共查询到20条相似文献，搜索用时 15 毫秒
1.
Shallow Radar (SHARAD) on board NASA’s Mars Reconnaissance Orbiter has successfully detected tens of reflectors in the subsurface of the north polar layered deposits (NPLD) of Mars. Radar reflections are hypothesized to originate from the same material interfaces that result in visible layering. As a first step towards verifying this assumption, this study uses signal analyses and geometric comparisons to quantitatively examine the relationship between reflectors and visible layers exposed in an NPLD outcrop. To understand subsurface structures and reflector geometry, reflector surfaces have been gridded in three dimensions, taking into account the influence of surface slopes to obtain accurate subsurface geometries. These geometries reveal reflector dips that are consistent with optical layer slopes. Distance–elevation profiling of subsurface reflectors and visible layer boundaries reveals that reflectors and layers demonstrate similar topography, verifying that reflectors represent paleosurfaces of the deposit. Statistical and frequency-domain analyses of the separation distances between successive layers and successive reflectors confirms the agreement of radar reflector spacing with characteristic spacing of certain visible layers. Direct elevation comparisons between individual reflectors and discrete optical layers, while necessary for a one-to-one correlation, are complicated by variations in subsurface structure that exist between the outcrop and the SHARAD observations, as inferred from subsurface mapping. Although these complications have prevented a unique correlation, a genetic link between radar reflectors and visible layers has been confirmed, validating the assumption that radar reflectors can be used as geometric proxies for visible stratigraphy. Furthermore, the techniques for conducting a stratigraphic integration have been generalized and improved so that the integration can be undertaken at additional locations. 相似文献
2.
现有标准设计的反光望远镜在拍摄CCD平场时,绝大多数都受残留散射光的影响.但是赤道式与地平式反光镜所受影响是不同的.在赤道式上做时间序列较差测光时,只要待测星永远位于CCD的固定像元上,不太准确的CCD平场也能得到高精度的测光结果.当需要0.1%～0.3%精度的平场时,则可以采用夜天平场.地平式的特点是,它的CCD相机必须置于旋转器上,在跟踪天体时不停地旋转,以抵消地球自转的影响.上述用于赤道式的方法失效,因此,在CCD平场时,消除散射光的影响比赤道式更为重要.一个典型的地平式反光镜的例子是NAOC兴隆天文台的EOS 1米镜.虽然该台已附加了防散射光的装置,但是对所有B、V、R、I滤光片,在不同旋转器位置拍摄的CCD平场,仍然有2%～3%的差别(主要是梯度).该文给出了改进的建议,必须满足下面两个条件:Cij=C(r);旋转器的中心与反光镜的光学中心重合.此问题的解决对所有地平式反光望远镜都有普遍意义. 相似文献
3.
S. K. Gupta P. V. Ramana Murthy S. C. Tonwar P. R. Vishwanath 《Astrophysics and Space Science》1985,115(1):163-183
Detection of cosmic sources of very high energy gamma rays based on the atmospheric Cerenkov technique is discussed. Very high energy gamma-rays initiate, on entering the terrestrial atmosphere, electron-photon cascade showers with in turn produce Cerenkov photons in the air. Parabolic reflectors are used to focus these photons onto fast photomultipliers. Two methods of deployment of parabolic reflectors are in vogue: one in which all the reflectors are located close to each other in a compact array and the other in which the reflectors are spread out farther apart forming a distributed array. In the latter mode, the arrival direction of individual showers can be determined accurately by using the measured relative arrival times between different detectors. Detailed studies with the distributed array helped us to understand the various parameters in the two designs and evaluate their relative merits in reaching the ultimate goals of lowering the energy threshold and improving the signal to background ratio for the detection of gamma-ray sources. It is found that the relative superiority among the two types of arrays is a function of the exponent assumed for the differential power law energy spectrum for the gamma ray source. It is also seen that with the type of reflectors commonly used in atmospheric Cerenkov work, lower energy thresholds can be achieved with use of larger aperture. 相似文献
4.
The design of the East West arm of a multi-element interferometer for solar noise observations in the band 2650–2950 mhz at the Algonquin Radio Observatory, Lake Traverse, Ont., Canada, is outlined. The completed first stage consists of an array of thirty-two 3-meter reflectors extending over 212 meters. In the second stage, four reflectors will be arranged symmetrically about the array for polarization studies, while in the third stage four additional reflectors will be placed to the East to form a compound interferometer. These configurations provide single lobed fan beams with half power widths of 1.5, 0.5 and 0.25 of arc respectively. Scans of the sun taken on April 26th, 1966 with the 32-element array located two widely separated enhancements of radio emission, which produced a small gradual rise and fall burst on the solar patrol record. 相似文献
5.
