The visitor instrument PIONIER provides VLTI with improved imaging capabilities and sensitivity. The in-
strument started routinely delivering scientic data in November 2010, that is less than 12 months after being
approved by the ESO Science and Technical Committee. We recall the challenges that had to be tackled to design, built and commission PIONIER. We summarize the typical performances and some astrophysical results obtained so far. We conclude this paper by summarizing lessons learned.
In this paper we compare the performance of multi and single-mode
interferometry for the estimation of the phase of the complex
visibility in presence of detector, photon and atmospheric noises. We show that, despite the loss of flux
occurring when injecting the light in the single-mode component, the spatial
filtering properties of such single-mode devices often enable
higher performance than multimode concepts. In
the high flux regime speckle noise dominated,
single-mode interferometry is always more efficient, and its performance is
significantly better when the correction provided
by adaptive optics becomes poor, by a factor of 2 and more when the Strehl
ratio is lower than 10%. In detector noise regime, multimode interferometry reaches better performance, yet the gain never exceeds 20%, which corresponds to the percentage of photon loss due to the injection in the guides.
We finally show that
single-mode interferometry is also more robust to the turbulence
in both cases of fringe tracking and phase referencing, at the exception of narrow field of views. We conclude that fringe trackers built using single-mode optics should be considered as a solution both practical and competitive.
In a few years, the second generation instruments of the Very Large Telescope Interferometer (VLTI) will routinely
provide observations with 4 to 6 telescopes simultaneously. To reach their ultimate performance, they will need
a fringe sensor capable to measure in real time the randomly varying optical paths differences. A collaboration
between LAOG (PI institute), IAGL, OCA and GIPSA-Lab has proposed the Planar Optics Phase Sensor concept
to ESO for the 2<sup>nd</sup> Generation Fringe Tracker. This concept is based on the integrated optics technologies,
enabling the conception of extremely compact interferometric instruments naturally providing single-mode spatial
filtering. It allows operations with 4 and 6 telescopes by measuring the fringes position thanks to a spectrally
dispersed ABCD method. We present here the main analysis which led to the current concept as well as the
expected on-sky performance and the proposed design.
PIONIER is a 4-telescope visitor instrument for the VLTI, planned to
see its first fringes in 2010. It combines four ATs or four UTs
using a pairwise ABCD integrated optics combiner that can also be
used in scanning mode. It provides low spectral resolution in H and K band. PIONIER is designed for
imaging with a specific emphasis on fast fringe recording to allow
closure-phases and visibilities to be precisely measured. In
this work we provide the detailed description of the instrument and
present its updated status.
The AMBER instrument, the three beams interferometric combiner of the VLTI, occasionally su.ers from a
fringing artifact, called "correlated noise", likely induced by electromagnetic radio frequencies present in the
lab. We analyze how this noise affects the extracted visibilities, becoming more important for fainter sources.
This unwanted effect can cause an overestimate of the instrumental <i>V</i><sup>2</sup> for low flux observations. We have
developed a software tool, called "AMBER Detector Cleaner" (AMDC), which successfully removes this artifact
and we present here the method on which it is based and some example results. Such software is made available
to the community, so that AMBER users can perform optimal data reduction even for faint sources.
