In this thesis work, we exploit the unique capabilities of long baseline interferometry to fill two gaps in exoplanet parameter space: 1) the discovery of new planets around stars more massive than the Sun (Project ARMADA), and 2) the characterization of known planets that are extremely close to their host star (Project PRIME). Current detection methods struggle to find exoplanets around hot (A/B-type) stars. We are pushing the astrometric limits of ground-based optical interferometers to carry out a survey of sub-arcsecond A/B-type binary systems with ARMADA. We are achieving astrometric precision at the few tens of micro-arcsecond level in short observations at CHARA/MIRC-X and VLTI/GRAVITY. This incredible precision allows us to probe the au-regime for giant planets orbiting individual stars of the binary system. We present the status of our survey, including our newly implemented etalon wavelength calibration method at CHARA, detection of new stellar mass companions, and non-detection limits down to a few Jupiter masses in some cases. With Project PRIME, we show that ground-based optical interferometry can be used to measure the orbit-dependent spectra of close-in “hot Jupiter”-type exoplanets with precision closure phases. Detecting the infrared spectra of such planets allows us to place useful constraints on atmosphere circulation models. We perform injection tests with MIRC-X and MYSTIC at CHARA for the hot Jupiter exoplanet Ups And b to show that we are reaching down to a contrast of 2e-4. The promise of both these methods demonstrate that optical interferometers are a valuable tool for probing unique regimes of exoplanet science.
Measuring the orbits of directly imaged exoplanets requires precise astrometry at the milliarcsec level over long periods of time due to their wide separation to the stars (≳10 au) and long orbital period (≳20 yr). To reach this challenging goal, a specific strategy was implemented for the instrument Spectro-Polarimetric High-contrast Exoplanet Research (SPHERE), the first dedicated exoplanet imaging instrument at the Very Large Telescope of the European Southern Observatory (ESO). A key part of this strategy relies on the astrometric stability of the instrument over time. We monitored for five years the evolution of the optical distortion, pixel scale, and orientation to the True North of SPHERE images using the near-infrared instrument IRDIS. We show that the instrument calibration achieves a positional stability of ∼1 mas over 2″ field of views. We also discuss the SPHERE astrometric strategy, issues encountered in the course of the on-sky operations, and lessons learned for the next generation of exoplanet imaging instruments on the Extremely Large Telescope being built by ESO.
Combining adaptive optics and interferometric observations results in a considerable contrast gain compared to single-telescope, extreme AO systems. Taking advantage of this, the ExoGRAVITY project is a survey of known young giant exoplanets located in the range of 0.1” to 2” from their stars. The observations provide astrometric data of unprecedented accuracy, being crucial for refining the orbital parameters of planets and illuminating their dynamical histories. Furthermore, GRAVITY will measure non-Keplerian perturbations due to planet-planet interactions in multi-planet systems and measure dynamical masses. Over time, repetitive observations of the exoplanets at medium resolution (R = 500) will provide a catalogue of K-band spectra of unprecedented quality, for a number of exoplanets. The K-band has the unique properties that it contains many molecular signatures (CO, H2O, CH4, CO2). This allows constraining precisely surface gravity, metallicity, and temperature, if used in conjunction with self-consistent models like Exo-REM. Further, we will use the parameter-retrieval algorithm petitRADTRANS to constrain the C/O ratio of the planets. Ultimately, we plan to produce the first C/O survey of exoplanets, kick-starting the difficult process of linking planetary formation with measured atomic abundances.
The new VLTI/GRAVITY instrument is a four telescope beam combiner installed at the VLT Interferometer. The principal novelty of this instrument is the availability of a dual field mode enabling narrow-angle relative astrometry at micro-arcsecond accuracy between two objects separated by several arcseconds. The fringe tracker (FT) stabilizes the interference fringes at up to 1 kHz frequency, allowing for long exposures with the science combiner (SC) as well as phase referenced imaging and differential astrometry (in dual field mode). The FT and SC beam combiners are integrated optics (IO) components, whose 24 outputs are (optionally) polarization-split and spectrally dispersed.
