Dissertations / Theses on the topic 'Very long baseline interferometry'
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1
Howard, P. D. "Very Long Baseline Interferometry and earth rotation parameters." Thesis, University of Nottingham, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.373809.
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Peterson, William M. "Very long baseline interferometry of evolved binary systems." Diss., University of Iowa, 2011. https://ir.uiowa.edu/etd/1056.
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Our understanding of the magnetic fields of stars is nearly monopolized by knowledge of our own sun. Very-long baseline radio interferometry (VLBI) provides a means of directly imaging the radio coronae of other stars at a high enough resolution to discern discrete features, something not possible with other observational methods. It also allows the highest positional accuracy of any observational method available. This thesis details the VLBI study of two nearby radio-loud stars in close binary systems. Both systems were observed at numerous epochs spaced over a range of several months with the global-scale radio interferometer array the VLBA-HSA. These observations were combined with archival data from the National Radioastronomy Observatory (NRAO) Archive. The resulting positions, as well as previously published positions, were used to calculate a least-squares solution for all the orbital and astrometric parameters of both systems. This made it possible to determine the position of the physical star with respect to the radio emission in each image. The images of both stars were found to conform to theoretical emission produced by a model dipolar magnetosphere with a filled or partially-filled coronal loop.
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Kemball, Athol James. "Data reduction techniques for Very Long Baseline Interferometric spectropolarimetry." Thesis, Rhodes University, 1993. http://hdl.handle.net/10962/d1005242.
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This thesis reports the results of an investigation into techniques for the calibration and imaging of spectral line polarization observations in Very Long Baseline Interferometry (VLBI). A review is given of the instrumental and propagation effects which need to be removed in the course of calibrating such obervations, with particular reference to their polarization dependence. The removal of amplitude and phase errors and the determination of the instrumental feed response is described. The polarization imaging of such data is discussed with particular reference to the case of poorly sampled cross-polarization data. The software implementation of the algorithms within the Astronomical Image Processing System (AlPS) is discussed and the specific case of spectral line polarization reduction for data observed using the MK3 VLBI system is considered in detail. VLBI observations at two separate epochs of the 1612 MHz OH masers towards the source IRC+ 10420 are reduced as part of this work. Spectral line polarization maps of the source structure are presented, including a discussion of source morphology and variability. The source is sigmficantly circularly polarized at VLBI resolution, but does not display appreciable linear polarization. A proper motion study of the circumstellar envelope is presented, which supports an ellipsoidal kinematic model with anisotropic radial outflow. Kinematic modelling of the measured proper motions suggests a distance to the source of ~ 3 kpc. The cirumstellar magnetic field strength in the masing regions is determined as 1-3 mG, assuming Zeeman splitting as the polarization mechanism.
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Momjian, Emmanuel. "Sensitive very long baseline interferometry studies of interacting/merging galaxies." Lexington, Ky. : [University of Kentucky Libraries], 2003. http://lib.uky.edu/ETD/ukyphys2003d00108/EMthesis.pdf.
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Thesis (Ph. D.)--University of Kentucky, 2003.Title from document title page (viewed onJune 1, 2004). Document formatted into pages; contains ix, 106 leaves : ill. (some col.). Includes abstract and vita. Includes bibliographical references (p. 99-104).
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Perez-Torres, Miguel ©¹ngel. "Radioemision dy supernovas y astrometria de alta precision = radio emission from supernovae and high precision astrometry / memoria presentada por Miguel Angel Perez Torres." Link to ADS abstract, 1999. http://adsabs.harvard.edu//abs/1999PhDT........11P.
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Richter, Laura. "A VLBI polarisation study of 43 GHZ SiO masers towards VY CMA /." Link to this resource, 2005. http://eprints.ru.ac.za/784/.
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Richter, Laura. "A VLBI polarisation study of 43 GHZ SiO masers towards VY CMA." Thesis, Rhodes University, 2006. http://hdl.handle.net/10962/d1005284.
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This thesis reports the calibration, imaging and analysis of one epoch of VLBI observations of the v (italics) = J (italics) = 1-0 transition of SiO towards VY CMa. Full polarisation information was recorded, allowing high resolution synthesis maps of each of the four Stokes parameters to be produced. A total of 81 maser components were extracted from the total intensity map, each approximately 1 mas in size. The emission spans approximately 100 x 80 mas in right ascension and declination and is concentrated to the east. The maser component positions were fitted to a ring of radius ~ 3.2R₊ (italics), or 7.2 x 1O¹⁴ cm for a stellar distance of 1.5 kpc. If the stellar position is assumed to be the centre of this ring then almost all of the maser components fall within the inner dust shell radius, which is at ~ 5R (italics)ϰ All of the maser components fall between 1.5R (italics)ϰ and 6R (italics)ϰ. A velocity gradient with position angle was observed in the sparsely filled western region of the maser ring. If interpreted as evidence of shell rotation, this gradient implies a rotational velocity of v (italics) rot (subscirpt) sin i (italics) = 18 km.s⁻¹. The fractional circular and linear polarisations of the maser spots were derived from the Stokes parameter maps. The mean fractional circular polarisation of the masers components was ~ 2 percent and the median fractional linear polarisation was ~ 6 percent, with many spots displaying over ~ 30 percent linear polarisation. The mean circular polarisation implies a magnetic field of ~ 4 G in the SiO maser region if the polarisation is due to Zeeman splitting. Two maser components display a rotation of linear polarisation position angle with velocity, possibly implying a connection between the magnetic field and the velocity field variations in the region of these components.
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Lenc, Emil. "Studies of radio galaxies and starburst galaxies using wide-field, high spatial resolution radio imaging." Swinburne Research Bank, 2009. http://hdl.handle.net/1959.3/48503.
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Thesis (Ph.D) - Swinburne University of Technology, Faculty of Information & Communication Technologies, 2009.A dissertation presented in fulfillment of the requirements for the degree of Doctor of Philosophy, [Faculty of Information and Communication Technologies], Swinburne University of Technology, 2009. Typescript. Bibliography p. 215-236.
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Shiomi, Tadashi. "Differential Very-Long-Baseline Interferometry Techniques for Precise Orbit Determination of a Geosynchronous Satellite." Kyoto University, 1986. http://hdl.handle.net/2433/162201.
