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Advanced signal processing for calibration and imaging in radio interferometry

English title Advanced signal processing for calibration and imaging in radio interferometry
Applicant Wiaux Yves
Number 140861
Funding scheme Project funding (Div. I-III)
Research institution Laboratoire de traitement des signaux 2 EPFL - STI - IEL - LTS2
Institution of higher education EPF Lausanne - EPFL
Main discipline Information Technology
Start/End 01.08.2012 - 31.07.2013
Approved amount 101'321.00
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All Disciplines (2)

Discipline
Information Technology
Astronomy, Astrophysics and Space Sciences

Keywords (5)

Signal processing: sparsity; Signal processing: inverse problems; Astronomy: imaging; Astronomy: radio interferometry; Signal processing: compressive sampling

Lay Summary (French)

Lead
Les défis scientifiques pour les décennies à venir en astronomie requièrent que les futurs radio-interféromètres permettent l'imagerie de phénomènes beaucoup plus ténus qu'aujourd'hui, et à plus haute résolution angulaire. Ces télescopes devront prendre en compte des données toujours plus complexes et volumineuses. Dans ce contexte, les techniques d'imagerie par interférométrie doivent tout simplement être réinventées.
Lay summary

Ce projet a permis des avancées algorithmiques importantes pour l'imagerie en radio-interférométrie. Une première version d'un logiciel de reconstruction d'images appelé PURIFY a été implémentée. Ce logiciel regroupe un ensemble d'algorithmes précédemment définis par notre groupe reposant sur la nouvelle théorie de l'échantillonnage compressif. Cette théorie  démontre qu'il est possible de reconstruire des signaux parcimonieux à partir de données incomplètes, et propose de nouvelles approches de reconstruction.

Nos résultats confirment la supériorité de nos algorithmes relativement à l'état de l'art, et le fait que nos structures algorithmiques offrent la possibilité de traiter des images de grande dimension comme celles attendues pour les futures télescopes.


Direct link to Lay Summary Last update: 18.07.2013

Responsible applicant and co-applicants

Employees

Publications

Publication
On Sparsity Averaging
Carrillo Rafael, McEwen Jason, Wiaux Yves (2013), On Sparsity Averaging, in SAMPTA 2013.
Sparsity Averaging for Compressive Imaging
Carrillo Rafael, McEwen Jason, Van De Ville Dimitri, Thiran Jean-Philippe, Wiaux Yves (2013), Sparsity Averaging for Compressive Imaging, in {IEEE} Signal Processing Letters, 20(6), 591-594.
Sparsity averaging for radio-interferometric imaging
Carrillo Rafael, McEwen Jason, Wiaux Yves (2013), Sparsity averaging for radio-interferometric imaging, in BASP Frontiers 2013.
The varying w spread spectrum effect for radio interferometric imaging
Wolz Laura, Abdalla Filipe, Carrillo Rafael, Wiaux Yves, McEwen Jason (2013), The varying w spread spectrum effect for radio interferometric imaging, in BASP Frontiers 2013.

Collaboration

Group / person Country
Types of collaboration
Astrophysics Group, University College London, UK: Dr J. McEwen & Dr F. Abdalla Switzerland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
Astrophysics Group, University of Cambridge: Prof. P. Alexander & Prof. M. Hobson Great Britain and Northern Ireland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
Physics & Astronomy, Southampton University, UK: Dr A. Scaife Great Britain and Northern Ireland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
National Radio Astronomy Observatory, Socorro, USA: Dr S. Bhatnagar & Dr U. Rau United States of America (North America)
- in-depth/constructive exchanges on approaches, methods or results

Scientific events

Active participation

Title Type of contribution Title of article or contribution Date Place Persons involved
SPARS 2013 08.07.2013 Switzerland
SAMPTA 2013 01.07.2013 Germany


Self-organised

Title Date Place
BASP Frontiers 20.01.2013 Switzerland

Associated projects

Number Title Start Funding scheme
146594 Next-generation calibration and imaging in radio interferometry 01.08.2013 Project funding (Div. I-III)
138311 Advanced signal processing on the sphere for high angular resolution diffusion magnetic resonance imaging 01.04.2012 Project funding (Div. I-III)
130359 Compressed sensing imaging techniques for radio interferometry 01.06.2010 Project funding (Div. I-III)