The processing of the photometric signals from the IO components is based on the pixel-to-visibility matrix (P2VM) formalism, that translates them into complex visibilities. The retrieval of the relative phase of the two objects subsequently relies on the combination of the phases measured from the FT, SC and the laser metrology. We will present the adopted algorithms, and an overview of the structure of the developed software. The calibration of the wavelength scales of the FT and SC at the required accuracy presents specific difficulties that we will briefly discuss. Examples of the reduction of on-sky data obtained during the commissioning will also be presented.
KEYWORDS: Sensors, Process control, Control systems, Cameras, Telescopes, Signal detection, Data acquisition, Computing systems, Interferometers, Interfaces
GRAVITY is the four-beam, near-infrared, AO-assisted, fringe tracking, astrometric and imaging instrument for the Very Large Telescope Interferometer (VLTI). It is requiring the development of one of the most complex instrument software systems ever built for an ESO instrument. Apart from its many interfaces and interdependencies, one of the most challenging aspects is the overall performance and stability of this complex system. The three infrared detectors and the fast reflective memory network (RMN) recorder contribute a total data rate of up to 20 MiB/s accumulating to a maximum of 250 GiB of data per night. The detectors, the two instrument Local Control Units (LCUs) as well as the five LCUs running applications under TAC (Tools for Advanced Control) architecture, are interconnected with fast Ethernet, RMN fibers and dedicated fiber connections as well as signals for the time synchronization. Here we give a simplified overview of all subsystems of GRAVITY and their interfaces and discuss two examples of high-level applications during observations: the acquisition procedure and the gathering and merging of data to the final FITS file.
We focus on the main algorithms of the data reduction software for the second generation VLTI instrument GRAVITY. From the interferometric data and the metrology signal, the pipeline recovers the complex visibility of the science target with an absolute phase with respect to the fringe tracker target. Visibilities are then calibrated and the relative astrometry is eventually computed when possible.
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.
This paper aims at giving an update on the most versatile Adaptive Optics fed instrument to date, the well
known and successful NACO*. Although NACO is only scheduled for about two more years† at the Very Large
Telescope (VLT), it keeps on evolving with additional operation modes bringing original astronomical results.
The high contrast imaging community uses it creatively as a test-bench for SPHERE‡ and other second generation
planet imagers. A new visible wavefront sensor (WFS) optimized for Laser Guide Star (LGS) operations has
been installed and tested, the cube mode is more and more required for frame selection on bright sources, a
seeing enhancer mode (no tip/tilt correction) is now offered to provide full sky coverage and welcome all kind
of extragalactic applications, etc. The Instrument Operations Team (IOT) and Paranal engineers are currently
working hard at maintaining the instrument overall performances but also at improving them and offering new
capabilities, providing the community with a well tuned and original instrument for the remaining time it is
being used. The present contribution delivers a non-exhaustive overview of the new modes and experiments that
have been carried out in the past months.
GRAVITY is an adaptive optics assisted Beam Combiner for the second generation VLTI instrumentation. The
instrument will provide high-precision narrow-angle astrometry and phase-referenced interferometric imaging in the
astronomical K-band for faint objects. We describe the wide range of science that will be tackled with this instrument,
highlighting the unique capabilities of the VLTI in combination with GRAVITY. The most prominent goal is to observe
highly relativistic motions of matter close to the event horizon of Sgr A*, the massive black hole at center of the Milky
Way. We present the preliminary design that fulfils the requirements that follow from the key science drivers: It includes
an integrated optics, 4-telescope, dual feed beam combiner operated in a cryogenic vessel; near-infrared wavefrontsensing
adaptive optics; fringe-tracking on secondary sources within the field of view of the VLTI and a novel metrology
concept. Simulations show that 10 μas astrometry within few minutes is feasible for a source with a magnitude of
mK = 15 like Sgr A*, given the availability of suitable phase reference sources (mK = 10). Using the same setup, imaging of mK = 18 stellar sources in the interferometric field of view is possible, assuming a full night of observations and the corresponding UV coverage of the VLTI.