Shoko Oshigami Shinya Okuno Yasushi Yamaguchi Makiko Ohtake Junichi Haruyama Takao Kobayashi Atsushi Kumamoto Takayuki Ono 《Icarus》2012,218(1):506-512
Comparison of the Lunar Radar Sounder (LRS) data to the Multiband Imager (MI) data is performed to identify the subsurface reflectors in Mare Serenitatis. The LRS is FM-CW radar (4–6 MHz) and the 2 MHz bandwidth leads to the range resolution of 75 m in a vacuum, whereas the sampling interval in the flight direction is about 75 m when an altitude of the spacecraft with polar orbit is nominal (100 km). Horizontally continuous reflectors were clearly detected by LRS in limited areas that consist of about 9% of the whole maria. The typical depth of the reflectors is estimated to be a few hundred meters. Layered structures of mare basalts are also discernible on some crater walls in the MI data of the visible bands (VIS). The VIS range has nine wavelengths of 415, 750, 900, 950, and 1000 nm, and their spatial resolution is 20 m/pixel at a nominal altitude. The stratigraphies around Bessel and Bessel-H craters in Mare Serenitatis are examined in this paper. It was revealed that the subsurface reflectors lie on the boundaries between basalt units with different chemical compositions. In addition, model calculations using the simplified radar equation indicate that the subsurface reflectors are not compositional interfaces but layer boundaries with a high-porosity contrast. These results suggest that the detected reflectors in Mare Serenitatis are regolith accumulated during so long hiatus of mare volcanisms enough for chemical composition of magma to change, not instantaneously. Therefore combination of the LRS and MI data has a potential to reveal characteristics of a series of magmatism forming each lithostratigraphic unit in Mare Serenitatis and other maria. 相似文献
6.
散射光对准确CCD平场的影响--修改某些反光镜挡光筒的必要性 总被引：1，自引：0，他引：1
继前文之后，进一步说明，晨昏蒙影平场与夜天露光的差别主要由于存在散射光．像Grundahl和Sorensen那样，上海天文台1．56米和南京大学63厘米反光镜的针孔像也已获得，直观地显示了散射光的存在，1．56米和63厘米反光镜都有标准设计的挡光筒，所有标准设计的反光镜都只有两个挡光筒，分别置于主镜和付镜前面．为了在得出准确平场时CCD不受散射光影响，这样的挡光筒是不够的．这是所有标准设计的档光筒共同的弱点．为了得到更准确的平场改正，修改所有类似的反光镜主镜挡光筒是极其重要的。 相似文献
7.
David H. DeVorkin 《Experimental Astronomy》2009,25(1-3):63-77
George Ellery Hale was a man of many dreams. One of his most persistent was to find the means to collect as much light as possible, but there is another element in his designs for the modern astrophysical observatory that has even greater significance, as it defines and distinguishes the practice of astrophysics from that of classical astronomy. Here we examine factors that either impeded or drove the acceptance of reflectors over refractors around the turn of the twentieth century at the outset of what may best be called the “Hale era.” This commenced in the late nineteenth century, when the first large multi-focus photographic reflectors emerged during the reign of the great refractors. It lasted through to the onset of World War II when astronomical practice was dominated by ten reflectors with mirrors between 60 and 100 in., and was about to add one more whose surface area would almost double that of all the rest combined. We will touch upon how design choice reflected both scientific priorities and technological limitations. 相似文献
8.
The construction and operation of a high resolution interferometer for solar noise observations at 10.7 cm for the Algonquin Radio Observatory is described. One element is the existig 32 element array of 3 m reflectors while the second is a grating element of 4 reflectors symmetrically placed on the same E-W line. The aperture of 665 m produces a fan beam of 0.5 E.W. by 2° N.S. Four Faraday rotators in the antenna feeds of the newly commissioned grating enable three Stoke's parameters to be observed. The expected antenna pattern is confirmed by making observations on the brighter radio sources. Observations of the Sun on 1972, January 6 show the range of phenomena to be observed and a detailed study of the association of eight radio emissive regions with all centers of solar activity is presented. 相似文献
9.
No single technology can meet the current science specifications for the Square Kilometre Array (SKA). In this paper a hybrid option is explored that uses cylindrical reflectors to satisfy low frequency sensitivity and field-of-view requirements and small parabolic dishes to work at frequencies above 0.5 GHz. 相似文献
10.
