Bertrand Bauvir

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.

The VLTI fringe sensors: FINITO and PRIMA FSU

FINITO is the first generation VLTI fringe sensor, optimised for three beam observations, recently installed at Paranal and currently used for VLTI optimisation. The PRIMA FSU is the second generation, optimised for astrometry in dual-feed mode, currently in construction. We discuss the constraints of fringe tracking at VLTI, the basic functions required for stabilised interferometric observations, and their different implementation in the two instruments, with remarks on the most critical technical aspects. We provide an estimate of the expected performance and describe some of their possible observing and calibration modes, with reference to the current scientific combiners.

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.

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.

Fringe tracking at VLTI : status report

FINITO (the VLTI three beam fringe-tracker) has been offered in September 2007 to the astronomical community for observations with the scientific instruments AMBER and MIDI. In this paper, we describe the last improvements of the fringe-tracking loop and its actual performance when operating with the 1.8m Auxiliary Telescopes. We demonstrate the gain provided to the scientific observations. Finally, we discuss how FINITO real-time data could be used in post-processing to enhance the scientific return of the facility.

E-ELT primary mirror control system

During the past year the control of the 42m segmented primary mirror of the E-ELT has been studied. This paper presents the progress in the areas of M1 figure control and control hardware implementation. The critical issue of coupling through the supporting structure has been considered in the controller design. Different control strategies have been investigated and from a tradeoff analysis modal control is proposed as a solution addressing the topics of wind rejection as well as sensor noise in the presence of cross-coupling through the supporting structure. Various implementations of the M1 Control System have been studied and a centralized architecture has been selected as baseline. This approach offers maximum flexibility for further iterations. The controller design and main parts of the control system are described.

Post-processing the VLTI fringe-tracking data: first measurements of stars

Context. At the Very Large Telescope Interferometer, the purpose of the fringe-tracker FINITO is to stabilize the optical path differences between the beams, allowing longer integration times on the scientific instruments AMBER and MIDI. Aims. Our goal is to demonstrate the potential of FINITO for providing H -band interferometric visibilities, simultaneously and in addition to its normal fringe-tracking role. Methods. We use data obtained during the commissioning of the Reflective Memory Network Recorder at the Paranal observatory. This device has permitted the first recording of all relevant real-time data needed for a proper data-reduction. Results. We show that post-processing the FINITO data allows valuable scientific visibilities to be measured. Over the several hours of our engineering experiment, the intrinsic transfer function is stable at the level of ± 2%. Such stability would lead to robust measurements of science stars even without the observation of a calibration star within a short period of time. We briefly discuss the current limitations and the potential improvements.

Field stabilization (tip/tilt control) of E-ELT

The image motion (tip/tilt) of the telescope is dominated by two types of perturbations: a) atmospheric b) wind load. The wind load effect on E-ELT can be an order of magnitude higher than the atmospheric effect. Part of the image motion due to the wind load on the telescope structure is corrected by the main axis control system (mainly large amplitude, low frequency errors). The residual tip/tilt is reduced by M5 and M4 mirror units. M5 with its large stroke and relative low bandwidth (higher than main axes) corrects for large amplitude and low frequency part of the image motion and M4 unit takes the higher frequency parts with smaller stroke availability. In this paper the two stage control strategy of the E-ELT field stabilization is introduced. The performance of the telescope due to the wind load and in the presence of the major imperfections in the control system is presented.

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.

Acceleration feedback control on an AT

he VLT observatory operated by ESO is located on Cerro Paranal in Chile and consists of four identical 8-m telescopes and four 1.8-m VLTI Auxiliary telescopes (ATs). In order to further improve the tracking axes performance of telescopes regarding wind rejection, different control techniques have been evaluated. Ongoing investigation and studies show that by measuring the acceleration and using that in appropriate control strategy the performance of telescope tracking in face of external perturbation can be improved. The acceleration signal contains the non filtered information (advanced phase compared to velocity and position) of the perturbation load, e.g. wind load. As a result the reaction of the control is faster and hence the perturbation rejection is more efficient. In this paper, two acceleration feedback techniques are discussed and the results of the measurement test on an AT telescope are presented.