SPICA (Stellar Parameters and Images with a Cophased Array) is a 6-telescope (6T) visible instrument for the CHARA Array (Center for High Angular Resolution in Astronomy) at Mount Wilson Observatory. It uses single mode fibers for feeding the interferometric spectrograph, which offers three different spectral resolutions: R=140, R=4000, and R=14000. CHARA/SPICA has been mainly designed for large programs (surveys) in the domain of stellar fundamental parameters but also permits fast imaging thanks to the 15 baselines and the large number of spectral channels (60 in low resolution mode). SPICA is made of the visible instrument SPICA-VIS and of a new H-band, 6T, ABCD combiner performing group delay and phase delay tracking. In this paper, we present the first light results of SPICA.
After the pause imposed by the pandemic, VLTI resumed science operations and restarted technical activities aiming to close commissionings of different modes. While the community develops projects of visiting instruments, the VLTI infrastructure is about to be significantly upgraded with new visible AO and laser guide star systems by the GRAVITY+ project. VLTI operations also evolve, in particular to support imaging programmes, but also towards a more automated and integrated model. In this context, we will present a review of current capabilities, ongoing activities and future plans for the VLTI.
ESO’s VLT interferometer (VLTI) is a general-user optical/infrared interferometric facility. Its operations scheme is fully integrated into the well-established scheme of all VLT instruments and profits enormously from this experience and the implemented infrastructure to offer a unique service to the community. Based on the greatly improved capabilities of the 2nd generation VLTI instruments and taking advantage of a further development of ESO’s Observation Handling Tools, we have evolved the VLTI operations scheme as well. We have offered to VLTI investigators the possibility to indicate baseline configurations in a more flexible way and have introduced nested scheduling containers to better formalize the observational strategy. We have prepared for dedicated support of different types of interferometric observations. For imaging observations specifically, we have introduced an improved workflow to fill the uv plane and to handle time-critical imaging.
With a possible angular resolution down to 0.1-0.2 millisecond of arc using the 330 m baselines and the access to the 600-900 nm spectral domain, the CHARA Array is ideally configured for focusing on precise and accurate fundamental parameters of stars. CHARA/SPICA (Stellar Parameters and Images with a Cophased Array) aims at performing a large survey of stars all over the Hertzsprung-Russell diagram. This survey will also study the effects of the different kinds of variability and surface structure on the reliability of the extracted fundamental parameters. New surface-brightness-colour relations will be extracted from this survey, for general purposes on distance determination and the characterization of faint stars. SPICA is made of a visible 6T fibered instrument and of a near-infrared fringe sensor. In this paper, we detail the science program and the main characteristics of SPICA-VIS. We present finally the initial performance obtained during the commissioning.
ESO’s La Silla Paranal Observatory uses a set of integrated tools for preparation and execution of Service and Visitor Mode (SM and VM, respectively) observations. The web interface for the observation preparation (p2) provides a versatile and robust environment for users to efficiently design their observations. The software architecture of p2 enabled implementation of new services, modeled according to the instruments’ specifications and operational standards. The automatic creation of Finding Charts is integrated within p2 and the Observation Preparation (ObsPrep) tool enables interactive observing strategy configuration including for example fine-tuning of the science field pointings, selection of blind offset and guide stars as well as selection of auxiliary stars for instruments using Adaptive Optics. Through the Visitor Execution Sequence, observers can plan and monitor in real-time their (on-site or remote) observations. For Service Mode runs the use of scheduling containers, recently extended to include nesting of containers, enables design of complex observing strategies that are machine readable, which allows programmatic preparation of short term scheduling for execution and planning of the night at the observatory.
