First high dynamic range and high resolution images of the sky obtained with a diffractive Fresnel array telescope     

Laurent Koechlin  Jean-Pierre Rivet  Paul Deba  Denis Serre  Truswin Raksasataya  René Gili  Jules David 《Experimental Astronomy》2012,33(1):129-140

This paper presents high contrast images of sky sources, obtained from the ground with a novel optical concept: Fresnel arrays. We demonstrate the efficiency of a small 20?cm prototype Fresnel array for making images with high brightness ratios, achieving contrasts up to 4 × 10⁵ on sky sources such as Mars and its satellites, and the Sirius?A?CB couple. These validation results are promising for future applications in space, for example the 4 m array we have proposed to ESA in the frame of the ??Call for a Medium-size mission opportunity for a launch in 2022??. Fresnel imagers are the subject of a topical issue of Experimental Astronomy published in 2011, but only preliminary results were presented at the time. Making images of astronomical bodies requires an optical component to focus light. This component is usually a mirror or a lens, the quality of which is critical for sharp and high contrast images. However, reflection on a mirror and refraction through a lens are not the only ways to focus light: an alternative is provided by diffraction through binary masks (opaque foils with multiple precisely etched sub-apertures). Our Fresnel arrays are such diffractive focusers, they offer weight, price and size advantages over traditional optics in space-based astronomical instruments. This novel approach requires only void apertures of special shapes in an opaque material to form sharp images, thus avoiding the wavefront distortion, diffusion and spectral absorption associated with traditional optical media. In our setup, lenses and/or mirrors are involved only downstream (at small sizes) for focal instrumentation and chromatic correction. Fresnel arrays produce high contrast images, the resolution of which reaches the theoretical limit of diffraction. Unlike mirrors, they do not require high precision polishing or positioning, and can be used in a large domain of wavelengths from far IR to far UV, enabling the study of many science cases in astrophysics from exoplanet surfaces and atmospheres to galaxy evolution.  相似文献   

Long-Term Changes in Population Dynamics of the Shrimp Palaemon longirostris in the Gironde Estuary     

Mélanie Béguer  Julien Bergé  Caroline Gardia-Parège  Laurent Beaulaton  Gérard Castelnaud  Michel Girardin  Philippe Bo?t 《Estuaries and Coasts》2012,35(4):1082-1099

Spanning 20?years (1979?C2007), this study is the longest time series pertaining to the resident shrimp species Palaemon longirostris in a European estuary. Data from monthly faunal surveys undertaken across the middle part of the Gironde estuary from April 1979 along with data from a statistical analysis of the commercial catches throughout the entire estuary and river were considered in order to explore their inter and intra-annual variability and long-term trends. Long-term densities series as well as environmental series (salinity, discharge, temperature and NAO) were decomposed and the effects of environmental variables were also examined using statistical models (GAMs). This revealed important spatio-temporal variability and a significant overall decrease in abundance of this species in the Gironde estuary since the beginning of the 1980s. This long-term decrease in abundance corresponded significantly to long-term decreases in both discharge and the NAO index, as well a long-term increase in temperature and salinity in the middle part of the estuary. However, models showed that environmental factors explained only a small part of the variability. The upstream shift of the population highlighted in this study, probably due to the intrusion of marine waters into the middle section of the estuary, may also have contributed to its decrease in abundance. Inter-annual variability of densities was also significantly linked with inter-annual fecundity fluctuations, and a significant decrease in both mean female size and fecundity was shown for preserved samples from 1992. Moreover, the breeding period has been temporally stretched out and began earlier in more recent years, potentially due to the increase in spring temperature.  相似文献   

Silver and lead in high-altitude lake sediments: Proxies for climate changes and human activities     

Marion Garçon  Catherine Chauvel  Emmanuel Chapron  Xavier Faïn  Mingfang Lin  Sylvain Campillo  Sarah Bureau  Marc Desmet  Marie-Christine Bailly-Maître  Laurent Charlet 《Applied Geochemistry》2012

High-altitude lake sediments are often used as archives for environmental changes and their chemical and isotopic compositions provide significant constraints on natural and anthropogenic long-term changes that have occurred in their catchment area. Here, trace-element concentrations and Pb isotopes are presented for two sedimentary cores from Lake Blanc Huez in the French Alps, to trace the impact of climate changes and human activities over the Holocene. Lead and Ag contents are very high and clearly dominated by input from a Pb–Ag vein located a few meters from the lakeshore, a vein that also buffers the Pb isotopes. Mining of this vein in medieval times is recorded in the corresponding lake sediments with high Ag content coupled with high Pb/U ratio. These chemical characteristics can be used to constrain the major Holocene climate changes. Significant advances of glaciers next to the lake produced sediments with Ag and Pb concentration peaks and high Pb/U ratios due to accelerated erosion of the Pb–Ag vein, similar to the effects of the medieval mining. In contrast, reduced glacier activity led to the formation of organic-rich sediments with high U and As contents and low Pb/U ratios. More generally, the observed combination of chemical changes could be used elsewhere to decipher environmental changes over long periods of time.  相似文献   

