987 research outputs found
Polarimetry from the Ground Up
Ground-based solar polarimetry has made great progress over the last decade.
Nevertheless, polarimetry is still an afterthought in most telescope and
instrument designs, and most polarimeters are designed based on experience and
rules of thumb rather than using more formal systems engineering approaches as
is common in standard optical design efforts. Here we present the first steps
in creating a set of systems engineering approaches to the design of
polarimeters that makes sure that the final telescope-instrument-polarimeter
system is more than the sum of its parts.Comment: To appear in proceedings of the Solar Polarization Workshop
A compact and robust method for full Stokes spectropolarimetry
We present an approach to spectropolarimetry which requires neither moving
parts nor time dependent modulation, and which offers the prospect of achieving
high sensitivity. The technique applies equally well, in principle, in the
optical, UV or IR. The concept, which is one of those generically known as
channeled polarimetry, is to encode the polarization information at each
wavelength along the spatial dimension of a 2D data array using static, robust
optical components. A single two-dimensional data frame contains the full
polarization information and can be configured to measure either two or all of
the Stokes polarization parameters. By acquiring full polarimetric information
in a single observation, we simplify polarimetry of transient sources and in
situations where the instrument and target are in relative motion. The
robustness and simplicity of the approach, coupled to its potential for high
sensitivity, and applicability over a wide wavelength range, is likely to prove
useful for applications in challenging environments such as space.Comment: 36 pages, 11 figures, 3 tables; accepted for publication in Applied
Optic
The measurement of dilational properties of liquid surfaces
For measurements of viscoelastic properties of monolayer covered interfaces, the longitudinal wave technique is used. The longitudinal wave is generated in the plane of the interface, using a horizontal oscillating barrier. The wave propagation depends on the values of the viscoelastic parameters of the monolayer. The present paper deals with a new method of determining the wave parameters. Using small tracer particles, the displacement of the monolayer, as a function of the distance to the oscillating barrier, is made visible and recorded by video cameras. From these recordings the wave parameters and viscoelastic properties are evaluated. A comparison is made between results obtained with this method and those obtained from the measurements of the varying surface tension. This latter method has until now normally been used to determine the wave parameters. Results on cholesterol and decanoic acid monolayers indicate that both methods lead to similar results within experimental error. It is shown that in case of reflected waves the combination of the results of the two methods, as described earlier by Crone et al., leads to significantly more accurate values of the wave parameters
Polarization properties of real aluminum mirrors; I. Influence of the aluminum oxide layer
In polarimetry it is important to characterize the polarization properties of
the instrument itself to disentangle real astrophysical signals from
instrumental effects. This article deals with the accurate measurement and
modeling of the polarization properties of real aluminum mirrors, as used in
astronomical telescopes. Main goals are the characterization of the aluminum
oxide layer thickness at different times after evaporation and its influence on
the polarization properties of the mirror. The full polarization properties of
an aluminum mirror are measured with Mueller matrix ellipsometry at different
incidence angles and wavelengths. The best fit of theoretical Mueller matrices
to all measurements simultaneously is obtained by taking into account a model
of bulk aluminum with a thin aluminum oxide film on top of it. Full Mueller
matrix measurements of a mirror are obtained with an absolute accuracy of ~1%
after calibration. The determined layer thicknesses indicate logarithmic growth
in the first few hours after evaporation, but it remains stable at a value of
4.12+/-0.08 nm on the long term. Although the aluminum oxide layer is
established to be thin, it is necessary to consider it to accurately describe
the mirror's polarization properties.Comment: accepted for publication in PAS
Instrumental polarisation at the Nasmyth focus of the E-ELT
The ~39-m European Extremely Large Telescope (E-ELT) will be the largest
telescope ever built. This makes it particularly suitable for sensitive
polarimetric observations, as polarimetry is a photon-starved technique.
However, the telescope mirrors may severely limit the polarimetric accuracy of
instruments on the Nasmyth platforms by creating instrumental polarisation
and/or modifying the polarisation signal of the object. In this paper we
characterise the polarisation effects of the two currently considered designs
for the E-ELT Nasmyth ports as well as the effect of ageing of the mirrors. By
means of the Mueller matrix formalism, we compute the response matrices of each
mirror arrangement for a range of zenith angles and wavelengths. We then
present two techniques to correct for these effects that require the addition
of a modulating device at the polarisation-free intermediate focus that acts
either as a switch or as a part of a two-stage modulator. We find that the
values of instrumental polarisation, Stokes transmission reduction and cross-
talk vary significantly with wavelength, and with pointing, for the lateral
Nasmyth case, often exceeding the accuracy requirements for proposed
polarimetric instruments. Realistic ageing effects of the mirrors after perfect
calibration of these effects may cause polarimetric errors beyond the
requirements. We show that the modulation approach with a polarimetric element
located in the intermediate focus reduces the instrumental polarisation effects
down to tolerable values, or even removes them altogether. The E-ELT will be
suitable for sensitive and accurate polarimetry, provided frequent calibrations
are carried out, or a dedicated polarimetric element is installed at the
intermediate focus.Comment: Accepted for publication in A&
- …