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Laser guide star (LGS) wave-front sensing (LGSWFS) is a key element of tomographic adaptive optics system. However, when considering Extremely Large Telescope (ELT) scales, the LGS spot elongation becomes so large that it challenges the standard recipes to design LGSWFS. For classical Shack-Hartmann wave-front sensor (SHWFS), which is the current baseline for all ELT LGS-assisted instruments, a trade-off between the pupil spatial sampling [number of sub-apertures (SAs)], the SA field-of-view (FoV) and the pixel sampling within each SA is required. For ELT scales, this trade-off is also driven by strong technical constraints, especially concerning the available detectors and in particular their number of pixels. For SHWFS, a larger field of view per SA allows mitigating the LGS spot truncation, which represents a severe loss of performance due to measurement biases. For a given number of available detectors pixels, the SA FoV is competing with the proper sampling of the LGS spots, and/or the total number of SAs. We proposed a sensitivity analysis, and we explore how these parameters impacts the final performance. In particular, we introduce the concept of super resolution, which allows one to reduce the pupil sampling per WFS and opens an opportunity to propose potential LGSWFS designs providing the best performance for ELT scales., Comment: 21 pages, 11 figures, 1 table, part of the 2022 JATIS Special Section on Extremely Large Telescopes

The MCAO Assisted Visible Imager and Spectrograph (MAVIS) is a facility-grade visible MCAO instrument, currently under development for the Adaptive Optics Facility at the VLT. The adaptive optics system will feed both an imager and an integral field spectrograph, with unprecedented sky coverage of 50% at the Galactic Pole. The imager will deliver diffraction-limited image quality in the V band, cover a 30" x 30" field of view, with imaging from U to z bands. The conceptual design for the spectrograph has a selectable field-of-view of 2.5" x 3.6", or 5" x 7.2", with a spatial sampling of 25 or 50 mas respectively. It will deliver a spectral resolving power of R=5,000 to R=15,000, covering a wavelength range from 380 - 950 nm. The combined angular resolution and sensitivity of MAVIS fill a unique parameter space at optical wavelengths, that is highly complementary to that of future next-generation facilities like JWST and ELTs, optimised for infrared wavelengths. MAVIS will facilitate a broad range of science, including monitoring solar system bodies in support of space missions; resolving protoplanetary- and accretion-disk mechanisms around stars; combining radial velocities and proper motions to detect intermediate-mass black holes; characterising resolved stellar populations in galaxies beyond the local group; resolving galaxies spectrally and spatially on parsec scales out to 50 Mpc; tracing the role of star clusters across cosmic time; and characterising the first globular clusters in formation via gravitational lensing. We describe the science cases and the concept designs for the imager and spectrograph.

A consortium of several Australian and European institutes – together with the European Southern Observatory (ESO) – has initiated the design of MAVIS, a Multi-Conjugate Adaptive Optics (MCAO) system for the ground- based 8-m Very Large Telescope (VLT). MAVIS (MCAO-assisted Visible Imager and Spectrograph) will deliver visible images and integral field spectrograph data with 2-3x better angular resolution than the Hubble Space Telescope, making it a powerful complement at visible wavelengths to future facilities like the space-based James Webb Space Telescope and the 30 to 40m-class ground-based telescopes currently under construction, which are all targeting science at near-infrared wavelengths. MAVIS successfully passed its Phase A in May 2020. We present the motivations, requirements, principal design choices, conceptual design, expected performance and an overview of the exciting science enabled by MAVIS.

MAORY is a post-focal adaptive optics module that forms part of the first light instrument suite for the ELT. The main function of MAORY is to relay the light beam from the ELT focal plane to the client instrument while compensating the effects of the atmospheric turbulence and other disturbances affecting the wavefront from the scientific sources of interest.

Adaptive optics (AO) techniques like laser tomography adaptive optics (LTAO) and multi-conjugated adaptive optics (MCAO) are becoming more and more used in the development of instrumentation for the existing 8-m class telescopes and future extremely large telescopes (ELT). Achieving the required level of AO correction over wide field of view requires the knowledge of the turbulence strength vs the altitude, first during the design phase of the instruments to ensure they match the needed performances, and then during the on-sky operation to optimize the AO control according to the atmospheric conditions. Obtaining reliable measurements of the turbulence in three dimensions is thus of high importance, today and in the future. The Paranal Observatory gives access to three atmosphere profilers and one atmosphere monitor on one single site. The possibility to obtain simultaneous data on independent systems is a great opportunity to compare their results and evaluate the advantages of each method. We present the results of statistical analysis of data coming from these systems, obtained in different periods and with various atmosphere conditions. From this analysis, we can draw conclusions on conditions needed for a useful comparison of the profilers and show the possible needed attempt to be made on aligning their assumptions and thus the provided parameters.