The VLTI Spectro Imager (VSI) was proposed as a second-generation instrument of the Very Large Telescope Interferometer
providing the ESO community with spectrally-resolved, near-infrared images at angular resolutions
down to 1.1 milliarcsecond and spectral resolutions up to R = 12000. Targets as faint as K = 13 will be imaged
without requiring a brighter nearby reference object; fainter targets can be accessed if a suitable reference is
available. The unique combination of high-dynamic-range imaging at high angular resolution and high spectral
resolution enables a scientific program which serves a broad user community and at the same time provides the
opportunity for breakthroughs in many areas of astrophysics. The high level specifications of the instrument are
derived from a detailed science case based on the capability to obtain, for the first time, milliarcsecond-resolution
images of a wide range of targets including: probing the initial conditions for planet formation in the AU-scale
environments of young stars; imaging convective cells and other phenomena on the surfaces of stars; mapping
the chemical and physical environments of evolved stars, stellar remnants, and stellar winds; and disentangling the central regions of active galactic nuclei and supermassive black holes. VSI will provide these new capabilities
using technologies which have been extensively tested in the past and VSI requires little in terms of new
infrastructure on the VLTI. At the same time, VSI will be able to make maximum use of new infrastructure as it
becomes available; for example, by combining 4, 6 and eventually 8 telescopes, enabling rapid imaging through
the measurement of up to 28 visibilities in every wavelength channel within a few minutes. The current studies
are focused on a 4-telescope version with an upgrade to a 6-telescope one. The instrument contains its own
fringe tracker and tip-tilt control in order to reduce the constraints on the VLTI infrastructure and maximize
the scientific return.
The AMBER instrument installed at the Very Large Telescope (VLT) combines three beams from as many
telescopes to produce spectrally dispersed fringes from milli-arcsecond angular scale in the near infrared. Two
years after installation, first scientific observations have been carried out during the Science Demonstration Time
and the Guaranteed Time mostly on bright sources due to some VLTI limitations. In this paper, we review these
first astrophysical results and we show which types of completely new information is brought by AMBER.
The first astrophysical results have been mainly focusing on stellar wind structure, kinematics, and its interaction
with dust usually concentrated in a disk. Because AMBER has dramatically increased the number of
measures per baseline, this instrument brings strong constraints on morphology and models despite a relatively
poor (<i>u,v</i>) coverage for each object.
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.
Second generation VLTI instruments will be able to use of the array full imaging capability with up to 8 telescopes. Such an instrument will allow astronomers to measure 28 visibilities and 21 independent closure-phases at the same time, providing therefore rapidly imaging abilities with a spatial resolution of one milliarcsecond in the near infrared range.
The VITRUV project is a proposition to achieve the VLTI interferometric combination thanks to single-mode planar optics (the so-called integrated optics, IO). IO technologies allow to design integrated combiners with remarkable stability and self allignement properties. In addition, modal filtering associated with photometric calibration will lead to accurate visibility and closure-phase measurements. In this paper we present a detailed analysis of beam combination concepts that takes into account several constraints: throughput, signal to noise ratio, interferometric efficiency, integrated optics circuit design constrains and astrophysical requirements for imaging mode.
Since several years, long baseline infrared interferometry succeeds in
providing handful of astrophysical results from model fitting of the visibility measurements alone. Continuing on these encouraging results, and thanks to the development of elaborated recombination scheme which allow to gather stellar light coming from 3 telescopes or more, recent (IOTA/IONIC, NPOI) and new (VLTI/AMBER) interferometers have also access to closure phase measurements as well as to a better (u,v) coverage. When the (u,v) coverage is still insufficient to perform image reconstruction. a least square fit approach is required, taking benefit of the closure phases together with visibility informations. Within this framework, and in the light of the AMBER experiment, we simulate realistic observations of star-planet systems. Computing the statistics of the observables, and then characterizing the performances of this instrument, we investigate the potential of AMBER to detect Jovian planets around sun-like stars
by computing Signal to Noise Ratio on the constrained parameters, i.e. the flux ratio and the separation. We focus here on the specific system sun-planet, knowing that the general case will be treated in a forthcoming paper. We particularly study how important is the contribution of the closure phase in the model-fitting process, relatively to the visibility.
AMBER is the focal near-infrared instrument of the VLTI combining
2 or 3 telescopes in the J, H and K bands with 3 spectral resolution modes. It uses single-mode fibers to ensure modal filtering and high measurement accuracies. AMBER has been integrated and tested in Grenoble during 2003. We report in this paper the lab performances of the instrument in terms of instrumental contrast, measurement accuracy and stability, and throughput.
