1,721,144 research outputs found
Analysis of diffraction from the occulter edges of a giant externally occulted solar coronagraph
No full textA technique is described for the calculation of the intensity of the light diffracted by the occulter of an externally
occulted solar coronagraph. This technique can be applied to an occulter of generic shape, but the attention
is here focused on a specific application; that is, the case of a giant space solar coronagraph, in which the
occulter is located at 100 m from the telescope aperture. By means of the code developed, it has been possible
to simulate the effects of various shapes of the occulter edge with the aim of analyzing in detail the best
apodization for the coronagraph. The results obtained show that an occulter with a circular serrated edge allows
a remarkable reduction of the amount of diffracted light on the coronagraph’s entrance aperture with
respect to a simpler circular disk case
A Multiplexed All-Reflective Static Fourier Transf..
No full textStatic Fourier transform spectrometry is a not widely used spectroscopic technique, that can be. particularly attractive for space-based applications in which only one emission line is analyzed. A key advantage over traditional dispersion methodologies, at the same resolving power and throughput, is the reduced instrumental volume and weight. This is due to the fact that the resolution power is not connected to the geometrical instrumental size, as in usual dispersion spectroscopy, but to the resolving power of the dispersive elements in the optical configuration. This peculiar property, considering the growing number of micro-satellite planned missions, can be a very attractive feature. Another important characteristics is the total absence of optical or mechanical moving parts: this assures the minimization of single-point failure-risk and consequently of costs. This instrument class, in visible range, usually reaches a resolution power of the order of 105. Consequently, assuming a reasonable number of sampling elements in the detector, the spectral band is limited to only a few nanometers: this explains why static Fourier transform spectrometry is presently the better choice only in limited number of spatial applications. The work here presented describes a possible optical configuration useful to increase the spectral band of these instruments. The improvement is of order of five-ten in band coverage and could greatly enlarge the applicability range of these spectrometers: for example to situations in which a medium spectral range visibility is needed, or in the not so rare cases in which simultaneous high resolution monitoring of correlated emission/absorption lines is required in not contiguous regions
Optical concept of a compressor for XUV pulses in the attosecond domain
No full textThe design of a grating compressor for attosecond pulses in the extreme-ultraviolet region is presented. The instrument adopts a double-grating configuration with grazing-incidence gratings in the off-plane mount. The configuration preserves the length of the optical paths of the different rays coming from the source at the same wavelength, while it realizes a controlled dispersion of the length of the optical paths of rays at different wavelengths. The corresponding group-delay dispersion can be either positive or negative, depending on the geometry of the system. As a test case, the application to the compression of an attosecond pulse in the 50-100 eV region is discussed
A high resolution XUV grating monochromator for the spectral Selection of Ultrashort harmonic pulses
Full text linkA new monochromator with high spectral resolution in the extreme ultraviolet (XUV) has been developed for high-order laser harmonics selection. The system has three optical elements-a cylindrical (or spherical) focusing mirror, a uniform-line-spaced plane grating, and a plane mirror. The last element is required to maintain the focus on a fixed vertical slit when the grating subtended angle is changed in order to minimize the spectral defocusing aberration. The parameters of the focusing mirror are determined to introduce a coma that compensates for the coma given by the grating. The possibility of using two interchangeable gratings made the set-up optimized for a broad energy range of 12-50 eV. As a design test case, the set-up has been applied to a selection of the discrete spectral lines emitted by a gas-discharge lamp as the XUV source, obtaining a resolving power E/ΔE > 3000
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
- …
