MATISSE is the second-generation mid-infrared spectrograph and imager for the Very Large Telescope Interferometer (VLTI) at Paranal. This new interferometric instrument will allow significant advances by opening new avenues in various fundamental research fields: studying the planet-forming region of disks around young stellar objects, understanding the surface structures and mass loss phenomena affecting evolved stars, and probing the environments of black holes in active galactic nuclei. As a first breakthrough, MATISSE will enlarge the spectral domain of current optical interferometers by offering the L and M bands in addition to the N band. This will open a wide wavelength domain, ranging from 2.8 to 13 μm, exploring angular scales as small as 3 mas (L band) / 10 mas (N band). As a second breakthrough, MATISSE will allow mid-infrared imaging - closure-phase aperture-synthesis imaging - with up to four Unit Telescopes (UT) or Auxiliary Telescopes (AT) of the VLTI. Moreover, MATISSE will offer a spectral resolution range from R ∼ 30 to R ∼ 5000. Here, we present one of the main science objectives, the study of protoplanetary disks, that has driven the instrument design and motivated several VLTI upgrades (GRA4MAT and NAOMI). We introduce the physical concept of MATISSE including a description of the signal on the detectors and an evaluation of the expected performances. We also discuss the current status of the MATISSE instrument, which is entering its testing phase, and the foreseen schedule for the next two years that will lead to the first light at Paranal.
OPTICON currently supports a Joint Research Activity (JRA) dedicated to providing easy to use image reconstruction algorithms for optical/IR interferometric data. This JRA aims to provide state-of-the-art image reconstruction methods with a common interface and comprehensive documentation to the community. These tools will provide the capability to compare the results of using different settings and algorithms in a consistent and unified way. The JRA is also providing tutorials and sample datasets to introduce the principles of image reconstruction and illustrate how to use the software products. We describe the design of the imaging tools, in particular the interface between the graphical user interface and the image reconstruction algorithms, and summarise the current status of their implementation.
Image reconstruction in optical interferometry has gained considerable importance for astrophysical studies during the last decade. This has been mainly due to improvements in the imaging capabilities of existing interferometers and the expectation of new facilities in the coming years. However, despite the advances made so far, image synthesis in optical interferometry is still an open field of research. Since 2004, the community has organized a biennial contest to formally test the different methods and algorithms for image reconstruction. In 2016, we celebrated the 7th edition of the "Interferometric Imaging Beauty Contest". This initiative represented an open call to participate in the reconstruction of a selected set of simulated targets with a wavelength-dependent morphology as they could be observed by the 2nd generation of VLTI instruments. This contest represents a unique opportunity to benchmark, in a systematic way, the current advances and limitations in the field, as well as to discuss possible future approaches. In this contribution, we summarize: (a) the rules of the 2016 contest; (b) the different data sets used and the selection procedure; (c) the methods and results obtained by each one of the participants; and (d) the metric used to select the best reconstructed images. Finally, we named Karl-Heinz Hofmann and the group of the Max-Planck-Institut fur Radioastronomie as winners of this edition of the contest.
This paper presents an extension of the spatio-spectral (“3D”) image reconstruction algorithm called PAINTER (Polychromatic opticAl INTErferometric Reconstruction software). The algorithm is able to solve large scale problems and relies on an iterative process, which alternates estimation of polychromatic images and of complex visibilities. The complex visibilities are not only estimated from squared moduli and closure phases, but also from differential phases, which helps to constrain the polychromatic reconstruction. Alternative methods to construct the specific differential phases used in PAINTER are proposed. Simulations on synthetic data illustrate the specificities of the proposed methods.
MATISSE is the mid-infrared spectrograph and imager for the Very Large Telescope Interferometer (VLTI) at Paranal. This second generation interferometry instrument will open new avenues in the exploration of our Universe. Mid-infrared interferometry with MATISSE will allow significant advances in various fundamental research fields: studies of disks around young stellar objects where planets form and evolve, surface structures and mass loss of stars in late evolutionary stages, and the environments of black holes in active galactic nuclei. MATISSE is a unique instrument. As a first breakthrough it will enlarge the spectral domain used by optical interferometry by offering the L & M bands in addition to the N band, opening a wide wavelength domain, ranging from 2.8 to 13 μm on angular scales of 3 mas (L/M band) / 10 mas (N band). As a second breakthrough, it will allow mid-infrared imaging – closure-phase aperture-synthesis imaging – with up to four Unit Telescopes (UT) or Auxiliary Telescopes (AT) of the VLTI. MATISSE will offer various ranges of spectral resolution between R~30 to ~5000. In this article, we present some of the main science objectives that have driven the instrument design. We introduce the physical concept of MATISSE including a description of the signal on the detectors and an evaluation of the expected performance and discuss the project status. The operations concept will be detailed in a more specific future article, illustrating the observing templates operating the instrument, the data reduction and analysis, and the image reconstruction software.
