Very Long Baseline Interferometry in Italy Wide-field VLBI imaging and astrometry and prospects for an Italian VLBI network including the Sardinia Radio Telescope

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.

Retorica e mito in Giambattista Vico

The purpose of this thesis is to show that Vico had the constant aim, remained unaltered in time, to support the usefulness of Rhetoric. The awareness of the strength acquired by the “anti-rhetorical” movements, particularly by the Cartesians and the Logicians from Port-Royal, obliged Vico to elaborate some strategies and counter-moves that, little by little, became more and more complex and refined, till they inspired his main work, The New Science. By taking into consideration the most significant aspects of Vico’s speculation (topic, tropes, the formation of language, the development of human thought, the new conception of philology ), this work shows that they are part of a strategy worked out by the Neapolitan philosopher, in order to demonstrate the validity, the usefulness and the modernity of Rhetoric. Every aspect of Vico’s thought reaches its peak when Vico formulates his theory about myths, which he interprets as an expression of the primitive man’s necessities and demands. The point of view expressed by this doctorate thesis emphasizes that, despite the inevitable modifications that have taken place through the centuries, Vico’s thought keeps its substantial unity. So those subjects that critical literature tends to separate are, in reality, linked and unified.