85 research outputs found
Extinction controlled adaptive phase-mask coronagraph
Context. Phase-mask coronagraphy is advantageous in terms of inner working
angle and discovery space. It is however still plagued by drawbacks such as
sensitivity to tip-tilt errors and chromatism. A nulling stellar coronagraph
based on the adaptive phase-mask concept using polarization interferometry is
presented in this paper. Aims. Our concept aims at dynamically and
achromatically optimizing the nulling efficiency of the coronagraph, making it
more immune to fast low-order aberrations (tip-tilt errors, focus, ...).
Methods. We performed numerical simulations to demonstrate the value of the
proposed method. The active control system will correct for the detrimental
effects of image instabilities on the destructive interference. The mask
adaptability both in size, phase and amplitude also compensates for
manufacturing errors of the mask itself, and potentially for chromatic effects.
Liquid-crystal properties are used to provide variable transmission of an
annulus around the phase mask, but also to achieve the achromatic {\pi} phase
shift in the core of the PSF by rotating the polarization by 180 degrees.
Results. We developed a new concept and showed its practical advantages using
numerical simulations. This new adaptive implementation of the phase-mask
coronagraph could advantageously be used on current and next-generation
adaptive optics systems, enabling small inner working angles without
compromising contrast.Comment: 7 pages, 6 figure
Extinction controlled Adaptive Mask Coronagraph Lyot and Phase Mask dual concept for wide extinction area
A dual coronagraph based on the Adaptive Mask concept is presented in this paper. A Lyot coronagraph with a variable diameter occulting disk and a nulling stellar coronagraph based on the Adaptive Phase Mask concept using polarization interferometry are presented in this work. Observations on sky and numerical simulations show the usefulness of the proposed method to optimize the nulling efficiency of the coronagraphs. In the case of the phase mask, the active control system will correct for the detrimental effects of image instabilities on the destructive interference (low-order aberrations such as tip-tilt and focus). The phase mask adaptability both in size, phase and amplitude also compensate for manufacturing errors of the mask itself, and potentially for chromatic effects. Liquid-crystal properties are used to provide variable transmission of an annulus around the phase mask, but also to achieve the achromatic π phase shift in the core of the PSF by rotating the polarization by 180°.A compressed mercury (Hg) drop is used as an occulting disk for the Lyot mask, its size control offers an adaptation to the seeing conditions and provides an optimization of the Tip-tilt correction
First results from fringe tracking with the PRIMA fringe sensor unit
The fringe sensor unit (FSU) is the central element of the phase referenced
imaging and micro-arcsecond astrometry (PRIMA) dual-feed facility for the Very
Large Telescope interferometer (VLTI). It has been installed at the Paranal
observatory in August 2008 and is undergoing commissioning and preparation for
science operation. Commissioning observations began shortly after installation
and first results include the demonstration of spatially encoded fringe sensing
and the increase in VLTI limiting magnitude for fringe tracking. However,
difficulties have been encountered because the FSU does not incorporate
real-time photometric correction and its fringe encoding depends on
polarisation. These factors affect the control signals, especially their
linearity, and can disturb the tracking control loop. To account for this,
additional calibration and characterisation efforts are required. We outline
the instrument concept and give an overview of the commissioning results
obtained so far. We describe the effects of photometric variations and
beam-train polarisation on the instrument operation and propose possible
solutions. Finally, we update on the current status in view of the start of
astrometric science operation with PRIMA.Comment: 12 pages, 11 figures, SPIE 2010 conference proceeding
Preferências estabelecidas e personalidade: uma abordagem comportamental
TCC (graduação) - Universidade Federal de Santa Catarina. Centro Sócio-Econômico. Economia.Este estudo tem o objetivo de contribuir para o entendimento da dissonância cognitiva que surge quando preferências estabelecidas com relação a produtos são alteradas à medida em que os consumidores se deparam com informação mais completa do que a existente na escolha inicial. Buscou-se verificar a probabilidade de abandono da escolha preliminar na amostra de consumidores analisada. A contribuição se refere à mediação da habilidade cognitiva e da personalidade nessas decisões. No experimento, os participantes com fraca preferência na escolha inicial – e baixa habilidade cognitiva, além de baixos escores em traços de honestidade-humildade, extroversão e conscienciosidade – tendem a abandonar a escolha inicial. Já os participantes com escores mais altos em todas essas características tendem a não abandonar a sua escolha preliminar
The ESPRI project: astrometric exoplanet search with PRIMA I. Instrument description and performance of first light observations
The ESPRI project relies on the astrometric capabilities offered by the PRIMA
facility of the Very Large Telescope Interferometer for the discovery and study
of planetary systems. Our survey consists of obtaining high-precision
astrometry for a large sample of stars over several years and to detect their
barycentric motions due to orbiting planets. We present the operation
principle, the instrument's implementation, and the results of a first series
of test observations. A comprehensive overview of the instrument infrastructure
is given and the observation strategy for dual-field relative astrometry is
presented. The differential delay lines, a key component of the PRIMA facility
which was delivered by the ESPRI consortium, are described and their
performance within the facility is discussed. Observations of bright visual
binaries are used to test the observation procedures and to establish the
instrument's astrometric precision and accuracy. The data reduction strategy
for astrometry and the necessary corrections to the raw data are presented.
