2,278 research outputs found
Ritchey-Chretien Telescope
A Ritchey-Chretien telescope is described which was designed to respond to images located off the optical axis by using two transparent flat plates positioned in the ray path of the image. The flat plates have a tilt angle relative to the ray path to compensate for astigmatism introduced by the telescope. The tilt angle of the plates is directly proportional to the off axis angle of the image. The plates have opposite inclination angles relative to the ray paths. A detector which is responsive to the optical image as transmitted through the plates is positioned approximately on the sagittal focus of the telescope
Fine guidance for a spaceborne telescope
Two transparent plates are mounted at equal and opposite angles in secondary optical-system housing, angles being set for optimum astigmatism correction. Rotation of secondary housing assembly and translation of detector are proportional to angular position of secondary image. Combined movement of two retains image within sagittal foci of secondary system
Global MRI with Braginskii viscosity in a galactic profile
We present a global-in-radius linear analysis of the axisymmetric
magnetorotational instability (MRI) in a collisional magnetized plasma with
Braginskii viscosity. For a galactic angular velocity profile we
obtain analytic solutions for three magnetic field orientations: purely
azimuthal, purely vertical and slightly pitched (almost azimuthal). In the
first two cases the Braginskii viscosity damps otherwise neutrally stable
modes, and reduces the growth rate of the MRI respectively. In the final case
the Braginskii viscosity makes the MRI up to times faster than its
inviscid counterpart, even for \emph{asymptotically small} pitch angles. We
investigate the transition between the Lorentz-force-dominated and the
Braginskii viscosity-dominated regimes in terms of a parameter \sim \Omega
\nub/B^2 where \nub is the viscous coefficient and the Alfv\'en speed.
In the limit where the parameter is small and large respectively we recover the
inviscid MRI and the magnetoviscous instability (MVI). We obtain asymptotic
expressions for the approach to these limits, and find the Braginskii viscosity
can magnify the effects of azimuthal hoop tension (the growth rate becomes
complex) by over an order of magnitude. We discuss the relevance of our results
to the local approximation, galaxies and other magnetized astrophysical
plasmas. Our results should prove useful for benchmarking codes in global
geometries.Comment: 14 pages, 5 figure
Novelty Search in Competitive Coevolution
One of the main motivations for the use of competitive coevolution systems is
their ability to capitalise on arms races between competing species to evolve
increasingly sophisticated solutions. Such arms races can, however, be hard to
sustain, and it has been shown that the competing species often converge
prematurely to certain classes of behaviours. In this paper, we investigate if
and how novelty search, an evolutionary technique driven by behavioural
novelty, can overcome convergence in coevolution. We propose three methods for
applying novelty search to coevolutionary systems with two species: (i) score
both populations according to behavioural novelty; (ii) score one population
according to novelty, and the other according to fitness; and (iii) score both
populations with a combination of novelty and fitness. We evaluate the methods
in a predator-prey pursuit task. Our results show that novelty-based approaches
can evolve a significantly more diverse set of solutions, when compared to
traditional fitness-based coevolution.Comment: To appear in 13th International Conference on Parallel Problem
Solving from Nature (PPSN 2014
Nonlinear growth of firehose and mirror fluctuations in turbulent galaxy-cluster plasmas
In turbulent high-beta astrophysical plasmas (exemplified by the galaxy
cluster plasmas), pressure-anisotropy-driven firehose and mirror fluctuations
grow nonlinearly to large amplitudes, dB/B ~ 1, on a timescale comparable to
the turnover time of the turbulent motions. The principle of their nonlinear
evolution is to generate secularly growing small-scale magnetic fluctuations
that on average cancel the temporal change in the large-scale magnetic field
responsible for the pressure anisotropies. The presence of small-scale magnetic
fluctuations may dramatically affect the transport properties and, thereby, the
large-scale dynamics of the high-beta astrophysical plasmas.Comment: revtex, 4 pages, 1 figure; replaced to match published versio
Excitability in autonomous Boolean networks
We demonstrate theoretically and experimentally that excitable systems can be
built with autonomous Boolean networks. Their experimental implementation is
realized with asynchronous logic gates on a reconfigurabe chip. When these
excitable systems are assembled into time-delay networks, their dynamics
display nanosecond time-scale spike synchronization patterns that are
controllable in period and phase.Comment: 6 pages, 5 figures, accepted in Europhysics Letters
(epljournal.edpsciences.org
Speeding up Simplification of Polygonal Curves using Nested Approximations
We develop a multiresolution approach to the problem of polygonal curve
approximation. We show theoretically and experimentally that, if the
simplification algorithm A used between any two successive levels of resolution
satisfies some conditions, the multiresolution algorithm MR will have a
complexity lower than the complexity of A. In particular, we show that if A has
a O(N2/K) complexity (the complexity of a reduced search dynamic solution
approach), where N and K are respectively the initial and the final number of
segments, the complexity of MR is in O(N).We experimentally compare the
outcomes of MR with those of the optimal "full search" dynamic programming
solution and of classical merge and split approaches. The experimental
evaluations confirm the theoretical derivations and show that the proposed
approach evaluated on 2D coastal maps either shows a lower complexity or
provides polygonal approximations closer to the initial curves.Comment: 12 pages + figure
Magnetofluid dynamics of magnetized cosmic plasma: firehose and gyrothermal instabilities
Both global dynamics and turbulence in magnetized weakly collisional cosmic
plasmas are described by general magnetofluid equations that contain pressure
anisotropies and heat fluxes that must be calculated from microscopic plasma
kinetic theory. It is shown that even without a detailed calculation of the
pressure anisotropy or the heat fluxes, one finds the macroscale dynamics to be
generically unstable to microscale Alfvenically polarized fluctuations. Two
instabilities are considered in detail: the parallel firehose instability
(including the finite-Larmor-radius effects that determine the fastest growing
mode) and the gyrothermal instability (GTI). The latter is a new result - it is
shown that a parallel ion heat flux destabilizes Alfvenically polarized
fluctuations even in the absence of the negative pressure anisotropy required
for the firehose. The main conclusion is that both pressure anisotropies and
heat fluxes trigger plasma microinstabilities and, therefore, their values will
likely be set by the nonlinear evolution of these instabilities. Ideas for
understanding this nonlinear evolution are discussed. It is argued that cosmic
plasmas will generically be "three-scale systems," comprising global dynamics,
mesoscale turbulence and microscale plasma fluctuations. The astrophysical
example of cool cores of galaxy clusters is considered and it is noted that
observations point to turbulence in clusters being in a marginal state with
respect to plasma microinstabilities and so it is the plasma microphysics that
is likely to set the heating and conduction properties of the intracluster
medium. In particular, a lower bound on the scale of temperature fluctuations
implied by the GTI is derived.Comment: 10 pages, MNRAS tex style, 1 figur
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