3,641 research outputs found
New technique to measure the cavity defects of Fabry-Perot interferometers
(Abridged):
We define and test a new technique to accurately measure the cavity defects
of air-spaced FPIs, including distortions due to the spectral tuning process
typical of astronomical observations. We further develop a correction technique
to maintain the shape of the cavity as constant as possible during the spectral
scan. These are necessary steps to optimize the spectral transmission profile
of a two-dimensional spectrograph using one or more FPIs.
We devise a generalization of the techniques developed for the so-called
phase-shifting interferometry to the case of FPIs. The technique is applicable
to any FPI that can be tuned via changing the cavity spacing (-axis), and
can be used for any etalon regardless of the coating' reflectivity. The major
strength of our method is the ability to fully characterize the cavity during a
spectral scan, allowing for the determination of scan-dependent modifications
of the plates. As a test, we have applied this technique to three 50 mm
diameter interferometers, with cavity gaps ranging between 600 micron and 3 mm,
coated for use in the visible range.
We obtain accurate and reliable measures of the cavity defects of air-spaced
FPIs, and of their evolution during the entire spectral scan. Our main, and
unexpected, result is that the relative tilt between the two FPI plates varies
significantly during the spectral scan, and can dominate the cavity defects; in
particular, we observe that the tilt component at the extremes of the scan is
sensibly larger than at the center of the scan. Exploiting the capability of
the electronic controllers to set the reference plane at any given spectral
step, we develop a correction technique that allows the minimization of the
tilt during a complete spectral scan. The correction remains highly stable over
long periods, well beyond the typical duration of astronomical observations.Comment: 15 pages, 20+ figures, accepted for publication in A&A. Two
additional movies are available in the online version of the pape
Solar Atmospheric Oscillations and the Chromospheric Magnetic Topology
We investigate the oscillatory properties of the quiet solar chromosphere in
relation to the underlying photosphere, with particular regard to the effects
of the magnetic topology. We perform a Fourier analysis on a sequence of
line-of-sight velocities measured simultaneously in a photospheric (Fe I 709.0
nm) and a chromospheric line (Ca II 854.2 nm). The velocities were obtained
from full spectroscopic data acquired at high spatial resolution with the
Interferometric BIdimensional Spectrometer (IBIS). The field of view
encompasses a full supergranular cell, allowing us to discriminate between
areas with different magnetic characteristics. We show that waves with
frequencies above the acoustic cut-off propagate from the photosphere to upper
layers only in restricted areas of the quiet Sun. A large fraction of the quiet
chromosphere is in fact occupied by ``magnetic shadows'', surrounding network
regions, that we identify as originating from fibril-like structures observed
in the core intensity of the Ca II line. We show that a large fraction of the
chromospheric acoustic power at frequencies below the acoustic cut-off,
residing in the proximity of the magnetic network elements, directly propagates
from the underlying photosphere. This supports recent results arguing that
network magnetic elements can channel low-frequency photospheric oscillations
into the chromosphere, thus providing a way to input mechanical energy in the
upper layers.Comment: 4 pages, 3 figure, A&A Letters in pres
Elastic moduli approximation of higher symmetry for the acoustical properties of an anisotropic material
The issue of how to define and determine an optimal acoustical fit to a set
of anisotropic elastic constants is addressed. The optimal moduli are defined
as those which minimize the mean squared difference in the acoustical tensors
between the given moduli and all possible moduli of a chosen higher material
symmetry. The solution is shown to be identical to minimizing a Euclidean
distance function, or equivalently, projecting the tensor of elastic stiffness
onto the appropriate symmetry. This has implications for how to best select
anisotropic constants to acoustically model complex materials.Comment: 20 page
Radiation detection properties of 4H-SiC Schottky diodes irradiated up to 10(16) n/cm(2) by 1 MeV neutrons
We report the results of an experimental study on the radiation hardness of 4H-SiC diodes used as alpha-particle detectors with 1 MeV neutrons up to a fluence of 8 x 10(15) n/cm(2). As the irradiation level approaches the range 10(15) n/cm(2), the material behaves as intrinsic due to a very high compensation effect and the diodes are still able to detect with a reasonable good Charge Collection Efficiency (CCE = 80%).For fluences > 10(15) n/cm(2) CCE decreases monotonically to approximate to 20 % at the highest fluence. Heavily irradiated SiC diodes have been studied, by means of Photo Induced Current Transien
On the generality of the effect of experiencing prior gains and losses on the Iowa gambling task: A study on young and old adults
Prospect Theory predicts that people tend to be more risk seeking if their reference point is perceived as a loss and more risk averse when the reference point is perceived as a gain. In line with this prediction, Franken, Georgieva, Muris and Dijksterhuis (2006) showed that young adults who had a prior experience of monetary gains make more safe choices on subsequent decisions than subjects who had an early experience of losses. There are no experimental studies on how experiencing prior gains and losses differently influences young and older adults on a subsequent decision-making task (the Iowa Gambling Task). Hence, in the current paper, adapting the methodology employed by Franken et al.’s (2006), we intended to test the generality of their effect across the life span. Overall, we found that subjects who experienced prior monetary gains or prior monetary losses did not display significant differences in safe/risky choices on subsequent performance in the Iowa Gambling task. Furthermore, the impact of prior gains and losses on risky/safe card selection did not significantly differ between young and older adults. These results showed that the effect found in the Franken et al.’s study (2006) is limited in its generality