Young Stellar Objects (YSOs) play a central role in the understanding
of stellar and planet formation, and progress in spatial resolution and sensitivity of long infrared interferometers made such instruments particularly well suited to probe the inner part of the disk where essential physical processes and interactions are believed to take place. The first astrophysical results obtained on
young stars arising from this technique are already giving a handful of informations about the structure of the extended component. However, model-fitting methods used to reduce the data prevent from definitive and unambiguous interpretations. Interferometric observations of Herbig Ae/Be stars is one of the most striking example. Whereas first results seemed to be in good agreement with accretion disk model, latest observations tend to favor the presence of a uniform ring with a inner radius set by dust sublimation temperature. Direct imaging of close environments around YSOs with infrared (IR) interferometers will allow to disentangle between current suggested models and to improve one step further the scenarios of stellar formation. Within this framework, we anticipate observations of YSOs with the VLTI and we investigate the potential of AMBER to recover images. Modelling their circumstellar environment, we simulate realistic observations of Herbig Ae/Be and TTauri stars. By using reconstruction technique specially dedicated
to infrared interferometry and to sparse ($u,v$) data coverage,
we analyze the quality of the recovered images, and we emphasize the critical points to take into account in the image reconstruction process. We conclude that it requires at least three nights of observations to perform imaging of YSOs with AMBER on the VLTI.
The science objectives of VITRUV is to investigate the morphology of compact astrophysical objects in optical wavelengths like the environment of AGN, star forming regions, stellar surfaces. This instrument will take full advantage of the VLTI site with 4 very large telescopes and 4 auxiliary telescopes. The instrument concept is to built aperture synthesis images like the millimeter-wave radiointerferometer of the IRAM Plateau de Bure. VITRUV coupled to the VLTI will have similar and even better resolution than ALMA. The astrophysical specifications although not yet finalized will be a temporal resolution of the order of 1 day, spectral resolution from 100 to 30,000, image dynamic from 100 to 1,000, a field of view of 1 arcsec for an initial wavlength coverage from 1 to 2.5 microns that could be extended from 0.5 to 5 microns. The technology that is contemplated at this stage is integrated optics.
We present here the general formalism and data processing steps used in the data reduction pipeline of the AMBER instrument. AMBER is a three-telescope interferometric beam combiner in J, H and K bands installed at ESO's Very Large Telescope Interferometer. The fringes obtained on the 3 pairs of telescopes are spatially coded and spectrally dispersed. These are monitored on a 512x512 infrared camera at frame rates up to 100 frames per second, and this paper presents the algorithm used to retrieve the complex coherent visibility of the science target and the subsequent squared visibility, differential phase and phase closure on the 3 bases and in the 3 spectral bands available in AMBER.
We report here the first visibility and closure-phase measurements done with the IONIC instrument at the IOTA interferometer. The IONIC
instrument is presented and preliminary analysis of the results
discussed. Future improvements of IONIC are envisioned.
Up until now and with the first generation of the next optical interferometers, only a few telescopes, such as 3 or 4, are enabled for gathering the starlight. In such a case, short night-time observational runs will only provide small number of measured baselines. The lack of spatial frequencies mapped in the uv plane thus prevents from a reconstruction of the studied object. It therefore implies to define an a-priori model and to constraint its parameters from the available measurements. We propose here to fit the model parameters from a least square minimization-type approach. We develop a general formalism that can be applied to the three common interferometric observables. These observables are the square visibility, the closure phase and the differential phase. We also pay attention on the formal analysis of the error on the estimated observables and on the resulting error on the model parameters. This approach allows us to run realistic pre-observational simulations, to estimate the performances of the telescope/recombiner instrument, and to sense the feasibility of the intended observation. We finally apply this theoretical study to the recombiner AMBER on the VLTI when pointing at a single star.