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In this paper we present the status of the Armazones Instrumentation Programme for ESO’s Extremely Large Telescope (ELT). While the ELT Construction Programme includes the first-generation instruments (MICADO, MAORY, HARMONI and METIS), the Armazones Programme covers the development of all future instrumentation for the ELT. As part of this Programme we have already completed 2 Phase-A studies for a high-resolution spectrograph (HIRES) and a multi-object spectrograph (MOSAIC). In this paper we report the status of the Programme, the complementarity of these new instruments with the ones already in construction, and the roadmap for future developments.

The ESO Extremely Large Telescope (ELT) has been in construction since 2014. In parallel with the construction of the telescope, ESO has entered into agreements with consortia in the ESO member states to build the first instruments for that telescope. To meet the telescope science goals, the ambitious instrument plan includes two instruments for first light: an optical to near-infrared integral field spectrograph with a dedicated adaptive optics system (HARMONI) and a near-infrared camera with simple spectrograph (MICADO) behind a multi-conjugate adaptive optics module (MAORY). The next instrument will be a mid-infrared imager and spectrograph (METIS). Plans to follow this first suite of instruments include a high-resolution spectrograph (HIRES) and a multi-object spectrograph (MOSAIC). Technology development is underway to prepare for building the ELT Planetary Camera and Spectrograph. An overview of the telescope and its instruments is given.

The FORS Upgrade project (FORS-Up) aims at bringing a new life to the highly demanded workhorse instrument attached to ESO's Very Large Telescope (VLT). FORS2 is a multimode optical instrument, which started regular science operations in 2000 and since then, together with its twin, FORS1, has been one of the most demanded and most productive instruments of the VLT. In order to ensure that a FORS shall remain operational for at least another 15 years, an upgrade has been planned. This is required as FORS2 is using technology and software that is now obsolete and cannot be put and maintained to the standards in use at the Observatory. The project - carried out as a collaboration between ESO and INAF-Astronomical Observatory of Trieste - aims at bringing to the telescope in 2023/2024 a refurbished instrument with a new scientific detector, an upgrade of the instrument control software and electronics, a new calibration unit, as well as additional filters and grisms. The new FORS will also serve as a test bench for the Extremely Large Telescope (ELT) standard technologies (among them the use of programmable logic controllers and of the features of the ELT Control Software). The project aims at minimising the downtime of the instrument by performing the upgrade on the currently decommissioned instrument FORS1 and retrofitting the Mask Exchange Unit and polarisation optics from FORS2 to FORS1., Comment: Author version of a paper presented at SPIE Ground-based and Airborne Instrumentation for Astronomy VIII; corrected one statement

The near-infrared GRAVITY instrument has become a fully operational spectro-imager, while expanding its capability to support astrometry of the key Galactic Centre science. The mid-infrared MATISSE instrument has just arrived on Paranal and is starting its commissioning phase. NAOMI, the new adaptive optics for the Auxiliary Telescopes, is about to leave Europe for an installation in the fall of 2018. Meanwhile, the interferometer infrastructure has continuously improved in performance, in term of transmission and vibrations, when used with both the Unit Telescopes and Auxiliary Telescopes. These are the highlights of the last two years of the VLTI 2nd generation upgrade started in 2015.

The ELT is a project led by the European Southern Observatory (ESO) for a 40-m class optical, near- and mid-infrared, ground-based telescope. When it will enter into operation, the ESO ELT will be the largest and most powerful optical telescope ever built. It will not only offer unrivalled light collecting power, but also exceedingly sharp images, thanks to its ability to compensate for the adverse effect of atmospheric turbulence on image sharpness. The basic optical solution for the ESO ELT is a folded three-mirror anastigmat, using a 39-m segmented primary mirror (M1), a 4-m convex secondary mirror (M2), and a 4-m concave tertiary mirror (M3), all active. Folding is provided by two additional flat mirrors sending the beams to either Nasmyth foci along the elevation axis of the telescope. The folding arrangement (flat M4 and M5 mirrors) is conceived to provide conveniently located flat surfaces for an adaptive shell (M4) and field stabilization (M5). This paper provides an update of the specifications, design, and manufacturing of the ESO ELT optical systems

ESO is undertaking a large upgrade of the infrastructure on Cerro Paranal in order to integrate the 2nd generation of interferometric instruments Gravity and MATISSE, and increase its performance. This upgrade started mid 2014 with the construction of a service station for the Auxiliary Telescopes and will end with the implementation of the adaptive optics system for the Auxiliary telescope (NAOMI) in 2018. This upgrade has an impact on the infrastructure of the VLTI, as well as its sub-systems and scientific instruments.

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.