Following the arrival of MATISSE, the second-generation of VLTI instrumentation is now complete and was simultaneously enhanced by a major facility upgrade including the NAOMI Adaptive Optics on the Auxiliary Telescopes. On the Unit Telescopes, significant efforts were also made to improve the injection stability into VLTI instruments. On top of GRAVITY's own evolution, its fringe tracker is now being used to allow coherent integrations on MATISSE (the so-called GRA4MAT project). Meanwhile, operations also evolved to be more flexible and make the most of an extended observing parameter space. In this context, we present an overview of the current VLTI performances. Finally, we will report on on-going improvements such as the extension of the longest baselines.
The Planet Formation Imager (PFI) is a near- and mid-infrared interferometer project with the driving science goal of imaging directly the key stages of planet formation, including the young proto-planets themselves. Here, we will present an update on the work of the Science Working Group (SWG), including new simulations of dust structures during the assembly phase of planet formation and quantitative detection efficiencies for accreting and non-accreting young exoplanets as a function of mass and age. We use these results to motivate two reference PFI designs consisting of a) twelve 3m telescopes with a maximum baseline of 1.2km focused on young exoplanet imaging and b) twelve 8m telescopes optimized for a wider range of young exoplanets and protoplanetary disk imaging out to the 150K H2O ice line. Armed with 4 x 8m telescopes, the ESO/VLTI can already detect young exoplanets in principle and projects such as MATISSE, Hi-5 and Heimdallr are important PFI pathfinders to make this possible. We also discuss the state of technology development needed to make PFI more affordable, including progress towards new designs for inexpensive, small field-of-view, large aperture telescopes and prospects for Cubesat-based space interferometry.
The Planet Formation Imager (PFI) project aims to provide a strong scientific vision for ground-based optical astronomy beyond the upcoming generation of Extremely Large Telescopes. We make the case that a breakthrough in angular resolution imaging capabilities is required in order to unravel the processes involved in planet formation. PFI will be optimised to provide a complete census of the protoplanet population at all stellocentric radii and over the age range from 0.1 to ~100 Myr. Within this age period, planetary systems undergo dramatic changes and the final architecture of planetary systems is determined. Our goal is to study the planetary birth on the natural spatial scale where the material is assembled, which is the "Hill Sphere" of the forming planet, and to characterise the protoplanetary cores by measuring their masses and physical properties. Our science working group has investigated the observational characteristics of these young protoplanets as well as the migration mechanisms that might alter the system architecture. We simulated the imprints that the planets leave in the disk and study how PFI could revolutionise areas ranging from exoplanet to extragalactic science. In this contribution we outline the key science drivers of PFI and discuss the requirements that will guide the technology choices, the site selection, and potential science/technology tradeoffs.
Here we present the results of the 6th biennial optical interferometry imaging beauty contest. Taking advantage of a unique opportunity, the red supergiant VY CMa and the Mira variable R Car were observed in the astronomical H-band with three 4-telescope configurations of the VLTI-AT array using the PIONIER instrument. The community was invited to participate in the subsequent image reconstruction and interpretation phases of the project. Ten groups submitted entries to the beauty contest, and we found reasonable consistency between images obtained from independent workers using quite different algorithms. We also found that significant differences existed between the submitted images, much greater than in past beauty contests that were all based on simulated data. A novel crowd-sourcing" method allowed consensus median images to be constructed, filtering likely artifacts and retaining real features." We definitively detect strong spots on the surfaces of both stars as well as distinct circumstellar shells of emission (likely water/CO) around R Car. In a close contest, Joel Sanchez (IAA-CSIC/Spain) was named the winner of the 2014 interferometric imaging beauty contest. This process has shown that new comers" can use publicly-available imaging software to interpret VLTI/PIONIER imaging data, as long as sufficient observations are taken to have complete uv coverage { a luxury that is often missing. We urge proposers to request adequate observing nights to collect sufficient data for imaging and for time allocation committees to recognise the importance of uv coverage for reliable interpretation of interferometric data. We believe that the result of the proposed broad international project will contribute to inspiring trust in the image reconstruction processes in optical interferometry.