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McAlpine, Kim. "A VLBI study of OH masers in a proto-planetary nebula OH 0.9+1.3." Thesis, Rhodes University, 2008. http://hdl.handle.net/10962/d1005285.
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This thesis reports the calibration, imaging and analysis of one epoch of VLBA observations of the 1612 MHz OH maser emission from the protoplanetary nebula OH 0.9+1.3. These are the first polarisation VLBI observations of this source and the spatial morphology of the OH emission is resolved on this scale. Proto-planetary nebulae represent the transition phase in the evolution of stars between the asymptotic giant branch (AGB) phase and their emergence as planetary nebulae. A long-standing astronomical question is how the predominantly spherical circumstellar envelopes of AGB stars evolve into the bipolar and axisymmetric structures that are commonly observed in planetary nebula. Proto-planetary nebulae offer a unique opportunity to study this transformation process. The high-resolution VLBI maps produced in this thesis were used to investigate the morphology and kinematics of OH 0.9+1.3 with a view to gaining insight into the development of asymmetries in the circumstellar material. The OH maser emission of OH 0.9+1.3 has a double-peaked profile with one peak blue-shifted and the other red-shifted with respect to the stellar velocity. The total intensity maser maps demonstrate a considerable degree of asymmetry with the blue- and red-shifted emission located in spatially distinct regions of the envelope. The blue-shifted emission is distributed preferentially along an axis at a projected position angle of » 135± ( North through East). The morphology of this source is not consistent with the standard symmetric thin-shell model and an attempt to fit the traditional OH/IR kinematic model of a simple expanding shell to the maser components was found to be unsatisfactory. No definitive evidence of a bipolar outflow was observed either. The high degree of asymmetry observed in the source is consistent with its status as a proto-planetary nebula. The source was imaged in all four Stokes parameters and the fractional linear and circular polarisations of the maser components were derived from the Stokes parameter maps. In all except one of the components the total fractional polarisation was found to be low ( < 15%). The mean fractional linear and circular polarisation were calculated to be 5.54% and 7.11% respectively. The absence of an identifiable Zeeman pair in the Stokes V map prohibited the estimation of the magnetic field in the circumstellar envelope of this source.
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Richter, Laura. "A comparative polarimetric study of the 43 GHz and 86 GHz SiO masers toward the supergiant star VY CMa." Thesis, Rhodes University, 2012. http://hdl.handle.net/10962/d1005239.
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The aim of this thesis is to perform observational tests of SiO maser polarisation and excitation models, using component-level comparisons of multiple SiO maser transitions in the 43 GHz and 86 GHz bands at milliarcsecond resolution. These observations reqwre very long baseline interferometric imaging with very accurate polarimetric calibration. The supergiant star VY CMa was chosen as the object of this study due to its high SiO maser luminosity, many detected SiO maser lines, and intrinsic scientific interest. Two epochs of full-polarisation VLBA observations of VY CMa were performed. The Epoch 2 observations were reduced using several new data reduction methods developed as part of this work, and designed specifically to improve the accuracy of circular polarisation calibration of spectral-line VLBI observations at millimetre wavelengths. The accuracy is estimated to be better than 1% using these methods. The Epoch 2 images show a concentration of v= l and v=2 J= 1-0 SiO masers to the east and northeast of the assumed stellar position. The v=l J=2-1 masers were more evenly distributed around the star, with a notable lack of emission in the northeast. There is appreciable spatial overlap between these three lines. The nature of the overlap is generally consistent with the predictions of hydrodynamical circumstellar SiO maser simulations. Where the v=l J = 1-0 and J =2-1 features overlap, the v=l J = 2-1 emission is usually considerably weaker. This is not predicted by current hydrodynamical models, but can be explained in the context of collisional pumping in a low density environment. Six observational tests of weak-splitting maser polarisation models were performed, including intercomparisons of linear polarisation in the v=l J=1-0 and J=2-1lines, linear polarisation versus saturation level, linear polarisation versus distance from the star, circular polarisation in the v= l J = 1-0 and J=2-1 lines, circular versus linear polarisation and modeling of ~ 900 electric-vector position angle rotations. The polarisation model tests generally do not support non-Zeeman circular polarisation mechanisms. For the linear polarisation tests, the results are more consistent with models that predict similar linear polarisation across transitions. The scientific importance of these tests is described in detail and avenues for future work are described.
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Tsioumis, Anastasios K. "VLBI studies of compact radio sources." Thesis, The University of Sydney, 1987. https://hdl.handle.net/2123/25997.
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Radio observations of a complete sample of 44 flat spectrum southern radio sources plus an additional 11 sources of particular interest are presented. Trends in the properties of the complete sample are discussed.
All sources of special interest and 19 of the sources in the complete sample were observed with the Southern Hemisphere VLBI Experiment (SHEVE). This was the first extensive VLBI array in the southern hemisphere and its characteristics are presented in some detail. The data reduction methods and, in particular, the calibration of the large SHEVE dataset are described as this is critical to the quality of the final images. The imaging techniques are illustrated by detailed imaging of the radio source 1934-638. Since few closure phase and closure amplitude relations are present self—calibration methods cannot be employed. It was found that the appropriate imaging method for the SHEVE data was modelfitting with Gaussian components. A detailed image of 1934—638 is presented and the source structure compared with previous VLBI observations. All sources observed in SHEVE were fitted with Gaussian component models. The models are presented in a table and source images shown to illustrate the source structure. The SHEVE data and the fits of the models are also shown.
All sources in the complete sample and six of the additional VLBI sources were mapped at low resolution using the Molonglo Observatory Synthesis Telescope (MOST) at 843 MHz and the Fleurs Synthesis Telescope at 1415 MHz. Special techniques were used at both instruments to gain direct access to the higher spatial frequency data and hence increased effective resolution. This is necessary since most of the sources in the sample are compact with little or no arcsecond-level structure. The results are summarised in tables and maps are presented at both frequencies for all sources. In addition, accurate radio positions were measured and were compared with other radio and optical positions. Significant pointing offsets were revealed for both instruments.
