Third generation calibrations for Meerkat Observation of Saraswati Supercluster
- Authors: Kincaid, Robert Daniel
- Date: 2022-10-14
- Subjects: Square Kilometre Array (Project) , Superclusters , Saraswati Supercluster , Radio astronomy , MeerKAT , Calibration
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/362916 , vital:65374
- Description: The international collaboration of the Square Kilometre Array (SKA), which is one of the largest and most challenging science projects of the 21st century, will bring a revolution in radio astronomy in terms of sensitivity and resolution. The recent launch of several new radio instruments, combined with the subsequent developments in calibration and imaging techniques, has dramatically advanced this field over the past few years, thus enhancing our knowledge of the radio universe. Various SKA pathfinders around the world have been developed (and more are planned for construction) that have laid down a firm foundation for the SKA in terms of science while additionally giving insight into the technological requirements required for the projected data outputs to become manageable. South Africa has recently built the new MeerKAT telescope, which is a SKA precursor forming an integral part of SKA-mid component. The MeerKAT instrument has unprecedented sensitivity that can cater for the required science goals of the current and future SKA era. It is noticeable from MeerKAT and other precursors that the data produced by these instruments are significantly challenging to calibrate and image. Calibration-related artefacts intrinsic to bright sources are of major concern since, they limit the Dynamic Range (DR) and image fidelity of the resulting images and cause flux suppression of extended sources. Diffuse radio sources from galaxy clusters in the form of halos, relics and most recently bridges on the Mpc scale, because of their diffuse nature combined with wide field of view (FoV) observations, make them particularly good candidates for testing the different approaches of calibration. , Thesis (MSc) -- Faculty of Science, Physics and Electronics, 2022
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- Date Issued: 2022-10-14
A Systematic Visualisation Framework for Radio-Imaging Pipelines
- Authors: Andati, Lexy Acherwa Livoyi
- Date: 2021-04
- Subjects: Radio interferometers , Radio astronomy -- Data processing , Radio astronomy -- Data processing -- Software , Jupyter
- Language: English
- Type: thesis , text , Masters , MSc
- Identifier: http://hdl.handle.net/10962/177338 , vital:42812
- Description: Pipelines for calibration and imaging of radio interferometric data produce many intermediate images and other data products (gain tables, etc.) These often contain valuable information about the quality of the data and the calibration, and can provide the user with valuable insights, if only visualised in the right way. However, the deluge of data that we’re experiencing with modern instruments means that most of these products are never looked at, and only the final images and data products are examined. Furthermore, the variety of imaging algorithms currently available, and the range of their options, means that very different results can be produced from the same set of original data. Proper understanding of this requires a systematic comparison that can be carried out both by individual users locally, and by the community globally. We address both problems by developing a systematic visualisation framework based around Jupyter notebooks, enriched with interactive plots based on the Bokeh and Datashader visualisation libraries. , Thesis (MSc) -- Faculty of Science, Department of Physics and Electronics, 2021
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- Date Issued: 2021-04
Addressing flux suppression, radio frequency interference, and selection of optimal solution intervals during radio interferometric calibration
- Authors: Sob, Ulrich Armel Mbou
- Date: 2020
- Subjects: CubiCal (Software) , Radio -- Interference , Imaging systems in astronomy , Algorithms , Astronomical instruments -- Calibration , Astronomy -- Data processing
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/147714 , vital:38663
- Description: The forthcoming Square Kilometre Array is expected to provide answers to some of the most intriguing questions about our Universe. However, as it is already noticeable from MeerKAT and other precursors, the amounts of data produced by these new instruments are significantly challenging to calibrate and image. Calibration of radio interferometric data is usually biased by incomplete sky models and radio frequency interference (RFI) resulting in calibration artefacts that limit the dynamic range and image fidelity of the resulting images. One of the most noticeable of these artefacts is the formation of spurious sources which causes suppression of real emissions. Fortunately, it has been shown that calibration algorithms employing heavy-tailed likelihood functions are less susceptible to this due to their robustness against outliers. Leveraging on recent developments in the field of complex optimisation, we implement a robust calibration algorithm using a Student’s t likelihood function and Wirtinger derivatives. The new algorithm, dubbed the robust solver, is incorporated as a subroutine into the newly released calibration software package CubiCal. We perform statistical analysis on the distribution of visibilities and provide an insight into the functioning of the robust solver and describe different scenarios where it will improve calibration. We use simulations to show that the robust solver effectively reduces the amount of flux suppressed from unmodelled sources both in direction independent and direction dependent calibration. Furthermore, the robust solver is shown to successfully mitigate the effects of low-level RFI when applied to a simulated and a real VLA dataset. Finally, we demonstrate that there are close links between the amount of flux suppressed from sources, the effects of the RFI and the employed solution interval during radio interferometric calibration. Hence, we investigate the effects of solution intervals and the different factors to consider in order to select adequate solution intervals. Furthermore, we propose a practical brute force method for selecting optimal solution intervals. The proposed method is successfully applied to a VLA dataset.
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- Date Issued: 2020