We present the latest update of the European Southern Observatory's Very Large Telescope interferometer (VLTI). The operations of VLTI have greatly improved in the past years: reduction of the execution time; better offering of telescopes configurations; improvements on AMBER limiting magnitudes; study of polarization effects and control for single mode fibres; fringe tracking real time data, etc. We present some of these improvements and also quantify the operational improvements using a performance metric. We take the opportunity of the first decade of operations to reflect on the VLTI community which is analyzed quantitatively and qualitatively. Finally, we present briefly the preparatory work for the arrival of the second generation instruments GRAVITY and MATISSE.
Claudia Paladini, Daniela Klotz, Stephane Sacuto, Josef Hron, Markus Wittkowski, Eric Lagadec, Tijl Verhoelst, Alain Jorissen, Andrea Richichi, Martin Groenewegen, Hans Olofsson, Franz Kerschbaum
The mass-loss process is a key ingredient for our understanding in many fields of astrophysics, including stellar evolution and the enrichment of the interstellar medium (ISM) via stellar yields. We combined the capability of the VLTI/MIDI and VLT/VISIR instruments with very recent Herschel/PACS observations to characterize the geometry of mass loss from evolved red giants on the Asymptotic Giant Branch (AGB) at various scales. This paper describes the sample of objects, the observing strategy, the tool for the interpretation, and preliminary MIDI results for two targets: U Ant and θ Aps.
KEYWORDS: Telescopes, Interferometers, Astatine, Interferometry, Large telescopes, Observatories, Systems engineering, Control systems, Mirrors, Sensors
The ESO Very Large Telescope Interferometer (VLTI) offers access to the four 8-m Unit Telescopes (UT) and
the four 1.8-m Auxiliary Telescopes (AT) of the Paranal Observatory located in the Atacama Desert in
northern Chile. The two VLTI instruments, MIDI and AMBER deliver regular scientific results. In parallel to the
operation, the instruments developments are pursued, and new modes are studied and commissioned to offer
a wider range of scientific possibilities to the community and increase sensitivity. New configurations of the
ATs have been offered and are frequently discussed with the science users of the VLTI and implemented to
optimize the scientific return. The PRIMA instrument, bringing astrometry capability to the VLTI and phase
referencing to the instruments is being commissioned. The visitor instrument PIONIER is now fully operational
and bringing imaging capability to the VLTI.
The current status of the VLTI is described with successes and scientific results, and prospects on future
evolution are presented.
The ESO Very Large Telescope Interferometer (VLTI) offers access to the four 8-m Unit Telescopes (UT) and the four
1.8-m Auxiliary Telescopes (AT) of the Paranal Observatory located in the Atacama Desert in northern Chile. The two
VLTI instruments, MIDI and AMBER deliver regular scientific results. In parallel to the operation, the instruments
developments are pursued, and new modes are studied and commissioned to offer a wider range of scientific possibilities
to the community. New configurations of the ATs array are discussed with the science users of the VLTI and
implemented to optimize the scientific return. The monitoring and improvement of the different systems of the VLTI is a
continuous work. The PRIMA instrument, bringing astrometry capability to the VLTI and phase referencing to the
instruments has been successfully installed and the commissioning is ongoing. The possibility for visiting instruments
has been opened to the VLTI facility.
Two interferometric instruments at ESO's Very Large Telescope Interferometer (VLTI) - MIDI and AMBER
operating in the mid-infrared (8-13 μm) and the near-infrared (JHK), respectively - have proven to be
very powerful to study the physical properties of the circumstellar material around evolved stars. With the
"spectro-interferometric" capability of MIDI and AMBER, we can disentangle spectral and spatial information
on the observed object. VLTI observations have confirmed our pictures on the circumstellar environment
of cool evolved stars in some cases but brought about entirely unexpected pictures in other cases. Here, we
present our recent results obtained with VLTI/MIDI.