The present results and other source information were then assembled to construct an extensive data base of source properties. Four new optical identifications were made and all objects but one have been identified except in the region obscured by the galactic plane. The database information was used to search for trends in the properties of the complete sample of radio sources. A brief account is given of the sources not in the complete sample and detailed analysis of two special sources is presentedin two appendices.
The thesis concludes with an examination of and suggestions for future VLBI observationsin Australia.
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Porter, David Gareth Jon. "Blazar jets imaged with Very Long Baseline Interferometry (VLBI) - Kinematics of helical trajectories in 3C273 and 3C345." Thesis, Cardiff University, 2009. http://orca.cf.ac.uk/54940/.
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In this work a large collection of VLBI (Very Long Baseline Interferometry) maps are presented based upon data from the VLBA (Very Long Baseline Array) for 29 sources. The maps are the result of three epochs of imaging. A kinematic study was undertaken with a view to understanding some interesting structure seen in the quasars 3C273 and 3C345. Fortunately, these two sources present a large number of components and it was possible to perform model fitting to pinpoint their positions. In addition, model fitting was performed upon all of the sources that were successfully imaged. The aim of this was to eventually derive component velocities for as many blazars studied as possible. The outcomes of the work are presented in tables in the main thesis. In order to further understand the 3C273 and 3C345 jet structures seen a physical model was developed with A. Papageorgiou. With this kinematic model it was possible to trace out a variety of helical jet structures. The free parameters are the injection velocity for new components, the period of jet precession, the viewing angle and the jet half angle. Using the Levenberg- Marquardt algorithm (references given below) it was possible to fit jet trajectories to my model fitted component data. The algorithm produced quantitative output for some of the free parameters in my physical model. Therefore I report average jet half angles for 3C273 and 3C345 of 2.968 0.153 degrees and 2.519 0.573 degrees respectively. In ad dition, I find the precessional periods of 3C273 and 3C345 to be 71.161 19.066 years and 48.478 3.385 years respectively.
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West, Craig James, and cwest@astro swin edu au. "Development of disk-based baseband recorders and software correlators for radio astronomy." Swinburne University of Technology. Centre for Astrophysics and Supercomputing, 2004. http://adt.lib.swin.edu.au./public/adt-VSWT20050804.143155.
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This thesis details work undertaken in the field of radio astronomy instrumentation. Specific components of the data collection and processing systems used by radio astronomers have been implemented using non-traditional approaches. Traditionally, the correlation of radio astronomy data has taken place on dedicated,
specific hardware. This thesis deals with the implementation of equivalent correlators
using software running on generic clusters of personal computers - the software approach to radio astronomy. Toward this end a system has been developed that records the raw telescope output onto computer hard drives, allowing
easy access to the data on cluster supercomputers. Part of this thesis describes the design, construction, testing and utilisation of these data recording systems. The correlator software used to process these data
on supercomputers is also fully described, including extensive tests of the software
and a detailed comparison between its output and the output of an existing hardware correlator. The software correlator is shown to produce output that agrees extremely well with the hardware correlator, verifying its accuracy and performance. Finally, results of on-going scientific investigations that use the software correlators described in this thesis are outlined, illustrating the flexibility and usefulness of the software approach to radio astronomy.
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Morgan, John <1981>. "Very Long Baseline Interferometry in Italy Wide-field VLBI imaging and astrometry and prospects for an Italian VLBI network including the Sardinia Radio Telescope." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2010. http://amsdottorato.unibo.it/2830/2/thesis.pdf.
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In this thesis the use of widefield imaging techniques and VLBI observations with a limited number of antennas are explored.
I present techniques to efficiently and accurately image extremely large UV datasets. Very large VLBI datasets must be reduced into multiple, smaller datasets if today’s imaging algorithms are to be used to image them. I present a procedure for accurately shifting the phase centre of a visibility dataset. This procedure has been thoroughly tested and found to be almost two orders of magnitude more accurate than existing techniques. Errors have been found at the level of one part in 1.1 million. These are unlikely to be measurable except in the very largest UV datasets.
Results of a four-station VLBI observation of a field containing multiple sources are presented. A 13 gigapixel image was constructed to search for sources across the entire primary beam of the array by generating over 700 smaller UV datasets. The source 1320+299A was detected and its astrometric position with respect to the calibrator J1329+3154 is presented. Various techniques for phase calibration and imaging across this field are explored including using the detected source as an in-beam calibrator and peeling of distant confusing sources from VLBI visibility datasets.
A range of issues pertaining to wide-field VLBI have been explored including; parameterising the wide-field performance of VLBI arrays; estimating the sensitivity across the primary beam both for homogeneous and heterogeneous arrays; applying techniques such as mosaicing and primary beam correction to VLBI observations; quantifying the effects of time-average and bandwidth smearing; and calibration and imaging of wide-field VLBI datasets. The performance of a computer cluster at the Istituto di Radioastronomia in Bologna has been characterised with regard to its ability to correlate using the DiFX software correlator. Using existing software it was possible to characterise the network speed particularly for MPI applications. The capabilities of the DiFX software correlator, running on this cluster, were measured for a range of observation parameters and were shown to be commensurate with the generic performance parameters measured.
The feasibility of an Italian VLBI array has been explored, with discussion of the infrastructure required, the performance of such an array, possible collaborations, and science which could be achieved.
Results from a 22 GHz calibrator survey are also presented. 21 out of 33 sources were detected on a single baseline between two Italian antennas (Medicina to Noto).
The results and discussions presented in this thesis suggest that wide-field VLBI is a technique whose time has finally come. Prospects for exciting new science are discussed in the final chapter.
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Morgan, John <1981>. "Very Long Baseline Interferometry in Italy Wide-field VLBI imaging and astrometry and prospects for an Italian VLBI network including the Sardinia Radio Telescope." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2010. http://amsdottorato.unibo.it/2830/.
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In this thesis the use of widefield imaging techniques and VLBI observations with a limited number of antennas are explored.