Following the arrival of MATISSE, the second-generation of VLTI instrumentation is now complete and was simultaneously enhanced by a major facility upgrade including the NAOMI Adaptive Optics on the Auxiliary Telescopes. On the Unit Telescopes, significant efforts were also made to improve the injection stability into VLTI instruments. On top of GRAVITY's own evolution, its fringe tracker is now being used to allow coherent integrations on MATISSE (the so-called GRA4MAT project). Meanwhile, operations also evolved to be more flexible and make the most of an extended observing parameter space. In this context, we present an overview of the current VLTI performances. Finally, we will report on on-going improvements such as the extension of the longest baselines.

The Adaptive Optics Module of MAVIS is a self-contained MCAO module, which delivers a corrected FoV to the postfocal scientific instruments, in the visible. The module aims to exploit the full potential of the ESO VLT UT4 Adaptive Optics Facility, which is composed of the high spatial frequency deformable secondary mirror and the laser guide stars launching and control systems. During the MAVIS Phase A, we evaluated, with the support of simulations and analysis at different levels, the main terms of the error budgets aiming at estimating the realistic AOM performance. After introducing the current opto-mechanical design and AO scheme of the AOM, we here present the standard wavefront error budget and the other budgets, including manufacturing, alignment of the module, thermal behavior and noncommon path aberrations, together with the contribution of the upstream telescope system.

The Very Large Telescope Interferometer Auxiliary Telescopes will soon be equipped with an adaptive optics system called NAOMI. The corrective optics deformable mirror is the commercial DM241 from ALPAO. Being part of an interferometer operating from visible to mid-infrared, the DMs of NAOMI face several challenges (high level of reliability, open-loop chopping, piston-free control, WFS/DM pupil rotation, high desired bandwidth and stroke). We here describe our extensive characterization of the DMs through measurements and simulations. We summarize the operational scenario we have defined to handle the specific mirror properties. We conclude that the ALPAO DMs have overall excellent properties that fulfill most of the stringent requirements and that deviations from specifications are easily handled. To our knowledge, NAOMI will be the first astronomical system with a command in true Zernike modes (allowing software rotation), and the first astronomical system in which a chopping is performed with the deformable mirror (5” sky, at 5 Hz).

The long commissioning of the Adaptive Optics Facility (AOF) project has been completed shortly after this conference, providing AO correction to two Very Large Telescope (VLT) foci supported by an adaptive secondary mirror and four laser guide stars. Four AO modes are delivered: a Single Conjugate AO (SCAO) system for commissioning purpose, wide field and medium field Ground Layer AO (GLAO) for seeing improvement and narrow field Laser Tomography AO (LTAO) for ultimate performance. This paper intends to describe the implemented AO baseline and to highlight the most relevant results and lessons learned. In particular, it will address the control and reconstruction strategy, the wavefront sensing baseline and the online telemetry used to optimize the system online, estimate the turbulence profile and calibrate the misregistrations. Focusing on the LTAO mode, we will describe the tomography optimization, by exploring the reconstruction parameter space. Finally, on sky performance results will be presented both in terms of strehl ratio and limiting magnitude.

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 New Adaptive Optics Module for Interferometry (NAOMI) is ready to be installed at the 1.8-metre Auxiliary Telescopes (ATs) at ESO Paranal. NAOMI will make the existing interferometer performance less dependent on the seeing conditions. Fed with higher and more stable Strehl, the fringe tracker will achieve the fringe stability necessary to reach the full performance of the second-generation instruments GRAVITY and MATISSE. All four ATs will be equipped between September and November 2018 with a Deformable mirror (ALPAO DM-241), a 4*4 Shack– Hartmann adaptive optics system operating in the visible and an RTC based on SPARTA Light. During the last 6 months thorough system test has been made in laboratory to demonstrate the Adaptive Optics and chopping capability of NAOMI.

In the framework of the preparation of the Darwin mission, Alcatel Space has been developing a Multi- Aperture Imaging Interferometer (MAI²) for ESA. The main purpose of this activity is to achieve a deep extinction rate, or "nulling", in the laboratory. The performance goal is a 106 nulling of a simulated star. A second objective is to image a planet-like source. The selected operating wavelength is centred around 1.55 microns, with a relative bandwidth of a few percent. The selected function conceptual designs can be updated to the Darwin spectral range (6-18μm).