The Phase Referenced Imaging and Micro Arcsecond Astrometry (PRIMA) facility for the Very Large Telescope Interferometer (VLTI), is being installed and tested in the observatory of Paranal. Most of the tests have been concentrated on the characterization of the Fringe Sensor Unit (FSU) and on the automation of the fringe tracking in preparation of dual-field observations. The status of the facility, an analysis of the FSU performance and the first attempts towards dual-field observations will be presented in this paper. In the FSU, the phase information is spatially encoded into four independent combined beams (ABCD) and the group delay comes from their spectral dispersion over 5 spectral channels covering the K-band. During fringe tracking the state machine of the optical path difference controller is driven by the Signal to Noise Ratio (SNR) derived from the 4 ABCD measurements. We will describe the strategy used to define SNR thresholds depending on the star magnitude for automatically detecting and locking the fringes. Further, the SNR as well as the phase delay measurements are affected by differential effects occurring between the four beams. We will shortly discuss the contributions of these effects on the measured phase and SNR noises. We will also assess the sensitivity of the group delay linearity to various instrumental parameters and discuss the corresponding calibration procedures. Finally we will describe how these calibrations and detection thresholds are being automated to make PRIMA as much as possible a user-friendly and efficient facility.

The Shack-Hartmann Phasing Sensor (SHAPS) has been integrated in the Active Phasing Experiment (APE) at ESO. It is currently under test in the laboratory. The tests on sky are foreseen for the end of 2008, when APE will be mounted at the Nasmyth focus of one of the VLT unit telescopes. SHAPS is based on the Shack-Hartmann principle: the lenslet array is located in a plane which is optically conjugated to the Active Segmented Mirror (ASM) of APE and is composed of two types of microlenses, circular and cylindrical, which give information about the wavefront slope and the piston steps, respectively. This proceeding contains a description of SHAPS and of the algorithms implemented for the wavefront reconstruction and for the phasing. The preliminary results obtained during the laboratory tests are discussed and compared with the theoretical predictions. The performances of SHAPS at the VLT and at the European Extremely Large Telescope (E-ELT) are estimated.

ESPRI is a project which aims at searching for and characterizing extra-solar planets by dual-beam astrometry with PRIMA@VLTI. Differential Delay Lines (DDL) are fundamental for achieving the micro-ar cseconds accuracy required by the scientific objective. Our Consortium, consisting of the Geneva Observatory, the Max-Planck Institut for Astronomy Heidelberg, and the Landessternwarte Heidelberg, in collaboration with ESO, has built and tested these DDLs successfully and will install them in summer 2008 at the VLTI. These DDLs consist of high quality cat’s eyes displaced on a parallel beam-mechanics and by means of a two-stage actuation with a precision of 5 nm over a stroke length of 70 mm. Over the full range, a bandwidth of about 400 Hz is achieved. The DDLs are operated in vacuum. We shall present, in this paper, their design and their exceptional performances. Keywords: Interferometry, near infrared, astrom etry, extrasolar planets, delay lines *Francesco.Pepe@obs.unige.ch

PRIMA, the instrument for Phase-Referenced Imaging and Micro-arcsecond Astrometry at the VLTI, is currently being developed at ESO. PRIMA will implement the dual-feed capability, at first for two UTs or ATs, to enable simultaneous interferometric observations of two objects that are separated by up to 1 arcmin. PRIMA is designed to perform narrow-angle astrometry in K-band with two ATs as well as phase-referenced aperture synthesis imaging with instruments like Amber and Midi. In order to speed up the full implementation of the 10 microarcsec astrometric capability of the VLTI and to carry out a large astrometric planet search program, a consortium lead by the Observatoire de Geneve, Max Planck Institute for Astronomy, and Landessternwarte Heidelberg, has built Differential Delay Lines for PRIMA and is developing the astrometric observation preparation and data reduction software. When the facility becomes fully operational in 2009, we will use PRIMA to carry out a systematic astrometric Exoplanet Search program, called ESPRI. In this paper, we describe the narrow-angle astrometry measurement principle, give an overview of the ongoing hardand software developments, and outline our anticipated astrometric exoplanet search program.

The purpose of the Active Phasing Experiment, designed at ESO, is to validate wavefront control concepts for ELT class telescopes. This instrument includes an Active Segmented Mirror, located in a pupil image. It will be mounted at a Nasmyth focus of one of the unit telescopes of the ESO VLT. The Active Phasing Experiment will compare four types of phasing sensor. One of them is based on the Shack-Hartmann principle. The lenslets in the array will be placed on intersegment borders for the measurement of piston steps, as well and inside the subapertures defined by the segments for the measurement of local slopes generated by the segments and the telescope optics. The paper describes the design of the sensor optics and the lenslet array, and discusses the expected performance of the sensor under laboratory conditions and in the telescope.