We describe the principles and potential of Color-Differential Astrometry (CDA), a high-resolution technique easily
implementable on the Science Coronographic Instrument (SCI) of the SPICA satellite, and aimed here at the direct
detection and spectroscopy of giant Extrasolar Planets (ESP). By measuring the photocentre of the source diffraction
pattern relatively between dispersed spectral channels, CDA gives access to flux ratio and angular information well
beyond the telescope resolution limit. Applied to known ESPs, it can yield the inclination (thus the mass) and spectrum
of the planet. Our estimates show that low-resolution spectroscopy of Jupiter-radius ESP can be measured within a few
hours for planets at orbital distances ranging from 0.05 AU to a few AUs, thus complementing the detection range
expected using the coronographic measurements. More generally, it may also apply to any unresolved source with some
wavelength-dependent asymmetry.
In addition to the ESP cases considered for the scientific signal and to their associated fundamental noises, we also
present the instrumental effects and a dedicated optical testbench. The combined effects of several instrumental noise
sources can be introduced into our numerical model (pointing errors, beam tip-tilt, optical aberations, variations of the
detector gain table), and then confronted to measurements from the experimental testbench.
We present the results of the fifth Interferometric Imaging Beauty Contest. The contest consists in blind imaging of test data sets derived from model sources and distributed in the OIFITS format. Two scenarios of imaging with CHARA/MIRC-6T were offered for reconstruction: imaging a T Tauri disc and imaging a spotted red supergiant. There were eight different teams competing this time: Monnier with the software package MACIM; Hofmann, Schertl and Weigelt with IRS; Thiebaut and Soulez with MiRA ; Young with BSMEM; Mary and Vannier with MIROIRS; Millour and Vannier with independent BSMEM and MiRA entries; Rengaswamy with an original method; and Elias with the radio-astronomy package CASA. The contest model images, the data delivered to the contestants and the rules are described as well as the results of the image reconstruction obtained by each method. These results are discussed as well as the strengths and limitations of each algorithm.
MATISSE is a mid-infrared spectro-interferometer combining the beams of up to four Unit Telescopes or Auxiliary
Telescopes of the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory.
MATISSE will constitute an evolution of the two-beam interferometric instrument MIDI. New characteristics present in
MATISSE will give access to the mapping and the distribution of the material, the gas and essentially the dust, in the
circumstellar environments by using the mid-infrared band coverage extended to L, M and N spectral bands. The four
beam combination of MATISSE provides an efficient uv-coverage: 6 visibility points are measured in one set and 4
closure phase relations which can provide aperture synthesis images in the mid-infrared spectral regime.
We give an overview of the instrument including the expected performances and a view of the Science Case. We present
how the instrument would be operated. The project involves the collaborations of several agencies and institutes: the
Observatoire de la Côte d’Azur of Nice and the INSU-CNRS in Paris, the Max Planck Institut für Astronomie of
Heidelberg; the University of Leiden and the NOVA-ASTRON Institute of Dwingeloo, the Max Planck Institut für
Radioastronomie of Bonn, the Institut für Theoretische Physik und Astrophysik of Kiel, the Vienna University and the
Konkoly Observatory.
We present here three recipes for getting better images with optical interferometers. Two of them, Low-
Frequencies Filling and Brute-Force Monte Carlo were used in our participation to the Interferometry Beauty
Contest this year and can be applied to classical imaging using V2 and closure phases. These two addition to image reconstruction provide a way of having more reliable images. The last recipe is similar in its principle
as the self-calibration technique used in radio-interferometry. We call it also self-calibration, but it uses the
wavelength-differential phase as a proxy of the object phase to build-up a full-featured complex visibility set
of the observed object. This technique needs a first image-reconstruction run with an available software, using
closure-phases and squared visibilities only. We used it for two scientific papers with great success. We discuss
here the pros and cons of such imaging technique.