Radar echos at 12.6 cm show that the particles of the rings of Saturn are very efficient radar reflectors. It follows that the particles are likely to be rough, with diameters on the order of a meter or larger. The data indicates that some of the reflecting objects may lie beyond the visible rings. 相似文献
11.
John D. Bunton 《Experimental Astronomy》2004,17(1-3):185-189
In the early days of radioastronomy cylindrical parabolic reflectors were a popular technology. This paper traces some of the factors that have hindered their use in recent times. These factors no longer apply and as the cost of electronics decreases their applicability to the SKA becomes more and more favourable. 相似文献
12.
Stefano Bianchi Giorgio Matt Kazushi Iwasawa 《Monthly notices of the Royal Astronomical Society》2001,322(3):669-680
The ASCA and BeppoSAX spectra of the Circinus galaxy and NGC 1068 are analysed and compared with photoionization models based on cloudy . In the case of Circinus, a single, mildly ionized reflector can account for the line spectrum, while in NGC 1068 at least three different reflectors (with different ionization states) are needed. We suggest that the reflector in Circinus and the low ionized one in NGC 1068 are the inner and visible part of the material responsible for the X-ray absorption. With this assumption, we estimate for the inner radius of the absorber a value of 0.2 pc for Circinus and of a few parsecs for NGC 1068. 相似文献
13.
We present observations of optical spectra of a flare in which blue line asymmetry was seen for more than 4 min close to the flare onset. The maximum blue asymmetry coincided with the maximum of a hard X-ray and microwave burst. We discuss possible interpretations of the blue asymmetry and conclude that the most plausible one is electron-beam heating with return current. Although this process predicts downflows in the lower transition region and upper chromosphere, its ultimate effect on the line profiles can be blue asymmetry: the upper layers moving away from us absorb the radiation of the red peak thus lowering its intensity in comparison to the blue one. 相似文献
14.
In the present paper revised distances of optical objects from radio sources of the 5C2 survey are given. A statistical investigation of the data is given for blue objects and galaxies by the statistical method of the “first neighbour”. The identification rate on blue plates for both the blue objects and galaxies amounts to about 40% out of the total number of 26 identifications. For every blue object and galaxy which are proposed as an identification the statistical reliability is given. 相似文献
15.
文中所作的主要工作是为云南天文台十米抛物面设计、制造一付好的对数周期偶极子天线（ＬＰＤＡ）作馈源。工作有：（１）设计一对交叉的ＬＰ－ＤＡ；（２）分析ＬＰＤＡ特性；（３）估算抛物面的电特性；（４）讨论线极化向园极化转换的问题；（５）天线测量。ＬＰＤＡ的测量结果比较满意。该天线的方向图、平均输入阻抗、三分贝和十分贝波瓣宽度都在从０．５－１．５ＧＨｚ的频率范围内比较一致和稳定，变化很小。天线理论上的许多成就都是与数子计算机有关的。在文中，我们充分利用了计算机的功能来设计天线，分析天线特性，并从测量所得的所有数据文件中计算天线的参数。所编ＦＯＲＴＲＡＮ程序使这一切工作变得容易。最后，我们还讨论了用功率合成器实现线极化向园极化转换的各种情况 相似文献
16.
The relations between the kinematics of satellites and the properties of hosts and satellites themselves are investigated in this paper. Our sample of hosts and satellites is selected from the Sloan Digital Sky Survey (SDSS) data by adopting a self-adapted method developed by van den Bosch et al. Consistent with the previous studies, the average velocity dispersion increases with the mass of host galaxy, and is larger for red hosts than for blue hosts. We find that, on average, the velocity dispersion is independent of satellite mass around red hosts, however it increases with the satellite mass around blue hosts, and red satellites have a larger velocity dispersion than their blue counterparts. Our further investigations show that in the same halo, the velocity dispersion is independent of satellite mass, regardless of the host color. Interestingly, around red hosts, the red satellites tend to have a smaller velocity dispersion than the blue ones. It implies some interesting processes. In addition, we also find that if host galaxies only have red or blue (high mass or low mass) satellites, the system with red (high mass) satellites has a larger velocity dispersion than that with blue (low mass) satellites. It suggests that satellite properties are important for the measurement of dark halo mass. 相似文献
17.
