1,721,002 research outputs found
Optical design study, testing and qualification of a Schwarzschild-Couder telescope for CTA and an assessment on the Intensity Interferometry capabilities with CTA
Full text linkThere is a growing common effort in the very high energy community towards the development of new research infrastructures to answer the fundamental questions of modern high-energy astrophysics and astroparticle physics. The Cherenkov Telescope Array Observatory (CTAO) is an international project aiming to deploy two separate arrays to observe the whole VHE sky between E = 20 GeV up to E = 300 TeV in a long term plan of about 30 years of operations. CTA is designed to increase the sensitivity by a factor 10 at 1 TeV, to enlarge the detection area, the angular resolution and the field of view over the facilities operating today. The observatory will be characterized by high flexibility, enhanced monitoring and deep survey capabilities, short time scale and simultaneous observations in multiple fields.
This PhD thesis addresses the optical design study and testing of dual mirror Imaging Atmospheric Cherenkov Telescopes (IACTs) for the incoming CTAO.
All of the IACTs facilities currently operating rely on single mirror solutions, which are mostly parabolic or Davies-Cotton optical designs, however there is a novel interest in the development of dual mirror configurations following the Schwarzschild-Couder optical design. This peculiar design, based on two highly aspherical mirrors promises wide-field, aplanatic telescopes characterized by small f-numbers and more compact structures. Dual mirror solutions allow use smaller camera pixels (3-6 mm) based on Silicon Photo Multiplier technology in substitution of the larger Photo Multiplier Tubes (1 inch) currently in use. The increased complexity in terms of optics manufacturing, replication and alignment is motivated from the attractive new capabilities of such configuration. In this context the Italian National Institute for Astrophysics (INAF) supported by the Italian Ministry of Education, University and Research (MIUR), is developing a small sized telescope prototype for CTA, named ASTRI, which is based upon the Schwarzschild-Couder optical design.
The present work deals with the challenging realization of this optical configuration that has never been applied to IACTs. After two introductory chapters on the gamma-ray astronomy and the ASTRI optical design and its main subsystems (chapters 1 and 2), the performances of this system are compared with those of the other common wide-field telescopes in use for Cherenkov observations and for other applications in astrophysics (chapter 3). This comparative study is based on a commercial ray tracing software into which the optical designs of the envisaged telescopes are reproduced.
Subsequently in chapter 4, an extended study of the ASTRI capabilities in relation to the performance and environmental requirements issued by CTA is presented in a detailed analysis of compliance supported by ray tracing simulations, finite element analysis and tolerance studies.
In chapter 5 the work on the qualification tests of the secondary mirror gives an insight into the complexity of the Schwarzschild-Couder optics. The realization of this optical element is challenging in relation to currently available technologies, in particular concerning the cost requirements imposed by the CTA project. These constraints and the large sagitta of the mirror (190 mm) requires the use of the hot slumping technique in substitution of the cold slumping and diamond milling approaches usually used in the manufacturing of mirrors for Cherenkov applications. The results of a careful and extended test campaign on a mirror prototype have indicated that this manufacturing technique can provide a reliable engineering process of production for such large, highly aspherical optics. With a perspective on the science with future large telescopes as those provided by CTA, an assessment study upon the potentialities of the Intensity Interferometry (II) technique is carried out in chapter 6. In particular, a new kind of observation based on II is explored; the method aims to estimate the direct distance of the celestial objects. The order of magnitudes of the problem parameters space and the sensitivity that CTA and other future large observatories should achieve is estimated by means of numerical simulations. A short-term concept of experiment to assess the reliability of this new method is also discussed in relation to a pilot measurement that could be pursued with the state of the art technology
Atmospheric modeling by SCRAT experiment during the 2010 ESA BEXUS 10 Flight Campaign
No full textThis paper discusses the atmospheric analyses that were carried out using pressure and temperature data collected during the flight of the SCRAT experiment onboard the BEXUS 10 balloon. Here are underlined the vast perspectives in terms of scientific data evaluation, analysis and modeling that can be retrieved by the use of simple commercial temperature and pressure sensors.
An environmental model accounting for radiative and convective energy transfer based on the temperature and pressure data has been developed. The diurnal effects on the stratospheric temperature and other related physical quantities as the lapse rate and the water vapor content have been derived. As a by product, the onboard temperature sensor used as a Sun sensor can infer many information on the attitude of the balloon by the study of the temperature oscillations induced by the balloon’s rotation during the floating phase
Data retrieved by SCRAT experiment during the 2010 ESA BEXUS 10 flight campaign
No full textThis paper describes the development of the SCRAT Experiment (Spherical Compact Rechargeable Air Thruster) for the evaluation of the performance of a cold-gas thruster on board the stratospheric balloon BEXUS 10. SCRAT is a two-stage cold gas micro-actuator, which employs atmospheric air as propellant to produce thrust and does not need propellant on board, since the gas is directly collected from the external environment. The air is then energized by two micro-compressors connected in parallel that fill a two-stage tank before ejecting the propellant to deliver the thrust; the two-stage configuration allows to modulate the thrust profile and the total impulse Different environmental conditions encountered during the stratospheric flight allowed to measure the actuator thrust in function of varying external pressure and temperature from 0 to 25 km of altitude. The raw data collected were submitted to extensive signal processing, to filter out the disturbances from the environment and the measurement system: a total of 101 thrusts were sampled in the 10-4 – 10-1 N range with a total impulse between 10-3 and 10-2 Ns each duty cycle. Compared to standard cold gas thrusters, with such performances a rechargeable actuator is able to save about 0.12 kg of propellant every unit of total impulse, which is more than 1 kg in a four-hour stratospheric flight and more than 2.7 kg on lower altitude equivalent flights
Conceptual design of a rechargeable air thruster for small aerial vehicles
No full textA lot of research in Aerospace Engineering aims both to miniaturization and development of economic projects. In this scenario the increasing employment of Lighter Than Air Vehicles such as blimps and balloons requires a new generation of attitude controllers with lower requirements in terms of mass and overall resources. This document analyzes the optimization of an innovative Rechargeable Cold Gas Actuator which is able to collect the propellant directly from the surrounding environment. In the first part of the paper the conceptual design of the actuator is introduced. After that, the optimization of thrust and total impulse parameters and to the development of a simplified technological demonstrator is discussed, and the last section analyzes future applications and niches of the marketplace
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
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