Astronomical aperture synthesis requires to combine beams coming from telescopes, with constraints on mechanical and thermal stability, accuracy on the measurement of the interferences visibility. One adapted way for solving the problem is integrated planar optics. A first two telescope beam combiner made by ion exchange technique on glass substrate and build with symmetric Y-junction provides laboratory white light interferograms simultaneously with photometric calibration. In order to increase the interferometric signal without loss of photometric output, we propose to replace symmetric Y-junctions by asymmetric ones. In this paper, we report the conception, the manufacturing and the characterization of asymmetric Y-junction realized by ion exchange on glass substrate. The specific application of astronomical interferometry required the characterization of such component in term of spectral behavior, so we report the simulation and the measurement of asymmetric Y-junction response versus wavelength.

This project, realized as part of a CNES/CSO Mesure contract, concerns the study and the industrial realization of a sensor aimed at the relative position control of space telescope mirrors.

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.

For two years starting in February 2014, the AO modules GRAAL for HAWK-I and GALACSI for MUSE of the Adaptive Optics Facility project have undergone System Testing at ESO's Headquarters. They offer four different modes: NGS SCAO, LGS GLAO in the IR, LGS GLAO and LTAO in the visible. A detailed characterization of those modes was made possible by the existence of ASSIST, a test bench emulating an adaptive VLT including the Deformable Secondary Mirror, a star simulator and turbulence generator and a VLT focal plane re-imager. This phase aimed at validating all the possible components and loops of the AO modules before installation at the actual VLT that comprises the added complexity of real LGSs, a harsher non-reproducible environment and the adaptive telescope control. In this paper we present some of the major results obtained and challenges encountered during the phase of System Tests, like the preparation of the Acquisition sequence, the testing of the Jitter loop, the performance optimization in GLAO and the offload of low-order modes from the DSM to the telescope (restricted to the M2 hexapod). The System Tests concluded with the successful acceptance, shipping, installation and first commissioning of GRAAL in 2015 as well as the acceptance and shipping of GALACSI, ready for installation and commissioning early 2017.

GRAAL is the adaptive optics module feeding the wide-field IR imager HAWK-I at the VLT observatory. As part of the adaptive optics facility, GRAAL is equipped with 4 Laser-guide star wave-front sensors and provides a large field-of-view, ground layer correction system to HAWK-I. After a successful testing in Europe, the module has been re-assembled in Chile and installed at the Nasmyth-A platform of Yepun, the fourth Unit telescope of the observatory. We report on the installation of GRAAL on the mountain and on its first testing in stand-alone and on-sky.

The MACAO curvature wavefront sensors have been designed as a generic adaptive optics sensor for the Very Large Telescope. Six systems have been manufactured and implemented on sky: four installed in the UTs Coude train as an AO facility for the VLTI, and two in UT’s instruments, SINFONI and CRIRES. The MACAO-VLTI have now been in use for scientific operation for more than a decade and are planned to be operated for at least ten more years. As second generation instruments for the VLTI were planned to start implementation in end of 2015, accompanied with a major upgrade of the VLTI infrastructure, we saw it as a good time for a rejuvenation project of these systems, correcting the obsolete components. This obsolescence correction also gave us the opportunity to implement improved capabilities: the correction frequency was pushed from 420 Hz to 1050 Hz, and an automatic vibrations compensation algorithm was added. The implementation on the first MACAO was done in October 2014 and the first phase of obsolescence correction was completed in all four MACAO-VLTI systems in October 2015 with the systems delivered back to operation. The resuming of the scientific operation of the VLTI on the UTs in November 2015 allowed to gather statistics in order to evaluate the improvement of the performances through this upgrade. A second phase of obsolescence correction has now been started, together with a global reflection on possible further improvements to secure observations with the VLTI.

International audience; GRAVITY is a near-infrared interferometric instrument that allows astronomers to combine the light of the four unit or four auxiliary telescopes of the ESO Very Large Telescope in Paranal, Chile. GRAVITY will deliver extremely precise relative astrometry and spatially resolved spectra. In order to study objects in regions of high extinction (e.g. the Galactic Center, or star forming regions), GRAVITY will use infrared wavefront sensors. The suite of four wavefront sensors located in the Coudé room of each of the unit telescopes are known as the Coudé Integrated Adaptive Optics (CIAO). The CIAO wavefront sensors are being constructed by the Max Planck Institute for Astronomy (MPIA) and are being installed and commissioned at Paranal between February and September of 2016. This presentation will focus on system tests performed in the MPIA adaptive optics laboratory in Heidelberg, Germany in preparation for shipment to Paranal, as well as on-sky data from the commissioning of the first instrument. We will discuss the CIAO instruments, control strategy, optimizations, and performance at the telescope.