In this talk, we will simply show the consequences of binary population synthesis for blue stragglers, such as the integrated spectral energy distribution (ISED), the color-magnitude diagram, the specific frequency, and the influences on colors. Blue stragglers have been found in all stellar populations and they are an important population component in both stellar evolution and star clusters. Much evidence shows that blue stragglers are relevant to primordial binaries. The binary population synthesis study shows that primordial binary evolution may produce blue stragglers at any given times. The specific frequency in this way decreases with time first, then increases when the age is larger than 10 Gyr, while that from angular momentum loss induced by magnetic braking in low-mass binaries increases with time and exceeds that of primordial binary evolution in a population older than 3 Gyr. Meanwhile, blue stragglers resulting from primordial binary evolution are dominant contributors to the ISEDs in ultraviolet and blue bands in a population between 0.3 and 2.0 Gyr. The mass fraction of the lost matter from the primary accreted by the secondary, β, significantly affects on the final results, e.g. the specific frequency of blue stragglers decreasing with β, blue stragglers produced from a high value of β being more massive, then contributing more to the ISEDs of the host clusters. For old open clusters, it is appropriate to adopt a higher value of β when the primary is in HG at the onset of mass transfer. Our study also shows that, for most Galactic open clusters, the specific frequency of blue stragglers obtained from our simulations is much lower than that of observations, which has been discussed in this talk. 相似文献
18.
Georgina V. Coldwell Diego G. Lambas Ilona K. Söchting Sebastián Gurovich 《Monthly notices of the Royal Astronomical Society》2009,399(1):88-96
We analyse the relation between active galactic nuclei (AGN) host properties and large-scale environment for a representative red and blue AGN host galaxy sample selected from the Data Release 4 Sloan Digital Sky Survey. A comparison is made with two carefully constructed control samples of non-active galaxies, covering the same redshift range and colour baseline. The cross-correlation functions show that the density distribution of neighbours is almost identical for blue galaxies, either active or non-active. Although active red galaxies inhabit environments less dense compared to non-active red galaxies, both reside in environments considerably denser than those of blue hosts. Moreover, the radial density profile of AGN relative to galaxy group centres is less concentrated than galaxies. This is particularly evident when comparing red AGN and non-active galaxies.
The properties of the neighbouring galaxies of blue and red AGN and non active galaxies reflect this effect. While the neighbourhood of the blue samples is indistinguishable, the red AGN environs show an excess of blue-star-forming galaxies with respect to their non-active counterpart. On the other hand, the active and non-active blue systems have similar environments but markedly different morphological distributions, showing an excess of blue early-type AGN, which are argued to be late-stage mergers. This comparison reveals that the observable differences between active red and blue host galaxy properties including star formation history and AGN activity depends on the environment within which the galaxies form and evolve. 相似文献
The properties of the neighbouring galaxies of blue and red AGN and non active galaxies reflect this effect. While the neighbourhood of the blue samples is indistinguishable, the red AGN environs show an excess of blue-star-forming galaxies with respect to their non-active counterpart. On the other hand, the active and non-active blue systems have similar environments but markedly different morphological distributions, showing an excess of blue early-type AGN, which are argued to be late-stage mergers. This comparison reveals that the observable differences between active red and blue host galaxy properties including star formation history and AGN activity depends on the environment within which the galaxies form and evolve. 相似文献
19.
Jacqueline D’Souza Annamma Mathew R. Rajamohan 《Journal of Astrophysics and Astronomy》1992,13(1):109-128
The effect of rotation on the observed colours of stars has been considered as a possible cause for the blue straggler phenomenon
in clusters listed by Mermilliod (1982). It appears that this phenomenon is definitely not real in the case of the late B
and early A spectral type blue stragglers that are intrinsic slow rotators. Among clusters containing the early B type blue
stragglers it is found that the anomalous position of the stragglers in NGC 6633, NGC 6475 and NGC 2516 cannot be accounted
for by rotation effects alone.
on leave from Assumption College, Changanacherry, Kerala. 相似文献
20.
A. Akhperjanian R. Kankanian V. Sahakian A. Heusler C.-A. Wiedner H. Wirth 《Experimental Astronomy》1998,8(2):135-152
The raytracing technique was used to derive a suitable design for the HEGRA system of Cherenkov telescopes, which is at present commissioned at La Palma. The reflectors with a diameter of 3.9 m consist of 30 spherical mirrors with focal lengths in the range of 4.88 – 4.94 m. It is shown that 93% of the photons from the Cherenkov light emitted by an extended air shower are contained in the camera pixels, 0.25° in diameter, for the full field of view of = ± 2.5°. The optical performance of the HEGRA design is compared to other layouts. 相似文献