1,721,648 research outputs found
Polarisation effects investigations in quasi-optical metal grid filters
No full textMany theories have been developed to describe metal grid filters that are commonly used for wavelength selection in far-infrared and millimetre wavelengths experiments. Application of these filters has shifted from simple photometric measurements to the more demanding requirements needed to accurately measure the polarisation. It has therefore become necessary to investigate residual polarisation effects inherent in these kinds of filter structures. In this work we present an alternative modelling approach, based on finite-element analysis software (HFSS) available commercially. To validate the model we have made Fourier transform spectrometer, FTS, measurements of a real capacitive grid component as a function of incidence angle and grid orientation for both s and p polarisations. The excellent agreement between model and data enable accurate predictions of performance of these structures when used in polarimetric experiments as well as determining the cause of parasitic effects. © 2005 Elsevier B.V. All rights reserved
Effects of quasi-optical components on feed-horn co- And cross-polarisation radiation patterns
No full textAstrophysical experiments dedicated to the study of the Cosmic Microwave Background are in needs of very well defined beam shape in order to get an accurate re-construction of the anisotropies power spectrum. These beams have to be carefully designed, but also properly characterised. Some of these instruments will be located in a cryostat for which filters and windows are necessary, and the effects of these additional optical components on the beam shape need to be taken into account. We present here, measurements of such effects on the co- and cross-polarisation radiation patterns of corrugated horns using a Vector Network Analyser
A review of metal mesh filters
No full textThe Astronomical Instrumentation Group at Cardiff University has been developing metal mesh optical filters for more than 30 years, which are currently in use in many ground-, balloon- and space-based instruments. Here we review the current state of the art with respect to these quasi-optical components (low-pass, high-pass and band-pass filters, dichroics and beam-dividers) as developed for the FIR and sub-millimetre wavelength region. We compare performance data with various modelling tools (HFSS, transmission line theory or Floquet mode analysis). These models assist with our understanding of the behaviour of these filters when used at non-normal incidence or in the diffraction region of the grid structures. Interesting artefacts, such as the Wood anomalies and behaviour with S and P polarisations, which dictate the usage of these components in polarisation sensitive instruments, will be discussed
Achromatic half-wave plate for submillimeter instruments in cosmic microwave background astronomy: Modeling and simulation
No full textWe adopted an existing formalism and modified it to simulate, with high precision, the transmission, reflection, and absorption of multiple-plate birefringent devices as a function of frequency. To validate the model, we use it to compare the measured properties of an achromatic five-plate device with a broadband antireflection coating to expectations derived from the material optical constants and its geometric configuration. The half-wave plate presented here is observed to perform well with a phase shift variation of <2° from the ideal 180° over a bandwidth of Δv/v ∼ 1 at millimeter wavelengths. This formalism represents a powerful design tool for birefringent polarization modulators and enables its optical properties to be specified with high accuracy. © 2006 Optical Society of America
Metal-mesh achromatic half-wave plate for use at submillimeter wavelengths
No full textA metal-mesh achromatic half-wave plate (HWP) has been designed, manufactured, and tested for potential use in millimeter and submillimeter astronomical instruments. The prototype device presented here is based on a 12-grid Shatrow [IEEE Trans. Antennas Propag. 43,109 (1995)] recipe to operate over the frequency range of 120-180 GHz. Transmission line modeling and finite-element analysis [Ansoft HFSS website: http://www.ansoft.com/hfss/1 were used to optimize the design geometrical parameters in terms of the device transmission, reflection, absorption, phase-shift, and cross-polarization as a function of frequency. The resulting prototype device was constructed and characterized using incoherent radiation from a polarizing Fourier transform spectrometer to explore its frequency and polarization behavior. These measurements are shown to be in excellent agreement with the models. Lists of the achieved HWP performance characteristics are reported. © 2008 Optical Society of America
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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