1,353 research outputs found
The Fringe Detection Laser Metrology for the GRAVITY Interferometer at the VLTI
Interferometric measurements of optical path length differences of stars over
large baselines can deliver extremely accurate astrometric data. The
interferometer GRAVITY will simultaneously measure two objects in the field of
view of the Very Large Telescope Interferometer (VLTI) of the European Southern
Observatory (ESO) and determine their angular separation to a precision of 10
micro arcseconds in only 5 minutes. To perform the astrometric measurement with
such a high accuracy, the differential path length through the VLTI and the
instrument has to be measured (and tracked since Earth's rotation will
permanently change it) by a laser metrology to an even higher level of accuracy
(corresponding to 1 nm in 3 minutes). Usually, heterodyne differential path
techniques are used for nanometer precision measurements, but with these
methods it is difficult to track the full beam size and to follow the light
path up to the primary mirror of the telescope. Here, we present the
preliminary design of a differential path metrology system, developed within
the GRAVITY project. It measures the instrumental differential path over the
full pupil size and up to the entrance pupil location. The differential phase
is measured by detecting the laser fringe pattern both on the telescopes'
secondary mirrors as well as after reflection at the primary mirror. Based on
our proposed design we evaluate the phase measurement accuracy based on a full
budget of possible statistical and systematic errors. We show that this
metrology design fulfills the high precision requirement of GRAVITY.Comment: Proc. SPIE in pres
GRAVITY: the Calibration Unit
We present in this paper the design and characterisation of a new sub-system
of the VLTI 2nd generation instrument GRAVITY: the Calibration Unit. The
Calibration Unit provides all functions to test and calibrate the beam combiner
instrument: it creates two artificial stars on four beams, and dispose of four
delay lines with an internal metrology. It also includes artificial stars for
the tip-tilt and pupil guiding systems, as well as four metrology pick-up
diodes, for tests and calibration of the corresponding sub-systems. The
calibration unit also hosts the reference targets to align GRAVITY to the VLTI,
and the safety shutters to avoid the metrology light to propagate in the
VLTI-lab. We present the results of the characterisation and validtion of these
differrent sub-units.Comment: 12 pages, 11 figures. Proceeding of SPIE 9146 "Optical and Infrared
Interferometry IV
The GRAVITY metrology system: modeling a metrology in optical fibers
GRAVITY is the second generation VLT Interferometer (VLTI) instrument for
high-precision narrow-angle astrometry and phase-referenced interferometric
imaging. The laser metrology system of GRAVITY is at the heart of its
astrometric mode, which must measure the distance of 2 stars with a precision
of 10 micro-arcseconds. This means the metrology has to measure the optical
path difference between the two beam combiners of GRAVITY to a level of 5 nm.
The metrology design presents some non-common paths that have consequently to
be stable at a level of 1 nm. Otherwise they would impact the performance of
GRAVITY. The various tests we made in the past on the prototype give us hints
on the components responsible for this error, and on their respective
contribution to the total error. It is however difficult to assess their exact
origin from only OPD measurements, and therefore, to propose a solution to this
problem. In this paper, we present the results of a semi-empirical modeling of
the fibered metrology system, relying on theoretical basis, as well as on
characterisations of key components. The modeling of the metrology system
regarding various effects, e.g., temperature, waveguide heating or mechanical
stress, will help us to understand how the metrology behave. The goals of this
modeling are to 1) model the test set-ups and reproduce the measurements (as a
validation of the modeling), 2) determine the origin of the non-common path
errors, and 3) propose modifications to the current metrology design to reach
the required 1nm stability.Comment: 20 pages, 19 figures. Proceeding of SPIE 9146 "Optical and Infrared
Interferometry IV
Palaeohistology and palaeopathology of an Aeolosaurini (Sauropoda: Titanosauria) from Morro do Cambambe (Upper Cretaceous, Brazil)
