7,433 research outputs found
Strategies for the characteristic extraction of gravitational waveforms
We develop, test, and compare new numerical and geometrical methods for improving the accuracy of extracting waveforms using characteristic evolution. The new numerical method involves use of circular boundaries to the stereographic grid patches which cover the spherical cross sections of the outgoing null cones. We show how an angular version of numerical dissipation can be introduced into the characteristic code to damp the high frequency error arising form the irregular way the circular patch boundary cuts through the grid. The new geometric method involves use of the Weyl tensor component Psi4 to extract the waveform as opposed to the original approach via the Bondi news function. We develop the necessary analytic and computational formula to compute the O(1/r) radiative part of Psi4 in terms of a conformally compactified treatment of null infinity. These methods are compared and calibrated in test problems based upon linearized waves
A fast and exact -stacking and -projection hybrid algorithm for wide-field interferometric imaging
The standard wide-field imaging technique, the -projection, allows
correction for wide-fields of view for non-coplanar radio interferometric
arrays. However, calculating exact corrections for each measurement has not
been possible due to the amount of computation required at high resolution and
with the large number of visibilities from current interferometers. The
required accuracy and computational cost of these corrections is one of the
largest unsolved challenges facing next generation radio interferometers such
as the Square Kilometre Array. We show that the same calculation can be
performed with a radially symmetric -projection kernel, where we use one
dimensional adaptive quadrature to calculate the resulting Hankel transform,
decreasing the computation required for kernel generation by several orders of
magnitude, whilst preserving the accuracy. We confirm that the radial
-projection kernel is accurate to approximately 1% by imaging the
zero-spacing with an added -term. We demonstrate the potential of our
radially symmetric -projection kernel via sparse image reconstruction, using
the software package PURIFY. We develop a distributed -stacking and
-projection hybrid algorithm. We apply this algorithm to individually
correct for non-coplanar effects in 17.5 million visibilities over a by
degree field of view MWA observation for image reconstruction. Such a
level of accuracy and scalability is not possible with standard -projection
kernel generation methods. This demonstrates that we can scale to a large
number of measurements with large image sizes whilst still maintaining both
speed and accuracy.Comment: 9 Figures, 19 Pages. Accepted to Ap
Virtual image out-the-window display system study. Volume 2 - Appendix
Virtual image out-the-window display system imaging techniques and simulation devices - appendices containing background materia
The Life and Times of the Parkes-Tidbinbilla Interferometer
The Parkes-Tidbinbilla took advantage of a real-time radio-link connecting
the Parkes and Tidbinbilla antennas to form the world's longest real-time
interferometer. Built on a minuscule budget, it was an extraordinarily
successful instrument, generating some 24 journal papers including 3 Nature
papers, as well as facilitating the early development of the Australia
Telescope Compact Array. Here we describe its origins, construction, successes,
and life cycle, and discuss the future use of single-baseline interferometers
in the era of SKA and its pathfinders.Comment: Accepted by Journal of Astronomical History & Heritage. arXiv admin
note: substantial text overlap with arXiv:1210.098
Present state of knowledge of the upper atmosphere: An assessment report; processes that control ozone and other climatically important trace gases
The state of knowledge of the upper atmosphere was assessed as of January 1986. The physical, chemical, and radiative processes which control the spatial and temporal distribution of ozone in the atmosphere; the predicted magnitude of ozone perturbations and climate changes for a variety of trace gas scenarios; and the ozone and temperature data used to detect the presence or absence of a long term trend were discussed. This assessment report was written by a small group of NASA scientists, was peer reviewed, and is based primarily on the comprehensive international assessment document entitled Atmospheric Ozone 1985: Assessment of Our Understanding of the Processes Controlling Its Present Distribution and Change, to be published as the World Meteorological Organization Global Ozone Research and Monitoring Project Report No. 16
Intraocular Forced Convection Mechanism Defect as Probable Cause of Normal-Tension Glaucoma
This paper describes several pathologies associated with pathological movements that can cause physical effort on the optic nerve and damage to vision. The accumulation of intraocular metabolic residues increases ocular globe mass and can change its position in the orbit, as well as increase the cornea and crystalline, accommodation resistance, in addition to being able to increase the aqueous humor output resistance. A series of discreet pathologies may result in optic nerve impairment: cyclotorsion and saccadic movement, position in the orbit, and increased intraocular pressure. The cyclotorsion movements can be stimulated by the superior visual field restriction, due to the metabolic residue accumulation in the light transmission regions of this visual field, preventing correct fusion of the images
Unsupervised Behaviour Analysis and Magnification (uBAM) using Deep Learning
Motor behaviour analysis is essential to biomedical research and clinical
diagnostics as it provides a non-invasive strategy for identifying motor
impairment and its change caused by interventions. State-of-the-art
instrumented movement analysis is time- and cost-intensive, since it requires
placing physical or virtual markers. Besides the effort required for marking
keypoints or annotations necessary for training or finetuning a detector, users
need to know the interesting behaviour beforehand to provide meaningful
keypoints. We introduce unsupervised behaviour analysis and magnification
(uBAM), an automatic deep learning algorithm for analysing behaviour by
discovering and magnifying deviations. A central aspect is unsupervised
learning of posture and behaviour representations to enable an objective
comparison of movement. Besides discovering and quantifying deviations in
behaviour, we also propose a generative model for visually magnifying subtle
behaviour differences directly in a video without requiring a detour via
keypoints or annotations. Essential for this magnification of deviations even
across different individuals is a disentangling of appearance and behaviour.
Evaluations on rodents and human patients with neurological diseases
demonstrate the wide applicability of our approach. Moreover, combining
optogenetic stimulation with our unsupervised behaviour analysis shows its
suitability as a non-invasive diagnostic tool correlating function to brain
plasticity.Comment: Published in Nature Machine Intelligence (2021),
https://rdcu.be/ch6p
- …