We present interferometric near-infrared observations of the Luminous Blue Variable (LBV) η Car using the
Very Large Telescope Interferometer (VLTI) and the AMBER instrument of the European Southern Observatory
(ESO). A high spatial resolution of 5 mas (~11.5 AU) and a high spectral resolution R = λ/Δλ=1500 and
12000 were obtained. Some of the data was recorded using the fringe tracker FINITO. The observations were
obtained in the wavelength range around both the He I 2.059 μm and the Brγ 2.166 μm emission lines. The
AMBER interferograms allow the investigation of the wavelength dependence of η Car's visibility, wavelength-differential phase, and closure phase. If we fit Hillier et al. model visibilities to the observations, we obtain
50% encircled-energy diameters of 4.2, 6.5 and 9.6 mas in the 2.17 μm continuum, the He I, and the Brγ emission
lines, respectively. In the continuum, an elongation along a position angle of 120° ± 15° was derived from the
visibilities. The VLTI observations support theoretical models of anisotropic winds from fast-rotating, luminous
hot stars with enhanced high-velocity mass loss near the pole.
The ESO Very Large Telescope Interferometer (VLTI) offers access to the four 8 m Unit Telescopes (UT) and the four
1.8 m Auxiliary Telescopes (AT) of the Paranal Observatory located in the Atacama Desert in northern Chile. The fourth
AT has been delivered to operation in December 2006, increasing the flexibility and simultaneous baselines access of the
VLTI. Regular science operations are now carried on with the two VLTI instruments, AMBER and MIDI. The FINITO
fringe tracker is now used for both visitor and service observations with ATs and will be offered on UTs in October
2008, bringing thus the fringe tracking facility to VLTI instruments. In parallel to science observations, technical periods
are also dedicated to the characterization of the VLTI environment, upgrades of the existing systems, and development
of new facilities. We will describe the current status of the VLTI and prospects on future evolution.
We present the first interferometric NIR observations of the LBV η Carinae with high spectral resolution. The observations were carried out with three 8.2 m VLTI Unit Telescopes in the K-band. The raw data are spectrally dispersed interferograms obtained with spectral resolutions of 1,500 (MR-K mode) and 12,000 (HR-K mode). The observations were performed in the wavelength range around both the He I 2.059 μm and the Brγ 2.166 μm emission lines. The spectrally dispersed AMBER interferograms allow the investigation of the wavelength dependence of the visibility, differential phase, and closure phase of η Car. In the K-band continuum, a diameter of 4.0±0.2 mas (Gaussian FWHM) was measured for η Car's optically thick wind region, whereas the Brγ and He I emission line regions are larger. If we fit Hillier et al. model visibilities to the observed AMBER visibilities, we obtain 50% encircled-energy diameters of 4.3, 6.5 and 9.6 mas in the 2.17 μm continuum, the He I, and the Brγemission lines, respectively. In the continuum near the Brγ line, an elongation along a position angle of 128° ± 15° was found, consistent with previous VLTI/VINCI measurements. We find good agreement between the measured visibilities and the predictions of the radiative transfer model of Hillier et al. For the interpretation of the non-zero differential and closure phases measured within the Brγ line, we present a simple geometric model of an inclined, latitude-dependent wind zone. Our observations support theoretical models of anisotropic winds from fast-rotating, luminous hot stars with enhanced high-velocity mass loss near the polar regions.
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 VLT Interferometer (VLTI) is a general-user facility and is operated in service mode (SM) for a large part of the available time. An important aspect of this SM observing mode is the definition of a set of critical observing conditions that must be met at the time of executing the requested observation. There are a number of observing constraints that are specific to interferometric observations, such as the choice of the array configuration and the hour angle at time of observation, which is processed during the scheduling. On the other hand, classical constraints such as the regular seeing or the lunar illumination are less critical for observations using VLTI instruments than for those using classical VLT instruments. In particular, the use of the adaptive optics system MACAO for VLTI observations employing the Unit Telescopes (UTs) ensures a very good image quality even for moderate environmental conditions. However, the exact dependence between environmental conditions, the performance of the MACAO systems, the wavefront quality at the interferometric instruments, and the accuracy of the final visibility, are not yet known in much detail. In order to investigate this dependence we have started to monitor routinely the environmental conditions, the quality of the MACAO systems, the quality of the acquisition images, and the final data product for all VLTI observations since June 2005. Here, we present the details of this study, as well as first statistics and results.
