421 research outputs found
A GPU based real-time software correlation system for the Murchison Widefield Array prototype
Modern graphics processing units (GPUs) are inexpensive commodity hardware
that offer Tflop/s theoretical computing capacity. GPUs are well suited to many
compute-intensive tasks including digital signal processing.
We describe the implementation and performance of a GPU-based digital
correlator for radio astronomy. The correlator is implemented using the NVIDIA
CUDA development environment. We evaluate three design options on two
generations of NVIDIA hardware. The different designs utilize the internal
registers, shared memory and multiprocessors in different ways. We find that
optimal performance is achieved with the design that minimizes global memory
reads on recent generations of hardware.
The GPU-based correlator outperforms a single-threaded CPU equivalent by a
factor of 60 for a 32 antenna array, and runs on commodity PC hardware. The
extra compute capability provided by the GPU maximises the correlation
capability of a PC while retaining the fast development time associated with
using standard hardware, networking and programming languages. In this way, a
GPU-based correlation system represents a middle ground in design space between
high performance, custom built hardware and pure CPU-based software
correlation.
The correlator was deployed at the Murchison Widefield Array 32 antenna
prototype system where it ran in real-time for extended periods. We briefly
describe the data capture, streaming and correlation system for the prototype
array.Comment: 11 pages, to appear in PAS
Spectral ageing in the era of big data : Integrated versus resolved models
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2017 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.Continuous injection models of spectral ageing have long been used to determine the age of radio galaxies from their integrated spectrum; however, many questions about their reliability remain unanswered. With various large area surveys imminent (e.g. LOw Frequency ARray, MeerKAT, MurchisonWidefield Array) and planning for the next generation of radio interferometers are well underway (e.g. next generationVLA, SquareKilometreArray), investigations of radio galaxy physics are set to shift away from studies of individual sources to the population as a whole. Determining if and how integrated models of spectral ageing can be applied in the era of big data is therefore crucial. In this paper, I compare classical integrated models of spectral ageing to recent well-resolved studies that use modern analysis techniques on small spatial scales to determine their robustness and validity as a source selection method. I find that integrated models are unable to recover key parameters and, even when known a priori, provide a poor, frequency-dependent description of a source's spectrum. I show a disparity of up to a factor of 6 in age between the integrated and resolved methods but suggest, even with these inconsistencies, such models still provide a potential method of candidate selection in the search for remnant radio galaxies and in providing a cleaner selection of high redshift radio galaxies in z - α selected samples.Peer reviewe
The lens and source of the optical Einstein ring gravitational lens ER 0047-2808
(Abridged) We perform a detailed analysis of the optical gravitational lens
ER 0047-2808 imaged with WFPC2 on the Hubble Space Telescope. Using software
specifically designed for the analysis of resolved gravitational lens systems,
we focus on how the image alone can constrain the mass distribution in the lens
galaxy. We find the data are of sufficient quality to strongly constrain the
lens model with no a priori assumptions about the source. Using a variety of
mass models, we find statistically acceptable results for elliptical
isothermal-like models with an Einstein radius of 1.17''. An elliptical
power-law model (Sigma \propto R^-beta) for the surface mass density favours a
slope slightly steeper than isothermal with beta = 1.08 +/- 0.03. Other models
including a constant M/L, pure NFW halo and (surprisingly) an isothermal sphere
with external shear are ruled out by the data. We find the galaxy light profile
can only be fit with a Sersic plus point source model. The resulting total
M/L_B contained within the images is 4.7 h_65 +/-0.3. In addition, we find the
luminous matter is aligned with the total mass distribution within a few
degrees. The source, reconstructed by the software, is revealed to have two
bright regions, with an unresolved component inside the caustic and a resolved
component straddling a fold caustic. The angular size of the entire source is
approx. 0.1'' and its (unlensed) Lyman-alpha flux is 3 x 10^-17 erg/s/cm^2.Comment: 13 pages, 5 figures. Revised version accepted for publication in
MNRA
DiFX2: A more flexible, efficient, robust and powerful software correlator
Software correlation, where a correlation algorithm written in a high-level
language such as C++ is run on commodity computer hardware, has become
increasingly attractive for small to medium sized and/or bandwidth constrained
radio interferometers. In particular, many long baseline arrays (which
typically have fewer than 20 elements and are restricted in observing bandwidth
by costly recording hardware and media) have utilized software correlators for
rapid, cost-effective correlator upgrades to allow compatibility with new,
wider bandwidth recording systems and improve correlator flexibility. The DiFX
correlator, made publicly available in 2007, has been a popular choice in such
upgrades and is now used for production correlation by a number of
observatories and research groups worldwide. Here we describe the evolution in
the capabilities of the DiFX correlator over the past three years, including a
number of new capabilities, substantial performance improvements, and a large
amount of supporting infrastructure to ease use of the code. New capabilities
include the ability to correlate a large number of phase centers in a single
correlation pass, the extraction of phase calibration tones, correlation of
disparate but overlapping sub-bands, the production of rapidly sampled
filterbank and kurtosis data at minimal cost, and many more. The latest version
of the code is at least 15% faster than the original, and in certain situations
many times this value. Finally, we also present detailed test results
validating the correctness of the new code.Comment: 28 pages, 9 figures, accepted for publication in PAS
Direction-Dependent Polarised Primary Beams in Wide-Field Synthesis Imaging
The process of wide-field synthesis imaging is explored, with the aim of
understanding the implications of variable, polarised primary beams for
forthcoming Epoch of Reionisation experiments. These experiments seek to detect
weak signatures from redshifted 21cm emission in deep residual datasets, after
suppression and subtraction of foreground emission. Many subtraction algorithms
benefit from low side-lobes and polarisation leakage at the outset, and both of
these are intimately linked to how the polarised primary beams are handled.