Abstract

Aperture synthesis in radio interferometry is a powerful technique that dates back to more than sixty years ago. It allows observations of the sky with otherwise inaccessible angular resolutions and sensitivities (i.e. dynamic range), providing a wealth of information for astrophysics and cosmology. The measurement equation for aperture synthesis provides incomplete linear information about the signal, thus defining an ill-posed inverse problem in the perspective of signal reconstruction. Under restrictive assumptions of narrow-band (i.e. monochromatic) non-polarized imaging on small fields of view, the visibilities measured identify with Fourier measurements. Already powerful calibration and imaging techniques have been developed in the field, which in essence regularize the inverse problem through an implicit sparsity assumption of the signal in the spatial dimensions.The new science envisaged for the next decades in astronomy requires that next-generation radio telescopes, such as the new LOw Frequency ARray (LOFAR), or the future Extended Very Large Array (EVLA) and Square Kilometer Array (SKA), achieve much higher dynamic range than current instruments, also at higher angular resolution. These telescopes will also have to consider wide-band (i.e. hyper-spectral) polarized imaging on wide fields of view on the celestial sphere. Direction-dependent effects further complicate the measurement equation, and will have to be accounted for in this high-dimensional imaging process, and calibrated. In this context, calibration and imaging techniques for radio interferometry literally need to be re-invented, thus triggering an intense research in the field.The now famous theory of compressive sampling deals with the recovery of sparse signals from incomplete linear measurements. It acknowledges the fact that natural signals often exhibit a sparse representation in multi-scale bases, and proposes both optimization of the acquisition techniques, and a large variety of regularization priors to solve an ill-posed inverse problem for image reconstruction, in particular through convex optimization. Dr Wiaux and Prof. P. Vandergheynst are renowned experts in signal processing, in particular compressive sampling. Dr Wiaux pioneered the field of compressive sampling for radio interferometry, as evident from his publications and the subsequent work triggered in the radio interferometry community.An ongoing project of the Swiss National Science Foundation (SNSF) conducted by the applicants has recently provided conceptual advances in the context of compressive sampling for improving wide-field imaging relatively to state-of-the-art techniques.The present three-year postdoctoral project proposed for funding to the SNSF aims at continuing the ongoing project by providing conceptual advances, still in a wide-field context, for wide-band imaging on one side, and calibration and imaging in the presence of direction-dependent effects on the other side. Beyond standard compressive sampling ideas for promoting signal sparsity, these advances will rely on the theory of matrix completion, one of the latest evolutions in the field compressive sampling, dealing with the recovery of low-rank matrices, i.e. sparse in their singular value decomposition, from incomplete linear measurements. Wide-band signals or calibration matrices bear a non-negligible degree of structure or correlation and thus represent very interesting targets.We will demonstrate the advantage in speed and versatility of convex optimization algorithms for wide-band imaging and calibration, enabling both enhanced signal reconstruction quality and scalability to high-dimensional problems. A tight comparison of our developments with the state-of-the-art calibration and imaging techniques will be performed in a realistic setting, in particular accounting for continuous measurement distributions, and considering the wide-field problem directly in natural spherical coordinates, rather than on the commonly adopted equatorial plane coordinates. Moreover, generalizing recent work by the applicants on the effect of the so-called w component, a spread spectrum phenomenon related to the convolutional nature of the direction-dependent effects will be highlighted. Its potential to enhance the imaging quality will be investigated, possibly paving the way to unexpected acquisition strategy optimizations.This proposal thus reinforces a unique attempt at the international level to bridge the gaps between the two pillars of signal processing and astronomy that are compressive sampling and aperture synthesis in radio interferometry. As such, and as suggested by the recognized expertise of the applicants, it will significantly advance the state-of-the-art calibration and imaging techniques in radio interferometry, thereby enhancing the quality of astrophysical and cosmological interpretation of the data.
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