Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Traditional radio astronomy imaging techniques assume that the interferometric array is
coplanar, with a small field of view, and that the two-dimensional Fourier relationship
between brightness and visibility remains valid, allowing the Fast Fourier Transform to be
used. In practice, to acquire more accurate data, the non-coplanar baseline effects need
to be incorporated, as small height variations in the array plane introduces the w spatial
frequency component. This component adds an additional phase shift to the incoming signals.
There are two approaches to account for the non-coplanar baseline effects: either the full
three-dimensional brightness and visibility model can be used to reconstruct an image, or the
non-coplanar effects can be removed, reducing the three dimensional relationship to that of the
two-dimensional one.
This thesis describes and implements the w-projection and w-stacking algorithms. The aim of
these algorithms is to account for the phase error introduced by non-coplanar synthesis arrays
configurations, making the recovered visibilities more true to the actual brightness distribution
model. This is done by reducing the 3D visibilities to a 2D visibility model. The algorithms
also have the added benefit of wide-field imaging, although w-stacking supports a wider field
of view at the cost of more FFT bin support. For w-projection, the w-term is accounted for in
the visibility domain by convolving it out of the problem with a convolution kernel, allowing
the use of the two-dimensional Fast Fourier Transform. Similarly, the w-Stacking algorithm
applies a phase correction in the image domain to image layers to produce an intensity model
that accounts for the non-coplanar baseline effects.
This project considers the KAT7 array for simulation and analysis of the limitations and
advantages of both the algorithms. Additionally, a variant of the Högbom CLEAN algorithm
was used which employs contour trimming for extended source emission flagging. The CLEAN
algorithm is an iterative two-dimensional deconvolution method that can further improve image
fidelity by removing the effects of the point spread function which can obscure source data. / AFRIKAANSE OPSOMMING: Tradisionele beeldvormingstegnieke in radio-astronomie aanvaar dat die interferometriese
skikking samevlakkig is. Dit beteken dat die twee-dimensionele Fourier verhouding tussen
helderheid en sigbaarheid geldig bly en dat die Vinnige Fourier Transform aangewend kan
word. Klein hoogtevariasies in die skikkingsvlak bring die w-ruimtelike frekwensiekomponent
mee, wat ’n faseverskuiwing in die inkomende seine tot gevolg het. Dus, in praktyk, moet
die bydrae van die nie-samevlakkige basislyneffekte in ag geneem word om sodoende
die akkuraatheid van die data te verhoog. Twee benaderings kan gevolg word om die
nie-samevlakkige basislyneffekte in ag te neem: Metodes wat die volle drie dimensionele
helderheid en sigbaarheidsmodel gebruik kan toegepas word om ’n beeld te herbou, andersins
kan die nie-samevlakkige effekte verwyder word om sodoende die drie-dimensionele
verhouding te verminder tot ’n twee-dimensionele verhouding.
Hierdie tesis beskryf en implementeer die ‘w-projeksie’ en ‘w-stapel’ algoritmes. Die doel van
hierdie algoritmes is om die fasefout wat deur nie-samevlakkige sinteseskikkingskonfigurasies
veroorsaak word, reg te stel. Hierdie regstelling maak die herwinde sigbaarheid van die beeld
meer getrou aan die werklike helderheidsverspreidingsmodel. ’n Bykomende voordeel van
die algoritmes is beeldvorming van wye-veld ruimtewaarnemings. In ‘w-projection’ word die
w-term in die sigbaarheidsdomein in ag geneem deur die ruimtelike frekwensiekomponent met
behulp van ’n konvolusiekern vanuit die probleem te verwyder. Die twee-dimensionele Vinnige
Fourier Transform kan gevolglik toegepas word. Soortgelyk hieraan, wend die ‘w-Stacking’
algoritme ’n fasekorreksie aan tot ’n reeks beeldlae, om sodoende ’n beeld te verkry wat die
nie-samevlakkige basislyneffekte in ag neem.
Die KAT7 teleskoop is gebruik in die simulasie en analiese van die tekortkominge en voordele
van beide algoritmes. ’n Hibriede weergawe van die Högbom CLEAN algoritme is bykomend
oorweeg. Hierdie algoritme is ’n iteratiewe twee-dimensionele dekonvolusiemetode wat die
betroubaarheid van beelde verbeter deur die verskansingseffek van puntverspreidingsfunksies
te verwyder. Verder gebruik die Högbom CLEAN algoritme kontoersnoeiing om uitgebreide
bron-emisies te identifiseer.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/96902 |
| Date | 03 1900 |
| Creators | Goodrick, Lee |
| Contributors | Davidson, David Bruce, Young, Andre, Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | xii, 73 pages : illustrations |
| Rights | Stellenbosch University |
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