I present techniques to efficiently and accurately image extremely large UV datasets. Very large VLBI datasets must be reduced into multiple, smaller datasets if today’s imaging algorithms are to be used to image them. I present a procedure for accurately shifting the phase centre of a visibility dataset. This procedure has been thoroughly tested and found to be almost two orders of magnitude more accurate than existing techniques. Errors have been found at the level of one part in 1.1 million. These are unlikely to be measurable except in the very largest UV datasets.
Results of a four-station VLBI observation of a field containing multiple sources are presented. A 13 gigapixel image was constructed to search for sources across the entire primary beam of the array by generating over 700 smaller UV datasets. The source 1320+299A was detected and its astrometric position with respect to the calibrator J1329+3154 is presented. Various techniques for phase calibration and imaging across this field are explored including using the detected source as an in-beam calibrator and peeling of distant confusing sources from VLBI visibility datasets.
A range of issues pertaining to wide-field VLBI have been explored including; parameterising the wide-field performance of VLBI arrays; estimating the sensitivity across the primary beam both for homogeneous and heterogeneous arrays; applying techniques such as mosaicing and primary beam correction to VLBI observations; quantifying the effects of time-average and bandwidth smearing; and calibration and imaging of wide-field VLBI datasets. The performance of a computer cluster at the Istituto di Radioastronomia in Bologna has been characterised with regard to its ability to correlate using the DiFX software correlator. Using existing software it was possible to characterise the network speed particularly for MPI applications. The capabilities of the DiFX software correlator, running on this cluster, were measured for a range of observation parameters and were shown to be commensurate with the generic performance parameters measured.
The feasibility of an Italian VLBI array has been explored, with discussion of the infrastructure required, the performance of such an array, possible collaborations, and science which could be achieved.
Results from a 22 GHz calibrator survey are also presented. 21 out of 33 sources were detected on a single baseline between two Italian antennas (Medicina to Noto).
The results and discussions presented in this thesis suggest that wide-field VLBI is a technique whose time has finally come. Prospects for exciting new science are discussed in the final chapter.
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Weston, Stuart Duncan. "Development of Very Long Baseline Interferometry (VLBI) techniques in New Zealand array simulation, image synthesis and analysis : a thesis submitted to Auckland University of Technology in fulfilment of the requirements for the degree of Master of Philosophy (MPhil), 2008 /." Click here to access this resource online, 2008. http://hdl.handle.net/10292/449.
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This thesis presents the design and development of a process to model Very Long Base Line Interferometry (VLBI) aperture synthesis antenna arrays. In line with the Auckland University of Technology (AUT) Institute for Radiophysics and Space Research (IRSR) aims to develop the knowledge, skills and experience within New Zealand, extensive use of existing radio astronomical software has been incorporated into the process namely AIPS (Astronomical Imaging Processing System), MIRIAD (a radio interferometry data reduction package) and DIFMAP (a program for synthesis imaging of visibility data from interferometer arrays of radio telescopes). This process has been used to model various antenna array configurations for two proposed New Zealand sites for antenna in a VLBI array configuration with existing Australian facilities and a passable antenna at Scott Base in Antarctica; and the results are presented in an attempt to demonstrate the improvement to be gained by joint trans-Tasman VLBI observation. It is hoped these results and process will assist the planning and placement of proposed New Zealand radio telescopes for cooperation with groups such as the Australian Long Baseline Array (LBA), others in the Pacific Rim and possibly globally; also potential future involvement of New Zealand with the SKA. The developed process has also been used to model a phased building schedule for the SKA in Australia and the addition of two antennas in New Zealand. This has been presented to the wider astronomical community via the Royal Astronomical Society of New Zealand Journal, and is summarized in this thesis with some additional material. A new measure of quality (“figure of merit”) for comparing the original model image and final CLEAN images by utilizing normalized 2-D cross correlation is evaluated as an alternative to the existing subjective visual operator image comparison undertaken to date by other groups. This new unit of measure is then used in the presentation of the results to provide a quantative comparison of the different array configurations modelled. Included in the process is the development of a new antenna array visibility program which was based on a Perl code script written by Prof Steven Tingay to plot antenna visibilities for the Australian Square Kilometre Array (SKA) proposal. This has been expanded and improved removing the hard coded fixed assumptions for the SKA configuration, providing a new useful and flexible program for the wider astronomical community. A prototype user interface using html/cgi/perl was developed for the process so that the underlying software packages can be served over the web to a user via an internet browser. This was used to demonstrate how easy it is to provide a friendlier interface compared to the existing cumbersome and difficult command line driven interfaces (although the command line can be retained for more experienced users).
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Hungwe, Faith. "Variability analysis of a sample of potential southern calibration sources." Thesis, Rhodes University, 2009. http://hdl.handle.net/10962/d1005281.
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A considerable number of Very Long Baseline Interferometry (VLBI) surveys have been conducted in the northern hemisphere and very few in the southern hemisphere mostly because of a lack of telescopes and therefore adequate baseline coverage. Thus there is a deficit of calibrator sources in the southern hemisphere. Further, some of the most interesting astronomical objects eg. the galactic centre and the nearest galaxies (the small and large Magellanic Clouds) lie in the southern hemisphere and these require high resolution studies. With a major expansion of radio astronomy observing capability on its way in the southern hemisphere (with the two SKA (Square Kilometre Array) precursors, meerKAT (Karoo Array Telescope) and ASKAP (Australian SKA Pathfinder), leading to the SKA itself) it is clear that interferometry and VLBI in the southern hemisphere need a dense network of calibration sources at different resolutions and a range of frequencies. This work seeks to help redress this problem by presenting an analysis of 31 southern sources to help fill the gaps in the southern hemisphere calibrator distribution. We have developed a multi-parameter method of classifying these sources as calibrators. From our sample of 31 sources, we have 2 class A sources (Excellent calibrators), 16 class B sources (Good calibrators), 9 class C sources (Poor calibrators) and 4 class D sources (Unsuitable calibrators).
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Spinks, M. J. "Low frequency long baseline interferometry." Thesis, University of Cambridge, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.372284.
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Merlano, Duncan Juan Carlos. "Phase synchronization scheme for very long baseline coherent arrays." Doctoral thesis, Universitat Politècnica de Catalunya, 2012. http://hdl.handle.net/10803/96673.
