942 research outputs found
Probing Fine-Scale Ionospheric Structure with the Very Large Array Radio Telescope
High resolution (~1 arcminute) astronomical imaging at low frequency (below
150 MHz) has only recently become practical with the development of new
calibration algorithms for removing ionospheric distortions. In addition to
opening a new window in observational astronomy, the process of calibrating the
ionospheric distortions also probes ionospheric structure in an unprecedented
way. Here we explore one aspect of this new type of ionospheric measurement,
the differential refraction of celestial source pairs as a function of their
angular separation. This measurement probes variations in the spatial gradient
of the line-of-sight total electron content (TEC) to 0.001 TECU/km accuracy
over spatial scales of under 10 km to over 100 km. We use data from the VLA
Low-frequency Sky Survey (VLSS; Cohen et al. 2007, AJ 134, 1245), a nearly
complete 74 MHz survey of the entire sky visible to the Very Large Array (VLA)
telescope in Socorro, New Mexico. These data comprise over 500 hours of
observations, all calibrated in a standard way. While ionospheric spatial
structure varies greatly from one observation to the next, when analyzed over
hundreds of hours, statistical patterns become apparent. We present a detailed
characterization of how the median differential refraction depends on source
pair separation, elevation and time of day. We find that elevation effects are
large, but geometrically predictable and can be "removed" analytically using a
"thin-shell" model of the ionosphere. We find significantly greater ionospheric
spatial variations during the day than at night. These diurnal variations
appear to affect the larger angular scales to a greater degree indicating that
they come from disturbances on relatively larger spatial scales (100s of km,
rather than 10s of km).Comment: Accepted for publication by The Astronomical Journa
Calibration Challenges for Future Radio Telescopes
Instruments for radio astronomical observations have come a long way. While
the first telescopes were based on very large dishes and 2-antenna
interferometers, current instruments consist of dozens of steerable dishes,
whereas future instruments will be even larger distributed sensor arrays with a
hierarchy of phased array elements. For such arrays to provide meaningful
output (images), accurate calibration is of critical importance. Calibration
must solve for the unknown antenna gains and phases, as well as the unknown
atmospheric and ionospheric disturbances. Future telescopes will have a large
number of elements and a large field of view. In this case the parameters are
strongly direction dependent, resulting in a large number of unknown parameters
even if appropriately constrained physical or phenomenological descriptions are
used. This makes calibration a daunting parameter estimation task, that is
reviewed from a signal processing perspective in this article.Comment: 12 pages, 7 figures, 20 subfigures The title quoted in the meta-data
is the title after release / final editing
Reionization and Cosmology with 21 cm Fluctuations
Measurement of the spatial distribution of neutral hydrogen via the
redshifted 21 cm line promises to revolutionize our knowledge of the epoch of
reionization and the first galaxies, and may provide a powerful new tool for
observational cosmology from redshifts 1<z<4 . In this review we discuss recent
advances in our theoretical understanding of the epoch of reionization (EoR),
the application of 21 cm tomography to cosmology and measurements of the dark
energy equation of state after reionization, and the instrumentation and
observational techniques shared by 21 cm EoR and post reionization cosmology
machines. We place particular emphasis on the expected signal and observational
capabilities of first generation 21 cm fluctuation instruments.Comment: Invited review for Annual Review of Astronomy and Astrophysics (2010
volume
The Murchison Widefield Array: the Square Kilometre Array Precursor at low radio frequencies
The Murchison Widefield Array (MWA) is one of three Square Kilometre Array
Precursor telescopes and is located at the Murchison Radio-astronomy
Observatory in the Murchison Shire of the mid-west of Western Australia, a
location chosen for its extremely low levels of radio frequency interference.
The MWA operates at low radio frequencies, 80-300 MHz, with a processed
bandwidth of 30.72 MHz for both linear polarisations, and consists of 128
aperture arrays (known as tiles) distributed over a ~3 km diameter area. Novel
hybrid hardware/software correlation and a real-time imaging and calibration
systems comprise the MWA signal processing backend. In this paper the as-built
MWA is described both at a system and sub-system level, the expected
performance of the array is presented, and the science goals of the instrument
are summarised.Comment: Submitted to PASA. 11 figures, 2 table
LOFAR Sparse Image Reconstruction
Context. The LOw Frequency ARray (LOFAR) radio telescope is a giant digital
phased array interferometer with multiple antennas distributed in Europe. It
provides discrete sets of Fourier components of the sky brightness. Recovering
the original brightness distribution with aperture synthesis forms an inverse
problem that can be solved by various deconvolution and minimization methods
Aims. Recent papers have established a clear link between the discrete nature
of radio interferometry measurement and the "compressed sensing" (CS) theory,
which supports sparse reconstruction methods to form an image from the measured
visibilities. Empowered by proximal theory, CS offers a sound framework for
efficient global minimization and sparse data representation using fast
algorithms. Combined with instrumental direction-dependent effects (DDE) in the
scope of a real instrument, we developed and validated a new method based on
this framework Methods. We implemented a sparse reconstruction method in the
standard LOFAR imaging tool and compared the photometric and resolution
performance of this new imager with that of CLEAN-based methods (CLEAN and
MS-CLEAN) with simulated and real LOFAR data Results. We show that i) sparse
reconstruction performs as well as CLEAN in recovering the flux of point
sources; ii) performs much better on extended objects (the root mean square
error is reduced by a factor of up to 10); and iii) provides a solution with an
effective angular resolution 2-3 times better than the CLEAN images.
