1,720,956 research outputs found
An iterative method to deconvolve coded-mask images
No full textEfficent astronomical imaging at energy greater than 20 keV is
mainly achieved through modulation, either time (i.e. HXMT) or spatial
(i.e. IBIS/INTEGRAL), techniques. Currently, the coded mask
technique is widely used with the true spatial intensity distribution
reconstructed from the data by the cross-correlation (CC) method.
As the sensitivity of instruments increases, so must the angular resolution
in order to avoid problems with source confusion. The IBIS 12’
angular resolution is clearly not sufficient to distinguish all the sources
in the crowded field of the Galactic Centre. One possibility to overcome
this problem is to change the deconvolution method.
The objective of this thesis is to evaluate the real imaging capability of
the Direct Demodulation (DD) method. It deconvolves incomplete and
noisy data by iteratively solving the image formation equation under
physical constraints. With the goal of exploiting the DD technique,
in the early of the 1990s the HXMT mission was designed, where the
imaging capability is obtained through the temporal modulation of the
detected counts by a set of mechanical collimators.
To achieve this goal, we developed the Lucy-Richardson (LR) code
to reconstruct directly hard-X/soft-
ray images. It assumes that the
data and the noise follow a Poisson distribution and it guarantees the
non-negativity of the restored images. For the moment, any kind of
regularization or constraint was implemented in the underlying optimization
problem, so this will be ill-posed yet.
Due to the general nature of the DD and the fact that HXMT has still
to fly, the IBIS/INTEGRAL data and its PSF were used to check our
own code. The pure geometrical PSF considering only the effects due
to the photon propagation from the mask to the detector was created.
Our CC code implements the same balanced cross-correlation as the
standard software for IBIS/INTEGRAL analysis. The CC deconvolved
images are the reference for the image quality obtained with the LR.
The great improvement in the theoretical angular resolution and location
precision is evident. It is independent on the source position in
the total FOV, the iteration number and the source flux. Within the
parameters of the simulations used, the LR statistical uncertainty was
found to be a factor of 10 smaller than that obtained with the CC.
Furthermore, the LR deconvolved images have less fluctuating reconstructed
background.
The main LR drawback is the flux evaluation of the reconstructed
source. It is mainly due to the choice of the correct iteration number.
The use of a-priori information about the unknown object allows a
complete regularization of the problem, so probably solving the problem
with the flux estimation.
Keywords: Coded-mask, Lucy, Richardson, INTEGRAL, IBI
Improving the angular resolution of coded aperture instruments using a modified Lucy-Richardson algorithm for deconvolution
No full textA problem with coded-mask telescopes is the achievable angular resolution. For example, with the standard cross-correlation (CC) analysis, the INTEGRAL IBIS/ISGRI angular resolution is about 13’. We are currently investigating an iterative Lucy-Richardson (LR) algorithm. The LR algorithm can be used effectively when the PSF is known, but little or no information is available for the noise. This algorithm maximizes the probability of the restored image, under the assumption that the noise is Poisson distributed, which is appropriate for photon noise in the data, and converges to the maximum likelihood solution. We have modified the classical LR algorithm, adding non-negative constraints. It doesn’t take into account of the features leading to a difference in PSF depending on position in the field of view (dead pixels, gaps between modules etc), which are easily corrected for in the classical CC analysis, so we must correct for these either after the
restoration of the image or by modifing the data before the sky reconstruction. We present some results using real IBIS data indicating the power of the proposed reconstruction algorithm
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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