Bertrand Koehler

The VLT Interferometer: a unique instrument for high-resolution astronomy

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.

The VLTI – A Status Report

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.

VLTI technical advances: present and future

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.

NACO performance: status after 2 years of operation

NACO is a VLT/Yepun instrument which provides adaptive optics corrected images in the near and thermal infrared. It is composed of the NAOS adaptive optics system and of an infrared imager CONICA. NACO has been operating since October 2001 and has already delivered a large amount of scientific results in various fields, eg the Solar System (Titan), the Interstellar Medium (outflows in Orion-OMC1), the Galactic Center, the central regions of AGN and ULIRG, ... We present the instrument performance in terms of image quality after two years of operation at Paranal. We first remind the system performance obtained from simulations, design, tests and compare them to the original specifications. We point out the telescope vibrations as a source of performance degradation. We then evaluate the impact of these vibrations on the Strehl ratio. We eventually analyze studies of the telescope vibrations to identify the systems that could excite the telescope vibration modes.

Exploring model based engineering for large telescopes: getting started with descriptive models

Large telescopes pose a continuous challenge to systems engineering due to their complexity in terms of requirements, operational modes, long duty lifetime, interfaces and number of components. A multitude of decisions must be taken throughout the life cycle of a new system, and a prime means of coping with complexity and uncertainty is using models as one decision aid. The potential of descriptive models based on the OMG Systems Modeling Language (OMG SysMLTM) is examined in different areas: building a comprehensive model serves as the basis for subsequent activities of soliciting and review for requirements, analysis and design alike. Furthermore a model is an effective communication instrument against misinterpretation pitfalls which are typical of cross disciplinary activities when using natural language only or free-format diagrams. Modeling the essential characteristics of the system, like interfaces, system structure and its behavior, are important system level issues which are addressed. Also shown is how to use a model as an analysis tool to describe the relationships among disturbances, opto-mechanical effects and control decisions and to refine the control use cases. Considerations on the scalability of the model structure and organization, its impact on the development process, the relation to document-centric structures, style and usage guidelines and the required tool chain are presented.

Atmospheric and internal turbulence measured on the Very Large Telescope Interferometer with VINCI

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.

The VLT Interferometer

One of the observing modes available with the ESO Very Large Telescope will be coherent combination of the light received by up to four 8m unit telescopes and several 1.8m auxiliary telescopes. The location of the main telescopes is fixed, while auxiliary telescopes can be moved among some 30 observing stations. The locations of these stations were chosen to augment the (µ, υ) coverage of the unit telescopes as well as to function as an independent interferometric array.

VLTI program: a status report

The VLTI (Very Large Telescope Interferometer) is one of the operating modes of the VLT, presently being built on Cerro Paranal, Chile. It aims at providing access to an observing mode at very high angular resolution and very high sensitivity (with respect to the currently operating astronomical interferometers). After a long period of conceptual, then detailed, studies, ESO is starting to build and to procure the main components of the interferometer in order to open this unpaired observing facility by the turn of the century.

VLTI auxiliary telescopes

As a complement to the coherent combination of the large 8-m VLT Unit Telescopes, the Very Large Telescope Interferometer will operate with a dedicated array of Auxiliary Telescopes (ATs). This array consists of three 1.8-m telescopes that can be relocated on thirty observing stations distributed on the top of the Paranal Observatory. The specificity of the AT requirements and design originates from its dedication to interferometric observations down to visible wavelengths. This calls for a mechanical stability at the nano-meter level together with the capability to relocate the telescope. The AT has entered into manufacturing phase in mid-1999. This paper presents the main specifications and the final design of the AT.

Narrow-Angle Astrometry with the VLT Interferometer

We discuss the merits of a narrow-angle (up to 40 arcsec) astrometric mode for the VLT Interferometer Sub-Array (VISA), which could perform with a precision of the order of 10 micro-arcsec. We present a concept for an astrometric instrument at the combined focus of VLTI.