Adaptive optics at short wavelengths     

Guido Agapito  Carmelo Arcidiacono  F. Quirós-Pacheco  Simone Esposito 《Experimental Astronomy》2014,37(3):503-523

The First Light Adaptive Optics (FLAO) system has been successfully commissioned at the Large Binocular Telescope. It delivers extreme adaptive optics performance using bright natural guide stars reaching 90 % Strehl Ratios in H-band. Observations with current adaptive optics systems are limited to the near infrared wavelengths, in these bands the diffraction limited resolution of the largest ground-based telescopes (8–10 meter class) is comparable to the one of the much smaller Hubble Space Telescope that observes in the visible bands. This study aims to demonstrate the feasibility of an adaptive optics system designed to achieve very high order correction at visible wavelengths (0.5 to 0.8 μ m) with significant sky coverage. Upgrading the FLAO design with a low noise CCD relaxes the reference magnitude limit needed to achieve greater performance. In particular, we demonstrate that a gain of 1–2 magnitudes is possible by upgrading the wavefront sensor with a very low read out noise CCD. For future AO systems, in addition to low noise CCDs, deformable (secondary) mirrors with a higher actuator density will be able to move the high order correction capability from the near infrared to the visible wavelengths (Strehl Ratio of 80 % in R (0.7 μ m), 60 % in V (0.5 μ m)). We investigate, by means of numerical simulation, the gain in imaging performance obtained at Near Infrared, Visible, and UV wavelengths. The results of these simulations have been used to derive the empirical relation between Strehl Ratio and magnitude of the reference star and we then use this relationship to perform a detailed sky coverage analysis based on astronomical catalog data. The detailed simulations of the Point Spread Functions allow us to compute Ensquared Energy and Strehl Ratio for the magnitude working range of such an Adaptive Optics system. We present the results of the instrumental isoplanatic angle determination. We then used these values to compute the relationship between correction level and the off-axis angle from the reference star. The Strehl Ratio relationship with the reference magnitude and the angular distance provides the information needed to perform the sky-coverage analysis, which demonstrates that the designed system is able to provide V and R bands correction on a not negligible few percent of the sky.  相似文献   

High Resolution Simulation of the Variability of Surface Energy Balance Fluxes Across Central London with Urban Zones for Energy Partitioning     

Thomas Loridan  Fredrik Lindberg  Oriol Jorba  Simone Kotthaus  Susanne Grossman-Clarke  C. S. B. Grimmond 《Boundary-Layer Meteorology》2013,147(3):493-523

The parameterization of surface heat-flux variability in urban areas relies on adequate representation of surface characteristics. Given the horizontal resolutions (e.g. $\approx $ 0.1–1 km) currently used in numerical weather prediction (NWP) models, properties of the urban surface (e.g. vegetated/built surfaces, street-canyon geometries) often have large spatial variability. Here, a new approach based on Urban Zones to characterize Energy partitioning (UZE) is tested within a NWP model (Weather Research and Forecasting model; WRF v3.2.1) for Greater London. The urban land-surface scheme is the Noah/Single-Layer Urban Canopy Model (SLUCM). Detailed surface information (horizontal resolution 1 km) in central London shows that the UZE offers better characterization of surface properties and their variability compared to default WRF-SLUCM input parameters. In situ observations of the surface energy fluxes and near-surface meteorological variables are used to select the radiation and turbulence parameterization schemes and to evaluate the land-surface scheme and choice of surface parameters. For radiative fluxes, improved performance (e.g. $>$ 25 W m $^{-2}$ root-mean-square error reduction for the net radiation) is attained with UZE parameters compared to the WRF v3.2.1 default for all three methods from the simplest to the most detailed. The UZE-based spatial fluxes reproduce a priori expectations of greater energy storage and less evaporation in the dense city centre compared to the residential surroundings. Problems in Noah/SLUCM partitioning of energy between the daytime turbulent fluxes are identified with the overestimation of the turbulent sensible heat and underestimation of the turbulent latent heat fluxes.  相似文献   

Properties of galaxy groups in the Sloan Digital Sky Survey – I. The dependence of colour, star formation and morphology on halo mass     

Simone M. Weinmann  Frank C. van den Bosch  Xiaohu Yang  H. J. Mo 《Monthly notices of the Royal Astronomical Society》2006,366(1):2-28

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The stellar velocity distribution in the solar neighbourhood     

Richard S. De Simone  Xiaoan Wu †  Scott Tremaine 《Monthly notices of the Royal Astronomical Society》2004,350(2):627-643

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The ineligible majority: Urbanizing the postcolony in Africa and Southeast Asia     

AbdouMaliq Simone 《Geoforum》2011,42(3):266-270

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Are metal-induced hypersensitivities in harbor seals associated with liver function?     