KEYWORDS: Telescopes, High dynamic range imaging, Imaging systems, Single mode fibers, Adaptive optics, Range imaging, Stars, Observatories, Space telescopes, Wavefronts
We present the laboratory demonstration of a very high-dynamic range imaging instrument FIRST (Fibered Imager foR
Single Telescope). FIRST combines the techniques for aperture masking and a single-mode fiber interferometer to
correct wavefront errors, which leads to a very high-dynamic range up to 106 around very near the central object (~ λ/D)
at visible to near-infrared wavelengths. Our laboratory experiments successfully demonstrated that the original image
can be reconstructed through a pupil remapping system. A first on-sky test will be performed at the Lick Observatory 3-
m Shane telescope for operational tests in the summer of 2010.
We present the second-generation VLTI instrument GRAVITY, which currently is in the preliminary design phase.
GRAVITY is specifically designed to observe highly relativistic motions of matter close to the event horizon of Sgr A*,
the massive black hole at center of the Milky Way. We have identified the key design features needed to achieve this
goal and present the resulting instrument concept. It includes an integrated optics, 4-telescope, dual feed beam combiner
operated in a cryogenic vessel; near infrared wavefront sensing adaptive optics; fringe tracking on secondary sources
within the field of view of the VLTI and a novel metrology concept. Simulations show that the planned design matches
the scientific needs; in particular that 10µas astrometry is feasible for a source with a magnitude of K=15 like Sgr A*,
given the availability of suitable phase reference sources.
Building on technological developments over the last 35 years, intensity interferometry now appears a feasible option by which to achieve diffraction-limited imaging over a square-kilometer synthetic aperture. Upcoming Atmospheric Cherenkov Telescope projects will consist of up to 100 telescopes, each with ~100m2 of light gathering area, and distributed over ~1km2. These large facilities will offer thousands of baselines from 50m to more than 1km and an unprecedented (u,v) plane coverage. The revival of interest in Intensity Interferometry has recently led to the formation of a IAU working group. Here we report on various ongoing efforts towards implementing modern Stellar Intensity Interferometry.
The VEGA spectrograph and polarimeter has been recently integrated on the visible beams of the CHARA
Array. With a spectral resolution up to 35000 and thanks to operation at visible wavelengths, VEGA brings
unique capabilities in terms of spatial and spectral resolution to the CHARA Array. We will present the main
characteristics of VEGA on CHARA, some results concerning the performance and a preliminary analysis of the
first science run.
KEYWORDS: High dynamic range imaging, Range imaging, Telescopes, Astronomy, Point spread functions, Visibility, Diffraction, Imaging systems, Exoplanets, Prototyping
We describe the present status of the development of a very high-dynamic range, diffraction limited imaging instrument FIRST (Fibered Imager foR Single Telescope), among which goals is the detection of nearby extra-solar planets at visible to near-infrared wavelengths from the ground. We have started to develop a prototype system which consists of a number of novel designs such as a segmented micro mirror array and silicon micro machined single-mode fiber arrays. Furthermore, we have proposed to build a FIRST instrument for the CFHT, which will be complementary to high-dynamic range instruments developed for 8m class telescopes at near-infrared wavelengths.
We describe a project for the installation of a visible focal instrument at the CHARA Array, named VEGA for Visible spEctroGraph and polArimeter. This new instrument will further open the visible domain and offer both spectral and polarimetric capabilities at the CHARA Array. It will create a new and unique scientific niche for the CHARA Array, especially in the context of international competition. The combination of the visible domain and high spectral resolution mode combined with a good sensitivity will allow VEGA/CHARA to carve out a new piece of observational phase space and compliment many existing or planned near-infrared interferometers. VEGA will help make CHARA the interferometer with the largest spectral and spatial resolution worldwide.
