471 research outputs found
Large amplitude tip/tilt estimation by geometric diversity for multiple-aperture telescopes
A novel method nicknamed ELASTIC is proposed for the alignment of
multiple-aperture telescopes, in particular segmented telescopes. It only needs
the acquisition of two diversity images of an unresolved source, and is based
on the computation of a modified, frequency-shifted, cross-spectrum. It
provides a polychromatic large range tip/tilt estimation with the existing
hardware and an inexpensive noniterative unsupervised algorithm. Its
performance is studied and optimized by means of simulations. They show that
with 5000 photo-electrons/sub-aperture/frame and 1024x1024 pixel images,
residues are within the capture range of interferometric phasing algorithms
such as phase diversity. The closed-loop alignment of a 6 sub-aperture mirror
provides an experimental demonstration of the effectiveness of the method.
Author accepted version. Final version is Copyright 2017 Optical Society of
America. One print or electronic copy may be made for personal use only.
Systematic reproduction and distribution, duplication of any material in this
paper for a fee or for commercial purposes, or modifications of the content of
this paper are prohibited.Comment: Final version:
https://www.osapublishing.org/josaa/abstract.cfm?uri=josaa-34-8-127
Performance of astrometric detection of a hotspot orbiting on the innermost stable circular orbit of the galactic centre black hole
The galactic central black hole Sgr A* exhibits outbursts of radiation in the
near infrared (so-called IR flares). One model of these events consists in a
hotspot orbiting on the innermost stable circular orbit (ISCO) of the hole.
These outbursts can be used as a probe of the central gravitational potential.
One main scientific goal of the second generation VLTI instrument GRAVITY is to
observe these flares astrometrically. Here, the astrometric precision of
GRAVITY is investigated in imaging mode, which consists in analysing the image
computed from the interferometric data. The capability of the instrument to put
in light the motion of a hotspot orbiting on the ISCO of our central black hole
is then discussed.
We find that GRAVITY's astrometric precision for a single star in imaging
mode is smaller than the Schwarzschild radius of Sgr A*. The instrument can
also demonstrate that a body orbiting on the last stable orbit of the black
hole is indeed moving. It yields a typical size of the orbit, if the source is
as bright as m_K=14.
These results show that GRAVITY allows one to study the close environment of
Sgr A*. Having access to the ISCO of the central massive black hole probably
allows constraining general relativity in its strong regime. Moreover, if the
hotspot model is appropriate, the black hole spin can be constrained.Comment: 13 pages, 11 figures ; accepted by MNRA
The tholeiites of the Valaisan domain (Versoyen, western Alps): a Carboniferous magma emplaced in a small oceanic basin
International audienceThe mafic-ultramafic assemblages of the Versoyen complex exposed in the Valaisan domain is close to the boundary between the Internal and the External domains of the western Alps. Zircons extracted from the Versoyen complex suggest an emplacement during Paleozoic times, and probably during the Visean (~337 Ma). The base of the Versoyen complex is formed of laccoliths and sills associated with black shales, while pillow basalts and tuffs predominate at the uppermost levels. Locally, basaltic dikelets intruded leucocratic gneiss. Ultramafic-mafic cumulates form the bottom of the thickest intrusions while diabases are present along the chilled margins. All these rocks have been affected by a polyphased metamorphism under eclogitic to blueschist and greenschist facies conditions. Magmatic textures have been destroyed and the igneous mineralogy is seldom preserved. The mafic rocks of the Versoyen complex show tholeiitic to alkali-transitional affinities. The pillow basalts and the sill cores have flat REE patterns characteristic of N-MORB and T-MORB. Their {varepsilon}Nd (assuming an age of 337 Ma) ratios range from + 5.7 to + 9 which suggest a mixing of N-MORB and OIB sources. The sill margins show Th, U and LREE-enrichments and negative {varepsilon}Nd ratios. These features are likely related to contamination when hot mafic magmas intruded unconsolidated sediments rich in water. The high Th, U, LREE abundances and low {varepsilon}Nd ratio of the basaltic dikelet are probably related to crustal contamination occurring during magma ascent. The geochemical characteristics of the Versoyen rocks are compatible with a tholeiitic magma emplaced into a small oceanic basin in the vicinity of a continent. The importance of pre-Mesozoic crustal thinning evidenced in one segment of the boundary between the Internal and External zones of the Alps suggests that the Pennine Front is an Alpine mega-thrust inherited from a Variscan suture
Correction of distortion for optimal image stacking in Wide Field Adaptive Optics: Application to GeMS data
The advent of Wide Field Adaptive Optics (WFAO) systems marks the beginning
of a new era in high spatial resolution imaging. The newly commissioned Gemini
South Multi-Conjugate Adaptive Optics System (GeMS) combined with the infrared
camera Gemini South Adaptive Optics Imager (GSAOI), delivers quasi