IBIS (Interferometric BIdimensional spectrometer)
IBIS is a new instrument for solar bidimensional spectroscopy, now under construction at the Arcetri Astrophysical Observatory. It essentially consists of two Fabry-Perot interferometers, piezo-scanned and capacity servo-controlled, which are used in classic mount and in axial-mode, in series with a narrow-band interference filter. This instrument will operate on a large field of view (80����) and on a large wavelength range (5800–8600 ˚A), with high spectral (λ/Δλ � 250000), spatial (�0.2) and temporal (� 5 frames s−1) resolution. When completed in
2002, it will be one of the leading instruments for solar research, well suited for a new generation telescopes such as THEMIS
Specific LTR-Retrotransposons Show Copy Number Variations between Wild and Cultivated Sunflowers
The relationship between variation of the repetitive component of the genome and domestication in plant species is not fully understood. In previous work, variations in the abundance and proximity to genes of long terminal repeats (LTR)-retrotransposons of sunflower (Helianthus annuus L.) were investigated by Illumina DNA sequencingtocompare cultivars and wild accessions. In this study, we annotated and characterized 22 specific retrotransposon families whose abundance varies between domesticated and wild genotypes. These families mostly belonged to the Chromovirus lineage of the Gypsy superfamily and were distributed overall chromosomes. They were also analyzed in respect to their proximity to genes. Genes close to retrotransposon were classified according to biochemical pathways, and differences between domesticated and wild genotypes are shown. These data suggest that structural variations related to retrotransposons might have occurred to produce phenotypic variation between wild and domesticated genotypes, possibly by affecting the expression of genes that lie close to inserted or deleted retrotransposons and belong to specific biochemical pathways as those involved in plant stress responses
Nonlinear response of single-molecule nanomagnets: equilibrium and dynamical
We present an experimental study of the {\em nonlinear} susceptibility of
Mn single-molecule magnets. We investigate both their
thermal-equilibrium and dynamical nonlinear responses. The equilibrium results
show the sensitivity of the nonlinear susceptibility to the magnetic
anisotropy, which is nearly absent in the linear response for axes distributed
at random. The nonlinear dynamic response of Mn was recently found to be
very large and displaying peaks reversed with respect to classical
superparamagnets [F. Luis {\em et al.}, Phys. Rev. Lett. {\bf 92}, 107201
(2004)]. Here we corroborate the proposed explanation -- strong field
dependence of the relaxation rate due to the detuning of tunnel energy levels.
This is done by studying the orientational dependence of the nonlinear
susceptibility, which permits to isolate the quantum detuning contribution.
Besides, from the analysis of the longitudinal and transverse contributions we
estimate a bound for the decoherence time due to the coupling to the phonon
bath.Comment: 13 pages, 8 figures, resubmitted to Phys. Rev. B with minor change
A knowledge-intensive approach to process similarity calculation
Process model comparison and similar processes retrieval are key issues to be addressed in many real world situations, and particularly relevant ones in some applications (e.g., in medicine), where similarity quantification can be exploited in a quality assessment perspective. Most of the process comparison techniques described in the literature suffer from two main limitations: (1) they adopt a purely syntactic (vs. semantic) approach in process activity comparison, and/or (2) they ignore complex control flow information (i.e., other than sequence). These limitations oversimplify the problem, and make the results of similarity-based process retrieval less reliable, especially when domain knowledge is available, and can be adopted to quantify activity or control flow construct differences. In this paper, we aim at overcoming both limitations, by introducing a framework which allows to extract the actual process model from the available process execution traces, through process mining techniques, and then to compare (mined) process models, by relying on a novel distance measure. The novel distance measure, which represents the main contribution of this paper, is able to address issues (1) and (2) above, since: (1) it provides a semantic, knowledge-intensive approach to process activity comparison, by making use of domain knowledge; (2) it explicitly takes into account complex control flow constructs (such as AND and XOR splits/joins), thus fully considering the different semantic meaning of control flow connections in a reliable way. The positive impact of the framework in practice has been tested in stroke management, where our approach has outperformed a state-of-the art literature metric on a real world event log, providing results that were closer to those of a human expert. Experiments in other domains are foreseen in the future
The impact of failures and successes on affect and self-esteem in young and older adults
Little is known about the impact of success and failure events on age-related changes in affect states and, particularly, in self-esteem levels. To fill this gap in the literature, in the present study changes in affect and self-esteem in 100 young (19 - 30 years) and 102 older adults (65-81 years) were assessed after participants experienced success and failure in a demanding cognitive task. Overall, the success-failure manipulation induced changes on affect states and on state self-esteem, not on trait self-esteem. Regarding age differences, older and young adults were affected to the same extent by experiences of successes and failures. Theoretical considerations of the empirical findings are provided in the general discussion
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