Interferometry has been intensively done at long wavelengths, starting with the radio interferometers in the years 50 since it was easier to guide radio wavelengths in cable while keeping the phase information or using a local oscillator and a correlator to recombine "a posteriori" the beams over intercontinental distances. In the optical a lot of work as been done at IR and near-IR wavelengths since it was technically easier, or we must say, less difficult to recombine directly the optical beams since the coherence length is larger and the turbulence slower than at shorter wavelengths.
Therefore, the visible domain of the electromagnetic spectrum is not covered at the same level than near or mid infrared.
Some very nice and important results have been however obtained with the GI2T interferometer in south of France, the
Mark III interferometer on the Mount Wilson, USA, the NPOI array in Flagstaff, USA or the SUSI interferometer in Australia. We will present in this paper the science cases of a new but already existing and tested instrument: the
REGAIN focal instrument which was designed and built for the GI2T. This instrument, in his CHARA adaptation, called VEGA will open new fields in a wide range of Astrophysical topics only addressable in the visible domain. It will provide a spectral resolution up to 30000 within the spectral range 0.4-0.9 micron and a spatial resolution of less than 1mas for up to 4 telescopes in its X-lambda special configuration. A polarimetric device (SPIN) measuring simultaneously the polarization in 2 directions either circular or linear is also implemented in this instrument. Since VEGA was already tested on the sky on 1.5 m telescopes it is also very well suited for the 1m CHARA array and will only need minor adaptations for the injection of the CHARA beams. This paper will focus on some of the most promising science drivers only possible with this visible instrument.
Using the FLUOR beam-combiner installed at the CHARA Array (Mt. Wilson, CA), we have obtained highprecision visibility measurements of Vega, one of the prototypic debris-disk stars, known to be surrounded by a large amount of cold dust in a ring-like structure at 80-100 AU. The combination of short and long baselines has allowed us to separately resolve the stellar photosphere and the close environment of the star (less than 8 AU). Our observations show a significant deficit in square visibility at short baselines with respect to the expected visibility of a simple UD stellar model (ΔV2 equal or equivalent to 2%), suggesting the presence of an extended source of emission around Vega. The sparse (u, v) plane coverage does not allow the discrimination between a point source and an extended circumstellar emission as the source of the extended emission. However, we show that the presence of a point-like source within the FLUOR field-of-view (1" in radius, i.e., 7.8 AU at the distance of Vega) is highly unlikely. The excess emission is most likely due to the presence of hot circumstellar dust in the inner part of Vega's debris disk, with a flux ratio of 1.29 plus or minus 0.19% between the integrated dust emission and the stellar photosphere. Complementing this result with archival photometric data in the near- and mid-infrared and taking into account a realistic photospheric model for the rapidly rotating Vega, we derive the expected physical properties of the circumstellar dust by modelling its Spectral Energy Distribution. The inferred properties suggest that the Vega system could be currently undergoing major dynamical perturbations.
We report on observations with MACAO-VLTI to feed the VLT Interferometer in November 2003. The purpose of this observing run was to optimize the feed to the VLTI by varying certain parameters of the curvature AO system and of the interferometer instrument VINCI. All along the main concern about this instrument combination was the differential piston introduced by 2 independent AO systems. A special so-called “piston removal algorithm” has been developed especially for this purpose. Each DM Influence Function is carefully characterized and a pure piston mode is defined to compensate piston over the pupil produced by a given voltage set. Piston is reduced by ~20 using this algorithm. It was found that decreasing the system main gain, while reducing strehl ratio, also reduces high frequency vibrations on the DM and therefore OPD variations. A control frequency of 420 Hz instead of the nominal 350 Hz was found to improve substantially the coupling by reducing the excitation of the DM resonance (~700Hz). On bright stars, an improvement of a factor of 30 in the flux injection into the VINCI fibers was measured. Following these tests a successful observation of the active nucleus of NGC 1068 was performed leading to a visibility of 40.4±5.4% on an average baseline of 45.84 m. The K magnitude in the 60 mas central source is 9.2±0.4. The results already put some interesting constraints on the inner torus and central engine of the nucleus of NGC 1068 but mostly show that the combination MACAO-VLTI and VINCI opens the realm of extragalactic astronomy to interferometry.
