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.
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.
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.
The ESO VLT Interferometer (VLTI) is a general-user facility and is operated in service mode (SM) for a large part of the available time. An important aspect of this SM observing mode is the definition of a set of critical observing conditions that must be met at the time of executing the requested observation. There are a number of observing constraints that are specific to interferometric observations, such as the choice of the array configuration and the hour angle at time of observation, which is processed during the scheduling. On the other hand, classical constraints such as the regular seeing or the lunar illumination are less critical for observations using VLTI instruments than for those using classical VLT instruments. In particular, the use of the adaptive optics system MACAO for VLTI observations employing the Unit Telescopes (UTs) ensures a very good image quality even for moderate environmental conditions. However, the exact dependence between environmental conditions, the performance of the MACAO systems, the wavefront quality at the interferometric instruments, and the accuracy of the final visibility, are not yet known in much detail. In order to investigate this dependence we have started to monitor routinely the environmental conditions, the quality of the MACAO systems, the quality of the acquisition images, and the final data product for all VLTI observations since June 2005. Here, we present the details of this study, as well as first statistics and results.
One of the key components of the planned VLTI dual feed facility PRIMA is the Fringe Sensor Unit (FSU). Its basic
function is the instantaneous measurement of the Optical Path Difference (OPD) between two beams. The FSU acts as
the sensor for a complex control system involving optical delay lines and laser metrology with the aim of removing any
OPD introduced by the atmosphere and the beam relay. We have initiated a cooperation between ESO and MPE with the
purpose of systematically testing this Fringe Tracking Control System in a laboratory environment. This testbed facility
is being built at MPE laboratories with the aim to simulate the VLTI and includes FSUs, OPD controller, metrology and
in-house built delay lines. In this article we describe this testbed in detail, including the environmental conditions in the
laboratory, and present the results of the testbed subsystem characterisation.
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.
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.
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.
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 mode as well as visitor mode to the whole astronomical community. Regular VLTI observations with the first scientific instrument, the mid-infrared instrument MIDI, have started in ESO observing period P73, for observations between April and September 2004. The efficient use of the VLTI as a general-user facility implies the need for a well-defined operations scheme. The VLTI follows the established general operations scheme of the other VLT instruments. Here, we present from a users' point of view the VLTI specific aspects of this scheme beginning from the preparation of the proposal until the delivery of the data.
The ARAL system of the VLTI is a multipurpose facility that helps to
have the interferometric instruments ready for night observations. It
consists of an artificial source (allowing a Mach-Zehnder mode of the
interferometric instruments for autotest), an alignment unit (verifying the position of the celestial target in the VLTI field-of-view), and an optical path router (controlling the optical switchyard and the instrument feeding-optics in the VLTI laboratory). With the multiplication of VLTI instruments and their specific features (wavelength coverage, number of beams), an upgrade of ARAL (from its November 2002 version) had to be carried out: the alignment unit has been redesigned, as well as the artificial source. This source will provide a point in the visible and in J, H, K and N infrared bands, split into four beams (with a zero optical path difference at the reference position). After a description of the optomechanics and of the computer architecture of ARAL, we detail the difficulties of building an interferometric artificial source with a wide spectral range.
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.
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.
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.
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.
The increasing number of digital control applications in the context of the VLT, and particularly the VLT Interferometer, brought the need to find a common solution to address the problems of performance and maintainability. Tools for Advanced Control (TAC) aims at helping both control and software engineers in the design and prototyping of real-time control applications by providing them with a set of standard functions and an easy way to combine them to create complex control algorithms. In this paper we describe the software architecture and design of TAC, the VLT standard for digital control applications. Algorithms are described at schematic level and take the form of a set of interconnected function blocks. Periodical execution of the algorithm as well as features like runtime modification of parameters and probing of internal data are also managed by TAC, allowing the application designers to avoid spending time writing low value software code and therefore focus on application-specific concerns. We also summarize the results achieved on the first actual applications using TAC, to manage real-time control or digital signal processing algorithms, currently deployed and being commissioned at Paranal Observatory.
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.
As the result of an analysis pursued from the very beginning, today the VLT Interferometer is the only interferometer allowing to have a 2 arcsec interferometric field of view (f.o.v) available at the instruments entrance. This accessible interferometric field is the direct result of a careful pupil transfer from the individual telescopes to the central laboratory, unique feature of the VLTI. For this goal it has been necessary to develop a new optical device, the Variable Curvature Mirror (VCM.), using large deformation theory of elasticity, and advanced techniques in optical fabrication.