A variety of instrumental effects can corrupt the observable quantities in optical or nulling stellar interferometry.
One such effect is parasitic interference, which can occur inside an interferometric instrument. Because of
diffraction effects related to beam propagation along finite size optics, or parasitic reflections inside transmitting
optics, a coherent crosstalk may occur between the beams and create a parasitic interference pattern superimposed
on the genuine one. We developed an analytical approach to describe the impact of this effect on the observables
of classical and nulling stellar interferometers. Considering classical interferometry, we show that differential
phase and closure phase are both corrupted, depending on the crosstalk level and the residual piston between
the beams. Considering typical specifications of piston correction of ground-based interferometers (≈ 100 nm),
the detection of hot Jupiter-like planets by differential phase implies a tolerance on the parasitic flux to about
5% of the incident intensity. Also, we show that the closure phase relation does not remove this parasitic
contribution. The corresponding corrupted closure phase is not zero for an unresolved source, and depends on
the residual piston. Considering nulling interferometry, we show that parasitic effects modify the transmission
map level, depending on the crosstalk level and the phase shift between primary and secondary beams. In the
extreme case of a pi-phase shift, the crosstalk effect implies a decrease of the final output signal-to-noise ratio.
Numerical simulations, adapted to handle consistently crosstalk, are then performed to estimate this degradation
and validate our theoretical study.
Unveiling the structure of the Broad Line Region (BLR) of AGNs is critical to understand the quasar phenomenon.
Resolving a few BLRs by optical interferometry will bring decisive information to confront, complement and calibrate
the reverberation mapping technique, seed of the mass-luminosity relation in quasars. BLRs are much smaller than the
angular resolution of the VLT and Keck interferometers and they can be resolved only by differential interferometry
very accurate measurements of differential visibility and phase. The latest yields the photocenter variation with λ, and constrains the size, position and velocity law of various regions of the BLR. AGNs are below the magnitude limit for
spectrally resolved interferometry set by currently available fringe trackers. A new “blind” observation method and a
data processing based on the accumulation of 2D Fourier power and cross spectra permitted us the first spectrally
resolved interferometric observation of a BLR, on the K=10 quasar 3C273. A careful bias analysis is still in progress, but
we report strong evidence that, as the baseline increases, the differential visibility decreases in the Paα line. Combined
with a differential phase certainly smaller than 3°, this yields an angular radius of the BLR larger than 0.4
milliarcseconds, or 1000 light days at the distance of 3C273, much larger than the reverberation mapping radius of 300
light days. Explaining the coexistence of these two different scales, and possibly structures and mechanisms, implies
very new insights about the BLR of 3C273.
Interferometric Closure Phase (CP) yields information on the asymmetries of the source brightness distribu tion. While accurate closure phases are the key for detecting, odeling and imaging low contrast features, their experimental accuracy is usually far from what it could be: in the case of the AMBER/VLTI instrument, the guaranteed accuracy calibration is between 3 and 5 degrees, while the theoretical limit is better than 0.01 deg for bright sources.
Closure phase should first be corrected for detection artifacts (mainly drifts in the detector and optics), using in our case the AMBER Beam Commutation Device. We show that closure phase is nevertheless contaminated by the pistons drifts of each baseline. This effect is attributed to a cross-talk between the fringes peaks, which cannot be completely avoided in a multi-axial beam combiner with a limited readout window.
We show that the variable bias on CP is a linear function of the external pistons. This relationship can be determined from the calibration source data and applied for correcting the science data. The global process both unbiases and stabilizes the average CP, yielding, with our measurements, an accuracy of 0.3 deg for 1 minute exposures with ATs, which is close to the fundamental limit for our K=4 source. It also allows to correct the chromatic OPD effect by comparison with a well chosen calibrator, displaying a CP vs. wavelength curve with aRMS error of 0.1 deg per spectral channel, about a factor 3 to 4 better than with a straight calibration.
This poster advertizes the Jean-Marie Mariotti Center software tools, databases and services aimed at facilitating the use of optical interferometry worldwide such as preparation of observations, data reduction and data analysis. Its mission and organization are presented before listing the current software suite. Finally some facts and perspectives are mentioned.