Building on previous contributions from a number of authors, in which
direction-dependent corrections are incorporated into visibility gridding
kernels, we consider the special characteristics of arrays of fixed dipole
antennas operating around 100-200 MHz, looking towards instruments such as the
Square Kilometre Array (SKA) and the Hydrogen Epoch of Reionization Arrays
(HERA). We show that integrating snapshots in the image domain can help to
produce compact gridding kernels, and also reduce the need to make complicated
polarised leakage corrections during gridding. We also investigate an
alternative form for the gridding kernel that can suppress variations in the
direction-dependent weighting of gridded visibilities by 10s of dB, while
maintaining compact support.Comment: 15 pages, 4 figures. Accepted for publication in JA
The Bias and Uncertainty of Redundant and Sky-based Calibration Under Realistic Sky and Telescope Conditions
The advent of a new generation of low frequency interferometers has opened a direct window into the Epoch of Reionisation (EoR). However, key to a detection of the faint 21-cm signal, and reaching the sensitivity limits of these arrays, is a detailed understanding of the instruments and their calibration. In this work we use simulations to investigate the bias and uncertainty of redundancy based calibration. Specifically, we study the influence of the flux distribution of the radio sky and the impact of antenna position offsets on the complex calibration solutions. We find that the position offsets introduce a bias into the phase component of the calibration solutions. This phase bias increases with the distance between bright radio sources and the pointing center, and with the flux density of these sources. This is potentially problematic for redundant calibration on MWA observations of EoR fields 1 and 2. EoR field 0, however, lacks such sources. We also compared the simulations with theoretical estimates for the bias and uncertainty in sky model based calibration on incomplete sky models for the redundant antenna tiles in the MWA. Our results indicate that redundant calibration outperforms sky based calibration due to the high positional precision of the MWA antenna tiles
Enabling a High Throughput Real Time Data Pipeline for a Large Radio Telescope Array with GPUs
The Murchison Widefield Array (MWA) is a next-generation radio telescope
currently under construction in the remote Western Australia Outback. Raw data
will be generated continuously at 5GiB/s, grouped into 8s cadences. This high
throughput motivates the development of on-site, real time processing and
reduction in preference to archiving, transport and off-line processing. Each
batch of 8s data must be completely reduced before the next batch arrives.
Maintaining real time operation will require a sustained performance of around
2.5TFLOP/s (including convolutions, FFTs, interpolations and matrix
multiplications). We describe a scalable heterogeneous computing pipeline
implementation, exploiting both the high computing density and FLOP-per-Watt
ratio of modern GPUs. The architecture is highly parallel within and across
nodes, with all major processing elements performed by GPUs. Necessary
scatter-gather operations along the pipeline are loosely synchronized between
the nodes hosting the GPUs. The MWA will be a frontier scientific instrument
and a pathfinder for planned peta- and exascale facilities.Comment: Version accepted by Comp. Phys. Com
Lensview: Software for modelling resolved gravitational lens images
We have developed a new software tool, Lensview, for modelling resolved
gravitational lens images. Based on the LensMEM algorithm, the software finds
the best fitting lens mass model and source brightness distribution using a
maximum entropy constraint. The method can be used with any point spread
function or lens model. We review the algorithm and introduce some significant
improvements. We also investigate and discuss issues associated with the
statistical uncertainties of models and model parameters and the issues of
source plane size and source pixel size.
We test the software on simulated optical and radio data to evaluate how well
lens models can be recovered and with what accuracy. For optical data, lens
model parameters can typically be recovered with better than 1% accuracy and
the degeneracy between mass ellipticity and power law is reduced. For radio
data, we find that systematic errors associated with using processed radio
maps, rather than the visibilities, are of similar magnitude to the random
errors. Hence analysing radio data in image space is still useful and
meaningful.
The software is applied to the optical arc HST J15433+5352 and the radio ring
MG1549+3047 using a simple elliptical isothermal lens model. For HST
J15433+5352, the Einstein radius is 0.525" +/- 0.015 which probably includes a
substantial convergence contribution from a neighbouring galaxy. For
MG1549+3047, the model has Einstein radius 1.105" +/- 0.005 and core radius
0.16" 0.03. The total mass enclosed in the critical radius is 7.06 x 10^{10}
Solar masses for our best model.Comment: 21 pages, 24 figures, appearing in MNRAS. Software available from
http://www.cfa.harvard.edu/~rwayth/lensview/Lensview_Home.htm
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