Conclusions. Sparse recovery gives a correct photometry on high dynamic and
wide-field images and improved realistic structures of extended sources (of
simulated and real LOFAR datasets). This sparse reconstruction method is
compatible with modern interferometric imagers that handle DDE corrections (A-
and W-projections) required for current and future instruments such as LOFAR
and SKAComment: Published in A&A, 19 pages, 9 figure
The 74MHz System on the Very Large Array
The Naval Research Laboratory and the National Radio Astronomy Observatory
completed implementation of a low frequency capability on the VLA at 73.8 MHz
in 1998. This frequency band offers unprecedented sensitivity (~25 mJy/beam)
and resolution (~25 arcsec) for low-frequency observations. We review the
hardware, the calibration and imaging strategies, comparing them to those at
higher frequencies, including aspects of interference excision and wide-field
imaging. Ionospheric phase fluctuations pose the major difficulty in
calibrating the array. Over restricted fields of view or at times of extremely
quiescent ionospheric ``weather'', an angle-invariant calibration strategy can
be used. In this approach a single phase correction is devised for each
antenna, typically via self-calibration. Over larger fields of view or at times
of more normal ionospheric ``weather'' when the ionospheric isoplanatic patch
size is smaller than the field of view, we adopt a field-based strategy in
which the phase correction depends upon location within the field of view. This
second calibration strategy was implemented by modeling the ionosphere above
the array using Zernike polynomials. Images of 3C sources of moderate strength
are provided as examples of routine, angle-invariant calibration and imaging.
Flux density measurements indicate that the 74 MHz flux scale at the VLA is
stable to a few percent, and tied to the Baars et al. value of Cygnus A at the
5 percent level. We also present an example of a wide-field image, devoid of
bright objects and containing hundreds of weaker sources, constructed from the
field-based calibration. We close with a summary of lessons the 74 MHz system
offers as a model for new and developing low-frequency telescopes. (Abridged)Comment: 73 pages, 46 jpeg figures, to appear in ApJ
The next detectors for gravitational wave astronomy
This paper focuses on the next detectors for gravitational wave astronomy
which will be required after the current ground based detectors have completed
their initial observations, and probably achieved the first direct detection of
gravitational waves. The next detectors will need to have greater sensitivity,
while also enabling the world array of detectors to have improved angular
resolution to allow localisation of signal sources. Sect. 1 of this paper
begins by reviewing proposals for the next ground based detectors, and presents
an analysis of the sensitivity of an 8 km armlength detector, which is proposed
as a safe and cost-effective means to attain a 4-fold improvement in
sensitivity. The scientific benefits of creating a pair of such detectors in
China and Australia is emphasised. Sect. 2 of this paper discusses the high
performance suspension systems for test masses that will be an essential
component for future detectors, while sect. 3 discusses solutions to the
problem of Newtonian noise which arise from fluctuations in gravity gradient
forces acting on test masses. Such gravitational perturbations cannot be
shielded, and set limits to low frequency sensitivity unless measured and
suppressed. Sects. 4 and 5 address critical operational technologies that will
be ongoing issues in future detectors. Sect. 4 addresses the design of thermal
compensation systems needed in all high optical power interferometers operating
at room temperature. Parametric instability control is addressed in sect. 5.
Only recently proven to occur in Advanced LIGO, parametric instability
phenomenon brings both risks and opportunities for future detectors. The path
to future enhancements of detectors will come from quantum measurement
technologies. Sect. 6 focuses on the use of optomechanical devices for
obtaining enhanced sensitivity, while sect. 7 reviews a range of quantum
measurement options
LOFAR sparse image reconstruction
The LOw Frequency ARray (LOFAR) radio telescope is a giant digital phased array interferometer with multiple antennas distributed in Europe. It provides discrete sets of Fourier components of the sky brightness. Recovering the original brightness distribution with aperture synthesis forms an inverse problem that can be solved by various deconvolution and minimization methods Aims. Recent papers have established a clear link between the discrete nature of radio interferometry measurement and the "compressed sensing" (CS) theory, which supports sparse reconstruction methods to form an image from the measured visibilities. Empowered by proximal theory, CS offers a sound framework for efficient global minimization and sparse data representation using fast algorithms. Combined with instrumental direction-dependent effects (DDE) in the scope of a real instrument, we developed and validated a new method based on this framework Methods. We implemented a sparse reconstruction method in the standard LOFAR imaging tool and compared the photometric and resolution performance of this new imager with that of CLEAN-based methods (CLEAN and MS-CLEAN) with simulated and real LOFAR data Results. We show that i) sparse reconstruction performs as well as CLEAN in recovering the flux of point sources; ii) performs much better on extended objects (the root mean square error is reduced by a factor of up to 10); and iii) provides a solution with an effective angular resolution 2-3 times better than the CLEAN images. Conclusions. Sparse recovery gives a correct photometry on high dynamic and wide-field images and improved realistic structures of extended sources (of simulated and real LOFAR datasets). This sparse reconstruction method is compatible with modern interferometric imagers that handle DDE corrections (A- and W-projections) required for current and future instruments such as LOFAR and SK
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