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Precise phase synchronization of clock or local oscillator signals over distributed systems is a recurring requirement in the design of a wide range of systems, with applications in telecommunications and, for example, in phased arrays. Addressing this requirement is a challenging task when the distance between the subsystems is large in terms of the wavelength and, in particular, when this electrical distance is time-varying.
Carrier phase synchronization is the most important challenge in order to implement distributed beamforming systems and to optimize their power efficiency. At the same time, phase synchronization is crucial for the implementation of distributed (bistatic / multistatic) radar systems.
In the last decades different techniques for remote phase synchronization have been proposed. In 1968 Thompson discussed, compared and classified the available techniques for reducing propagation induced phase fluctuations in frequency distribution systems and presented the principle of round trip stabilization systems. These phase synchronization schemes are Phase Locked Loops (PLL) in which the phase offset is measured directly for the signal which travels along the two-way path, and a direct feedback action closes the loop.
The most important challenge for the design of these systems is the implementation of a method for the discrimination between the waves that travel in opposite directions in the transmission channel. One approach that has been proposed in previous works to fulfill this requirement is the use of different frequencies.
This Ph.D. thesis dissertation presents a new scheme for remote carrier phase synchronization in which a set of tones with similar frequencies are simultaneously used to synchronize two separated stations. In the proposed scheme, the bandwidth required by the set of tones traveling along the loop can be reduced wherewith the phase fluctuations generated by the dispersive effects of the medium can be drastically reduced.
The proposed scheme was prototyped and tested in the laboratory, with satisfactory results, and a possible application of the phase synchronization prototype has been evaluated.
In order to show the excellent behavior of the phase synchronization prototype a simple distance measurement experiment has been carried out. In that experiment the variations of a distance are obtained from the variations of a measured phase using the synchronization prototype.
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Grellmann, Rebekka. "Massive star formation as seen by infrared long-baseline interferometry." Diss., lmu, 2012. http://nbn-resolving.de/urn:nbn:de:bvb:19-149346.
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Torres, Rosa M. "Midiendo regiones de formacion estelar cercanas con el VLBA : de la distancia a la dinamica = Measuring nearby star forming regions with the VLBA: from the distance to the dynamics /." Morelia, Michoacan, Mexico : Universidad Nacional Autonoma de Mexico, 2009. http://arxiv.org/abs/1001.5144.
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Kok, Yitping. "Phase-referenced Interferometry and Narrow-angle Astrometry with SUSI." Thesis, The University of Sydney, 2013. http://hdl.handle.net/2123/10028.
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This thesis describes the development of an astrometric facility at the Sydney University Stellar Interferometer (SUSI) with an aim to measure at high precision the relative astrometry of bright close binary stars and ultimately to detect the presence of exoplanets within those binary star systems through observations of the systems’ perturbed motion. At the core of the facility is a new beam combiner that is phase-referenced to an existing primary beam combiner in the visible wave- length regime. The latter provides post-processed fringe-tracking information to the former for fringe stabilization and coherent integration of pre-recorded stellar fringes using newly developed data reduction software. Interference fringe packets of a binary star are recorded alternately; first the fringe packet of the primary, then the secondary, finally back to the primary again. The measurement of the fringe packet separation is facilitated by an air-filled differential delay line and a network of interferometer-based metrology systems. Characterizations and initial astronomical observations carried out with the dual beam combiner setup demonstrated for the first time the success of the dual-star phase-referencing technique in visible (~1μm) wavelengths. The current astrometric precision is larger than 100μas while the long term astrometric accuracy is yet to be characterized. In a parallel development, a complementary observing method using only the primary beam combiner is also demonstrated in this thesis. Relative astrometry of binary stars up to ~0.8” separation with this technique has been demonstrated to have precision of better than 100μas. A simple detection limit analysis based on a list of target binary stars estimates up to two exoplanet detections can be achieved with SUSI if the new astrometric facility attains precision of 10μas while the primary beam combiner operates at its designed peak performance. Finally, one new stellar companion was resolved and a preliminary astrometry for another suspected companion was estimated from the astronomical observation data collected throughout the course of this thesis.
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Grellmann, Rebekka [Verfasser], and Thomas [Akademischer Betreuer] Preibisch. "Massive star formation as seen by infrared long-baseline interferometry / Rebekka Grellmann. Betreuer: Thomas Preibisch." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2012. http://d-nb.info/1027842380/34.
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Cox, Henry G. "Interagency Arraying." International Foundation for Telemetering, 1987. http://hdl.handle.net/10150/615319.
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International Telemetering Conference Proceedings / October 26-29, 1987 / Town and Country Hotel, San Diego, CaliforniaVoyager ground aperture requirements for Neptune encounter in August 1989 exceed the expected capabilities of the Jet Propulsion Laboratory's Deep Space Network (DSN) 70- and 34-meter antennas. Agreements have been consummated with the National Science Foundation to array the National Radio Astronomy Observatory's Very Large Array in New Mexico and with the Commonwealth Scientific and Industrial Research Organization's Parkes Radio Telescope in Australia with the DSN. This technique, which was demonstrated during Voyager's Uranus encounter, will provide a greater return of imaging and non-imaging science data. The arrays consist of the normal facility receiving equipment at each location, augmented by special receiving, combining, recording, and monitor and control equipment. This equipment has been designed, is being implemented, and will be operated during the Neptune encounter to effectively double the available antenna aperture over the western United States and Australia.
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Bowsher, Emily Collins. "Measuring the Effective Wavelength of CHARA Classic." Digital Archive @ GSU, 2010. http://digitalarchive.gsu.edu/phy_astr_theses/8.
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This thesis presents an engineering project measuring the effective wavelength of the CHARA Classic beam combiner on the CHARA Array. Knowing the actual effective wavelength of light observed is very important because that value is necessary for determining astrophysical parameters of stars. Currently, the value used for CHARA Classic data comes from a model of the system and is based on numbers published by the manufacturer of the filter; it is not derived from measurements done on the system directly. We use two data collection methods to observe standard stars of different spectral types and calculate the wavelength of light recorded by the instrument for each star. We find the best estimate of the effective wavelength for the CHARA Classic K′-band configuration to be 2.138±0.003μm, a 0.56% decrease from the previously adopted value of 2.150μm. Our result establishes the first estimate of the uncertainty in the effective wavelength.
