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721.
We report laboratory and telescopic observations with a polarimetric spectral imager based on an acousto-optic tunable filter (AOTF) where we demonstrate simultaneous acquisition of orthogonally polarized images at a spectral resolving power on the order of 103 and at a Rayleigh criterion spatial resolution of 100 line pairs per mm. This matches the spatial resolution of most digital arrays. An AOTF is shown to be usable as a fast shutter with a contrast of over 104 on a sub-millisecond time scale while providing an high transmittance of about 75% (both polarizations summed) when illuminated by white light. The polarization contrast between the orthogonally polarized images exceeds 104 across the field of view, permitting accurate measurement of the polarization parameters of incident light. These characteristics are now being used in a program of high spatial resolution imaging in narrow spectral bands. Some peculiarities and limitations of AOTFs are indicated.This research was supported by NASA under Grant NAGW-122 and under contract with the Jet Propulsion Laboratory under NASA grant NAS 7-918.Visiting Astronomer, Mauna Kea Observatory. 相似文献
722.
A. Glindemann J. Algomedo R. Amestica P. Ballester B. Bauvir E. Bugueño S. Correia F. Delgado F. Delplancke F. Derie Ph. Duhoux E. di Folco A. Gennai B. Gilli P. Giordano Ph. Gitton S. Guisard N. Housen A. Huxley P. Kervella M. Kiekebusch B. Koehler S. Lévêque A. Longinotti S. Ménardi S. Morel F. Paresce T. Phan Duc A. Richichi M. Schöller M. Tarenghi A. Wallander M. Wittkowski R. Wilhelm 《Astrophysics and Space Science》2003,286(1-2):35-44
The Very Large Telescope (VLT) Observatory on Cerro Paranal (2635 m) in Northern Chile is approaching completion. After the
four 8-m Unit Telescopes (UT) individually saw first light in the last years, two of them were combined for the first time
on October 30, 2001 to form a stellar interferometer, the VLT Interferometer. The combination in pairs of all four UTs was
completed in September 2002. In this article, we will describe the subsystems of the VLTI and the planning for the following
years.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
723.
724.
Stefan J. Wijnholds A. Ger De Bruyn Jaap D. Bregman Jan Geralt Bij De Vaate 《Experimental Astronomy》2004,17(1-3):59-64
The thousand element array (THEA) system is a phased array system consisting of 1 m2 tiles having 64 Vivaldi elements each, arranged on a regular 8-by-8 grid, which has been developed as a demonstrator of technology and applicability for SKA. In this paper we present imaging results of Galactic neutral hydrogen with THEA. Measurements have been taken using a dense 2-by-2 array of four tiles as a four tile adder. The results are compared with results from the Leiden-Dwingeloo Survey, showing qualitative agreement, but also indicating that further studies are needed on the instrumental characteristics. 相似文献
725.
726.
J.R. Barnes A. Collier Cameron D.J. James D. Steeghs 《Monthly notices of the Royal Astronomical Society》2001,326(3):1057-1066
We present two images of intermediate and low axial inclination G dwarfs (AP 149 and AP 193) in the young open cluster α Persei, and compare these with previous images of intermediate and high axial inclination objects in this cluster. All stars show starspots at high latitudes, with one star exhibiting a strong polar spot. Although low-latitude features are found on all stars to some degree, the detection of spots on AP 193 is marginal. The apparent difference in starspot morphology from one object to the next is probably the result of a stellar magnetic cycle, although the exact effect on the starspot distribution throughout a cycle is unknown.
Polar spots are found in many Doppler images of rapidly rotating cool stars. In the past, their existence has been called into question, and it has been suggested that they could be the manifestations of NLTE (e.g. chromospheric filling in of line profiles) effects rather than real photospheric features. We assume the polar spots to be real photospheric features, and conclude that the flat-bottomed nature of the profile shape can be attributed to photospheric polar spots. The degree of truncation of the profile depends not only on spot size and strength, but also on the effective foreshortening of the polar region, a function of axial inclination.
H α is in emission on AP 149 which shows a double peak at most phases. The time-series of the profile shows an s-wave pattern as the position of these peaks changes throughout the rotation cycle. We attribute this to coronal clouds located above the stellar surface in synchronous orbit. A maximum-entropy tomogram is derived revealing four distinct emission regions located near and above the corotation radius. 相似文献
Polar spots are found in many Doppler images of rapidly rotating cool stars. In the past, their existence has been called into question, and it has been suggested that they could be the manifestations of NLTE (e.g. chromospheric filling in of line profiles) effects rather than real photospheric features. We assume the polar spots to be real photospheric features, and conclude that the flat-bottomed nature of the profile shape can be attributed to photospheric polar spots. The degree of truncation of the profile depends not only on spot size and strength, but also on the effective foreshortening of the polar region, a function of axial inclination.
H α is in emission on AP 149 which shows a double peak at most phases. The time-series of the profile shows an s-wave pattern as the position of these peaks changes throughout the rotation cycle. We attribute this to coronal clouds located above the stellar surface in synchronous orbit. A maximum-entropy tomogram is derived revealing four distinct emission regions located near and above the corotation radius. 相似文献
727.
This work is intended to investigate the influence of temperature-dependent metamorphism of ice on the shape of small depressions in the surface of cometary nuclei. We are mainly interested in the role of initial cohesivity of a nucleus. For this purpose we simulate sublimation of ice from the facets of initially cylindrical depressions in ice of different initial structure. The simulations account for the diurnal and orbital changes of insolation and its dependence on the current shape of the depressions. Our model includes heat transport in the cometary material and metamorphism of ice. We present the results obtained for the nucleus of the Comet 67P/Churyumov-Gerasimenko, target of the ESA cornerstone mission Rosetta. 相似文献
728.
Arecibo radar observations of Comet P/2005 JQ5 (Catalina) have produced the first delay-Doppler images of a comet nucleus and the first radar detection of large-grain ejection from a Jupiter-family comet. The nucleus is small (1.4 km diameter), rough, and rapidly rotating. The large (>cm) grains have low velocities (∼1 m/s) and a low production rate. 相似文献
729.
730.