Kakuschke A  Valentine-Thon E  Griesel S  Fonfara S  Siebert U  Prange A 《Marine pollution bulletin》2011,62(8):1891-1894

Environmental exposure to metals is believed to affect marine mammal health adversely including immunosuppression or acute as well as chronic inflammatory processes leading to hypersensitivities or autoimmune diseases. Metal-specific hypersensitivities were found in several pinnipeds of the North Sea. However, hypersensitivity is a complex phenomenon whose characteristics are still not completely understood; in particular, effects on health are not well established. In the present study, we compared basic hematological and biochemical parameters of seals with and without metal-specific hypersensitivities. We found altered hematological parameters and liver enzyme patterns in seals with a metal-induced hypersensitivity, including a reduction in macrophages, an increase in lymphocytes, and elevated levels of lactate dehydrogenase. These findings support the suggestion of a chronic influence of metal pollutants on the health of marine mammals of the North Sea.  相似文献   

Shifting sands on Mars: insights from tropical intra‐crater dunes     

Paul E. Geissler  Nicholas W. Stantzos  Nathan T. Bridges  Mary C. Bourke  Simone Silvestro  Lori K. Fenton 《地球表面变化过程与地形》2013,38(4):407-412

Evidence for sand motion is found in repeated observations of sand dunes at three sites in the Martian tropics by the High Resolution Imaging Science Experiment on Mars Reconnaissance Orbiter. An eroding outcrop of layered sediments is identified as a possible source of the sand in Pasteur crater. Ancient layered sediments in Becquerel crater are actively being carved into flutes and yardangs by the blowing sands. Dunes in an un‐named crater in Meridiani near the Mars Exploration Rover Opportunity landing site advanced as much as 50 cm over an interval of one Martian year. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

A Comparison of Data‐Driven Groundwater Vulnerability Assessment Methods     

Alessandro Sorichetta  Cristiano Ballabio  Marco Masetti  Gilpin R. Robinson Jr.  Simone Sterlacchini 《Ground water》2013,51(6):866-879

Increasing availability of geo‐environmental data has promoted the use of statistical methods to assess groundwater vulnerability. Nitrate is a widespread anthropogenic contaminant in groundwater and its occurrence can be used to identify aquifer settings vulnerable to contamination. In this study, multivariate Weights of Evidence (WofE) and Logistic Regression (LR) methods, where the response variable is binary, were used to evaluate the role and importance of a number of explanatory variables associated with nitrate sources and occurrence in groundwater in the Milan District (central part of the Po Plain, Italy). The results of these models have been used to map the spatial variation of groundwater vulnerability to nitrate in the region, and we compare the similarities and differences of their spatial patterns and associated explanatory variables. We modify the standard WofE method used in previous groundwater vulnerability studies to a form analogous to that used in LR; this provides a framework to compare the results of both models and reduces the effect of sampling bias on the results of the standard WofE model. In addition, a nonlinear Generalized Additive Model has been used to extend the LR analysis. Both approaches improved discrimination of the standard WofE and LR models, as measured by the c‐statistic. Groundwater vulnerability probability outputs, based on rank‐order classification of the respective model results, were similar in spatial patterns and identified similar strong explanatory variables associated with nitrate source (population density as a proxy for sewage systems and septic sources) and nitrate occurrence (groundwater depth).  相似文献   

Preface to the special issue ��Triggered and induced seismicity: probabilities and discrimination��     

Simone Cesca  Bernard Dost  Adrien Oth 《Journal of Seismology》2013,17(1):1-4

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Dune fields on Mars: Recorders of a climate change?     

Emilie Gardin  Pascal Allemand  Cathy Quantin  Simone Silvestro  Christophe Delacourt 《Planetary and Space Science》2012,60(1):314-321

Dunes have similar morphologies on the Earth and Mars. The main differences between Martian and terrestrial dunes are their size, which is larger on Mars, and their duration of formation, which is longer on Mars. As the characteristic time of Martian dunes is in the same order as that of the Martian climatic oscillations, Martian dunes could be recorders of past winds regimes and past climates. In order to test this hypothesis, we performed a morphological study of 550 dune fields with high resolution images and we inferred the directions of the dune formative winds from the orientation of the dune slip faces. Our study shows that 310 dune fields record one to four distinct wind directions with some geometric patterns that do not exist on the Earth such as barchans built by opposite wind directions coexisting in the same dune field. Our study demonstrates that the inferred formative wind directions are only partially in agreement with the current wind-patterns predicted by General Circulation Models (GCM). Several possible causes for the misalignment between dunes and GCM outputs are discussed: these include the local variation of the global circulation due to local topographic effects or the possibility that these dunes could be in a transient geometry or fossil. Such bedforms are considered indeed to be not in equilibrium with the present-day atmospheric conditions. This latter hypothesis is supported by the presence, in some ergs, of closely spaced dunes showing nearly opposite slip face orientations. Therefore, we propose that Martian dune fields are constituted, in some cases, by active and fossil dunes and therefore have the potential to preserve information on paleoclimates over extensive periods.  相似文献