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O'Brien, David P. "The Self-Calibration Method for Multiple Systems at the CHARA Array." Digital Archive @ GSU, 2011. http://digitalarchive.gsu.edu/phy_astr_diss/48.
Full textAbstract:
The self-calibration method, a new interferometric technique using measurements in the K′-band (2.1 μm) at the CHARA Array, has been used to derive orbits for several spectroscopic binaries. This method uses the wide component of a hierarchical triple system to calibrate visibility measurements of the triple’s close binary system through quasi-simultaneous observations of the separated fringe packets of both. Prior to the onset of this project, the reduction of separated fringe packet data had never included the goal of deriving visibilities for both fringe packets, so new data reduction software has been written. Visibilities obtained with separated fringe packet data for the target close binary are run through both Monte Carlo simulations and grid search programs in order to determine the best-fit orbital elements of the close binary.
Several targets, with spectral types ranging from O to G and luminosity classesfrom III to V, have been observed in this fashion, and orbits have been derived for the close binaries of eight targets (V819 Her B, Kappa Peg B, Eta Vir A, Eta Ori Aab, 55 UMa A, 13 Ceti A, CHARA 96 Ab, HD 129132 Aa). The derivation of an orbit has allowed for the calculation of the masses of the components in these systems. The magnitude differences between the components can also be derived, provided that the components of the close binary have a magnitude difference of Delta K < 2.5 (CHARA’s limit). Derivation of the orbit also allows for the calculation of the mutual inclination (Phi), which is the angle between the planes of the wide and close orbits. According to data from the Multiple Star Catalog, there are 34 triple systems other than the 8 studied here for which the wide and close systems both have visual orbits. Early formation scenarios for multiple systems predict coplanarity (Phi < 15 degrees), but only 6 of these 42 systems are possibly coplanar. This tendency against coplanarity may suggest that the capture method of multiple system formation is more important than previously believed.
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Blind, Nicolas. "Etude de concept d'instruments cophaseur pour l'imagerie interférométrique infrarouge. Observation de binaires en interaction à très haute résolution angulaire." Phd thesis, Université de Grenoble, 2011. http://tel.archives-ouvertes.fr/tel-00684251.
Full textAbstract:
Malgré sa capacité unique à discerner des détails qu'aucun instrument "classique" ne peut voir, l'interférométrie optique est fortement handicapée par l'atmosphère. Celle-ci limite drastiquement les temps de pose des interféromètres au sol et les empèche d'accumuler suffisamment de photons pour observer des sources toujours plus faibles, limitant de facto l'échantillon des astres observables. Les suiveurs de franges sont des instruments développés spécifiquement dans le but de compenser ces perturbations atmosphériques, et ainsi de repousser les limites de l'univers visible par les interféromètres optiques. Le but premier de cette thèse est d'étudier et d'améliorer ces instruments, dans le contexte des technologies actuelles et des nouvelles générations d'interféromètres combinant 4 télescopes et plus. La seconde grande partie de cette thèse s'attachera quant à elle à montrer l'intérêt de l'interférométrie optique dans l'étude des binaires en interaction, astres en mesure d'apporter des réponses à un vaste panel de champs d'étude du fait de la diversité des processus physiques en jeu en leur sein.
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Raghavan, Deepak. "A Survey of Stellar Families: Multiplicity of Solar-type Stars." unrestricted, 2009. http://etd.gsu.edu/theses/available/etd-04212009-165714/.
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Thesis (Ph. D.)--Georgia State University, 2009.Title from file title page. Harold A. McAlister, committee chair; Russel J. White, Brian D. Mason, Douglas R. Gies, David W. Latham, A.G. Unil Perera, Todd J. Henry, committee members. Description based on contents viewed Aug. 24, 2009. Includes bibliographical references (p. 391-410).
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Perianhes, Roberto Vitoriano. "Utilizando algoritmo de cross-entropy para a modelagem de imagens de núcleos ativos de galáxias obtidas com o VLBA." Universidade Presbiteriana Mackenzie, 2017. http://tede.mackenzie.br/jspui/handle/tede/3466.
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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
The images obtained by interferometers such as VLBA (Very Long Baseline Array) and VLBI (Very Long Baseline Interferometry), remain the direct evidence of relativistic jets and outbursts associated with supermassive black holes in active galactic nuclei (AGN). The study of these images are critical tools to the use of information from these observations, since they are one of the main ingredients for synthesis codes7 of extragalactic objects. In this thesis is used both synthetic and observed images. The VLBA images show 2-dimensional observations generated from complex 3-dimensional astrophysical processes. In this sense, one of the main difficulties of the models is the definition of parameters of functions and equations to reproduce macroscopic and dynamic physical formation events of these objects, so that images could be study reliably and on a large scale. One of the goals of this thesis is to elaborate a generic8 form of observations, assuming that the formation of these objects had origin directly by similar astrophysical processes, given the information of certain parameters of the formation events. The definition of parameters that reproduce the observations are key to the generalization formation of sources and extragalactic jets. Most observation articles have focus on few or even unique objects. The purpose of this project is to implement an innovative method, more robust and efficient, for modeling and rendering projects of various objects, such as the MOJAVE Project, which monitors several quasars simultaneously offering a diverse library for creating models (Quasars9 and Blazars10: OVV11 and BL Lacertae12). In this thesis was implemented a dynamic way to study these objects. Presents in this thesis the adaptation of the Cross-Entropy algorithm for the calibration of the parameters of astrophysical events that summarize the actual events of the VLBA observations. The development of the code of the adaptation structure includes the possibility of extension to any image, assuming that these images are dispose in intensities (Jy/beam) distributed in Right Ascension (AR) and Declination (DEC) maps. The code is validating by searching for self-convergence to synthetic models with the same structure, i.e, realistics simulations of components ejection, in milliarcsecond, similar to the observations of the MOJAVE project in 15.3 GHz. With the use of the parameters major semi-axis, angle of position, eccentricity and intensity applied individually to each observed component, it was possible to calculate the structure of the sources, the velocities of the jets, as well as the conversion in flux density to obtain light curves. Through the light curve, the brightness temperature, the Doppler factor, the Lorentz factor and the observation angle of the extragalactic objects can be estimated with precision. The objects OJ 287, 4C +15.05, 3C 279 and 4C +29.45 are studied in this thesis due the fact that they have different and complex morphologies for a more complete study.
