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| Paper: |
Astronomical Applications for “Radial Polarimetry'' |
| Volume: |
449, Astronomical Polarimetry 2008: Science from Small to Large Telescopes |
| Page: |
21 |
| Authors: |
Snik, F. |
| Abstract: |
Many objects on the sky exhibit a centrosymmetric polarization pattern,
particularly in cases involving single scattering around a central
source. Utilizing a novel liquid crystal device (the “theta cell'')
that transforms the coordinate system of linear polarization in an image
plane from Cartesian to polar, the observation of centrosymmetric
polarization patterns can be improved: instead of measuring Stokes Q
and U on the sky, one only needs to measure Stokes Q' in the new
instrument coordinate system. This reduces the effective exposure time
by a factor of two and simplifies the polarization modulator
design. According to the manufacturer's specifications and to
measurements in the lab, the liquid crystal device can be applied in the
visible and NIR wavelength range. Astronomical science cases for
a“radial polarimeter'' include exoplanet detection, imaging of
circumstellar disks, reflection nebulae and light echos,
characterization of planetary atmospheres and diagnostics of the solar
K-corona. The first astronomical instrument that utilizes a theta cell
for radial polarimetry is the S5T (Small Synoptic Second Solar
Spectrum Telescope), which accurately measures scattering polarization
signals near the limb of the sun. These observations are crucial for
understanding the nature and origin of weak, turbulent magnetic fields
in the solar photosphere and elsewhere in the universe. A “radial
polarimeter'' observing a slightly defocused point source performs
one-shot full linear polarimetry. With a theta cell in a pupil plane, a
beam's linear polarization properties (e.g. for calibration purposes)
can be fully controlled through pupil masking. |
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