Real-Time Calibration of the Murchison Widefield Array

Mitchell, D. A.; Greenhill, L. J.; Wayth, R. B.; Sault, R. J.; Lonsdale, Colin J.; Cappallo, R. J.; Morales, M. F.; Ord, S. M.

doi:10.1109/jstsp.2008.2005327

Cited by 206 publications

(269 citation statements)

References 24 publications

(40 reference statements)

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“…We assume that, with an accurate beam model, this percentage could decrease to an average of 5% over the whole field of view. If we also assume that the polarized signal has |RM| = 3 rad m −2 , which is the highest value observed in the 3C 196 field, the leaking signal will generate rms fluctuations of approximately 14 mK over a bandwidth of 4 MHz, where a coherent EoR signal is expected (Morales & Hewitt 2004). Since the cosmological signal is expected to be ∼5-10 mK as a function of frequency (Mellema et al 2006) such a contamination would pose very serious problems in extracting the EoR signal.…”

Section: Discussionmentioning

confidence: 99%

“…This can be done through the procedure known as "peeling": direction-dependent calibration solutions are computed for each source that has to be peeled from the data (see, for instance, Mitchell et al 2008;and Intema et al 2009, as useful references for the peeling procedure). Peeling was implemented as follows.…”

Section: Calibration Of the 3c 196 Fieldmentioning

confidence: 99%

“…Several authors have studied the foreground contamination of the EoR signal together with the problem of removing it through various techniques (Di Matteo et al 2004;A&A 522, A67 (2010) Morales & Hewitt 2004;Santos et al 2005;Morales et al 2006;Wang et al 2006;McQuinn et al 2006;Gleser et al 2008;Jelić et al 2008;Bowman et al 2009;Liu et al 2009;Harker et al 2009;Liu et al 2009;Parsons et al 2010). Since none of these methods is fully blind but includes assumptions about the foreground properties, a further observational characterization of foregrounds is mandatory in order to improve these strategies.…”

Section: Introductionmentioning

confidence: 99%

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Foregrounds for observations of the cosmological 21 cm line

Bernardi

Bruyn

Harker

et al. 2010

A&A

123

141

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Context. In the coming years a new insight into galaxy formation and the thermal history of the Universe is expected to come from the detection of the highly redshifted cosmological 21 cm line. Aims. The cosmological 21 cm line signal is buried under Galactic and extragalactic foregrounds which are likely to be a few orders of magnitude brighter. Strategies and techniques for effective subtraction of these foreground sources require a detailed knowledge of their structure in both intensity and polarization on the relevant angular scales of 1-30 arcmin. Methods. We present results from observations conducted with the Westerbork telescope in the 140-160 MHz range with 2 arcmin resolution in two fields located at intermediate Galactic latitude, centred around the bright quasar 3C 196 and the North Celestial Pole. They were observed with the purpose of characterizing the foreground properties in sky areas where actual observations of the cosmological 21 cm line could be carried out. The polarization data were analysed through the rotation measure synthesis technique. We have computed total intensity and polarization angular power spectra. Results. Total intensity maps were carefully calibrated, reaching a high dynamic range, 150 000:1 in the case of the 3C 196 field. No evidence of diffuse Galactic emission was found in the angular power spectrum analysis on scales smaller than ∼10 arcmin in either of the two fields. On these angular scales the signal is consistent with the classical confusion noise of ∼3 mJy beam −1 . On scales greater than 30 arcmin we found an excess of power attributed to the Galactic foreground with an rms of 3.4 K and 5.5 K for the 3C 196 and the NCP field respectively. The intermediate angular scales suffered from systematic errors which prevented any detection. Patchy polarized emission was found only in the 3C 196 field whereas the polarization in the NCP area was essentially due to radio frequency interference. The polarized signal in the 3C 196 field is close to the thermal noise for angular scales smaller than ∼10 arcmin. On scales greater than 30 arcmin it has an rms value of 0.68 K. The polarized signal appears mainly at rotation measure values smaller than 4 rad m −2 . Conclusions. In regard of the detection of the cosmological 21 cm line, we conclude that Galactic total intensity emission lacks small-scale power, which is below the confusion noise level at the angular resolution of 2 arcmin. Galactic polarization, given its relative weakness and its small rotation measure values, is less severe than expected as a contaminant of the cosmological 21 cm line.

