Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........25s&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF ALBERTA (CANADA), 1995.Source: Dissertation Abstracts International, Volume: 56-11, Section: B, pa
Astronomy and Astrophysics
Astrophysics
1
Scientific paper
Polarimetry has been added to the wide-field aperture synthesis radio telescope at the Dominion Radio Astrophysical Observatory. This addition has significantly extended the ability of this telescope to explore the astrophysics of large low brightness objects at arcminute resolution at 21 cm wavelength. A unique combination of equatorial antenna mounts and circularly polarized non-rotatable feed antennas preclude the option of using calibration techniques established for other synthesis telescopes. A directional coupler is shown to be a good model for a polarization transducer. A transducer formed by a four port network that is matched, lossless and reciprocal will permit the measurement of polarized intensity without the need for special calibration techniques. Although low in ohmic losses, practical networks suffer from mismatch losses, and require calibration and correction for non -orthogonality. Calibration of transducer orthogonality is readily accomplished for this type of instrument but determination of the array polarization is not. However it is shown that the polarization of the antennas need not be known exactly, providing that the feed polarization is predominately circular and the source polarization is linear. Polarization of the antenna feeds was measured in an antenna range and confirmed adequate to allow polarimetry with orthogonality calibration only. The phase-amplitude method for antenna measurement was employed and proved to be of great utility in situations where preparation and set-up are a major undertaking. A technique to calibrate the field probes involved in this method of measurement is presented which allows the use of probes with both arbitrary and unknown polarization. A technique for calibrating the orthogonality of the array was implemented and its operation verified by a number of astronomical test observations. The results are presented. Observations made of the extended supernova remnant DA530 placed at two different positions in the field of view demonstrate that the telescope is capable of polarimetric imaging of extended objects up to 0.5 degree in angular size and has a usable field of view, for polarimetry, of at least 1.5 degrees.
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