In a framework of ELT design study our group is building an Active Phasing Experiment (APE), the main goals of which is to demonstrate the non-adaptive wavefront control scheme and technology for Extremely Large Telescope (ELT). The experiment includes verification and test of different phasing sensors and integration of a phasing wavefront sensor into a global scheme of segmented telescope active control. After a sufficient number of tests in the laboratory APE will be mounted and tested on sky at a Nasmyth focus of a VLT unit telescope. The paper presents APE as a demonstrator of particular aspects of ELT and provides a general understanding concerning the strategy of segmented mirrors active control.

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 purpose of the Active Phasing Experiment, designed under the lead of ESO, is to validate wavefront control concepts for ELT class telescopes. This instrument includes an Active Segmented Mirror, located in a pupil image. It will be mounted at a Nasmyth focus of one of the Unit Telescopes of the ESO VLT. APE contains four different types of phasing sensors, which are developed by Istituto Nazionale di Astrofisica in Arcetri, Instituto Astrofisica Canarias, Laboratoire d'Astrophysique de Marseille and ESO. These phasing sensors can be compared simultaneously under identical optical and environmental conditions. All sensors receive telecentric F/15 beams with identical optical quality and intensity. Each phasing sensor can measure segmentation errors of the active segmented mirror and correct them in closed loop. The phasing process is supervised by an Internal Metrology system developed by FOGALE Nanotech and capable of measuring piston steps with an accuracy of a few nanometers. The Active Phasing Experiment is equipped with a turbulence generator to simulate atmospheric seeing between 0.45 and 0.85 arcsec in the laboratory. In addition, the Active Phasing Experiment is designed to control simultaneously with the phasing corrections the guiding and the active optics of one of the VLT Unit Telescopes. This activity is supported by the European Community (Framework Programme 6, ELT Design Study, contract No 011863).

The completed VLTI with eight Delay Lines and eight ATs forms the basis for the second generation instrumentation. We describe the events up to first fringes with the test instrument VINCI using the siderostats, and the planning for the immediate future. Multi beam combination for ‘smoother images’ will be briefly discussed as well as artificial guide stars for fringe tracking. New technological developments like fiber optics amplifiers and integrated optics in combination with STJ open the door for a new type of interferometric arrays. Baselines as long as a a few kilometres come into reach. Examples of these second generation interferometers will be given.

The future European Extremely Large Telescope will be composed of one or two giant segmented mirrors (up to 100 m of diameter) and of several large monolithic mirrors (up to 8 m in diameter). To limit the aberrations due to misalignments and defective surface quality it is necessary to have a proper active optics system. This active optics system must include a phasing system to limit the degradation of the PSF due to misphasing of the segmented mirrors. We will present the lastest design and development of the Active Phasing Experiment that will be tested in laboratory and on-sky connected to a VLT at Paranal in Chile. It includes an active segmented mirror, a static piston plate to simulate a secondary segmented mirror and of four phasing wavefront sensors to measure the piston, tip and tilt of the segments and the aberrations of the VLT. The four phasing sensors are the Diffraction Image Phase Sensing Instrument developed by Instituto de Astrofisica de Canarias, the Pyramid Phasing Sensor developed by Arcetri Astrophysical Observatory, the Shack-Hartmann Phasing Sensor developed by the European Southern Observatory and the Zernike Unit for Segment phasing developed by Laboratoire d'Astrophysique de Marseille. A reference measurement of the segmented mirror is made by an internal metrology developed by Fogale Nanotech. The control system of Active Phasing Experiment will perform the phasing of the segments, the guiding of the VLT and the active optics of the VLT. These activities are included in the Framework Programme 6 of the European Union.

In the framework of the Phase-Referenced Imaging and Micro-arcsecond Astrometry facility (PRIMA) developed for the Very Large Telescope Interferometer (VLTI), a sophisticated opto-mechanical system has been developed by TNO-TPD. It will be placed at the Coude focus of the telescopes and will allow picking up two stars anywhere in a 2 arcmin field-of-view and collimating their light into two beams that will propagate through the rest of the interferometer toward the instrument. These Star Separator systems have a very high optical quality, fast and accurate pointing and chopping, independent high speed remote control of the beam tip-tilt and of the pupil position. They are very rigid, accurate mechanical systems non-sensitive to temperature variations The Star Separator systems are described in this paper.