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Section: Discussionmentioning

confidence: 99%

Section: Calibration Of the 3c 196 Fieldmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Foregrounds for observations of the cosmological 21 cm line

Bernardi

Bruyn

Harker

et al. 2010

A&A

123

141

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“…All currently mooted schemes of DDE calibration for LOFAR (Nijboer & Noordam 2007), the MWA (Mitchell et al 2008) and the ionosphere in general (Intema et al 2009;Cotton et al 2004) revolve around the use of "beacon sources" to probe the ionosphere and/or the primary beam. It is rather difficult to envisage a closed-loop scheme without beacons (how else would one sample a DDE?…”

Section: Discussionmentioning

confidence: 99%

Revisiting the radio interferometer measurement equation

Smirnov

2011

A&A

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Context. Papers I and II of this series have extended the radio interferometry measurement equation (RIME) formalism to the full-sky case, and provided a RIME-based description of calibration and the problem of direction-dependent effects (DDEs). Aims. This paper aims to provide a practical demonstration of a RIME-based approach to calibration, via an example of extremely high-dynamic range calibration of WSRT observations of 3C 147 at 21 cm, with full treatment of DDEs. Methods. A version of the RIME incorporating differential gains has been implemented in MeqTrees, and applied to the 3C 147 data. This was used to perform regular selfcal, then solve for interferometer-based errors and for differential gains. Results. The resulting image of the field around 3C 147 is thermal noise-limited, has a very high dynamic range (1.6 million), and none of the off-axis artefacts that plague regular selfcal. The differential gain solutions show a high signal-to-noise ratio, and may be used to extract information on DDEs and errors in the sky model. Conclusions. The differential gain approach can eliminate DDE-related artefacts, and provide information for iterative improvements of sky models. Perhaps most importantly, sources as faint as 2 mJy have been shown to yield meaningful differential gain solutions, and thus can be used as potential calibration beacons in other DDE-related schemes.

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“…In the calibration stage of the imaging pipeline (Mitchell et al 2008), the Jones matrices of the individual antenna elements are solved for, which essentially allows G and D to be obtained, but they are incorporated in the fitted Jones matrix and not decomposed. The 32T calibration scheme attempts to solve for the elements of the diagonal G matrix, and the rest of the matrices are assumed to be the same for all tiles.…”

Section: Building and Applying The Weightsmentioning

confidence: 99%

Interferometric Imaging with the 32 Element Murchison Wide-Field Array

Ord¹,

Mitchell²,

Wayth³

et al. 2010

PUBL ASTRON SOC PAC

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ABSTRACT. The Murchison Wide-Field Array (MWA) is a low-frequency radio telescope, currently under construction, intended to search for the spectral signature of the epoch of reionization (EOR) and to probe the structure of the solar corona. Sited in western Australia, the full MWA will comprise 8192 dipoles grouped into 512 tiles and will be capable of imaging the sky south of 40°declination, from 80 MHz to 300 MHz with an instantaneous field of view that is tens of degrees wide and a resolution of a few arcminutes. A 32 station prototype of the MWA has been recently commissioned and a set of observations has been taken that exercise the whole acquisition and processing pipeline. We present Stokes I, Q, and U images from two ∼4 hr integrations of a field 20°wide centered on Pictoris A. These images demonstrate the capacity and stability of a real-time calibration and imaging technique employing the weighted addition of warped snapshots to counter extreme wide-field imaging distortions.

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Real-Time Calibration of the Murchison Widefield Array

Cited by 206 publications

References 24 publications

Foregrounds for observations of the cosmological 21 cm line

Foregrounds for observations of the cosmological 21 cm line

Revisiting the radio interferometer measurement equation

Interferometric Imaging with the 32 Element Murchison Wide-Field Array

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