Calibration artefacts in radio interferometry – I. Ghost sources in Westerbork Synthesis Radio Telescope data

Grobler, T. L.; Nunhokee, Chuneeta D.; Smirnov, O.; Zyl, Gusti van; Bruyn, A. G. de

doi:10.1093/mnras/stu268

Cited by 57 publications

(48 citation statements)

References 32 publications

(32 reference statements)

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“…This behaviour has already been observed in previous works, even in the DIE calibration case, and is known as "ghost sources" (see e.g. Grobler et al (2014)). …”

Section: Dynamic Range Of the Imagesupporting

confidence: 72%

Non-convex optimization for self-calibration of direction-dependent effects in radio interferometric imaging

Repetti

Birdi²,

Dabbech³

et al. 2017

Monthly Notices of the Royal Astronomical Society

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Radio interferometric imaging aims to estimate an unknown sky intensity image from degraded observations, acquired through an antenna array. In the theoretical case of a perfectly calibrated array, it has been shown that solving the corresponding imaging problem by iterative algorithms based on convex optimization and compressive sensing theory can be competitive with classical algorithms such as CLEAN. However, in practice, antenna-based gains are unknown and have to be calibrated. Future radio telescopes, such as the SKA, aim at improving imaging resolution and sensitivity by orders of magnitude. At this precision level, the direction-dependency of the gains must be accounted for, and radio interferometric imaging can be understood as a blind deconvolution problem. In this context, the underlying minimization problem is non-convex, and adapted techniques have to be designed. In this work, leveraging recent developments in non-convex optimization, we propose the first joint calibration and imaging method in radio interferometry, with proven convergence guarantees. Our approach, based on a block-coordinate forward-backward algorithm, jointly accounts for visibilities and suitable priors on both the image and the direction-dependent effects (DDEs). As demonstrated in recent works, sparsity remains the prior of choice for the image, while DDEs are modelled as smooth functions of the sky, i.e. spatially band-limited. Finally, we show through simulations the efficiency of our method, for the reconstruction of both images of point sources and complex extended sources. MATLAB code is available on GitHub.

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“…This behaviour has already been observed in previous works, even in the DIE calibration case, and is known as "ghost sources" (see e.g. Grobler et al (2014)). …”

Section: Dynamic Range Of the Imagesupporting

confidence: 72%

Non-convex optimization for self-calibration of direction-dependent effects in radio interferometric imaging

Repetti

Birdi²,

Dabbech³

et al. 2017

Monthly Notices of the Royal Astronomical Society

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“…Nevertheless, sky models remain essentially always incomplete at some level, as source catalogues are limited in depth, source characterization and -often -sky coverage. [24], [95] and [25] show that incomplete catalogues used as sky models bias the calibration and eventually lead to artifacts in the form of ghost-like sources in interferometric images, most of the times fainter than the image noise level. The ghost pattern is stronger for regularly spaced arrays and if the sky model is less complete.…”

Section: Interferometric Calibration and 21 CM Observationsmentioning

confidence: 99%

21 cm Observations: Calibration, Strategies, Observables

Bernardi¹

2019

The Cosmic 21-Cm Revolution

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This chapter aims to provide a review of the basics of 21 cm interferometric observations and its methodologies. A summary of the main concepts of radio interferometry and their connection with the 21 cm observables -power spectra and images -is presented. I then provide a review of interferometric calibration and its interplay with foreground separation, including the current open challenges in calibration of 21 cm observations. Finally, a review of 21 cm instrument designs in the light of calibration choices and observing strategies follows. arXiv:1909.11938v1 [astro-ph.IM] 26 Sep 2019 I D (l, m, ν) =S V =S * Ṽ = PSF(l, m, ν) * Ī(l, m, ν), (5.11)where the tilde indicates the Fourier transform, * the convolution operation and PSF is the Point Spread Function, i.e. the response of the interferometric array to a point sources which, in our case, is also the Fourier transform of the uv coverage. The sampling function always effectively reduces the integral over a finite (often not contiguous) area of the uv plane. In particular, the sampled uv plane is restricted between a

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“…Multi-direction algorithms may subtract flux from sources that are not included in the source model in their quest to minimize the difference between data and model (Kazemi & Yatawatta 2013;Grobler et al 2014). Furthermore, the Moon is a moving source (in the celestial coordinates frame in which astrophysical sources are fixed).…”

Section: Faint Source Removalmentioning

confidence: 99%

Lunar occultation of the diffuse radio sky: LOFAR measurements between 35 and 80 MHz

Vedantham¹,

Koopmans²,

Bruyn³

et al. 2015

Monthly Notices of the Royal Astronomical Society

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We present radio observations of the Moon between 35 and 80 MHz to demonstrate a novel technique of interferometrically measuring large-scale diffuse emission extending far beyond the primary beam (global signal) for the first time. In particular, we show that (i) the Moon appears as a negative-flux source at frequencies 35 < ν < 80 MHz since it is 'colder' than the diffuse Galactic background it occults, (ii) using the (negative) flux of the lunar disc, we can reconstruct the spectrum of the diffuse Galactic emission with the lunar thermal emission as a reference, and (iii) that reflected RFI (radio-frequency interference) is concentrated at the center of the lunar disc due to specular nature of reflection, and can be independently measured. Our RFI measurements show that (i) Moon-based Cosmic Dawn experiments must design for an Earth-isolation of better than 80 dB to achieve an RFI temperature < 1 mK, (ii) Moon-reflected RFI contributes to a dipole temperature less than 20 mK for Earthbased Cosmic Dawn experiments, (iii) man-made satellite-reflected RFI temperature exceeds 20 mK if the aggregate cross section of visible satellites exceeds 80 m 2 at 800 km height, or 5 m 2 at 400 km height. Currently, our diffuse background spectrum is limited by sidelobe confusion on short baselines (10-15% level). Further refinement of our technique may yield constraints on the redshifted global 21-cm signal from Cosmic Dawn (40 > z > 12) and the Epoch of Reionization (12 > z > 5).

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Calibration artefacts in radio interferometry – I. Ghost sources in Westerbork Synthesis Radio Telescope data

Cited by 57 publications

References 32 publications

Non-convex optimization for self-calibration of direction-dependent effects in radio interferometric imaging

Non-convex optimization for self-calibration of direction-dependent effects in radio interferometric imaging

21 cm Observations: Calibration, Strategies, Observables

Lunar occultation of the diffuse radio sky: LOFAR measurements between 35 and 80 MHz

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