51,675 research outputs found
Inferring telescope polarization properties through spectral lines without linear polarization
We present a technique to determine the polarization properties of a
telescope through observations of spectral lines that have no intrinsic linear
polarization signals. For such spectral lines, any observed linear polarization
must be induced by the telescope optics. We apply the technique to observations
taken with the SPINOR at the DST and demonstrate that we can retrieve the
characteristic polarization properties of the DST at three wavelengths of 459,
526, and 615 nm. We determine the amount of crosstalk between the intensity
Stokes I and the linear and circular polarization states Stokes Q, U, and V,
and between Stokes V and Stokes Q and U. We fit a set of parameters that
describe the polarization properties of the DST to the observed crosstalk
values. The values for the ratio of reflectivities X and the retardance tau
match those derived with the telescope calibration unit within the error bars.
Residual crosstalk after applying a correction for the telescope polarization
stays at a level of 3-10%. We find that it is possible to derive the parameters
that describe the polarization properties of a telescope from observations of
spectral lines without intrinsic linear polarization signal. Such spectral
lines have a dense coverage (about 50 nm separation) in the visible part of the
spectrum (400-615 nm), but none were found at longer wavelengths. Using
spectral lines without intrinsic linear polarization is a promising tool for
the polarimetric calibration of current or future solar telescopes such as
DKIST.Comment: 22 pages, 24 figures, accepted for publication in A&
Arrest of flow and emergence of activated processes at the glass transition of a suspension of particles with hard sphere-like interactions
By combining aspects of the coherent and self intermediate scattering
functions, measured by dynamical light scattering on a suspension of hard
sphere-like particles, we show that the arrest of particle number density
fluctuations spreads from the position of the main structure factor peak.
Taking the velocity auto-correlation function into account we propose that as
density fluctuations are arrested the system's ability to respond to diffusing
momentum currents is impaired and, accordingly, the viscosity increases. From
the stretching of the coherent intermediate scattering function we read a
quantitative manifestation of the undissipated thermal energy, the source of
those, ergodicity restoring, processes that short-circuit the sharp transition
to a perfect glass.Comment: 9 pages, 4 figure
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