We present the results of the fourth Optical/IR Interferometry Imaging Beauty Contest. The contest consists
of blind imaging of test data sets derived from model sources and distributed in the OI-FITS format. The test
data consists of spectral data sets on an object "observed" in the infrared with spectral resolution. There were 4
different algorithms competing this time: BSMEM the Bispectrum Maximum Entropy Method by Young, Baron
& Buscher; RPR the Recursive Phase Reconstruction by Rengaswamy; SQUEEZE a Markov Chain Monte Carlo
algorithm by Baron, Monnier & Kloppenborg; and, WISARD theWeak-phase Interferometric Sample Alternating
Reconstruction Device by Vannier & Mugnier. The contest model image, the data delivered to the contestants
and the rules are described as well as the results of the image reconstruction obtained by each method. These
results are discussed as well as the strengths and limitations of each algorithm.
This paper presents some methods being developped for relaxing the underdetermination of the image reconstruction
from interferometric data. We consider, in a first part, the advantages of using spectro-differential data
for having a more accurate and complete set of complexe visibilities. We formulate some regularization criteria
along the spectral dimension, in order to express some prior knowledge on the correlation between the brightness
distributions in different wavelength. These spectral prior terms are inspired by, and combinable with, some
spatial regularization functions already in use in existing Image Reconstruction sofwares.
We also show that the interferometric image reconstruction problem can benefit from being reformulated as
a sparse approximation problem in redundant dictionaries. The dictionary is composed from union of representation
bases, whose atoms correspond to geometric features of the image. Different bases (e.g. impulsions,
wavelets, discrete cosine transform) correspond to different features. The sparse approximation approach consists
in selecting the geometrical features that best explain the interferometric data, by imposing that only a few
such features should be necessary to reconstruct the image. Simulations showing images reconstructed using this
method are presented.
LITpro is a software for fitting models on data obtained from various stellar optical interferometers, like the VLTI. As a
baseline, for modeling the object, it provides a set of elementary geometrical and center-to-limb darkening functions, all
combinable together. But it is also designed to make very easy the implementation of more specific models with their
own parameters, to be able to use models closer to astrophysical considerations. So LITpro only requires the modeling
functions to compute the Fourier transform of the object at given spatial frequencies, and wavelengths and time if needed.
From this, LITpro computes all the necessary quantities as needed (e.g. visibilities, spectral energy distribution, partial
derivatives of the model, map of the object model). The fitting engine, especially designed for this kind of optimization, is
based on a modified Levenberg-Marquardt algorithm and has been successfully tested on real data in a prototype version.
It includes a Trust Region Method, minimizing a heterogeneous non-linear and non-convex criterion and allows the user
to set boundaries on free parameters. From a robust local minimization algorithm and a starting points strategy, a global
optimization solution is effectively achieved. Tools have been developped to help users to find the global minimum. LITpro
is also designed for performing fitting on heterogeneous data. It will be shown, on an example, how it fits simultaneously
interferometric data and spectral energy distribution, with some benefits on the reliability of the solution and a better
estimation of errors and correlations on the parameters. That is indeed necessary since present interferometric data are
generally multi-wavelengths.
The amdlib AMBER data reduction software is meant to produce AMBER data products from the raw data files
that are sent to the PIs of different proposals or that can be found in the ESO data archive. The way defined
by ESO to calibrate the data is to calibrate one science data file with a calibration one, observed as close in
time as possible. Therefore, this scheme does not take into account instrumental drifts, atmospheric variations
or visibility-loss corrections, in the current AMBER data processing software, amdlib.
In this article, we present our approach to complement this default calibration scheme, to perform the final
steps of data reduction, and to produce fully calibrated AMBER data products. These additional steps include:
an overnight view of the data structure and data quality, the production of night transfer functions from the
calibration stars observed during the night, the correction of additional effects not taken into account in the
standard AMBER data reduction software, and finally, the production of fully calibrated data products. All these new features are implemented in the modular pipeline script amdlibPipeline, written to complement the amdlib software.
MATISSE is foreseen as a mid-infrared spectro-interferometer combining the beams of up to four UTs/ATs of the Very
Large Telescope Interferometer (VLTI) of the European Southern Observatory. The related science case study
demonstrates the enormous capability of a new generation mid-infrared beam combiner.
