in 200 to 350 individual antennas. For each measured air shower, we have fitted a conical, spherical, and hyperboloid shape to the arrival times. The fit quality and a likelihood analysis show that a hyperboloid is the best parameterization. Using a non-planar wavefront shape gives an improved angular resolution, when reconstructing the shower arrival direction. Furthermore, a dependence of the wavefront shape on the shower geometry can be seen. This suggests that it will be possible to use a wavefront shape analysis to get an additional handle on the atmospheric depth of the shower maximum, which is sensitive to the mass of the primary particle. 相似文献
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Tetsuya Tokano 《Planetary and Space Science》2010,58(5):814-829
The orientation of the atmospheric angular momentum vector of Titan and its temporal variation predicted by a general circulation model are analysed and interpreted. The atmospheric angular momentum vector is tilted by a few degrees from the polar axis and the vector rotates (precesses) westward with a constant period of 1 Titan day. The fast westward rotation is likely to be caused by migrating diurnal thermal tides. The tilt is almost cancelled out in the troposphere by the wavenumber 2 pattern of Saturn's gravitational tide, but is more pronounced in the stratosphere, where thermal tides are significant. The predicted tilt angle and the equatorial angular momentum vary with season and maximize when the hemispheric asymmetry of the axial angular momentum or superrotation attains its peak. 相似文献
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Mark Chun Richard Wilson Remy Avila Tim Butterley Jose-Luis Aviles Don Wier Sam Benigni 《Monthly notices of the Royal Astronomical Society》2009,394(3):1121-1130
We present the results of an 18-month study to characterize the optical turbulence in the boundary layer and in the free atmosphere above the summit of Mauna Kea in Hawaii. This survey combined the Slope-Detection and Ranging (SLODAR) and Low-Layer SCIntillation Detection And Ranging (SCIDAR) (LOLAS) instruments into a single manually operated instrument capable of measuring the integrated seeing and the optical turbulence profile within the first kilometre with spatial and temporal resolutions of 40–80 m and 1 min (SLODAR) or 10–20 m and 5 min (LOLAS). The campaign began in the fall of 2006 and observed for roughly 50–60 h per month. The optical turbulence within the boundary layer is found to be confined within an extremely thin layer (≤80 m), and the optical turbulence arising within the region from 80 to 650 m is normally very weak. Exponential fits to the SLODAR profiles give an upper limit on the exponential scaleheight of between 25 and 40 m. The thickness of this layer shows a dependence on the turbulence strength near the ground, and under median conditions the scaleheight is <28 m. The LOLAS profiles show a multiplicity of layers very close to the ground but all within the first 40 m. The free-atmosphere seeing measured by the SLODAR is 0.42 arcsec (median) at 0.5 μm and is, importantly, significantly better than the typical delivered image quality at the larger telescopes on the mountain. This suggests that the current suite of telescopes on Mauna Kea is largely dominated by a very local seeing either from internal seeing, seeing induced by the flow in/around the enclosures, or from an atmospheric layer very close to the ground. The results from our campaign suggest that ground-layer adaptive optics can be very effective in correcting this turbulence and, in principle, can provide very large corrected fields of view on Mauna Kea. 相似文献
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P. N. Brandt 《Solar physics》1970,13(1):243-246
An investigation being carried out at the Fraunhofer Institut is described; it has two aims: to establish a simple method for the quantitative measurement of solar seeing effects and to obtain informations of their dependence on telescopic aperture.Mitteilungen aus dem Fraunhofer Institut Nr. 99. 相似文献
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L. V. Ksanfomality 《Astronomy Letters》2005,31(11):767-785
We took electronic photographs of Mercury on the side of the planet that was not photographed from the Mariner-10 spacecraft in 1973–1975 by the millisecond-exposure method in ground-based observations. Based on these photographs, we synthesized resolved images of the surface of unknown regions of the planet. The capabilities of the method are limited by the small angular size of the planetary disk (only 7.3 arcsec at average quadrature), specific difficulties of Mercury’s ground-based observations, their very limited duration, and the laboriousness of the subsequent computer-aided observational data processing. The millisecond-exposure method is complex, but a sufficient number of primary electronic photographs can be taken under good seeing conditions for the subsequent synthesis of Mercurian images with a resolution of no worse than the diffraction limit. A giant basin about 2000 km in diameter and other large structures are distinguished in the synthesized images of the planet. In the regions where radar data are available, these structures can be identified with previously found ones. In some measure, the synthesized images allow the relief of the longitude sector 210°–290° W to be reconstructed on Mercury. It can be asserted with caution that the large relief features are distributed asymmetrically over the surface of Mercury, much as observed on other terrestrial planets, the Moon, and many satellites of giant planets. 相似文献
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The power spectrum and the rms-value of the granular intensity fluctuations were studied using granulation photographs of excellent quality obtained during the JOSO site testing campaign 1979 at Izaña. The observed power spectrum was corrected using various effective modulation transfer functions of the system: telescope+aberrations+atmospheric seeing, assuming different contributions of the atmospheric seeing. With this procedure a lower and upper limit for the ‘true’ power spectrum of the granular intensity fluctuations and thus for the rms-value could be derived: 7.2% <I rms <12% at λ = 550 nm, with a most probable value of I rms = 10.5%. We checked the validity of the upper limit by applying to our data a MTF (Deubner and Mattig, 1975), which certainly must lead to an overcorrection. This procedure lead to I rms = 13.4%. Thus we can state that the true rms-value of the granular intensity fluctuations does certainly not exceed 13% at λ = 550 nm. 相似文献
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Franz Ludwig Deubner 《Solar physics》1968,3(3):439-447
It is shown that the short-period fluctuations of photospheric velocity records can be explained by the scanning effect of atmospheric seeing (image motion) and the velocity gradients present on the solar surface. Some observations supporting this explanation are presented.Mitteilungen aus dem Fraunhofer Institut Nr. 76. 相似文献
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在文献」１「４ 基础上，根据近年文选址实践中提出的问题，先进一步讨论了大气相干直径和时间的的物理和意义，然后强调了自由大气湍流的天文成像效应及选址中进行研究和测量的重要性。最后对差分像运动方法测量视宁度的定标和系统误差等问题作了分析。 相似文献
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