16,464 research outputs found
Image Reconstruction with Analytical Point Spread Functions
The image degradation produced by atmospheric turbulence and optical
aberrations is usually alleviated using post-facto image reconstruction
techniques, even when observing with adaptive optics systems. These techniques
rely on the development of the wavefront using Zernike functions and the
non-linear optimization of a certain metric. The resulting optimization
procedure is computationally heavy. Our aim is to alleviate this
computationally burden. To this aim, we generalize the recently developed
extended Zernike-Nijboer theory to carry out the analytical integration of the
Fresnel integral and present a natural basis set for the development of the
point spread function in case the wavefront is described using Zernike
functions. We present a linear expansion of the point spread function in terms
of analytic functions which, additionally, takes defocusing into account in a
natural way. This expansion is used to develop a very fast phase-diversity
reconstruction technique which is demonstrated through some applications. This
suggest that the linear expansion of the point spread function can be applied
to accelerate other reconstruction techniques in use presently and based on
blind deconvolution.Comment: 10 pages, 4 figures, accepted for publication in Astronomy &
Astrophysic
Compressive Sensing for Spectroscopy and Polarimetry
We demonstrate through numerical simulations with real data the feasibility
of using compressive sensing techniques for the acquisition of
spectro-polarimetric data. This allows us to combine the measurement and the
compression process into one consistent framework. Signals are recovered thanks
to a sparse reconstruction scheme from projections of the signal of interest
onto appropriately chosen vectors, typically noise-like vectors. The
compressibility properties of spectral lines are analyzed in detail. The
results shown in this paper demonstrate that, thanks to the compressibility
properties of spectral lines, it is feasible to reconstruct the signals using
only a small fraction of the information that is measured nowadays. We
investigate in depth the quality of the reconstruction as a function of the
amount of data measured and the influence of noise. This change of paradigm
also allows us to define new instrumental strategies and to propose
modifications to existing instruments in order to take advantage of compressive
sensing techniques.Comment: 11 pages, 9 figures, accepted for publication in A&
Teaching students about chemical elements using daily-life contexts
Learning the names and symbols for chemical elements is a task that students often find dull, although it is of crucial importance for understanding chemistry. In this respect, the use of games or similar play activities could make the learning experience more enjoyable. This paper presents the results of a study in which two tasks involving play (TIPs) and based on daily-life contexts (football and the home) were used to teach the names and symbols of chemical elements. The experimental group comprised 38 year-10 students who studied this topic through a teaching unit built around the TIPs. A control group of 67 year-10 students followed a traditional teaching approach to the same topic. The effectiveness of the TIPs was assessed using three items, administered pre- and post-test, that explored students’ knowledge about metallic and non-metallic elements and their ability to identify them in their everyday environment. Following the TIP-based teaching unit, students in the experimental group gave a higher percentage of appropriate answers, with the Wilcoxon test indicating significant post-test differences for all three items. However, the Kolgomorov-Smirnov test indicated that the experimental and control groups only differed significantly at post-test in their ability to give the names and symbols of non-metals, with the experimental group performing better. Memorising the names and symbols of chemical elements is a complex task for students, and identifying their presence in everyday environments appears to be particularly difficult. However, the results suggest that the use of TIPs linked to daily-life contexts could help students with their learning of this topic.Universidad de Málaga. Campus de Excelencia Internacional AndalucÃa Tech. Proyecto ‘I+D Excelencia’ EDU2013-41952-
An Active Attack on a Multiparty Key Exchange Protocol
The multiparty key exchange introduced in Steiner et al.\@ and presented in
more general form by the authors is known to be secure against passive attacks.
In this paper, an active attack is presented assuming malicious control of the
communications of the last two users for the duration of only the key exchange
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