Adaptive optics observations with NACO are used as an independent verification
of PRIMA astrometric observations. The PRIMA facility was used to carry out
tests of astrometric observations. The astrometric performance in terms of
precision is limited by the atmospheric turbulence at a level close to the
theoretical expectations and a precision of 30 micro-arcseconds was achieved.
In contrast, the astrometric accuracy is insufficient for the goals of the
ESPRI project and is currently limited by systematic errors that originate in
the part of the interferometer beamtrain which is not monitored by the internal
metrology system. Our observations led to the definition of corrective actions
required to make the facility ready for carrying out the ESPRI search for
extrasolar planets.Comment: 32 pages, 39 figures, Accepted for publication in Astronomy and
Astrophysic
Improving the astrometric performance of VLTI-PRIMA
In the summer of 2011, the first on-sky astrometric commissioning of
PRIMA-Astrometry delivered a performance of 3 m'' for a 10 '' separation on
bright objects, orders of magnitude away from its exoplanet requirement of 50
{\mu}'' ~ 20 {\mu}'' on objects as faint as 11 mag ~ 13 mag in K band. This
contribution focuses on upgrades and characterizations carried out since then.
The astrometric metrology was extended from the Coud\'e focus of the
Auxillary Telescopes to their secondary mirror, in order to reduce the baseline
instabilities and improve the astrometric performance. While carrying out this
extension, it was realized that the polarization retardance of the star
separator derotator had a major impact on both the astrometric metrology and
the fringe sensors. A local compensation of this retardance and the operation
on a symmetric baseline allowed a new astrometric commissioning. In October
2013, an improved astrometric performance of 160 {\mu}'' was demonstrated,
still short of the requirements. Instabilities in the astrometric baseline
still appear to be the dominating factor.
In preparation to a review held in January 2014, a plan was developed to
further improve the astrometric and faint target performance of PRIMA
Astrometry. On the astrometric aspect, it involved the extension of the
internal longitudinal metrology to primary space, the design and implementation
of an external baseline metrology, and the development of an astrometric
internal fringes mode. On the faint target aspect, investigations of the
performance of the fringe sensor units and the development of an AO system
(NAOMI) were in the plan. Following this review, ESO decided to take a proposal
to the April 2014 STC that PRIMA be cancelled, and that ESO resources be
concentrated on ensuring that Gravity and Matisse are a success. This proposal
was recommended by the STC in May 2014, and endorsed by ESO.Comment: 12 pages, 9 figures, 2 tables, Proceeding of SPIE conference in
Montrea
First direct detection of an exoplanet by optical interferometry; Astrometry and K-band spectroscopy of HR8799 e
To date, infrared interferometry at best achieved contrast ratios of a few
times on bright targets. GRAVITY, with its dual-field mode, is now
capable of high contrast observations, enabling the direct observation of
exoplanets. We demonstrate the technique on HR8799, a young planetary system
composed of four known giant exoplanets. We used the GRAVITY fringe tracker to
lock the fringes on the central star, and integrated off-axis on the HR8799e
planet situated at 390 mas from the star. Data reduction included
post-processing to remove the flux leaking from the central star and to extract
the coherent flux of the planet. The inferred K band spectrum of the planet has
a spectral resolution of 500. We also derive the astrometric position of the
planet relative to the star with a precision on the order of 100as. The
GRAVITY astrometric measurement disfavors perfectly coplanar stable orbital
solutions. A small adjustment of a few degrees to the orbital inclination of HR
8799 e can resolve the tension, implying that the orbits are close to, but not
strictly coplanar. The spectrum, with a signal-to-noise ratio of
per spectral channel, is compatible with a late-type L brown dwarf. Using
Exo-REM synthetic spectra, we derive a temperature of \,K and a
surface gravity of cm/s. This corresponds to a radius
of and a mass of , which is an independent confirmation of mass estimates from evolutionary
models. Our results demonstrate the power of interferometry for the direct
detection and spectroscopic study of exoplanets at close angular separations
from their stars.Comment: published in A&
