Bruno Gilli

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.

The Very Large Telescope Interferometer: 2010 edition

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.

The Very Large Telescope Interferometer: an update

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.

Recent progress at the Very Large Telescope Interferometer

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.

The Very Large Telescope Interferometer v2012

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.

A status update of the VLTI control system

In the last two years the Very Large Telescope Interferometer (VLTI) has, on one hand grown with the addition of new subsystems, on the other hand matured with experience from commissioning and operation. Two adaptive optics systems for the 8-m unit telescopes have been fully integrated in the VLTI infrastructure. The first scientific instrument, MIDI, has been commissioned and is now being offered to the community. A second scientific instrument AMBER is currently being commissioned. The performance of the interferometer is being enhanced by the installation of a dedicated fringe sensor, FINITO, and a tip-tilt sensor in the interferometric laboratory, IRIS, and the associated control loops. Four relocatable auxiliary 1.8 m telescopes and three additional delay lines are being added to the infrastructure. At the same time the design and development of the dual feed PRIMA facility, which will have major impact on the existing control system, is in full swing. In this paper we review the current status of the VLTI control system and assess the impact on complexity and reliability caused by this explosion in size. We describe the applied methods and technologies to maximize the performance and reliability in order to keep VLTI and its control system a competitive, reliable and productive facility.

Integration tests of the VLT telescope control system

The installation of the complete VLT telescope control system on the observatory is a complex task. It is important that the various components of the system have been carefully tested and integrated before. This paper presents the ESO strategy to pre-installation testing. In particular, results and experiences from pre-erection tests of the telescope structure are presented. In these tests, the complete telescope structure, including both axes with encoders and drives, has been built up at the premises of the European manufacturer (in Milan, Italy). These tests provide valuable input for the erection on Paranal. To this system, ESO added control electronics and software, which was tested with the telescope. The complete positioning of both main axes is under test, including slewing and tracking performance tests, as far as this is possible without using the sky. The VLT control software and most parts of the VLT control electronics have also been tested on the NTT on La Silla. Since the NTT upgrade software is practically a subset of the VLT software, the NTT tests have provided invaluable feedback for the VLT. The NTT tests are described in a separate paper presented at this conference. The paper also briefly discusses subsystem tests, and presents results from some of the subsystem tests performed in Europe.

VLT tracking and guiding software

The VLT must provide high quality images in a wide range of operating conditions and for a number of different modes. Performances of tracking and guiding impacts heavily on the final image quality. Control of the tracking axis is implemented in VME controllers, one per axis. They are accurately synchronized with a central time distribution system. This allows each axis to operate independently from the others, deriving its own position from the star coordinates and the time. This coordinate transformation includes basic corrections like refraction and pointing model compensations. To improve tracking performances, guiding devices (guiding cameras located in the adapter, or even instrument detector itself) can be used to measure position errors and derive small corrections for tracking. These corrections can be applied either to the secondary mirror, or to the tracking axis. To coordinate all these activities, the VMEs are connected through several LANs together with supervising workstations. While the basic functionality is controlled by applications implemented in the VMEs running a real time commercial operating system: VxWorks, the presentation layer and the non time critical operations can be implemented on workstations running standard Unix. The distribution of control software on VMEs or workstations is then dictated by real time constraints and/or availability of proved solutions for one of the operating system. This paper presents the choices in the hardware and software architectures, as well as the design concepts, made to support the different operational modes and fulfill the performance requirements.

Workstation Environment for VLT

The VLT Control system is distributed on a number of different computing units. While the real time aspects is dealt with VME-based Local Control Units (LCU) running VxWorks, the high-level control is performed by several workstations running standard Unix. Each workstation will coordinate a group of LCUs logically related by the functions they control.