As imagens obtidas por interferômetros, tais como VLBA (Very Long Baseline Array) e VLBI (Very Long Baseline Interferometry), são evidências diretas de jatos relativísticos associados a buracos negros supermassivos em núcleos ativos de galáxias (AGN). O estudo dessas imagens é fundamental para o aproveitamento das informações dessas observações, já que é um dos principais ingredientes para os códigos de síntese1 de objetos extragalácticos. Utiliza-se nesta tese, tanto imagens sintéticas quanto observadas. As imagens de VLBA mostram observações em 2 dimensões de processos astrofísicos complexos ocorrendo em 3 dimensões. Nesse sentido, uma das principais dificuldades dos modelos é a definição dos parâmetros das funções e equações que reproduzam de forma macroscópica e dinâmica os eventos físicos de formação desses objetos, para que as imagens sejam estudadas de forma confiável e em grande escala. Um dos objetivos desta tese é elaborar uma forma genérica2 de observações, supondo que a formação desses objetos é originada por processos astrofísicos similares, com a informação de determinados parâmetros da formação dos eventos. A definição de parâmetros que reproduzam as observações são elementos chave para a generalização da formação de componentes em jatos extragalácticos. Grande parte dos artigos de observação são voltados para poucos ou únicos objetos. Foi realizada nesta tese a implementação um método inovador, robusto e eficiente para a modelagem e reprodução de vários objetos, como por exemplo nas fontes do Projeto MOJAVE, que monitora diversos quasares simultaneamente, oferecendo uma biblioteca diversificada para a criação de modelos (Quasares3 e Blazares4: OVV5 e BL Lacertae6). Com essas fontes implementou-se uma forma dinâmica para o estudo desses objetos. Apresenta-se, nesta tese, a adaptação do algoritmo de Cross-Entropy para a calibração dos parâmetros dos eventos astrofísicos que sintetizem os eventos reais das observações em VLBA. O desenvolvimento da estrutura de adaptação do código incluiu a possibilidade de extensão para qualquer imagem, supondo que as mesmas estão dispostas em intensidades (Jy/beam) distribuídas em mapas de Ascensão Reta (AR) e Declinação (DEC). A validação do código foi feita buscando a auto convergência para modelos sintéticos com as mesmas estruturas, ou seja, de simulações realísticas de ejeção de componentes, em milissegundos de arco, similares às observações do projeto MOJAVE, em 15,3 GHz. Com a utilização dos parâmetros semieixo maior, ângulo de posição, excentricidade e intensidade aplicados individualmente a cada componente observada, é possível calcular a estrutura das fontes, as velocidades dos jatos, bem como a conversão em densidade de fluxo para obtenção de curvas de luz. Através da curva de luz estimou-se com precisão a temperatura de brilhância, o fator Doppler, o fator de Lorentz e o ângulo de observação dos objetos extragalácticos. Os objetos OJ 287, 4C +15.05, 3C 279 e 4C +29.45 são estudados nesta tese pois têm morfologias diferentes e complexas para um estudo mais completo.
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Searle, Anthony. "Signal processing and analysis of VLBI data using frequency-agile bandwidth synthesis /." 2008. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&res_dat=xri:pqdiss&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&rft_dat=xri:pqdiss:MR38824.
Full textAbstract:
Thesis (M.Sc.)--York University, 2008. Graduate Programme in Physics and Astronomy.Typescript. Includes bibliographical references (leaves 108-110). Also available on the Internet. MODE OF ACCESS via web browser by entering the following URL: http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&res_dat=xri:pqdiss&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&rft_dat=xri:pqdiss:MR38824
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Luca, Peter. "High sensitivity VLBI study of the quasar 3C454.3 /." 2007. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&res_dat=xri:pqdiss&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&rft_dat=xri:pqdiss:MR38804.
Full textAbstract:
Thesis (M.Sc.)--York University, 2007. Graduate Programme in Physics and Astronomy.Typescript. Includes bibliographical references (leaves 105-109). Also available on the Internet. MODE OF ACCESS via web browser by entering the following URL: http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&res_dat=xri:pqdiss&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&rft_dat=xri:pqdiss:MR38804
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Legge, David(David Robert). "Accurate astrometry of southern pulsars." Thesis, 2002. https://eprints.utas.edu.au/20524/1/whole_LeggeDavidRobert2002_thesis.pdf.
Full textAbstract:
This thesis outlines the astrometric measurements of 10 pulsars using phase-referencing Very Long Baseline Interferometry (VLBI) employing telescopes in the Australian Long Baseline Array (LI3A) and Hartbeesthoek observatory. Observations spanned from 1993 to 2001.
A parallax was measured for the Vela pulsar of π = 2.46 ± 0.24 mas placing the pulsar at a distance of 406=44-36 pc. This constitutes the first model independent measurement of the pulsar's distance. This is below the early distance estimates to the pulsar of 500 pc, and above some other more recent estimates. The proper motion was found to be μα cos δ = -50.3 ± 0.4 mas yr-1 and μδ = 29.0 ± 0.2 mas yr-1. This lies between proper motion measurements made using both optical astrometry and phase-referencing VLBI.
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Hailemariam, Mekuanint Kifle. "New calibration sources for very long baseline interferometry at 1.6 GHz." Diss., 2016. http://hdl.handle.net/2263/57244.
Full textAbstract:
I present new 1.6 GHz VLBI observations of sample of 43 radio sources in the Southern
celestial Hemisphere. My goals were firstly to establish the suitability of the 43 sources
as calibrators for 1.6 GHz VLBI observations, and secondly to determine, based on some
selected sources from the sample, how the properties of the sources seen at 8.4 GHz are
related to those seen at 1.6 GHz. I used seven telescopes; ASKAP, ATCA, Ceduna,
Hobart, Mopra and Parkes from Australia, and HartRAO from South Africa.