Security and party politics in Cape Town     

Laurent Fourchard 《Geoforum》2012,43(2):199-206

Despite a long academic debate on the patrimonial dimension of the state in Africa and a more recent interest in African political parties, the effect of patronage and party politics on governability in Africa’s cities remains poorly addressed in the academic literature. This includes the case in South Africa when one looks at the security sector, which to a certain extent, looks like a depoliticised field of expertise. Popular claims for security seem to be a side issue in the literature on social movements, while vigilante specialists and policing experts do not place party politics at the core of security issue challenges, especially in poor townships. The provision of security in poor neighbourhoods is an important resource in the struggle for political support however. This is examined through two case studies in Cape Town Coloured townships, considering the role played by political leaders, NGO leaders and key officials in grassroots mobilisations for security. These mobilisations are not only about politicking however; ‘ordinary members’ of local security organisations also get involved for motivations, which have nothing to do with confrontational party politics. These different agendas between ordinary members and local leaders cannot be read as the manifestation of a fundamental opposition between the popular classes and a westernised elite as suggested by Charterjee. It reveals instead prevalent and ambivalent relationships between partisan politics and popular mobilisations for security in a context of high insecurity.  相似文献   

Stratigraphy and Palaeopedology of the Palaeolithic Cave Site of Combe‐Saunière,Southwest France     

Aurlie Ajas  Pascal Bertran  Laurent Leme  Alain Queffelec 《Geoarchaeology》2013,28(5):432-449

The sequence of Combe‐Saunière (southwest France) has been studied with a large suite of methods, including micromorphology, geochemistry, magnetic susceptibility, and organic matter (OM) analysis in order to document the post‐depositional processes that occurred during the last climatic cycle at the cave entrance. This study shows that intense weathering together with the accumulation of OM and phosphates by cave animals and others vectors (including humans) occurred during the phases of low clastic sedimentation. During the latter part of Marine Isotope Stage (MIS) 3, the Aurignacian and Gravettian humus‐bearing levels, which may be classified as cryoturbated leptosols, show incomplete decalcification and are dominated by poorly evolved forms of phosphates (bone fragments, coprolites). Phosphatized cambic B‐horizons formed at the beginning of MIS 3 and the Early Glacial and may reflect longer periods of pedological modification. The molecules identified by the analysis of the OM extracted from the Châtelperronian level derive mostly from the degradation of vegetal matter, most notably from conifers and mosses, in addition to bacteria and fungi. The Last Interglacial s.l. (MIS 5) is typified by the development of a phosphatized illuviated B‐horizon. Intense weathering together with phosphate leaching and precipitation took place in this soil, forming thick apatite–CaOH crusts. Although the poor chronological resolution prevents us from drawing clear conclusions as to the respective roles of the factors involved, phosphate accumulation and weathering seem to be controlled primarily by the duration of the soil‐forming periods. This study strongly suggests that a climatic signal has been recorded in the deposits.  相似文献   

Contraintes,déformation et déplacement dans Г avant-pays Nord-pyrénéen du Languedoc méditerranéen     

François Arthaud  Philippe Laurent 《Geodinamica Acta》2013,26(3):142-157

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Structure and Evolution of Supergranulation from Local Helioseismology     

Johann Hirzberger  Laurent Gizon  Sami K. Solanki  Thomas L. Duvall Jr. 《Solar physics》2008,251(1-2):417-437

Supergranulation is visible at the solar surface as a cellular pattern of horizontal outflows. Although it does not show a distinct intensity pattern, it manifests itself indirectly in, for example, the chromospheric network. Previous studies have reported significant differences in the inferred basic parameters of the supergranulation phenomenon. Here we study the structure and temporal evolution of a large sample of supergranules, measured by using local helioseismology and SOHO/MDI data from the year 2000 at solar activity minimum. Local helioseismology with f modes provides maps of the horizontal divergence of the flow velocity at a depth of about 1 Mm. From these divergence maps supergranular cells were identified by using Fourier segmentation procedures in two dimensions and in three dimensions (two spatial dimensions plus time). The maps that we analyzed contain more than 10⁵ supergranular cells and more than 10³ lifetime histories, which makes possible a detailed analysis with high statistical significance. We find that the supergranular cells have a mean diameter of 27.1 Mm. The mean lifetime is estimated to be 1.6 days from the measured distribution of lifetimes (three-dimensional segmentation), with a clear tendency for larger cells to live longer than smaller ones. The pair and mark correlation functions do not show pronounced features on scales larger than the typical cell size, which suggests purely random cell positions. The temporal histories of supergranular cells indicate a smooth evolution from their emergence and growth in the first half of their lives to their decay in the second half of their lives (unlike exploding granules, which reach their maximum size just before they fragment).  相似文献   

Qualitative and Quantitative Description of Multibeam Echosounder Systematic Errors on Rocky Areas     

Nathalie Debese  Nicolas Seube  Laurent Heydel 《Marine Geodesy》2013,36(3):306-321

This paper deals with the problem of systematic depth errors made in surveying dumped rocks with multibeam echosounder. These errors may induce dangers for navigation in very shallow water areas or huge costs for coastal engineering contractors who perform rock dumping operation and surveying. We analyze results from four different multibeam echosounder systems and compare those data to a reference digital terrain model of a dumped rocks area obtained from a 3D terrestrial laser scanner. The systematic depth errors are statistically described and analysed on local areas. Finally, we propose a look-up table linking the error amplitude with the rock size, resulting from our error analysis.  相似文献   

Indirect and direct recharges in a tropical forested watershed: Mule Hole,India     

Jean-Christophe Maréchal  Murari R.R. Varma  Jean Riotte  Jean-Michel Vouillamoz  M.S. Mohan Kumar  Laurent Ruiz  M. Sekhar  Jean-Jacques Braun 《Journal of Hydrology》2009,364(3-4):272-284

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Mobility of arsenic in the sub-surface environment: An integrated hydrogeochemical study and sorption model of the sandy aquifer materials     

Bibhash Nath  Sudipta Chakraborty  André Burnol  Doris Stüben  Debashis Chatterjee  Laurent Charlet 《Journal of Hydrology》2009,364(3-4):236-248

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