diffraction-limited images over a field of 2 arc-minutes across. However,
despite this excellent performance, some variable residues still limit the
quality of the analyses. In particular, distortions severely affect GSAOI and
become a critical issue for high-precision astrometry and photometry. In this
paper, we investigate an optimal way to correct for the distortion following an
inverse problem approach. Formalism as well as applications on GeMS data are
presented.Comment: 10 pages, 6 figure
Imaging the spotty surface of Betelgeuse in the H band
This paper reports on H-band interferometric observations of Betelgeuse made
at the three-telescope interferometer IOTA. We image Betelgeuse and its
asymmetries to understand the spatial variation of the photosphere, including
its diameter, limb darkening, effective temperature, surrounding brightness,
and bright (or dark) star spots. We used different theoretical simulations of
the photosphere and dusty environment to model the visibility data. We made
images with parametric modeling and two image reconstruction algorithms: MIRA
and WISARD. We measure an average limb-darkened diameter of 44.28 +/- 0.15 mas
with linear and quadratic models and a Rosseland diameter of 45.03 +/- 0.12 mas
with a MARCS model. These measurements lead us to derive an updated effective
temperature of 3600 +/- 66 K. We detect a fully-resolved environment to which
the silicate dust shell is likely to contribute. By using two imaging
reconstruction algorithms, we unveiled two bright spots on the surface of
Betelgeuse. One spot has a diameter of about 11 mas and accounts for about 8.5%
of the total flux. The second one is unresolved (diameter < 9 mas) with 4.5% of
the total flux. Resolved images of Betelgeuse in the H band are asymmetric at
the level of a few percent. The MOLsphere is not detected in this wavelength
range. The amount of measured limb-darkening is in good agreement with model
predictions. The two spots imaged at the surface of the star are potential
signatures of convective cells.Comment: 10 pages, 10 figures, accepted for publication in A&A, references
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Elaboration and characterization of Fe1âxO thin films sputter deposited from magnetite target
Majority of the authors report elaboration of iron oxide thin films by reactive magnetron sputtering from an iron target with ArâO2 gas mixture. Instead of using the reactive sputtering of a metallic target we report here the preparation of Fe1âxOthin films, directly sputtered froma magnetite target in a pure argon gas flow with a bias power applied. This oxide is generally obtained at very low partial oxygen pressure and high temperature.We showed that bias sputtering which can be controlled very easily can lead to reducing conditions during deposition of oxide thin film on simple glass substrates. The proportion of wustite was directly adjusted bymodifying the power of the substrate polarization. Atomic force microscopy was used to observe these nanostructured layers. Mössbauer measurements and electrical properties versus bias polarization and annealing temperature are also reported
Optimizing the use of pressurized bladders for the assembly of HL-LHC MQXFB magnets
The use of pressurized bladders for stress control of superconducting magnets
was firstly proposed at Lawrence Berkeley National Laboratory (LBNL) in the
early 2000s. Since then, the so-called bladders and keys procedure has become
one of the reference techniques for the assembly of high-field accelerator
magnets and demonstrators. Exploiting the advantages of this method is today of
critical importance for Nb3Sn-based accelerator magnets, whose production
requires the preservation of tight stress targets in the superconducting coils
to limit the effects of the strain sensitivity and brittleness of the
conductor. The present manuscript reports on the results of an experimental
campaign focused on the optimization of the bladders and keys assembly process
in the MQXFB quadrupoles. These 7.2 m long magnets shall be among the first
Nb3Sn cryomagnets to be installed in a particle accelerator as a part of the
High Luminosity upgrade of the LHC. One of the main practical implications of
the bladders technique, especially important when applied to long magnets like
MQXFB, is that to insert the loading keys, the opening of a certain clearance
in the support structure is required. The procedure used so far for MQXF
magnets involved an overstress in the coils during bladder inflation. The work
presented here shows that such an overshoot can be eliminated thanks to
additional bladders properly positioned in the structure. This optimized method
was validated in a short model magnet and in a full-length mechanical model,
becoming the new baseline for the series production at CERN. Furthermore, the
results are supported by numerical predictions using Finite Element models
LP 349-25: a new tight M8V binary
We present the discovery of a tight M8V binary, with a separation of only 1.2
astronomical units, obtained with the PUEO and NACO adaptive optics systems,
respectively at the CFHT and VLT telescopes. The estimated period of LP 349-25
is approximately 5 years, and this makes it an excellent candidate for a
precise mass measurement.Comment: Accepted by Astronomy and Astrophysics Letter
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