2,024 research outputs found
On the computation of directional scale-discretized wavelet transforms on the sphere
We review scale-discretized wavelets on the sphere, which are directional and
allow one to probe oriented structure in data defined on the sphere.
Furthermore, scale-discretized wavelets allow in practice the exact synthesis
of a signal from its wavelet coefficients. We present exact and efficient
algorithms to compute the scale-discretized wavelet transform of band-limited
signals on the sphere. These algorithms are implemented in the publicly
available S2DW code. We release a new version of S2DW that is parallelized and
contains additional code optimizations. Note that scale-discretized wavelets
can be viewed as a directional generalization of needlets. Finally, we outline
future improvements to the algorithms presented, which can be achieved by
exploiting a new sampling theorem on the sphere developed recently by some of
the authors.Comment: 13 pages, 3 figures, Proceedings of Wavelets and Sparsity XV, SPIE
Optics and Photonics 2013, Code is publicly available at http://www.s2dw.org
Nanoscale Magnetic Imaging using Circularly Polarized High-Harmonic Radiation
This work demonstrates nanoscale magnetic imaging using bright circularly
polarized high-harmonic radiation. We utilize the magneto-optical contrast of
worm-like magnetic domains in a Co/Pd multilayer structure, obtaining
quantitative amplitude and phase maps by lensless imaging. A
diffraction-limited spatial resolution of 49 nm is achieved with iterative
phase reconstruction enhanced by a holographic mask. Harnessing the unique
coherence of high harmonics, this approach will facilitate quantitative,
element-specific and spatially-resolved studies of ultrafast magnetization
dynamics, advancing both fundamental and applied aspects of nanoscale
magnetism.Comment: Ofer Kfir and Sergey Zayko contributed equally to this work.
Presented in CLEO 2017 (Oral) doi.org/10.1364/CLEO_QELS.2017.FW1H.
Colloidal dynamics in polymer solutions: Optical two-point microrheology measurements
We present an extension of the two-point optical microrheology technique
introduced by Crocker \textit{et al.} [Phys. Rev. Lett. \textbf{85}, 888
(2000)] to high frequencies. The correlated fluctuations of two probe spheres
held by a pair of optical tweezers within a viscoelastic medium are determined
using optical interferometry. A theoretical model is developed to yield the
frequency-dependent one- and two-particle response functions from the
correlated motion. We demonstrate the validity of this method by determining
the one- and two-particle correlations in a semi-dilute solution of polystyrene
in decalin. We find that the ratio of the one- and two-particle response
functions is anomalous which we interpret as evidence for a slip boundary
condition caused by depletion of polymer from the surface of the particle
Second-Generation Curvelets on the Sphere
Curvelets are efficient to represent highly anisotropic signal content, such as a local linear and curvilinear structure. First-generation curvelets on the sphere, however, suffered from blocking artefacts. We present a new second-generation curvelet transform, where scale-discretized curvelets are constructed directly on the sphere. Scale-discretized curvelets exhibit a parabolic scaling relation, are well localized in both spatial and harmonic domains, support the exact analysis and synthesis of both scalar and spin signals, and are free of blocking artefacts. We present fast algorithms to compute the exact curvelet transform, reducing computational complexity from O(L5) to O(L3 log2 L) for signals band limited at L. The implementation of these algorithms is made publicly available. Finally, we present an illustrative application demonstrating the effectiveness of curvelets for representing directional curve-like features in natural spherical images
Parallel Spherical Harmonic Transforms on heterogeneous architectures (GPUs/multi-core CPUs)
Spherical Harmonic Transforms (SHT) are at the heart of many scientific and
practical applications ranging from climate modelling to cosmological
observations. In many of these areas new, cutting-edge science goals have been
recently proposed requiring simulations and analyses of experimental or
observational data at very high resolutions and of unprecedented volumes. Both
these aspects pose formidable challenge for the currently existing
implementations of the transforms.
This paper describes parallel algorithms for computing SHT with two variants
of intra-node parallelism appropriate for novel supercomputer architectures,
multi-core processors and Graphic Processing Units (GPU). It also discusses
their performance, alone and embedded within a top-level, MPI-based
parallelisation layer ported from the S2HAT library, in terms of their
accuracy, overall efficiency and scalability. We show that our inverse SHT run
on GeForce 400 Series GPUs equipped with latest CUDA architecture ("Fermi")
outperforms the state of the art implementation for a multi-core processor
executed on a current Intel Core i7-2600K. Furthermore, we show that an
MPI/CUDA version of the inverse transform run on a cluster of 128 Nvidia Tesla
S1070 is as much as 3 times faster than the hybrid MPI/OpenMP version executed
on the same number of quad-core processors Intel Nahalem for problem sizes
motivated by our target applications. Performance of the direct transforms is
however found to be at the best comparable in these cases. We discuss in detail
the algorithmic solutions devised for major steps involved in the transforms
calculation, emphasising those with a major impact on their overall
performance, and elucidates the sources of the dichotomy between the direct and
the inverse operations
Analytical method for perturbed frozen orbit around an Asteroid in highly inhomogeneous gravitational fields : A first approach
This article provides a method for nding initial conditions for perturbed frozen orbits around inhomogeneous fast rotating asteroids. These orbits can be used as reference trajectories in missions that require close inspection of any rigid body. The generalized perturbative procedure followed exploits the analytical methods of relegation of the argument of node and Delaunay normalisation to arbitrary order. These analytical methods are extremely powerful but highly computational. The gravitational potential of the heterogeneous body is rstly stated, in polar-nodal coordinates, which takes into account the coecients of the spherical harmonics up to an arbitrary order. Through the relegation of the argument of node and the Delaunay normalization, a series of canonical transformations of coordinates is found, which reduces the Hamiltonian describing the system to a integrable, two degrees of freedom Hamiltonian plus a truncated reminder of higher order. Setting eccentricity, argument of pericenter and inclination of the orbit of the truncated system to be constant, initial conditions are found, which evolve into frozen orbits for the truncated system. Using the same initial conditions yields perturbed frozen orbits for the full system, whose perturbation decreases with the consideration of arbitrary homologic equations in the relegation and normalization procedures. Such procedure can be automated for the first homologic equation up to the consideration of any arbitrary number of spherical harmonics coefficients. The project has been developed in collaboration with the European Space Agency (ESA)
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