MATISSE will constitute an evolution of the two-beam interferometric instrument MIDI. MIDI is a very successful
instrument which offers a perfect combination of spectral and angular resolution. New characteristics present in
MATISSE will give access to the mapping and the distribution of the material (typically dust) in the circumstellar
environments by using a wide mid-infrared band coverage extended to L, M and N spectral bands. The four beam
combination of MATISSE provides an efficient UV-coverage : 6 visibility points are measured in one set and 4 closure
phase relations which can provide aperture synthesis images in the mid-infrared spectral regime.
The first astrophysical results of the VLTI focal instrument AMBER have shown the importance of the differential
and closure phase measures, which are supposed to be much less sensitive to atmospheric and instrumental biases
than the absolute visibility. However there are artifacts limiting the accuracy of these measures which can be
substantially overcome by a specific calibration technique called Beam Commutation. This paper reports the
observed accuracies on AMBER/VLTI phases in different modes, discusses some of the instrumental biases and
shows the accuracy gain provided by Beam Commutation on the Differential Phase as well as on the Closure
Phase.
Performed in November 2007 as a part of the MIDI Guaranteed Time Observation exoplanet program, the
observation of the hot Jupiter-like exoplanet Gliese 86b constituted the first attempt of exoplanet detection with
the VLTI instrument MIDI. It is also a technical achievement as the first VLTI observation using AMBER and
MIDI simultaneously. Fringes were obtained for both instruments with the aim to correct the phase in N-band
from the dispersion using the fringes in K-band.
In N-band, the parent star has an estimated magnitude of 3.8, and a flux ratio planet/star of about 10-3
is expected. After simulating the effect of the data reduction process of MIDI (EWS), it appears that the
theoretical interferometric phase spectrum is a curved-like function with an amplitude (that we call arrow) of
about 0.05°. According to the phase spectra of the calibrator HD9362, taken during the first night of observation,
we estimate that a precision on the curvature measurement of about 0.33° is currently reached. Consequently,
we are at least at a factor 6 from a possible detection.
The AMBER data, obtained in parallel, were too noisy to be used to extrapolate and remove the corresponding
dispersion in N band at the required level of precision.
Recent site testing (see: http://www-luan.unice.fr/Concordiastro/indexantartic.html) has shown that Dome C in Antarctica might have a high potential for stellar interferometry if some solutions related to the surface atmospheric layer are found. A demonstrator interferometer could be envisioned in order to fully qualify the site and prepare the future development of a large array. We analyse the performances of a prototype interferometer for Dome C made with 3 telescopes of 40 cm diameter. It assumes classical Michelson recombination. The most recent atmospheric and environmental conditions measured at Dome C are considered (see K. Agabi "First whole atmosphere night-time seeing measurements at Dome C, Antarctica"[1]). We also study the possible science reachable with such a demonstrator. Especially we evaluate that even such small aperture interferometer could allow the detection and low resolution spectroscopy of the most favorable pegaside planets.
The VLTI has been operating for about 5 years using the VINCI instrument first, and later MIDI. In October 2005
(Period 76) the first Science Operations with the AMBER instrument started, with 14 Open Time proposals in
the observing queues submitted by the astronomical community. AMBER, the near-infrared/red focal instrument
of the VLTI, operates in the bands J, H, and, K (i.e. 1.0 to 2.5 micrometers) with three beams, thus enabling the
use of closure phase techniques. Light was fed from the 8m Unit Telescopes (UT). The Instrument was offered
with the Low Resolution Mode (JHK) and the Medium Resolution Mode in K-band on the UTs. We will present
how the AMBER VLTI Science Operations currently are performed and integrated into the general Paranal
Science Operations, using the extensive experience of Service Mode operations performed by the Paranal Science
operations and in particular applying the know-how learned from the two years of MIDI Science Operations. We
will also be presenting the operational statistics from these first ever Open Time observations with AMBER.
The ESO Very Large Telescope Interferometer (VLTI) is the first general-user interferometer that offers near- and mid-infrared long-baseline interferometric observations in service and visitor mode to the whole astronomical community. Over the last two years, the VLTI has moved into its regular science operation mode with the two science instruments, MIDI and AMBER, both on all four 8m Unit Telescopes and the first three 1.8m Auxiliary Telescopes. We are currently devoting up to half of the available time for science, the rest is used for characterization and improvement of the existing system, plus additional installations. Since the first fringes with the VLTI on a star were obtained on March 17, 2001, there have been five years of scientific observations, with the different instruments, different telescopes and baselines. These observations have led so far to more than 40 refereed publications. We describe the current status of the VLTI and give an outlook for its near future.