By evaluating the sources radial extents, flux density of the central components of the
sources and their brightness, I classified the sources into very good, good, intermediate
and bad calibrators. Among the 43 sources, I found that 38 sources fell into the good
or very good calibrator classes. Among 35 of our sources, which are known to be good
calibrators at 8.4 GHz, I found that 32 sources fell into the good or very good calibrator
classes. Of the basis of my sample therefore, I can say that 91 percent of the good
calibrators at 8.4 GHz are also safe to use at 1.6 GHz.Dissertation (MSc)--University of Pretoria, 2016.
tm2016
Physics
MSc
Unrestricted
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Ransom, Ryan R. "VLBI observations of the candidate guide stars and their reference sources for the spaceborne NASA/Stanford gyroscope relativity mission (Gravity Probe B)." 1997. http://wwwlib.umi.com/cr/yorku/fullcit?pMQ27373.
Full textAbstract:
Thesis (M. Sc.)--York University, 1997. Graduate Programme in Physics and Astronomy.Typescript. Includes bibliographical references (leaves 137-141). Also available on the Internet. MODE OF ACCESS via web browser by entering the following URL: http://wwwlib.umi.com/cr/yorku/fullcit?pMQ27373.
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Tingay, Steven J. "Parsec and sub-parsec-scale structure and evolution in nearby compact radio sources and relationships to emission at other wavelengths." Phd thesis, 1996. http://hdl.handle.net/1885/143997.
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Iles, EJL. "Scheduling for next era operations of geodetic VLBI : simulations with the AuScope array." Thesis, 2018. https://eprints.utas.edu.au/30165/1/Iles_whole_thesis_ex_pub_mat.pdf.
Full textAbstract:
The technique of Very Long Baseline Interferometry (VLBI) is an important method for the collection of geodetic and astrometric data. At least two telescopes simultaneously observe signals from a range of extra-galactic radio sources, coordinated by a well-optimised observing schedule. Improving this scheduling process is an important focus as we move into the next generation of geodetic VLBI. This project considered Earth Orientation Parameters (EOPs) and station coordinates, in order to develop our understanding of the capabilities and limitations of the traditional scheduling method, as well as the new, automated scheduling mode ‘dynamic scheduling’. Scheduling with the Australian AuScope VLBI array, in particular, was considered through a regime of repeated scheduling, simulation and analysis. However, the findings are widely applicable to other stations and networks hoping to maximise their effective operations in line with the global goals of the geodetic community.
We find that short interruptions to a standard 24 hour schedule will not significantly impact the precision of results. This is true even if the total time lost is up to half the total observing time. Conversely, reducing the length of the observing period results in an exponential-like degradation of precision. The dynamic scheduling mode is shown to be capable of producing results consistent with current scheduling. However, it is significantly more flexible, with close to real-time adaptability. It is shown that the baseline-limited AuScope array can also be augmented dynamically to improve EOP precision through an intermittent contribution from only 2 additional global stations. The results presented here confirm that a positive impact on efficiency and precision in geodetic VLBI can be achieved through effective scheduling. In light of this, the current scheduling practice can be adapted immediately for greater operational capability.
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Ramessur, Arvind. "Fourier-plane modeling of the jet in the nucleus of galaxy M81." Diss., 2017. http://hdl.handle.net/10500/23817.
Full textAbstract:
The mildly active nuclear region in the galaxy M81 (henceforth, M81‹) is one of
the nearest low-luminosity active galactic nuclei (LLAGN) whose structure is marginally
resolved when probed with Very Long Baseline Interferometry (VLBI). Motivated by the
way resolved radio sources usually appear on the smallest scales, i.e., a core with a one-
sided jet structure, we developed a strictly one-sided, asymmetric triangular model, which
we call ASYM, with brightness distribution along a line segment on the sky, with maximum
brightness at one end of the segment fading linearly to zero at the other end. The ASYM
model is compared and contrasted with an elliptical Gaussian model (hereafter, GAUS), by
fitting existing VLBI data of M81‹ at 39 epochs between 1993 and 2003 at 8.4 and 5.0 GHz
with the two models. Contrary to what we envisioned, we find that for 77% of our epochs,
a simple GAUS model fits the visibility data of M81‹ at 8.4 GHz better (i.e., has a lower
reduced 2) than the ASYM model. We conclude that M81‹ is not strictly a one-sided,
asymmetric jetted source; as is thought to be the case for the majority of AGN observed
at VLBI scales. Our results imply that M81‹ is mostly symmetrical with a significant jet
counterpart which cannot be overlooked.School of Interdisciplinary Research and Graduate Studies (SIRGS)
M. Sc. (Astronomy)
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Hall, Evan Drew. "Long-Baseline Laser Interferometry for the Detection of Binary Black-Hole Mergers." Thesis, 2017. https://thesis.library.caltech.edu/10031/1/evan-hall-thesis.pdf.
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Late in 2015, gravitational physics reached a watershed moment with the first direct detections of gravitational waves. Two events, each from the coalescence of a binary black hole system, were detected by the Laser Interferometer Gravitational-wave Observatory (LIGO). At present, LIGO comprises two 4 km laser interferometers, one in Washington and the other in Louisiana; a third detector is planned to be installed in India. These interferometers, known as Advanced LIGO, belong to the so-called “second generation” of gravitational-wave detectors. Compared to the first-generation LIGO detectors (Initial and Enhanced LIGO), these instruments use multi-stage active seismic isolation, heavier and higher-quality mirrors, and more laser power to achieve an unprecedented sensitivity to gravitational waves. In 2015, both Advanced LIGO detectors achieved a strain sensitivity better than 10-23/Hz1/2 at a few hundred hertz; ultimately, these detectors are designed to achieve a sensitivity of a few parts in 10-24/Hz1/2 at a few hundred hertz.
This thesis covers several topics in gravitational physics and laser interferometry. First, it presents the design, control scheme, and noise performance of the Advanced LIGO detector in Washington during the first observing run (O1). Second, it discusses some issues relating to interferometer calibration, and the impact of calibration errors on astrophysical parameter estimation. Third, it discusses the prospects for using terrestrial and space-based laser interferometers as dark matter detectors.
This thesis has the internal LIGO document number P1600295.