Implementation of a calibration algorithm for efficient modeling of direction-dependent and baseline-dependent effects for interferometric imaging arrays in radio astronomy

Hokwana, Anathi (2017-03)

Thesis (MEng)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: This thesis presents the implementation of the H ¨ ogbom Clean and the AStacking algorithms. Radio interferometric imaging arrays are subject to various direction-dependent effects, such as manufacturing tolerances of the antennas, mutual coupling and ionospheric delays. Since these direction-dependent effects are different for every baseline, the Fourier relationship between the intensity function and the visibility function breaks down, as the measured visibilities are no longer related to the same apparent sky. This break down in the Fourier relationship implies that the efficiency of the Fast Fourier Transform(FFT) in computing the backward calculation (visibility to sky) can no longer be employed due to the effects introduced in the gain matrix. A linear model called A-Stacking is used to correct for these effects and provides a trade-off between accuracy and computational efficiency. In this work, FEKO simulations for LOFAR-like and PAPER-like interferometer arrays have been computed with special interests in direction-dependent effects due to mutual coupling of these arrays. The efficiency of the H ¨ ogbom Clean algorithmhave been investigated and as expected, found to perform better for point source-like structure than extended structure. The perfomance of the A-Stacking algorithm has also been investigated, and a simple trade off between accuracy and computational cost when computing the forward calculation (sky to visibility) is shown.

AFRIKAANSE OPSOMMING: In hierdie werkstuk word die implementasie van die “H ¨ ogbom Clean” en die “A-Stacking” algoritmes verduidelik. Radio interferometriese beelding samestellings word beïnvloed deur verskeie nigting afhanklike effekte, soos vervaardigingstoleransie van die antennas, wedersydse koppeling en ionosferiese vertragings. Bogenoemde effekte is anders vir elke basislyn en veroorsaak dat die Fourier transformasie tussen die intensiteit en sigbaarheid funksies afbreek. Die rede hiervoor is dat die gemete sigbaarheid nie meer verwant is aan die dieselfde o ¨ enskynlike ruimte nie. Die afbreek van die Fourier transformasie verhouding impliseer dat die effektiwiteit van die Vinnige Fourier Transformasie (VFT) nie meer van toepassing is nie, hoofsaaklik weens effekte wat in die aanwins matriks voorgestel word. ‘n Lineêremodel, genaamd die “A-Stacking” algoritme word gebruik omhierdie effekte te korrigeer. In hierdie navorsing word FEKO simulasies van die LOFAR en PAPER tipe antenna samestellings gebruik om nigting afhanklike effekte, voorgestel weens wedersydse koppeling, te analiseer. Die effektiwiteit van die “ H ¨ ogbom Clean” algoritme word ook ondersoek en daar word gevind dat dit beter werk vir puntbron tipe, eerder as uitgebreide strukture. Die gedrag van die “A-Stacking” algoritme beeld ook ‘n goeie balans tussen akkuraatheid en berekenings-koste (in termevan rekenaargeheue en looptyd) uit, spesifiek tydens die berekening van voorwaartse stap (beeld tot sigbaarheid).

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/101145
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