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.
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 ESO Very Large Telescope Interferometer (VLTI) is the first general-user interferometer that offers near- and mid-infrared long-baseline interferometric observations in service mode as well as visitor mode to the whole astronomical community. Regular VLTI observations with the first scientific instrument, the mid-infrared instrument MIDI, have started in ESO observing period P73, for observations between April and September 2004. The efficient use of the VLTI as a general-user facility implies the need for a well-defined operations scheme. The VLTI follows the established general operations scheme of the other VLT instruments. Here, we present from a users' point of view the VLTI specific aspects of this scheme beginning from the preparation of the proposal until the delivery of the data.
The ESO Data Flow Operations group (also called Quality Control group) is dedicated to look into the performance of the different VLT instruments, to verify the quality of the calibration and scientific data, to control and monitor them on different time scales. At ESO headquarters in Garching, Germany, one QC scientist is dedicated to these tasks for the VLTI instruments: VINCI, MIDI, AMBER, and (eventually) PRIMA.
In this paper, we focus on MIDI. In this presentation, we define the tasks of the Quality Control scientist and describe the lessons learned on quality control and instrument trending with the commissioning instrument VINCI. We then illustrate the different aspects of the MIDI Data Flow Operations supported by the QC scientist such as data management issues (data volume, distribution to the community), processing of the data, and data quality control.
Dusty tori have been suggested to play a crucial role in determining the physical characteristics of active galactic nuclei (AGN), but investigation of their properties has stalled for lack of high resolution mid-IR imaging. Recently, a long-awaited breakthrough in this field was achieved: NGC 1068, a nearby AGN, was the first extragalactic object to be observed with a mid-IR interferometer, thereby obtaining the needed angular resolution to study the alleged torus. The instrument used was MIDI mounted on the ESO's VLT interferometer. The resulting 8-13 micron interferometric spectra indicated the presence of a thick (3 x 4 parsec) configuration of warm dust surrounding a hot ~1 pc component, marginally elongated in the direction perpendicular to the main orientation of the warm component. The structure of the 10 micron "silicate" absorption feature hinted at the presence of non-typical dust.
In this proceeding, first the field of AGN research is briefly reviewed, with an emphasis on models of dusty tori. Second, the general properties of the key object NGC 1068 are discussed. Third, the MIDI data set is presented together with a first attempt to interpret this data in the context of tori models. Fourth, preliminary MIDI interferometric spectra of the nucleus of the nearby starbursting galaxy Circinus are presented. The apparent observed absence of both a hot component as well as a sharp absorption feature suggest that we view the torus more edge-on than is the case for NGC 1068. Finally, we briefly discuss the prospects of ESA's Darwin mission for observing nearby and distant AGN. The required capabilities for Darwin's first goal -- the search for and subsequent characterization of earth-like planets orbiting nearby stars -- are such that for its second goal -- high resolution astrophysical imaging -- the sensitivity will be similar to JWST and the angular resolution 1-2 orders better. This will allow detailed mapping of tori of low luminosity AGN such as NGC 1068 up to redshifts of 1 - 2 and more luminous AGN up to redshift of 10 and beyond.
We present K-band commissioning observations of the Mira star prototype o Cet obtained at the ESO Very Large Telescope Interferometer (VLTI) with the VINCI instrument and two siderostats.
