155 research outputs found
Resolving gravitational microlensing events with long-baseline optical interferometry. Prospects for the ESO Very Large Telescope Interferometer
Until now, the detailed interpretation of the observed microlensing events
has suffered from the fact that the physical parameters of the phenomenon
cannot be uniquely determined from the available astronomical measurements,
i.e. the photometric lightcurves. The situation will change in the near-future
with the availability of long-baseline, sensitive optical interferometers,
which should be able to resolve the images of the lensed objects into their
components. For this, it will be necessary to achieve a milliarcsecond
resolution on sources with typical magnitudes K \ga 12. Indeed, brighter
events have never been observed up to now by micro-lensing surveys. We discuss
the possibilities opened by the use of long baseline interferometry in general,
and in particular for one such facility, the ESO VLT Interferometer, which will
attain the required performance. We discuss the expected accuracy and limiting
magnitude of such measurements. On the basis of the database of the events
detected by the OGLE experiment, we estimate the number of microlenses that
could be available for measurements by the VLTI. We find that at least several
tens of events could be observed each year. In conjunction with the photometric
data, our ability to measure the angular separation between the microlensed
images will enable a direct and unambiguous determination of both their masses
and locations.Comment: Accepted for publication in Astronomy & Astrophysic
ONE STEP ELECTRODEPOSITION OF CuInSe2 THIN FILMS
Formation of CuInSe2 (CIS) thin films from aqueous solution containing citrate as complexing agent is reported. The surface morphology and the composition of the deposited films are characterized by scanning electron microscopy (SEM). The texture of the deposits and their compositions are analyzed by X-ray diffraction and transmission electron microscopy (TEM). Annealing of the films at 350°C in flowing argon electrodeposited at potentials in the range [-0.24, -0.4 (V vs Ag/AgCl)] resulted in the formation of alpha-Cu 2 Se (JCPDS 24-1131) and CuSe (JCPDS 6-0427). On the contrary, annealing in the same conditions of the films electrodeposited between -0.4 and -0.6 V vs Ag/AgCl led to the formation of chalcopyrite CuInSe 2 (JCPDS 23-209) with alpha-Cu 2 Se (JCPDS 24-1131) as secondary phase. The formation of CuInSe 2 films with a chalcopyrite structure and good stoichiometry is observed.Formation of CuInSe2 (CIS) thin films from aqueous solution containing citrate as complexing agent is reported. The surface morphology and the composition of the deposited films are characterized by scanning electron microscopy (SEM). The texture of the deposits and their compositions are analyzed by X-ray diffraction and transmission electron microscopy (TEM). Annealing of the films at 350°C in flowing argon electrodeposited at potentials in the range [-0.24, -0.4 (V vs Ag/AgCl)] resulted in the formation of alpha-Cu 2 Se (JCPDS 24-1131) and CuSe (JCPDS 6-0427). On the contrary, annealing in the same conditions of the films electrodeposited between -0.4 and -0.6 V vs Ag/AgCl led to the formation of chalcopyrite CuInSe 2 (JCPDS 23-209) with alpha-Cu 2 Se (JCPDS 24-1131) as secondary phase. The formation of CuInSe 2 films with a chalcopyrite structure and good stoichiometry is observed
Mid-infrared interferometry with K band fringe-tracking I. The VLTI MIDI+FSU experiment
Context: A turbulent atmosphere causes atmospheric piston variations leading
to rapid changes in the optical path difference of an interferometer, which
causes correlated flux losses. This leads to decreased sensitivity and accuracy
in the correlated flux measurement. Aims: To stabilize the N band
interferometric signal in MIDI (MID-infrared Interferometric instrument), we
use an external fringe tracker working in K band, the so-called FSU-A (fringe
sensor unit) of the PRIMA (Phase-Referenced Imaging and Micro-arcsecond
Astrometry) facility at VLTI. We present measurements obtained using the newly
commissioned and publicly offered MIDI+FSU-A mode. A first characterization of
the fringe-tracking performance and resulting gains in the N band are
presented. In addition, we demonstrate the possibility of using the FSU-A to
measure visibilities in the K band. Methods: We analyzed FSU-A fringe track
data of 43 individual observations covering different baselines and object K
band magnitudes with respect to the fringe-tracking performance. The N band
group delay and phase delay values could be predicted by computing the relative
change in the differential water vapor column density from FSU-A data.
Visibility measurements in the K band were carried out using a scanning mode of
the FSU-A. Results: Using the FSU-A K band group delay and phase delay
measurements, we were able to predict the corresponding N band values with high
accuracy with residuals of less than 1 micrometer. This allows the coherent
integration of the MIDI fringes of faint or resolved N band targets,
respectively. With that method we could decrease the detection limit of
correlated fluxes of MIDI down to 0.5 Jy (vs. 5 Jy without FSU-A) and 0.05 Jy
(vs. 0.2 Jy without FSU-A) using the ATs and UTs, respectively. The K band
visibilities could be measured with a precision down to ~2%.Comment: 11 pages, 13 figures, Accepted for publication in A&
Image reconstruction in optical interferometry: Benchmarking the regularization
With the advent of infrared long-baseline interferometers with more than two
telescopes, both the size and the completeness of interferometric data sets
have significantly increased, allowing images based on models with no a priori
assumptions to be reconstructed. Our main objective is to analyze the multiple
parameters of the image reconstruction process with particular attention to the
regularization term and the study of their behavior in different situations.
The secondary goal is to derive practical rules for the users. Using the
Multi-aperture image Reconstruction Algorithm (MiRA), we performed multiple
systematic tests, analyzing 11 regularization terms commonly used. The tests
are made on different astrophysical objects, different (u,v) plane coverages
and several signal-to-noise ratios to determine the minimal configuration
needed to reconstruct an image. We establish a methodology and we introduce the
mean-square errors (MSE) to discuss the results. From the ~24000 simulations
performed for the benchmarking of image reconstruction with MiRA, we are able
to classify the different regularizations in the context of the observations.
We find typical values of the regularization weight. A minimal (u,v) coverage
is required to reconstruct an acceptable image, whereas no limits are found for
the studied values of the signal-to-noise ratio. We also show that
super-resolution can be achieved with increasing performance with the (u,v)
coverage filling. Using image reconstruction with a sufficient (u,v) coverage
is shown to be reliable. The choice of the main parameters of the
reconstruction is tightly constrained. We recommend that efforts to develop
interferometric infrastructures should first concentrate on the number of
telescopes to combine, and secondly on improving the accuracy and sensitivity
of the arrays.Comment: 15 pages, 16 figures; accepted in A&
ESPRI data-reduction strategy and error budget for PRIMA
The Exoplanet Search with PRIma (ESPRI) will use the PRIMA dual-feed astrometric capability on the Very Large Telescope Interferometer (VLTI) to perform astrometric detections of extra-solar planets. We present an overview of our data-reduction strategy for achieving 10-μarcsecond accuracy narrow-angle astrometry using the PRIMA instrument. We discuss the error budget for astrometric measurements, and those aspects of our strategy which are designed to minimise the astrometric measurement error
The ESPRI project: narrow-angle astrometry with VLTI-PRIMA
We describe the ongoing hardware and software developments that shall enable the ESO VLTI to perform narrow-angle differential delay astrometry in K-band with an accuracy of up to 10 μarcsec. The ultimate goal of these efforts is to perform an astrometric search for extrasolar planets around nearby star
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