MATISSE is the second-generation mid-infrared spectrograph and imager for the Very Large Telescope Interferometer (VLTI) at Paranal. This new interferometric instrument will allow significant advances by opening new avenues in various fundamental research fields: studying the planet-forming region of disks around young stellar objects, understanding the surface structures and mass loss phenomena affecting evolved stars, and probing the environments of black holes in active galactic nuclei. As a first breakthrough, MATISSE will enlarge the spectral domain of current optical interferometers by offering the L and M bands in addition to the N band. This will open a wide wavelength domain, ranging from 2.8 to 13 um, exploring angular scales as small as 3 mas (L band) / 10 mas (N band). As a second breakthrough, MATISSE will allow mid-infrared imaging - closure-phase aperture-synthesis imaging - with up to four Unit Telescopes (UT) or Auxiliary Telescopes (AT) of the VLTI. Moreover, MATISSE will offer a spectral resolution range from R ~ 30 to R ~ 5000. Here, we present one of the main science objectives, the study of protoplanetary disks, that has driven the instrument design and motivated several VLTI upgrades (GRA4MAT and NAOMI). We introduce the physical concept of MATISSE including a description of the signal on the detectors and an evaluation of the expected performances. We also discuss the current status of the MATISSE instrument, which is entering its testing phase, and the foreseen schedule for the next two years that will lead to the first light at Paranal., Comment: SPIE Astronomical Telescopes and Instrumentation conference, June 2016, 11 pages, 6 Figures

Rather than using an adaptive optics (AO) system to correct a telescope s entire pupil, it can instead be used to more finely correct a smaller sub-aperture. Indeed, existing AO systems can be used to correct a sub-aperture 1/3 to 1/2 the size of a 5-10 m telescope to extreme adaptive optics (ExAO) levels. We discuss the potential performance of a clear off-axis well-corrected sub-aperture (WCS), and describe our initial imaging results with a 1.5 m diameter WCS on the Palomar Observatory s Hale telescope. These include measured Strehl ratios of 0.92-0.94 in the infrared (2.17 microns), and 0.12 in the B band, the latter allowing a binary of separation 0.34 arc sec to be easily resolved in the blue. Such performance levels enable a variety of novel observational modes, such as infrared ExAO, visible-wavelength AO, and high-contrast coronagraphy. One specific application suggested by the high Strehl ratio stability obtained (1%) is the measurement of planetary transits and eclipses. Also described is a simple dark-hole experiment carried out on a binary star, in which a comatic phase term was applied directly to the deformable mirror, in order to shift the diffraction rings to one side of the point spread function., Comment: accepted by ApJ

Using the 3-telescope IOTA interferometer on Mt. Hopkins, we report results from the first near-infrared (lambda=1.65 mu) closure-phase survey of Young Stellar Objects (YSOs). These closure phases allow us to unambiguously detect departures from centrosymmetry (i.e., skew) in the emission pattern from YSO disks on the scale of ~4 milliarcseconds, expected from generic ``flared disk'' models. Six of fourteen targets showed small, yet statistically-significant, non-zero closure phases, with largest values from the young binary system MWC 361-A and the (pre-main sequence?) Be star HD 45677. Our observations are quite sensitive to the vertical structure of the inner disk and we confront the predictions of the ``puffed-up inner wall'' models of Dullemond, Dominik, and Natta (DDN). Our data support disks models with curved inner rims because the expected emission appear symmetrically-distributed around the star over a wide range of inclination angles. In contrast, our results are incompatible with the models possessing vertical inner walls because they predict extreme skewness (i.e., large closure phases) from the near-IR disk emission that is not seen in our data. In addition, we also present the discovery of mysterious H-band ``halos'' (~5-10% of light on scales 0.01-0.50 arcsec) around a few objects, a preliminary ``parametric imaging'' study for HD 45677, and the first astrometric orbit for the young binary MWC 361-A., Comment: Accepted for publication in Astrophysical Journal

In the frame of ESA's DARWIN mission technological studies, Alcatel Space, supported by ESA, is designing and manufacturing a nulling interferometer whose aim is to demonstrate in laboratory the extinction of a point source with a stable nulling ratio of 10 -6 . The results of the first integration phase of this breadboard is presented in this paper.

