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Tecniche di taratura di stereocamere per missioni planetarie
Full text linkOptical instruments for space missions work in hostile environment, it’s thus necessary to accurately study the effects of ambient parameters variations on the equipment.
In particular, optical instruments are very sensitive to ambient conditions, especially temperature. This variable can cause dilatation and misalignment of the optical elements, and can also lead to rise of dangerous stresses in the optics. Their displacements degrade the quality of the sampled images.
In this work the optics and mountings of a stereo-camera for the BepiColombo mission are modelled and processed by a thermo-mechanical FEM analysis, reproducing expected operative conditions. The output is elaborated into a MATLAB optimisation code, based on non-linear least square algorithm to determine the equation of the best fitting nth polynomial or spherical surfaces of the deformed lenses and mirrors; model accuracy is 10-8 m.
The obtained mathematical surface representations are then directly imported into ZEMAX for sequential raytrace analysis. The results are spot diagrams, chief ray coordinates on the detector, MTF curves and Diffraction Encircled Energy variations due to simulated thermal loads.
This helps also to design and compare different optical housing systems for a feasible solution. Different types of lenses and prisms constraints have been designed and analysed. The results show the preferable use of kinematic constraints instead of glue to correctly operate the instrument to maintaine in focus in orbit around Mercury considering an operative temperature between -20°C and +30°C.Gli strumenti ottici che vengono utilizzati nelle missioni spaziali risentono delle variazione delle condizioni ambientali, per questo è necessario studiare l’effetto di queste ultime sull’equipaggiamento.
In particolare gli strumenti ottici sono molto sensibili alle variazioni di temperatura, perché questa grandezza può causare la deformazione e il disalinneamento delle ottiche, inoltre può comportare l’insorgere di tensioni rilevanti che possono provocare la loro rottura.
In questa tesi sono state effettuate delle analisi termo-elastiche utilizzando un software ad elementi finite (Nastran) riproducendo le condizioni operative in cui si troverà la stereocamera coinvolta nella missione BepiColombo. I risultati delle analisi sono stati elaborati in MATLAB per determinare le equazioni matematiche delle superfici degli elementi ottici deformati utilizzando un’ottimizzazione non lineare ai minimi quadrati, e considerando equazioni polinomiali, sferiche e planari.
Le superfici matematiche sono state importate in un software raytrace (ZEMAX) per poter verificare la performance ottica dello strumento. I risultati mostrano come le variazioni di temperatura influenzino gli Spot Diagrams, la Diffraction Ensquare Energy e le curve MTF.
Per migliorare la risposta del telescopio ai carichi termici sono stati ideati dei vincoli cinematici, il loro utilizzo compromette molto meno la performance della stereocamera rispetto a vincoli rigidi per le ottiche.
È stata valutata l’influenza delle variazioni dei parametri ottici (focale, spostamento del centro ottico, spostamento degli Spot Diagrams sul piano immagine e distorsioni) sulla ricostruzione della profondità della superficie di Mercurio utilizzando la propagazione dell’incertezza secondo le metodologie GUM e Monte Carlo.
Infine è stato ideato unsetup strumentale per determinare gli spostamenti e le rotazioni di alcuni elementi ottici della sterocamera in camera di vuoto riproducendo le condizioni operative dello strumento
EFFECTS OF THERMAL DEFORMATION ON OPTICAL INSTRUMENTS FORSPACE APPLICATION
No full textOptical instruments for space missions work in hostile environment, it’s thus necessary to accurately study the
effects of ambient parameters variations on the equipment.
In particular optical instruments are very sensitive to ambient conditions, especially temperature. This variable
can cause dilatations and misalignments of the optical elements, and can also lead to rise of dangerous stresses
in the optics. Their displacements and the deformations degrade the quality of the sampled images.
In this work a method for studying the effects of the temperature variations on the performance of imaging
instrument is presented. The optics and their mountings are modeled and processed by a thermo-mechanical
Finite Element Model (FEM) analysis, then the output data, which describe the deformations of the optical
element surfaces, are elaborated using an ad hoc MATLAB routine: a non-linear least square optimization
algorithm is adopted to determine the surface equations (plane, spherical, nth polynomial) which best fit the data.
The obtained mathematical surface representations are then directly imported into ZEMAX for sequential
raytracing analysis. The results are the variations of the Spot Diagrams, of the MTF curves and of the
Diffraction Ensquared Energy due to simulated thermal loads.
This method has been successfully applied to the Stereo Camera for the BepiColombo mission reproducing
expected operative conditions.
The results help to design and compare different optical housing systems for a feasible solution and show that
it is preferable to use kinematic constraints on prisms and lenses to minimize the variation of the optical
performance of the Stereo Camera
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
Method for studying the effects of thermal deformations on optical systems for space application
No full textIn this paper, the results of the thermo-elastic analysis performed on the stereo channel of the imaging
system Integrated Observatory System for the BepiColombo European Space Agency mission to Mercury
are presented. The aim of the work is to determine the effects of ambient parameter variations on the
equipment performance; the optical performance is changing during the mission lifetime primarily
because of the optics misalignments and deformations induced by temperature variations. The camera
optics and their mountings are modeled and processed by a thermo-mechanical finite element model
(FEM) program, which reproduces the expected optics and structure thermo-elastic deformations in
the instrument foreseen operative temperature range, i.e., between −20 °C and 30 °C. The FEM outputs
are elaborated using a MATLAB optimization routine: an algorithm based on nonlinear least square data
fitting is adopted to determine the surface equation (plane, spherical, nth polynomial) which best fits the
deformed optical surfaces. The obtained surfaces are then directly imported into a ZEMAX code for
sequential ray-tracing analysis. Variations of the optical spot diagrams, modulation transfer function
curves, and ensquared energy are then computed. The overall analysis shows that the preferred solution
for mounting the optical elements is adopting the kinematic constraints instead of using the classical glue
solution
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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
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