11,696 research outputs found
Step energies and equilibrium shape of strained monolayer islands
Using a simple atomistic model of anharmonic nearest-neighbors interaction,
we have calculated the step energies of strained hexagonal monolayer islands.
These have been found to decrease with the absolute value of the misfit due to
the strain relaxation at steps. The effect is significantly more pronounced in
the case of positive misfit owing to the stronger repulsive interatomic forces.
Furthermore, (111)-faceted steps are favored at positive misfit (compressed
islands) and, to a lesser extent, (100)-faceted steps at negative misfits
(tensile islands). The result is rationalized in terms of the different bonding
geometries at step edges and a comparison with experiments is included. Thus,
the equilibrium shape transforms from regular hexagons at zero misfit to
threefold symmetric hexagons with increasing misfit.Comment: 6 pages, 5 figures, 1 table. Improved, finally accepted version
including a new figure, a new table and several minor modifications resulting
from discussions with referee
Modeling a Slicer Mirror Using Zemax User-Defined Surface
A slicer mirror is a complex surface composed by many tilted and decentered
mirrors sub-surfaces. The major difficulty to model such a complex surface is
the large number of parameters used to define it. The Zemax's
multi-configuration mode is usually used to specify each parameters (tilts,
curvatures, decenters) for each mirror sub-surface which are then considered
independently. Otherwise making use of the User-Defined Surface (UDS-DLL) Zemax
capability, we are able to consider the set of sub-surfaces as a whole surface.
In this paper, we present such a UDS-DLL tool comparing its performance with
those of the classical multi-configuration mode. In particular, we explore the
use of UDS-DLL to investigate the cross-talk due to the diffraction on the
slicer array mirrors which has been a burden task when using
multi-configuration mode.Comment: Submitted to the proceedings of the Durham Integral Field
Spectroscopy Workshop July 4th-8th 200
Modal logic S4 as a paraconsistent logic with a topological semantics
In this paper the propositional logic LTop is introduced, as an extension of classical propositional logic by adding a paraconsistent negation. This logic has a very natural interpretation in terms of topological models. The logic LTop is nothing more than an alternative presentation of modal logic S4, but in the language of a paraconsistent logic. Moreover, LTop is a logic of formal inconsistency in which the consistency and inconsistency operators have a nice topological interpretation. This constitutes a new proof of S4 as being "the logic of topological spaces", but now under the perspective of paraconsistency
Image Slicer Performances from a Demonstrator for the SNAP/JDEM Mission - Part I: Wavelength Accuracy
A well-adapted visible and infrared spectrograph has been developed for the
SNAP (SuperNova/Acceleration Probe) experiment proposed for JDEM. The
instrument should have a high sensitivity to see faint supernovae but also a
good redshift determination better than 0.003(1+z) and a precise
spectrophotometry (2%). An instrument based on an integral field method with
the powerful concept of imager slicing has been designed. A large prototyping
effort has been performed in France which validates the concept. In particular
a demonstrator reproducing the full optical configuration has been built and
tested to prove the optical performances both in the visible and in the near
infrared range. This paper is the first of two papers. The present paper focus
on the wavelength measurement while the second one will present the
spectrophotometric performances. We adress here the spectral accuracy expected
both in the visible and in the near infrared range in such configuration and we
demonstrate, in particular, that the image slicer enhances the instrumental
performances in the spectral measurement precision by removing the slit effect.
This work is supported in France by CNRS/INSU/IN2P3 and by the French spatial
agency (CNES) and in US by the University of California.Comment: Submitted to PAS
Seeing, Wind and Outer Scale Effects on Image Quality at the Magellan Telescopes
We present an analysis of the science image quality obtained on the twin 6.5
metre Magellan telescopes over a 1.5 year period, using images of ~10^5 stars.
We find that the telescopes generally obtain significantly better image quality
than the DIMM-measured seeing. This is qualitatively consistent with
expectations for large telescopes, where the wavefront outer scale of the
turbulence spectrum plays a significant role. However, the dominant effect is
found to be wind speed with Magellan outperforming the DIMMs most markedly when
the wind is strongest. Excluding data taken during strong wind conditions (>10
m/s), we find that the Magellan telescopes still significantly outperform the
DIMM seeing, and we estimate the site to have L_0 ~ 25 m on average. We also
report on the first detection of a negative bias in DIMM data. This is found to
occur, as predicted, when the DIMM is affected by certain optical aberrations
and the turbulence profile is dominated by the upper layers of the atmosphere.Comment: Accepted for publication in PASP. 10 pages, 12 figures
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