Altres ajuts: CAISEP (Comisión de Ayudas a la Investigación de la Sociedad Española de PaleontologÃa) project #2018-07153A recent publication of fossil bones of titanosaurs assigned to Aeolosaurini from the Morro do Cambambe site (Mato Grosso state, Brazil, Upper Cretaceous) reported anomalous growth in some of them. Here, we present osteohistological sections of elements to understand not only the microstructure and growth of such bones, but also the nature of those anomalies. The primary bone of all specimens consisted of a variation of the fibrolamellar complex, with the inner cortex being rich in woven bone with dispersed longitudinal canals, while the outer cortex was parallel-fibred with rows of longitudinal canals, interlayered by Lines of Arrested Growth (LAGs). We identified a maximum of two LAGs in the cervical rib and haemal arch, and four in the dorsal rib. The haemal arch shows an External Fundamental System (EFS) in most sections. The advanced remodelling and variation of the fibrolamellar bone in the cortex suggests that all the specimens represent individuals that reached sexual maturity. However, the haemal arch was distinct due to the wide distribution of EFS. The dorsal rib exhibited periosteal and endosteal outgrowth. Such microstructure was assigned to a reactive bone due to an intra-thoracic infection (a pneumonia, probably related to a tuberculosis), which is the first report in a non-avian dinosaur. The microstructure resembles the medullary bone recovered in dinosaurs, which suggests that further studies of medullary bone in thoracic bones should also regard the pathological cases.En una reciente publicación de los huesos fósiles de titanosaurios asignados al clado Aeolosaurini provenientes del yacimiento de Morro do Cambambe (estado de Mato Grosso, Brasil, Cretácico Superior), se reconocieron anormalidades en el crecimiento de algunos de ellos. En el presente trabajo presentamos cortes osteohistológicos de elementos para entender no sólo la microestructura y crecimiento de los mismos, sino también la naturaleza de aquellas anomalÃas. Entre ellos, seleccionamos una costilla cervical y una costilla dorsal media posterior, asà como un arco hemal. El hueso primario de todos los especÃmenes comprendÃa una variación del complejo fibrolamelar, siendo la corteza interna rica en tejido reticular óseo con canales longitudinales dispersos, mientras que la corteza externa tenÃa fibras paralelas con hileras de canales longitudinales, intercaladas por lÃneas de crecimiento detenido. Identificamos un máximo de dos lÃneas de crecimiento detenido tanto en la costilla cervical como en el arco hemal, y cuatro en la costilla dorsal. El arco hemal muestra un Sistema Externo Fundamental en la mayorÃa de las secciones. La remodelación avanzada y la variación del hueso fibrolamelar en la corteza, se sugiere que todas los especÃmenes alcanzaron la madurez sexual. Sin embargo, el arco hemal fue distinto debido a la amplia distribución de Sistema Externo Fundamental. Con base en la microestructura, identificamos un semaforonte subadulto, y probablemente a un adulto. La costilla dorsal mostró una excrecencia perióstica y endosteal. Dicha microestructura se ha identificado con un hueso reactivo a una infección intratorácica (una neumonÃa, probablemente relacionada con una tuberculosis), que es el primer informe de un dinosaurio no aviano. La microestructura se asemeja al hueso medular recuperado en los dinosaurios, lo que sugiere que los estudios posteriores del hueso medular en los huesos torácicos también deberÃan considerar los casos patológicos
Public crises, public futures
This article begins to map out a novel approach to analyzing contemporary contexts of public crisis, relationships between them and possibilities that these scenes hold out for politics. The article illustrates and analyses a small selection of examples of these kinds of contemporary scenes and calls for greater attention to be given to the conditions and consequences of different forms and practices of public and political mediation. In offering a three-fold typology to delineate differences between ‘abject’, ‘audience’ and ‘agentic’ publics the article begins to draw out how political and public futures may be seen as being bound up with how the potentialities, capacities and qualities that publics are imagined to have and resourced to perform. Public action and future publics are therefore analysed here in relation to different versions of contemporary crisis and the political concerns and publics these crises work to articulate, foreground and imaginatively and practically support
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