The VLTI now has performed three years of science operations using the
VINCI instrument since the first fringes on a star were obtained on March 17, 2001. Since December 5th, 2001, shared risk science observations have been performed with VINCI. In April 2004 (period 73) we have started science operations with the MIDI instrument. Subsequently both the AMBER instrument and the Auxiliary Telescopes (ATs) will be also running under the science Operations at Paranal and offered to the astronomical community.
We will present how the VLTI Science operations currently are performed and integrated into the general Paranal Science Operations scheme, using the extensive experience of Service Mode operations performed by the Paranal Science operations group. We focus on the execution of the Service mode operations, how they are planned, performed, evaluated, and processed and the data finally sent to ESO Garching. The near future developments are also presented and how the new instruments and telescopes will be integrated into the Paranal Science Operations.
The Very Large Telescope Interferometer (VLTI) on Cerro Paranal (2635 m) in Northern Chile reached a major milestone in September 2003 when the mid infrared instrument MIDI was offered for scientific observations to the community. This was only nine months after MIDI had recorded first fringes. In the meantime, the near infrared instrument AMBER saw first fringes in March 2004, and it is planned to offer AMBER in September 2004.
The large number of subsystems that have been installed in the last two years - amongst them adaptive optics for the 8-m Unit Telescopes (UT), the first 1.8-m Auxiliary Telescope (AT), the fringe tracker FINITO and three more Delay Lines for a total of six, only to name the major ones - will be described in this article. We will also discuss the next steps of the VLTI mainly concerned with the dual feed system PRIMA and we will give an outlook to possible future extensions.
The Very Large Telescope Interferometer (VLTI) makes in its final configuration use of four 1.8m Auxiliary Telescopes, which can be located on 30 different stations. These four telescopes can theoretically be arranged in more than 25,000 different configurations. Of course, operational constraints will allow only some dozens of these configurations to be realized over the entire lifetime of the interferometer. Furthermore, there are restrictions on sky accessibility posed by both physical limits of the delay lines and vignetting by the 8.2m telescope enclosures. We describe criteria for an optimum selection of configurations and propose a subset of AT stations to be offered for science operations with the VLTI.
MIDI (MID-infrared Interferometric instrument) gave its first N-band (8 to 13 micron) stellar interference fringes on the VLTI (Very Large Telescope Interferometer) at Cerro Paranal Observatory (Chile) in December 2002. An lot of work had to be done to transform it, from a successful physics experiment, into a premium science instrument which is offered to the worldwide community of astronomers since September 2003. The process of "paranalization", carried out by the European Southern Observatory (ESO) in collaboration with the MIDI consortium, has aimed to make MIDI simpler to use, more reliable, and more efficient. We describe in this paper these different aspects of paranalization (detailing the improvement brought to the observation software) and the lessons we have learnt. Some general rules, for bringing an interferometric instrument into routine operation in an observatory, can be drawn from the experience with MIDI. We also report our experience of the first "service mode" run of an interferometer (VLTI + MIDI) that took place in April 2004.
The ARAL system of the VLTI is a multipurpose facility that helps to
have the interferometric instruments ready for night observations. It
consists of an artificial source (allowing a Mach-Zehnder mode of the
interferometric instruments for autotest), an alignment unit (verifying the position of the celestial target in the VLTI field-of-view), and an optical path router (controlling the optical switchyard and the instrument feeding-optics in the VLTI laboratory). With the multiplication of VLTI instruments and their specific features (wavelength coverage, number of beams), an upgrade of ARAL (from its November 2002 version) had to be carried out: the alignment unit has been redesigned, as well as the artificial source. This source will provide a point in the visible and in J, H, K and N infrared bands, split into four beams (with a zero optical path difference at the reference position). After a description of the optomechanics and of the computer architecture of ARAL, we detail the difficulties of building an interferometric artificial source with a wide spectral range.
The near-infrared instrument AMBER at the VLTI allows, among other interferometric observables, the simultaneous measurement of the phase between various spectral channels. Color-differential phase thus yields spatial and spectral information on unresolved sources, and could lead to such ambitious goals as the spectroscopy of nearby hot, giant exoplanets. This will require, though, an extreme stability on the measurement, which is likely to be affected by chromatic effects at the various stages of the light path. We present how AMBER has been designed to minimize and to calibrate such effects. We give estimates of their contributions from different origins, and present recent measurements of the instrumental stability. We discuss the possibility to supress the residual chromatic effects in post-data treatment in order to reach a precision limited by the photon noise on the differential phase.
