Development and Implementation of an Ultra High Speed Astronomical Polarimeter: The Galway Astronomical Stokes Polarimeter (GASP)
Collins, Patrick P.
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Optical polarization is a powerful diagnostic tool in astrophysics, allowing the investigation of asymmetries in source regions of astronomical objects, magnetic field configurations, and magnetic field strengths. Polarized sources that vary on short time scales such as Pulsars can be better understood with high-time resolution observations and, for example, may assist in finding the connection between the optical pulses and the giant radio pulses. Current astronomical optical polarimetry can at best record either the linear or circular polarization at resolutions of microseconds, but the entire stokes vector cannot be measured in a single exposure. In this thesis we demonstrate that a division of amplitude polarimeter (DOAP) can be modified and designed to measure, in a single exposure the entire Stokes vector. DOAP polarimetry is where light is initially split in two, one portion has a quarter wave of retardance added and then both portions are split in two by a polarizing beamsplitter, these four portions of the beams are measured and they are linearly related to the input stokes vector. The development of our polarimeter begins with the enhancement of a retarding beam splitter (RBS) prism, initially used by Compain and Drevillon , adds a quarter wave of retardance and splits the input beam in two. This RBS prism was redesigned to be achromatic by changing its geometry and choosing a more suitable glass. It was also modified for imaging polarimetry. Multiple optical designs of the polarimeter were made before arriving at the final folded layout that could record four images onto the one detector and whose polarization could then be measured. Calibration of astronomical polarimeters are difficult and we detail a method, known as the Eigenvalue Calibration Method (ECM) that can unambiguously do so. We present various laboratory and astronomical trails that test the performance and capability of the polarimeter, investigate how the errors propagate through polarimetry, conduct some Monte Carlo test for situations not possible in the laboratory and then finally present recommendations for future enhancement of this polarimeter.
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