In the frame of ESA's DARWIN mission technological studies, Alcatel Space, supported by ESA, is designing and manufacturing a nulling interferometer whose aim is to demonstrate in laboratory the extinction of a point source with a stable nulling ratio of 10 -6 . This breadboard is presented with emphasis on the integrated optics beam combiner.

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. After the first fringes obtained in 2011 with the commissioning instrument VINCI and with siderostats, the VLTI has seen an important number of systems upgrades, paving the path towards reaching the infrastructure level and scientific results it had been designed for. The current status of the VLTI operation all year round with up to four telescopes simultaneously and real imaging capability demonstrates the powerful interferometric infrastructure that has been delivered to the astronomical community. Reaching today’s level of robustness and operability of the VLTI has been a long journey, with a lot of lessons learned and gained experience. In 2007, the Paranal Observatory recognized the need for a global system approach for the VLTI, and a dedicated system engineering team was set to analyse the status of the interferometer, identify weak points and area where performances were not met, propose and apply solutions. The gains of this specific effort can be found today in the very good operability level with faster observations executions, in the decreased downtime, in the improved performances, and in the better reliability of the different systems. We will present an historical summary of the system engineering effort done at the VLTI, showing the strategy used, and the implemented upgrades and technical solutions. Improvements in terms of scientific data quality will be highlighted when possible. We will conclude on the legacy of the VLTI system engineering effort, for the VLTI and for future systems.

MATISSE is the mid-infrared spectrograph and imager for the Very Large Telescope Interferometer (VLTI) at Paranal. This second generation interferometry instrument will open new avenues in the exploration of our Universe. Mid-infrared interferometry with MATISSE will allow significant advances in various fundamental research fields: studies of disks around young stellar objects where planets form and evolve, surface structures and mass loss of stars in late evolutionary stages, and the environments of black holes in active galactic nuclei. MATISSE is a unique instrument. As a first breakthrough it will enlarge the spectral domain used by optical interferometry by offering the L & M bands in addition to the N band, opening a wide wavelength domain, ranging from 2.8 to 13 μm on angular scales of 3 mas (L/M band) / 10 mas (N band). As a second breakthrough, it will allow mid-infrared imaging – closure-phase aperture-synthesis imaging – with up to four Unit Telescopes (UT) or Auxiliary Telescopes (AT) of the VLTI. MATISSE will offer various ranges of spectral resolution between R~30 to ~5000. In this article, we present some of the main science objectives that have driven the instrument design. We introduce the physical concept of MATISSE including a description of the signal on the detectors and an evaluation of the expected performance and discuss the project status. The operations concept will be detailed in a more specific future article, illustrating the observing templates operating the instrument, the data reduction and analysis, and the image reconstruction software.

The ESO Very Large Telescope Interferometer (VLTI) using the Unit Telescope (UT) was strongly affected by vibrations since the first observations. Investigation by ESO on that subject had started in 2007, with a considerable effort since mid 2008. An important number of investigations on various sub-systems (On telescope: Guiding, Passive supports, Train Coude, insulation of electronics cabinets; On Instruments: dedicated campaign on each instruments with a special attention on the ones equipped with Close Cycle Cooler) were realized. Vibrations were not only recorded and analyzed using the usual accelerometers but also using on use sub-systems as InfRared Image Sensor (IRIS) and Multiple Applications Curvature Adaptive Optics (MACAO) and using a specific tool developed for vibrations measurements Mirror vibrAtion Metrology systeM for the Unit Telescope (MAMMUT). Those tools and systems have been used in order to improve the knowledge on telescope by finding sources. The sources whenever it was possible were damped. As known for years, instruments are still the principal sources of vibrations, for the majority of the UT. A special test in which 2 UTs instruments were completely shut down was realized to determine the minimum Optical Path Length (OPL) achievable. Vibrations is now a part of the instruments interface document and during the installation of any new instrument (KMOS) or system (AOF) a test campaign is realized. As a result some modifications (damping of CCC) can be asked in case of non-compliance. To ensure good operational conditions, levels of vibrations are regularly recorded to control any environmental change.

