Abstract
All astronomical sources are polarized to some degree. Polarimetry is therefore a powerful astronomical technique. It furnishes unique diagnostics of e.g. magnetic fields and scattering media. This thesis presents new polarimetric concepts, instruments, and measurements targeting astronomical science questions. The first part of the thesis describes three novel polarimetric concepts.
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-A dedicated passive liquid crystal device known as a theta cell is introduced to enable one-shot observations of astronomical targets exhibiting a centrosymmetric polarization pattern. -A new passive measurement concept for broad-band linear polarization is introduced. It is based on a sinusoidal modulation of the spectrum, and is particularly suitable for instruments for which classical spatial and/or temporal polarization modulation is unfavorable. -Calibration of polarimetric instruments is usually limited by non-ideal effects of the calibration optics themselves. A mathematical frame-work based on Fourier analysis is introduced to tackle various non-ideal effects in polarimetric calibration. The second part of the thesis presents the designs and first results of three very different astronomical polarimeters. -The ultra-stable high-resolution HARPS spectrograph is successfully upgraded with a dual-beam polarimetric module. It furnishes direct observations of magnetic fields on stars. -The Small Synoptic Second Solar Spectrum Telescope (S5T) is designed to accurately monitor the variation of weak, turbulent magnetic fields on the Sun during a solar cycle. Such measurements are crucial for the understanding of local dynamo action in the solar photosphere. The prototype shows the feasibility of the instrument concept. -The Spectropolarimeter for Planetary EXpolaration (SPEX) is designed to study a planet's or moon's atmosphere from orbit. The additional information from the polarization measurement of scattered sunlight allows for determination of microphysical properties of atmospheric aerosols. The third part of the thesis presents new polarimetric measurements and observations. -A precise ellipsometer was constructed to measure the polarization properties of various optical components. The instrument was employed to measure the complete Mueller matrix of an aluminum mirror at various durations after evaporation. It is concluded that the aluminum oxide layer has a significant effect on the polarization properties, which is constant in time after a day from evaporation. -Next, an aluminized mirror was contaminated with dust. This also has a significant effect on the measured Mueller matrix, for which a linear model is presented as a function of dust absorption. -Observations of solar scattering polarization at high spatial resolution were obtained with the Hinode space telescope. A significant difference of ~10% in scattering polarization was detected between granules and intergranules, which constrains models of quiet Sun magnetoconvection.
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