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 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.
We present in this paper new and accurate calibrations of the surface brightness-color relations that can be used to predict accurately the angular diameter of dwarf stars and subgiants. These stars present significant advantages as calibrators for interferometric observations. In many cases, they are more stable than giants and supergiants, as they are steadily burning their hydrogen. They are present in large numbers in the solar neighborhood, and offer a broad variety of colors. Their proximity allows to minimize the problems related to interstellar extinction in estimating their true magnitudes and colors. Excluding multiple stars, fast rotators and highly variable stars, it is possible to select reliable calibrators from spectro-photometric observations. Moreover, as opposed to the giant and supergiant stars, the photospheric diameter of dwarf stars is well defined and less sensitive to the assumed atmosphere models. In particular, the limb darkening related problems are alleviated at infrared wavelengths.
Though interferometric techniques are now used routinely around the world, the processing of interferometric data is still the subject of active research. In particular, the corruption of the interferometric fringes by the turbulent atmosphere is currently the most critical limitation to the precision of the ground-based interferometric measurements. In this paper, we discuss the data acquisition and processing procedures of the VINCI/VLTI instrument. Optimal data acquisition parameters and wavelets based processing allow us to remove a posteriori part of the data corrupted by atmospheric turbulence. A relative precision better than 0.1% on the instrumental visibility (for a 5 minutes observation) was already achieved on bright stars without fringe stabilization. Using a dedicated fringe tracker, an even better precision, of the order of a few 10-4, appears to be within reach. However, we show in this paper that the calibration of the instrumental visibility measurements can easily be the source of significant systematic errors beyond this statistical precision.
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.
Installed at the heart of the Very Large Telescope Interferometer (VLTI), VINCI combines coherently the infrared light coming from two telescopes. The first fringes were obtained in March 2001 with the VLTI test siderostats, and in October of the same year with the 8 meters Unit Telescopes (UTs). After more than one year of operation, it is now possible to evaluate its behavior and performances with a relatively long timescale. During this period, the technical downtime has been kept to a very low level. The most important parameters of the instrument (interferometric efficiency, mechanical stability,...) have been followed regularly, leading to a good understanding of its performances and characteristics. In addition to a large number of laboratory measurements, more than 3000 on-sky observations have been recorded, giving a precise knowledge of the behavior of the system under various conditions. We report in this paper the main characteristics of the VINCI instrument hardware and software. The differences between observations with the siderostats and the UTs are also briefly discussed.
Several scientific topics linked to the observation of extended structures around astrophysical sources (dust torus around AGN, disks around young stars, envelopes around AGBs) require imaging capability with milli-arcsecond spatial resolution. The current VLTI instruments, AMBER and MIDI, will provide in the coming months the
required high angular resolution, yet without actual imaging. As a rule of thumb, the image quality accessible with an optical interferometer is directly related to the number of telescopes used simultaneously: the more the apertures, the better and the faster the reconstruction of the image. We propose an instrument concept to
achieve interferometric combination of N telescopes (4 ≤ N ≤ 8) thanks to planar optics technology: 4 x 8-m telescopes in the short term and/or 8 x 1.8-m telescopes in the long term. The foreseen image reconstruction quality in the visible and/or in the near infrared will be equivalent to the one achieved with millimeter radio interferometers. Achievable spatial resolution will be better than the one foreseen with ALMA. This instrument would be able to acquire routinely 1 mas resolution images. A 13 to 20 magnitude sensitivity in spectral ranges from 0.6 to 2.5 μm is expected depending on the choice of the phase referencing guide source. High dynamic range, even on faint objects, is achievable thanks to the high accuracy provided by integrated optics
for visibility amplitude and phase measurements. Based on recent validations of integrated optics presented here an imaging instrument concept can be proposed. The results obtained using the VLTI facilities give a demonstration of the potential of the proposed technique.