Coordinated changes in energy intake and expenditure following hypothalamic administration of neuropeptides involved in energy balance
OBJECTIVE: The hypothalamic control of energy balance is regulated by a complex network of neuropeptide-releasing neurons. Whilst the effect of these neuropeptides on individual aspects of energy homeostasis has been studied, the coordinated response of these effects has not been comprehensively investigated. We have simultaneously monitored a number of metabolic parameters following ICV administration of 1nmol and 3nmol of neuropeptides with established roles in the regulation of feeding, activity and metabolism. Ad libitum fed rats received the orexigenic neuropeptides neuropeptide Y (NPY), agouti-related protein (AgRP), melanin-concentrating hormone (MCH) or orexin-A. Overnight food deprived rats received an ICV injection of the anorectic peptides α-MSH, corticotrophin releasing factor (CRF) or neuromedin U (NMU). RESULTS: Our results reveal the temporal sequence of the effects of these neuropeptides on both energy intake and expenditure, highlighting key differences in their function as mediators of energy balance. NPY and AgRP increased feeding and decreased oxygen consumption, with the effects of AgRP being more prolonged. In contrast, orexin-A increased both feeding and oxygen consumption, consistent with an observed increase in activity. The potent anorexigenic effects of CRF were accompanied by a prolonged increase in activity whilst NMU injection resulted in significant but short-lasting inhibition of food intake, ambulatory activity and oxygen consumption. Alpha-MSH injection resulted in significant increases in both ambulatory activity and oxygen consumption, and reduced food intake following administration of 3nmol of the peptide. CONCLUSION: We have for the first time, simultaneously measured several metabolic parameters following hypothalamic administration of a number of neuropeptides within the same experimental system. This work has demonstrated the interrelated effects of these neuropeotides on activity, energy expenditure and food intake thus facilitating comparison between the different hypothalamic systems
First light for GRAVITY: Phase referencing optical interferometry for the Very Large Telescope Interferometer
This is the author accepted manuscript. the final version is available from EDP Sciences via the DOI in this recordGRAVITY is a new instrument to coherently combine the light of the European Southern Observatory Very Large Telescope Interferometer to form a telescope with an equivalent 130 m diameter angular resolution and a collecting area of 200 m2. The instrument comprises fiber fed integrated optics beam combination, high resolution spectroscopy, built-in beam analysis and control, near-infrared wavefront sensing, phase-tracking, dual-beam operation, and laser metrology. GRAVITY opens up to optical/infrared interferometry the techniques of phase referenced imaging and narrow angle astrometry, in many aspects following the concepts of radio interferometry. This article gives an overview of GRAVITY and reports on the performance and the first astronomical observations during commissioning in 2015/16. We demonstrate phase-tracking on stars as faint as mK ≈ 10 mag, phase-referenced interferometry of objects fainter than mK ≈ 15 mag with a limiting magnitude of mK ≈ 17 mag, minute long coherent integrations, a visibility accuracy of better than 0.25%, and spectro-differential phase and closure phase accuracy better than 0.5°, corresponding to a differential astrometric precision of better than ten microarcseconds (μas). The dual-beam astrometry, measuring the phase difference of two objects with laser metrology, is still under commissioning. First observations show residuals as low as 50 μas when following objects over several months. We illustrate the instrument performance with the observations of archetypical objects for the different instrument modes. Examples include the Galactic center supermassive black hole and its fast orbiting star S2 for phase referenced dual-beam observations and infrared wavefront sensing, the high mass X-ray binary BP Cru and the active galactic nucleus of PDS 456 for a few μas spectro-differential astrometry, the T Tauri star S CrA for a spectro-differential visibility analysis, ξ Tel and 24 Cap for high accuracy visibility observations, and η Car for interferometric imaging with GRAVITY.Agence Nationale de la RechercheAlexander von Humboldt FoundationEuropean Union, Seventh Framework Programm
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