The possibility with the VLTI to use various baselines, from 8 to 200 m with UTs or ATs, leads to severe conditions on the VCM curvature range. A given delay-line, and its associated VCM, should be able to transfer a pupil to the interferometric laboratory from a very far or relatively close position of an ATs. Considering the f.o.v required in the VLTI (2 arcsec), the delay-lines strokes or the OPD to compensate for, and the various locations of the UTs and ATs stations, the curvature of the VCM has to be continuously variable within a range from 84 mm<sup>-1</sup> to 2800 mm<sup>-1</sup>. The location of the VCM in the delay-line system, on the piezo-translator used for small OPD compensation, led to minimize its dimensions and to realize a small active mirror with a 16mm diameter. With this small optical aperture, the VCM range of curvature corresponds to a f ratio from f/∞ to f/2.625.
The two first VCM complete systems (mirror, mechanics and control command software) have been achieved in 2001/2002 and will be installed in the VLTI delay-lines during fall 2002. Their final performances (optical quality, pupil transfer accuracy, etc.) are 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.
After having established routine science operations for four 8 m single dish telescopes and their first set of instruments at the Paranal Observatory, the next big engineering challenge for ESO has been the VLT Interferometer. Following an intense integration period at Paranal, first fringes were obtained in the course of last year, first with two smaller test siderostats and later with two 8 m VLT telescopes. Even though optical interferometry today may be considered more experimental than single telescope astronomy, we have aimed at developing a system with the same requirements on reliability and operability as for a single VLT telescope. The VLTI control system is responsible for controlling and co-ordinating all devices making up VLTI, where a telescope is just one out of many subsystems. Thus the pure size of the complete system increases the complexity and likelihood of failure. Secondly, some of the new subsystems introduced, in particular the delay lines and the associated fringe-tracking loop, have more demanding requirements in terms of control loop bandwidth, computing power and communication. We have developed an innovative generic multiprocessor controller within the VLT framework to address these requirements. Finally, we have decided to use the VLT science operation model, whereby the observation is driven by observation blocks with minimum human real-time interaction, which implies that VLTI is seen as one machine and not as a set of telescopes and other subsystems by the astronomical instrument. In this paper we describe the as-built architecture of the VLTI control and data flow system, emphasising how new techniques have been incorporated, while at the same time the investments in technology and know-how obtained during the VLT years have been protected. The result has been a faster development cycle, a robustness approaching that of VLT single dish telescopes and a "look and feel" identical to all other ESO observing facilities. We present operation, performance and development cost data to confirm this. Finally we discuss the plans for the coming years, when more and more subsystems will be added in order to explore the full potential of the VLTI.
The first VLT unit telescope, Antu, saw first light in May 1998 and started science operation in April 1999, roughly at the same time as first light of unit telescope 2, Kueyen, was achieved. The time between first light and science operation is used to verify, quantify, qualify and optimize the functionality and performance of the telescopes and their instruments.
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.
Already in the early stages of the design of ESO's Very Large Telescope (VLT) and its instruments, it became clear that the network technology in use at the La Silla Observatory could not be simply extrapolated to cope with the VLT requirements. The new generation of instruments has needs for much higher throughput, and also the `remote' operation of four telescopes from a central Control Building forced a new approach.
The NTT project had the principal aim of field testing the VLT control system prior to installation on UT1 on Paranal. In July 1996 we began installing the control electronics and software. The telescope was stripped down to the field electronics and completely rewired. First light was achieved 2 months later and the integration of the system was completed 4 months after that. The VLT control system has been proven to be functional and fundamentally sound. Over 80% of the VLT control system is mirrored on the NTT with deviations only allowed where the hardware made it impossible to reuse the code prepared for Paranal. The NTT will operate with the complete control system executing service observations following the shake down period in early 1997.
A considerable risk is involved in developing a large complex control system, with a long lead time to integration and commissioning. To reduce this risk for VLT, ESO decided in late 1993, to make use of NTT as a testbed for the VLT control system. By upgrading the system using VLT components, e.g. standardized VME I/O boards, VLT common software, and whenever possible VLT application software, these components are tested in real-life conditions well before the start of VLT commissioning. In order to minimize disturbance to scientific operation, the strategy has been to field test subsystem by subsystem during short test periods, and afterwards restoring the original system. Each of these field tests allows an early verification of VLT control architecture and a possibility to correct direction of development. The project culminates in July 1996, when the telescope will be shut down for scientific operation and the complete control system replaced and recommissioned. The new system will then provide a VLT identical high level interface, allowing continued prototyping of operational procedures, scheduling, and science operations. The paper reports on the status of the NTT upgrade project with emphasis on how VLT has and will gain from this prototyping activity.