Image Formation by Self-Calibration in Radio Astronomy

Pearson, T. J.; Readhead, A. C. S.

doi:10.1146/annurev.aa.22.090184.000525

Cited by 365 publications

(226 citation statements)

References 1 publication

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“…In a second step the FITS files were imported to the Caltech DIFMAP 2 software-package (Shepherd 1997). Visibility data for both frequency bands were self-calibrated, Fourier inverted, and CLEANed using the hybrid-mapping technique (Pearson & Readhead 1984), employing DIFMAP in an automatic mode. A point-source model was used as a starting model for this iterative procedure in all cases.…”

Section: Observations and Data Reductionmentioning

confidence: 99%

VLBI observations of optically-bright extragalactic radio sources for the alignment of the radio frame with the future Gaia frame

Bourda

Collioud

Charlot

et al. 2011

A&A

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Context. The European space astrometry mission Gaia, to be launched by 2012, will construct a dense optical QSO-based celestial reference frame which will need to be linked to the International Celestial Reference Frame (ICRF; the IAU fundamental frame), with the highest accuracy. However, it has been found that only 10% of the ICRF sources (70 sources) are suitable to establish this link. The remaining sources are not useful either because they are not bright enough at optical wavelengths or because they have significant extended radio emission which precludes reaching the highest astrometric accuracy. Aims. In order to improve the accuracy of this alignment, we have developed a program of VLBI observations based on three steps to detect, image and measure astrometric positions of weak extragalactic radio sources, with bright optical counterparts, from a sample of 447 candidate sources. Methods. The experiments devoted to VLBI detection, carried out with the European VLBI Network (EVN) in June and October 2007, were very successful, with 398 sources detected at both S-and X-bands. From these, 105 sources were observed in March 2008 with a global VLBI array (EVN and VLBA; Very Long Baseline Array) for imaging their VLBI structures. Results. All sources were successfully imaged in both bands and about 50% (47 sources) were found to be point-like on VLBI scales. These images are available at http://www.obs.u-bordeaux1.fr/BVID/GC030/. VLBI positions of these sources will be measured accurately in future astrometric experiments.

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Section: Observations and Data Reductionmentioning

confidence: 99%

VLBI observations of optically-bright extragalactic radio sources for the alignment of the radio frame with the future Gaia frame

Bourda

Collioud

Charlot

et al. 2011

A&A

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“…The data were then phase self-calibrated using a point source model and coherently averaged to 10 s. After editing, amplitude calibration was improved by Ðtting and applying a simple Gaussian source model to the compact source 1709]096 to determine a time-independent amplitude gain correction for each BBC at each antenna. The data were then self-calibrated following the hybrid-mapping technique (Pearson & Readhead 1984) to correct for residual amplitude and phase errors. The data were initially mapped using uniform weighting followed by natural weighting after several iterations.…”

Section: Observations and Data Analysismentioning

confidence: 99%

Anisotropic Interstellar Scattering toward the Cygnus Region

Desai¹,

Fey²

2001

ASTROPHYS J SUPPL S

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We were unable to reliably determine scattering disk parameters for 2021]317 at frequencies from 8.5 GHz down to 1.67 GHz because of its complex intrinsic source structure. The scattering disks are elliptical with axial ratios of about 0.75 : 1 with little measurable variation between these sources. We interpret our measurements as due to the e †ects of anisotropic interstellar turbulence. The anisotropy parameters, axial ratio and position angle, for those sources for which we have data at multiple frequencies appear to be frequency-independent making refractive distortions of isotropic turbulence an unlikely explanation. Our estimates of b, the power-law index of the power spectrum of electron density Ñuctuations, based upon direct model Ðts to the data on a source-by-source and frequency-by-frequency basis, are consistent with b \ 4 for most of the data but are strongly a †ected by our estimate of the intrinsic structure of the sources. Two of these sources, 2005]403 and 2021]317, exhibit signiÐcant intrinsic source structure at frequencies as low as 1.67 GHz, while the other two, 2008]332 and 2048]312, lack discernible source structure at frequencies lower then 2.3 GHz. The frequency scalings of the scattered angular sizes are also consistent with b \ 4. We discuss the implications of our measurements for the inner and outer scales of the turbulence.

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“…In this situation, a single phase term for each antenna is sufficient for capturing the effects of the ionosphere. Calibration algorithms such as phase referencing (Fomalont & Perley 1999) and self-calibration (Pearson & Readhead 1984;Cornwell & Fomalont 1999), which solve for a single phase correction term per antenna, are effective at removing ionospheric phases from the data. In contrast, the widefield nature of next-generation instruments implies that each antenna sees a large patch of the ionosphere, and the assumption of local uniformity breaks down (Cotton et al 2004;Lonsdale 2005;Intema 2009).…”

Section: Observations Data Reduction and Analysismentioning

confidence: 99%

Quantifying ionospheric effects on time-domain astrophysics with the Murchison Widefield Array

Loi

Murphy

Bell

et al. 2015

Mon. Not. R. Astron. Soc.

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Refraction and diffraction of incoming radio waves by the ionosphere induce time variability in the angular positions, peak amplitudes and shapes of radio sources, potentially complicating the automated cross-matching and identification of transient and variable radio sources. In this work, we empirically assess the effects of the ionosphere on data taken by the Murchison Widefield Array (MWA) radio telescope. We directly examine 51 hours of data observed over 10 nights under quiet geomagnetic conditions (global storm index K p < 2), analysing the behaviour of short-timescale angular position and peak flux density variations of around ten thousand unresolved sources. We find that while much of the variation in angular position can be attributed to ionospheric refraction, the characteristic displacements (10-20 arcsec) at 154 MHz are small enough that search radii of 1-2 arcmin should be sufficient for cross-matching under typical conditions. By examining bulk trends in amplitude variability, we place upper limits on the modulation index associated with ionospheric scintillation of 1-3% for the various nights. For sources fainter than ∼1 Jy, this variation is below the image noise at typical MWA sensitivities. Our results demonstrate that the ionosphere is not a significant impediment to the goals of time-domain science with the MWA at 154 MHz.

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Image Formation by Self-Calibration in Radio Astronomy

Cited by 365 publications

References 1 publication

VLBI observations of optically-bright extragalactic radio sources for the alignment of the radio frame with the future Gaia frame

VLBI observations of optically-bright extragalactic radio sources for the alignment of the radio frame with the future Gaia frame

Anisotropic Interstellar Scattering toward the Cygnus Region

Quantifying ionospheric effects on time-domain astrophysics with the Murchison Widefield Array

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