Modeling low order aberrations in laser guide star adaptive optics systems

When using a laser guide star (LGS) adaptive optics (AO) system, quasi-static aberrations are observed between the measured wavefronts from the LGS wavefront sensor (WFS) and the natural guide star (NGS) WFS. These LGS aberrations, which can be as much as 1200 nm RMS on the Keck II LGS AO system, arise due to the finite height and structure of the sodium layer. The LGS aberrations vary significantly between nights due to the difference in sodium structure. In this paper, we successfully model these LGS aberrations for the Keck II LGS AO system. We use this model to characterize the LGS aberrations as a function of pupil angle, elevation, sodium structure, uplink tip/tilt error, detector field of view, the number of detector pixels, and seeing. We also employ the model to estimate the LGS aberrations for the Palomar LGS AO system, the planned Keck I and the Thirty Meter Telescope (TMT) LGS AO systems. The LGS aberrations increase with increasing telescope diameter, but are reduced by central projection of the laser compared to side projection

Cross-Border Mergers and acquisitions: Financial and institutional forces

Cross-border mergers and acquisitions (M&As) sharply increased over the last two decades. It is often pointed out that cross-border capital reallocation is partly the result of financial liberalization policies, government policies and regional agreements. In this paper, we identify some of the main forces driving cross-border M&As using a unique database on bilateral cross-border M&As at the sectoral level (in manufacturing and services) over the period 1985-2004. We focus on the role of institutional and financial developments with a special attention to the role played by the European Integration process. We identify the impact of (i) joining the European Union and (ii) joining the Euro on cross-border M&As. We show that EU and EMU have almost doubled M&As in manufacturing towards their members from all over the globe, with an additional 50% increase within EMU countries. Conversely, the service sector did not exploit the opportunity offered by the single currency. We also show how cross-border M&As are linked to the acquirer expected profitability and provide insights on the effectiveness of policies to attract foreign capital (such as corporate tax incentives, and interventions to improve the country's financial system and product market regulations). JEL Classification: F30, F36, F41, G11Cross Mergers and Acquisitions, Euro, Gravity Equation

Quasi-static aberrations induced by laser guide stars in adaptive optics

Laser Guide Star Adaptive Optics (LGS AO) systems use the return from an artificial guide star to measure the wavefront aberrations in the direction of the science object. We observe quasi-static differences between the measured wavefront and the wavefront aberration of the science object. This paper quantifies and explains the source of the difference between the wavefronts measured using an LGS and a natural guide star at the W. M. Keck Observatory, which can be as high as 1000 nm RMS

Gymnopus Beltraniae, a new species of section Vestipedes (Agaricales) from the Canary Islands (Spain)

Gymnopus beltraniae, collected on the Canary Islands in 'monteverde'forest as well as mixed monteverde /Canary pine forest, is proposed as a new species belonging to sect. Vestipedes subsect. Vestipedes. It is compared with species from North and South America considered to represent the closest taxa in its section

Gymnopus Beltraniae, a new species of section Vestipedes (Agaricales) from the Canary Islands (Spain)

Gymnopus beltraniae, collected on the Canary Islands in 'monteverde'forest as well as mixed monteverde /Canary pine forest, is proposed as a new species belonging to sect. Vestipedes subsect. Vestipedes. It is compared with species from North and South America considered to represent the closest taxa in its section

Revisiting energy release rates in brittle fracture

International audienceWe revisit in a 2d setting the notion of energy release rate, which plays a pivotal role in brittle fracture. Through a blow-up method, we extend that notion to crack patterns which are merely closed sets connected to the crack tip. As an application, we demonstrate that, modulo a simple meta-stability principle, a moving crack cannot generically kink while growing continuously in time. This last result potentially renders obsolete in our opinion a longstanding debate in fracture mechanics on the correct criterion for kinking