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A sounding-rocket program called the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is proposed to be
launched in the summer of 2014. CLASP will observe the solar chromosphere in Ly-alpha (121.567 nm), aiming to detect
the linear polarization signal produced by scattering processes and the Hanle effect for the first time. The polarimeter of
CLASP consists of a rotating half-waveplate, a beam splitter, and a polarization analyzer. Magnesium Fluoride (MgF2) is
used for these optical components, because MgF2 exhibits birefringent property and high transparency at ultraviolet
wavelength.
The development and comprehensive testing program of the optical components of the polarimeter is underway using the
synchrotron beamline at the Ultraviolet Synchrotron Orbital Radiation Facility (UVSOR). The first objective is deriving
the optical constants of MgF2 by the measurement of the reflectance and transmittance against oblique incident angles for
the s-polarized and the p-polarized light. The ordinary refractive index and extinction coefficient along the ordinary and
extraordinary axes are derived with a least-square fitting in such a way that the reflectance and transmittance satisfy the
Kramers-Krönig relation. The reflection at the Brewster's Angle of MgF2 plate is confirmed to become a good polarization
analyzer at Ly-alpha. The second objective is the retardation measurement of a zeroth-order waveplate made of MgF2. The
retardation of a waveplate is determined by observing the modulation amplitude that comes out of a waveplate and a
polarization analyzer. We tested a waveplate with the thickness difference of 14.57 um. The 14.57 um waveplate worked as
a half-waveplate at 121.74 nm. We derived that a waveplate with the thickness difference of 15.71 um will work as a
half-waveplate at Ly-alpha wavelength.
We developed a prototype of CLASP polarimeter using the MgF2 half-waveplate and polarization analyzers, and
succeeded in obtaining the modulation patterns that are consistent with the theoretical prediction. We confirm that the
performance of the prototype is optimized for measuring linear polarization signal with the least effect of the crosstalk
from the circular polarization.
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