The observations were carried out between 2001 October and December, in 2002 January and December, and in 2003 January. Rosseland angular radii are derived from the measured visibilities by fitting theoretical visibility functions obtained from center-to-limb intensity variations (CLVs) of Mira star models. Using the derived Rosseland angular radii and the spectral energy distributions (SEDs) reconstructed from available photometric and spectrophotometric data, we find effective temperatures ranging from T_eff=3192 +/- 200 K at phase 0.13 to 2918 +/- 183 K at phase 0.26. Comparison of these Rosseland radii, effective temperatures, and the shape of the observed visibility functions with model predictions suggests that o Cet is a fundamental mode pulsator. Furthermore, we investigated the variation of visibility function and diameter with phase. The Rosseland angular diameter of o Cet increased from 28.9 +/- 0.3 mas
(corresponding to a Rosseland radius of 332 +/- 38 Rsun for a distance of D=107 +/- 12 pc) at phase 0.13 to 34.9 +/- 0.4 mas (402 +/- 46 Rsun) at phase 0.4. The observational error of the Rosseland linear radius almost entirely results from the error of the parallax, since the error of the angular diameter is only approximately 1%.
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.
Science interferometry instruments are now available at the Very Large Telescope for observations in service mode; the MID-Infrared interferometry instrument, MIDI, started commissioning and has been opened to observations in 2003 and the AMBER 3-beam instrument shall follow in 2004. The Data Flow System is the VLT end-to-end software system for handling astronomical observations from the initial observation proposal phase through to the acquisition, archiving, processing, and control of the astronomical data. In this paper we present the interferometry specific components of the Data Flow System and the software tools which are used for the VLTI.
The Very Large Telescope (VLT) Observatory on Cerro Paranal (2635 m) in Northern Chile is approaching completion. After the four 8-m Unit Telescopes (UT) individually saw first light in the last years, two of them were combined for the first time on October 30, 2001 to form a stellar interferometer, the VLT Interferometer. The remaining two UTs will be integrated into the interferometric array later this year. In this article, we will describe the subsystems of the VLTI and the planning for the following years.
The VLT interferometer has been operating since the time of first fringes in March 2001 with a pair of 40 cm diameter siderostats at baselines of 16 and 66 m and a pair of 8 m diameter telescopes (UT1 and UT3) with a baseline of 102 m using the test camera VINCI operating in the K band. A fair fraction of its commissioning time has been devoted to observing a number of objects of scientific interest around the southern sky bright enough to allow high precision visibilities to be obtained on a routine basis. A large number of stellar sources with correlated magnitudes brighter than K approximately 6 and K approximately 3 with the 8 m and 40 cm telescopes respectively have been observed over this time period with limited, u,v plane coverage. In this paper, the most interesting results on sources never observed before at these spatial resolutions and on known sources for which the VLTI data allow the establishment of tighter constraints on theoretical models will be reviewed.
We discuss the potential of interferometric studies of nearby galactic nuclei with long-baseline interferometric facilities.
Information on the morphology of galactic centers has so far been
limited to angular sizes corresponding to the diffraction limit
of 6-10 m class telescopes. Optical and near-infrared interferometry
could in principle be used to reach significantly higher angular resolution, but has so far only been used for bright objects due to the small collecting areas of existing interferometers. Right now, the first interferometers consisting of 8-10 m class telescopes are starting operations and, hence, will soon allow us for the first time to study galactic centers on angular scales which are of an order of magnitude smaller than ever before, i.e. on scales corresponding to baselines of up to 100 m. We discuss these facilities and report on the observational techniques and strategies which are relevant for interferometric observations of these objects.
We review imaging results of nearby galactic centers with highest angular resolution so far, with an emphasis on our bispectrum speckle interferometry studies of the core of the Seyfert galaxy NGC 1068. Employing these results, we analyze how near-infrared interferometry can discriminate between the different scenarios which are consistent with our current knowledge based on observations. In particular, characteristic sizes of the circumnuclear dusty torus can be derived with higher precision, additional dust components and the inner part of the jet can be identified, and radiative transfer models of the torus can be better constrained. Furthermore, the flux contribution of central source components can be separated from those of the torus, and thus they can be modeled in more detail. These investigations may ultimately result in a refinement of the unification scheme of galactic nuclei.