AMBER, Astronomical Multi BEam combineR, is the near-infrared focal instrument dedicated to the VLTI. It is designed to combine three of the VLTI Telescopes and to work simultaneously in the J, H and K spectral bands (1.0 to 2.4 μm).
The project successfully passed the Preliminary Acceptance in Europe in November 2003, resulting in the validation of the instrument laboratory performance1, of the compliance with the initial scientific specifications, and of the acceptance of ESO for AMBER to be part of the VLTI. After the transportation of the instrument to Paranal, Chile in January 2004, the Assembly Integration and Verification phase occurred mid-March to succeed with the first fringes observing bright stars with the VLTI siderostats.
This paper describes the different steps of the AIV and the first results in terms of instrumental stability, estimated visibility and differential phase.
AMBER is the General User near infrared focal instrument of the Very Large Telescope Interferometer. Its a single mode, dispersed fringes, three telescopes instrument. A limiting magnitude of the order of H=13 will allow to tackle a fair sample of extra galactic targets. A very high accuracy, in particular in color differential phase and closure phase modes gives good hope for very high dynamic range observation, possibly including hot extra solar planets. The relatively high maximum spectral resolution, up to 10000, will allow some stellar activity observations. Between this extreme goals, AMBER should have a wide range of applications including Young Stellar Objects, Evolved Stars, circumstellar material and many others. This paper tries to introduce AMBER to its future users with
information on what it measures, how it is calibrated and hopes
to give the readers ideas for applications.
AMBER, Astronomical Multi BEam combineR, is the near-infrared focal instrument dedicated to the VLTI. It is designed to combine three of the VLTI Telescopes and to work simultaneously in the J, H and K spectral bands (1.1 to 2.4 μm).
The instrumental concept and its opto-mechanic specifications were defined in order to reach the ambitious scientific requirements
to satisfy the core astrophysical programs.
The project passed the Final Design Review in May 2001, phase which marks the acceptation of the instrument final design and
the beginning of the construction and tests. After this phase, optics and mechanical systems have been receptioned since February 2002, for the laboratory tests and alignments.
The cooled spectrograph and its cryostat is assembled at the Osservatorio di Arcetri in Firenze, Italy and the cooled detector at the Max-Planck-Institut fur Radioastronomie in Bonn, Deutschland. The warm optics, including spatial filter in K and artificial sources injection system, have been pre-aligned at the
Observatoire de la Cote d'Azur in Nice, France to validate most of the alignment procedure, the required element accuracies and the
associated degrees of freedom. The whole instrument is then currently fully assembled and optimized at the Laboratoire
d'Astrophysique de l'Observatoire de Grenoble, France. Its sensitivity and final performance will be evaluated in order to reach the Preliminary Acceptance in Europe, scheduled beginning 2003.
This paper gives the results of the warm optics laboratory commissioning.
The differential phase yields high dynamics and/or super resolution information. This mode will be available with AMBER at VLTI in autumn 2003 and has a wide range of scientific applications. The goal of our study is to keep the instrumental effects under the level of fundamental noises. This might be achieved, with 2 telescopes, thanks to a proper stabilization of the chromatic instrumental effects and/or a fast calibration, which are both presented together with the expected residual chromatic effects. With (at least) 3 telescopes, closure phase with AMBER together with spatial modulation should allow to make low-resolution spectroscopy on many nearby EGPs, but would be restrictive for non-resolved objects.
We evaluate the potential of differential interferometry using the AMBER and MIDI instruments of the VLT interferometer for the direct detection and the study of atmospheric characteristics of hot giant extrasolar planets around nearby stars. Differential interferometry has been shown to allow unbiased measurements of phase and/or visibility variations with wavelengths. For a star and planet system (as for any binary), these measurements at several orbital phases yield the angular separation and the spectrum of the components. We present an evaluation of the fundamental measurement uncertainty resulting from source and sky photon noise and detector noise. This shows that, with two 8 meters telescopes and at least an 80 meters baseline, some hot Jupiter like planets could be detected, by differential interferometry, if the semi-major axis of their orbit is less than 0.1 astronomical units and assuming a solar type star at 10 pc.
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