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.
Knowledge of the environmental conditions in the Very Large Telescope Interferometer (VLTI) is fundamental for assessing the performance of the scientific instruments and sub-systems of the VLTI, as well as for the calibration of measurement biases (e.g. in astrometry). Therefore, four temperature and humidity sensors were installed in the VLTI delay line tunnel and in the VLTI laboratory in September 2004.
First results from the analysis of the long-term (18 months) humidity and temperature data measured by this network will be presented, along with a comparison of the temperatures monitored by the VLTI temperature sensors network and correlations with the external data of the Paranal weather station.
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.
PRIMA, the Phase-Referenced Imaging and Micro-arcsecond Astrometry facility for the Very Large Telescope Interferometer, is now nearing the end of its manufacturing phase. An intensive test period of the various sub-systems (star separators, fringe sensor units and incremental metrology) and of their interactions in the global system will start in Garching as soon as they are delivered. The status and performances of the individual sub-systems are presented in this paper as well as the proposed observation and calibration strategy to reach the challenging goal of high-accuracy differential astrometry at 10 μas level.
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.
Two competitive design studies for the Ground-based European Nulling Interferometer Experiment (GENIE) have been initiated by the European Space Agency and the European Southern Observatory in November 2003. The GENIE instrument will most probably consist of a two-telescope Bracewell interferometer, using the 8-m Unit Telescopes and/or the 1.8-m Auxiliary Telescopes of the VLTI, and working in the infrared L' band (3.5 - 4.1 microns). A critical issue affecting the overall performance of the instrument is its capability to compensate for the phase and intensity fluctuations produced by the atmospheric turbulence. In this paper, we present the basic principles of phase and intensity control by means of real-time servo loops in the context of GENIE. We then propose a preliminary design for these servo loops and estimate their performance using GENIEsim, the science simulation software for the GENIE instrument.
The prime objective of GENIE (Ground-based European Nulling Interferometry Experiment) is to obtain experience with the design, construction and operation of an IR nulling interferometer, as a preparation for the DARWIN / TPF mission. In this context, the detection of a planet orbiting another star would provide an excellent demonstration of nulling interferometry. Doing this through the atmosphere, however, is a formidable task. In this paper we assess the prospects of detecting with nulling interferometry on ESO's VLTI, low-mass companions in orbit around their parent stars. With the GENIE science simulator (GENIEsim) we can model realistic detection scenarios for the GENIE instrument operating in the VLTI environment, and derive detailed requirements on control-loop performance, IR background subtraction and the accuracy of the photometry calibration. We analyse the technical feasibility of several scenarios for the detection of low-mass companions in the L'-band.
Darwin is one of the most challenging space projects ever considered by the European Space Agency (ESA). Its principal objectives are to detect Earth-like planets around nearby stars and to characterise their atmospheres. Darwin is conceived as a space "nulling interferometer" which makes use of on-axis destructive interferences to extinguish the stellar light while keeping the off-axis signal of the orbiting planet. Within the frame of the Darwin program, the European Space Agency (ESA) and the European Southern
Observatory (ESO) intend to build a ground-based technology demonstrator called GENIE (Ground based European Nulling Interferometry Experiment). Such a ground-based demonstrator built
around the Very Large Telescope Interferometer (VLTI) in Paranal will
test some of the key technologies required for the Darwin Infrared Space Interferometer. It will demonstrate that nulling interferometry can be achieved in a broad mid-IR band as a precursor to the next phase of the Darwin program. The instrument will operate in the L' band around 3.8 μm, where the thermal emission from the telescopes and the atmosphere is reduced. GENIE will be able to operate in two different configurations, i.e. either as a single Bracewell nulling interferometer or as a double-Bracewell nulling interferometer with an internal modulation scheme.
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.
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.