In March 2001, the commissioning instrument of the VLTI, VINCI, succeeded in obtaining its first fringes by linking two 40 cm aperture siderostats on a 16 m baseline. During the first year of operation, thousands of interferometric observations on different baselines were carried out, with the technical goal of characterizing this complex system. We report in this paper these first measurements and estimate the main parameters of the atmospheric and internal turbulence along the complete light path.
We first illustrate the degradation of the visibility accuracy caused by the differential piston and evaluate the contribution of the internal optical path fluctuations with respect to the atmospheric ones. The stability of the VLTI complex is demonstrated, which enabled us to record easily fringes with Unit Telescopes (UTs) on baselines as long as 102.5 m (November 2001). In the last part, infrared measurements of the atmospheric differential piston are reported. They were obtained with the siderostats on two different baselines ranging from 16m to 66m.
Estimations of the coherence time at Cerro Paranal are derived from these commissioning data and compared to the values predicted by the Astronomic Site Monitor (ASM). Finally, constraints on the outer scale length are discussed.
We report on direct interferometric measurements of stellar intensity
profiles obtained with the Navy Prototype Optical Interferometer (NPOI) and the Very Large Telescope Interferometer (VLTI). These measurements need to make use of weak fringes, i.e. low visibility values, on resolved stars. We describe techniques that were used to obtain these low visibility values with high precision. They include the methods of baseline bootstrapping and wavelength bootstrapping, as well as, lately, coherent integration using phase bootstrapping. In addition, we developed methods to compensate photon and detection biases. We present recent measurements on the giant star γ Sge, obtained with the NPOI, which succeeded not only in discriminating between uniform disks and limb-darkened disks, but also in constraining Kurucz model atmosphere parameters. We present first VLTI measurements of visibility values beyond the first minimum which were taken on the giant star Ψ Phe. Here, the capabilities to synthesize baselines of different lengths and
to use different aperture sizes were used for the first time with the VLTI. We close with an outlook on the future potential on studies of stellar surface structure with the six-way beam combination at the NPOI and with the completed VLTI. This includes for instance direct measurements of the limb-darkened profiles of a large number of different types of stars, and of starspots which may for instance be caused by magnetic fields or large-scale photospheric convection.
On March 17, 2001, the VLT interferometer saw for the first time interferometric fringes on sky with its two test siderostats on a 16m baseline. Seven months later, on October 29, 2001, fringes were found with two of the four 8.2m Unit Telescopes (UTs), named Antu and Melipal, spanning a baseline of 102m. First shared risk science operations with VLTI will start in October 2002. The time between these milestones is used for further integration as well as for commissioning of the interferometer with the goal to understand all its characteristics and to optimize performance and observing procedures. In this article we will describe the various commissioning tasks carried out and present some results of our work.
The VLTI Calibrators Program is a common project between ESO and NEVEC. The main goal is to establish a network of measurements of calibrator objects with an accuracy high enough to fully exploit the different VLTI instruments. We started this project in 2001 by defining a list of objects to be used during the observations with the commissioning instrument VINCI. During the first year of observation (18th March 2001 - 18th March 2002), a total of 5060 observations have been recorded on 156 astronomical objects. More than 60% of the observations have been done on 63 calibrator objects. These calibrator data are currently analyzed to refine the measurements of the adopted diameters. After a brief description of the instrument and of the data reduction process, we describe the criteria used to establish a list of calibrators suitable for the commissioning instrument VINCI with baselines of up to 200m. We define a strategy to observe and analyze the data for the commissioning of the VLTI and of several baselines. We emphasize the difficulties of instrumental calibration to an accuracy of a few 0.1% and the necessity of a long term effort.