This paper will provide details of the outstanding performance of the VLTi Delay Lines in Blind Tracking and Fringe Tracking modes. This OPD control performance was demonstrated during the first fringe event in October last year where two UTs (ANTU and MELIPAL) were combined by the interferometer. Already in March 2001 the fringes were observed by the VLT interferometer equipped, at this time, with two Siderostats. The stellar light was collected and trasported via the Delay Lines to the VINCI instrument, also used as fringe detector. The excellent results were achieved after several missions on the mountain, managed by ESO and DUTCH SPACE personnel, to install and to verify the tracking performance of the first three Delay Lines over the full length of the tracks and to align the science beams over the Delay Lines reflectors. The first performance tests were done in open loop (without a fringe sensor feedback to the Delay Line) by imposing a global trajectory on the active Delay Line. During the commissioning period with the Siderostats, this procedure was a success and provided already encouraging results. In a later stage of the Delay Line commissioning, the Fringe Acquisition and Tracking Mode enabling to keep closed loop performance for longer periods each night replaced the Blind Tracking Mode. Performance monitoring during various occasions after installation revealed that OPD errors are not degrading in time; extensive preventive maintenance activities are not required to keep the performance levels within specifications, proving the robustness of the controller of the Delay Line.

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 -1 to 2800 mm -1 . 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.

Two siderostats having 40 cm input pupil have been developed for the early commissioning phase of the VLT Interferometer at Paranal. Performances, design and development of the system are briefly introduced. First results obtained in Europe are discussed.

The VLTI Delay Lines have been designed for two objectives: the Optical Path Length compensation and the pupil transfer between the VLT telescopes to the VLT Interferometer instruments. The system performance and the technical solutions are presented. The development approach is briefly introduced too.

The pupil transfer, from the individual telescopes to the interferometric laboratory, is an unique feature of the VLT Interferometer allowing to have a 2 arcsec interferometric field available at the instruments entrance. This capability is the result of a careful analysis pursued from the very beginning of the VLTI until today in the interferometric laboratory layout. For this goal it has been necessary to develop a new optical device, the Variable Curvature Mirror (VCM), and also to design all the optical systems located after the delay-lines, as the beam compressors for instance, according to these interferometric field-of-view and pupil transfer requirements. This pupil transfer and the role/design of the various optical systems are presented for the major configurations of the VLTI. A special section is dedicated to the VCM system as this component is the most critical one and required special studies, using large deformation theory of elasticity, and advanced techniques in optical fabrication. The final performances of the VCM are reviewed. As these performances had an important influence ont he design of the other systems in the interferometric laboratory, the trade-off between the instruments requirements and the VCM capabilities is presented.

A variable curvature mirror is a powerful device that can increase the field of view of optical interferometers. Such a mirror has being developed for the coherent combined focus of the European Southern Observatory Very Large Telescope Interferometer. The variable focal length permits positioning of the pupil image of an individual telescope at a precise location after the delay-line. This property is necessary to exactly remap homothetically the output pupil configuration at the image beam combiner. Given the large zoom range that is needed in the delay line, when the mirror is not stressed the optical surface is a plane while it is convex with f/2.5 at maximum stress. The mirror itself is a very small stainless steel meniscus, with a 300 μm thickness, because only the high elasticity of such material allows to achieve the full domain of curvature. The thickness distribution of the meniscus is calculated using elasticity theory in the case of a large deformation. The realization of this micro-optic active device requires advanced techniques in optical fabrication and in particular high precision manufacturing with numerical command lathe. This article also presents the testing of this highly Variable Curvature Mirror (VCM) and the surface quality obtained within the full curvature range.

The interferometric mode of the ESO very large telescope (VLT) permits coherent combination of stellar light beams collected by four telescopes with 8 m diameter and by several auxiliary telescopes of the 2 m class. While the position of the 8 m telescopes is fixed, auxiliary telescopes can be moved on rails, and can operate from 30 stations distributed on the top of the observatory site for efficient UV coverage. Coherent beam combination can be achieved with the 8 m telescopes alone, with the auxiliary telescopes alone, or with any combination, up to eight telescopes in total. A distinct feature of the interferometric mode is the high sensitivity due to the 8 m pupil of the main telescopes, with the potential for adaptive optics compensation in the near- infrared spectral regime. The VLT interferometer is conceived as an evolutionary program where a significant fraction of the interferometer's functionality is initially funded, and more capability may be added later while experience is gained and further funding becomes available. The scientific program is now defined by a team which consists of a VLTI scientist at ESO and fifteen astronomers from the VLT community. ESO has recently decided to resume the construction of the VLTI which was delayed in December 1993, in order to achieve first interferometric fringes with two of the 8 m telescopes around the year 2000, and routine operation with 2 m auxiliary telescopes from 2003 onwards. This paper presents an overview of the recent evolution of the project and its future development.