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.
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 IRSI/DARWIN spatial interferometer of the European Space Agency (ESA) is aimed at detecting extrasolar planets. The high difference in flux emission between the star and the planet is tackled by using nulling interferometry as a coronographic method. By star light extinction, one can retrieve the planet signal, and thus have access to high resolution imaging by interferometric measurements. Critical technological solutions are to be developed in order to reach the high level performances of such instruments. This is the scope of the Multi-Aperture Imaging Interferometer (MAI2) breadboard developed by Alcatel Space in an ESA contract. The goal of this laboratory experiment, based on integrated optics (IO) beam combination, is to obtain stable rejection of a star signal at a level of 106.
In its current configuration, the VLT Interferometer (VLTI) combines the light collected by two telescopes and directs it towards the commissioning instrument called VINCI. In an interferometer, the optical path ranging from a telescope to the point where beams are combined is referred as an arm of the interferometer. This arm contains a large number of optics that have to be aligned at installation time and kept aligned during the period of use of the interferometer. The method used to perform the initial alignment is reported in a separate article. This paper is focussed on the methods used to assess the stability of the image alignment of each interferometer arm. Collected data sets are presented and interpreted.
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.
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.
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.
An update of the current status and schedule of PRIMA (Phase-Referenced Imaging and Micro-arcsecond Astrometry) developed for the Very Large Telescope Interferometer (VLTI) is given, with emphasis on the astrometric objectives, performances and technological challenges. PRIMA will allow to observe simultaneously two fields separated by 2 to 60 arcsec, to detect and track the fringes on the brightest object, to detect the fringes on the faintest, and to measure the phase of the secondary set of fringes relative to the primary one, with an accuracy of (lambda) /1000 at 2 micrometers .
We report interferometric observations of the classical galactic Cepheid (zetz) Gem with FLUOR (Fiber Linked Unit for Optical Recombination), installed at the IOTA (Infrared Optical Telescope Array) interferometer. Thanks to the high precision of the visibility measurements with FLUOR, it has been possible to estimate its mean uniform disk angular diameter to a relative precision of 5% ((theta) ud equals 1.98 +/- 0.09 mas). Variations of the angular diameter of (zetz) Gem were marginally detected, with an amplitude of (Delta) (theta) ud equals 0.38 +/- 0.23 mas, but further observations should allow a more precise estimation of this value. The feasibility of the observation of Cepheids with the VLTI is also evaluated. When in operation, the VLTI will allow the precise measurement of the angular diameters of a large number of Cepheids. The zero point of the period- luminosity relation could then be set with a precision of less than 0.1 mag.
The Very Large Telescope Interferometer (VLTI) is a complex system, made of a large number of separated elements. To prepare an early successful operation, it will require a period of extensive testing and verification to ensure that the many devices involved work properly together, and can produce meaningful data. This paper describes the concept chosen for the VLTI commissioning instrument, LEONARDO da VINCI, and details its functionalities. It is a fiber based two-way beam combiner, associated with an artificial star and an alignment verification unit. The technical commissioning of the VLTI is foreseen as a stepwise process: fringes will first be obtained with the commissioning instrument in an autonomous mode (no other parts of the VLTI involved); then the VLTI telescopes and optical trains will be tested in autocollimation; finally fringes will be observed on the sky.
The Very Large Telescope (VLT) Observatory on Cerro Paranal (2635 m) in Northern Chile is approaching completion in this year when the fourth of the 8-m Unit Telescopes will see first light. At the same time, the preparation for first fringes of the VLT Interferometer (VLTI) is advancing rapidly with the goal of having the first fringes with two siderostats within this year. In this article we describe the status of the VLTI and its subsystems, we discuss the planning for first fringes with the different telescopes and instruments. Eventually, we present an outlook for the future of interferometry with Very Large Telescopes.
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