KEYWORDS: Visibility, Signal to noise ratio, Algorithm development, Atmospheric modeling, Observatories, Data integration, Detection and tracking algorithms, Data modeling, Interferometers, Stars
We have developed an algorithm to determine precise fringe phases
in the presence of atmospheric turbulence. We use phase bootstrapping
to improve parameter estimates of weak fringes observed on long
baselines through coherent integration. With data from the Navy
Prototype Optical Interferometer (NPOI) on γ Sagittae, we demonstrate the importance of this method for the study of limb-darkening of stars.
KEYWORDS: Interferometry, Telescopes, Interferometers, Calibration, Visibility, Data archive systems, Data processing, Observatories, Signal processing, Space telescopes
In this article we present the Data Flow System (DFS) for the Very Large Telescope Interferometer (VLTI). The Data Flow System is the VLT end-to-end software system for handling astronomical observations from the initial observation proposal phase through the acquisition, processing and control of the astronomical data. The Data Flow system is now in the process of installation and adaptation for the VLT Interferometer. The DFS was first installed for VLTI first fringes utilising the siderostats together with the VINCI instrument and is constantly being upgraded in phase with the VLTI commissioning. When completed the VLT Interferometer will make it possible to coherently combine up to three beams coming from the four VLT 8.2m telescopes as well as from a set of initially three 1.8m Auxiliary Telescopes, using a Delay Line tunnel and four interferometry instruments. Observations of objects with some scientific interest are already being carried out in the framework of the VLTI commissioning using siderostats and the VLT Unit Telescopes, making it possible to test tools under realistic conditions. These tools comprise observation preparation, pipeline processing and further analysis systems. Work is in progress for the commissioning of other VLTI science instruments such as MIDI and AMBER. These are planned for the second half of 2002 and first half of 2003 respectively. The DFS will be especially useful for service observing. This is expected to be an important mode of observation for the VLTI, which is required to cope with numerous observation constraints and the need for observations spread over extended periods of time.
The Data Flow System is the VLT end-to-end system for handling astronomical observations from the initial observation proposal phase through the acquisition, processing and control of the astronomical data. The VLT Data Flow System has been in place since the opening of the first VLT Unit Telescope in 1998. When completed the VLT Interferometer will make it possible to coherently combine up to three beams coming from the four VLT 8.2m telescopes as well as from a set of initially three 1.8m Auxiliary Telescopes, using a Delay Line tunnel and four interferometry instruments. The Data Flow system is now in the process of installation and adaptation for the VLT Interferometer. Observation preparation for a multi-telescope system, handling large data volume of several tens of gigabytes per night are among the new challenges offered by this system. This introduction paper presents the VLTI Data Flow system installed during the initial phase of VLTI commissioning. Observation preparation, data archival, and data pipeline processing are addressed.
We present K-band observations of five Mira stars with the IOTA interferometer. The interferograms were obtained with the FLUOR fiber optics beam combiner which provides high- accuracy visibility measurements in spite of time-variable atmospheric conditions. For the Mira stars X Oph, R Aql, RU Her, R Ser, and V CrB we derived the uniform-disk diameters 11.7 mas, 10.9 mas, 8.4 mas, 8.1 mas, and 7.9 mas (+/- 0.3 mas), respectively. Simultaneous photometric observations yielded the bolometric fluxes. The derived angular Rosseland radii and the bolometric fluxes allowed the determination of effective temperatures. For instance, the effective temperature of R Aql was determined to be 3072 K +/- 161 K. A Rosseland radius for R Aql of 250 R. +/- 63 R. was derived from the angular Rosseland radius of 5.5 mas +/- 0.2 mas and the HIPPARCOS parallax of 4.73 mas +/- 1.19 mas. The observations were compared with theoretical Mira star models (D/P model Rosseland radius equals 255 R.; measured R Aql Rosseland radius equals 250 R. +/- 63 R.).
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