We present the latest update of the European Southern Observatory's Very Large Telescope interferometer (VLTI). The operations of VLTI have greatly improved in the past years: reduction of the execution time; better offering of telescopes configurations; improvements on AMBER limiting magnitudes; study of polarization effects and control for single mode fibres; fringe tracking real time data, etc. We present some of these improvements and also quantify the operational improvements using a performance metric. We take the opportunity of the first decade of operations to reflect on the VLTI community which is analyzed quantitatively and qualitatively. Finally, we present briefly the preparatory work for the arrival of the second generation instruments GRAVITY and MATISSE., Comment: 10 pages, 7 figures, Proceedings of the SPIE, 9146-19

We present a new Adaptive Phase Mask (APM) coronagraph design enabling Amplitude and Phase Modulation control (APM). The Adaptive Phase mask coronagraph is a technique proposed to provide both high dynamic and high angular resolution imaging of faint sources around bright objects. Discriminating faint sources from static speckles is a challenging problem. Our new system is based on synchronous demodulation that allows high dynamic range detection of a faint target immersed in a background. The APM^2 uses the coherence of speckles to discriminate them from proper companions, using the mask itself as the electric field modulator. Synchronous detection in the radio frequency range is used to side-step the effect of atmospheric turbulence and enable the detection of low amplitude signals. The APM^2 concept offers high dynamic range detection and provides a time- and cost-effective method to quantify the probability of presence of a faint object close to the central star.

We present the concept of an instrument that will create 4 circular sub-pupils of 3 m in diameter. Each sub-pupil path will be corrected by a high order adaptive optics system (SR~80% in H) without spider and M2 obstruction. These four independent channels, obviously all pointed towards the same field, allows the possibility of covering totally different parts of the electromagnetic spectrum simultaneously without compromising Signal to Noise Ratio. Each channel can be dedicated to very specialized but complementary purposes: high contrast imaging, pseudo-wide field imaging, high precision multi-color photometry, medium-resolution spectroscopy, polarimetry and sparse-aperture masking.

A dual coronagraph based on the Adaptive Mask concept is presented in this paper. A Lyot coronagraph with a variable diameter occulting disk and a nulling stellar coronagraph based on the Adaptive Phase Mask concept using polarization interferometry are presented in this work. Observations on sky and numerical simulations show the usefulness of the proposed method to optimize the nulling efficiency of the coronagraphs. In the case of the phase mask, the active control system will correct for the detrimental effects of image instabilities on the destructive interference (low-order aberrations such as tip-tilt and focus). The phase mask adaptability both in size, phase and amplitude also compensate for manufacturing errors of the mask itself, and potentially for chromatic effects. Liquid-crystal properties are used to provide variable transmission of an annulus around the phase mask, but also to achieve the achromatic π phase shift in the core of the PSF by rotating the polarization by 180°.A compressed mercury (Hg) drop is used as an occulting disk for the Lyot mask, its size control offers an adaptation to the seeing conditions and provides an optimization of the Tip-tilt correction.

The visitor instrument PIONIER provides VLTI with improved imaging capabilities and sensitivity. The instrument started routinely delivering scientific data in November 2010, that is less than 12 months after being approved by the ESO Science and Technical Committee. We recall the challenges that had to be tackled to design, built and commission PIONIER. We summarize the typical performances and some astrophysical results obtained so far. We conclude this paper by summarizing lessons learned., Comment: To appear in the proceedings of the 2012 SPIE Conference "Astronomical Telescopes and Instrumentation"

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. Since January 2011 the integration and individual testing of the different subsystem has come to a necessary minimum. At the same time the astrometric commissioning phase has begun. In this contribution we give an update on the status of the facility and present some highlights and difficulties on our way from first dual-feed fringe detection to first astrometric measurements. We focus on technical and operational aspects. In particular, within the context of the latter we are going to present a modified mode of operation that scans across the fringes. We will show that this mode, originally only intended for calibration purposes, facilitates the detection of dual-fringes.

MATISSE is a mid-infrared spectro-interferometer combining the beams of up to four Unit Telescopes or Auxiliary Telescopes of the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory. MATISSE will constitute an evolution of the two-beam interferometric instrument MIDI. New characteristics present in MATISSE will give access to the mapping and the distribution of the material, the gas and essentially the dust, in the circumstellar environments by using the mid-infrared band coverage extended to L, M and N spectral bands. The four beam combination of MATISSE provides an efficient uv-coverage: 6 visibility points are measured in one set and 4 closure phase relations which can provide aperture synthesis images in the mid-infrared spectral regime. We give an overview of the instrument including the expected performances and a view of the Science Case. We present how the instrument would be operated. The project involves the collaborations of several agencies and institutes: the Observatoire de la Cote d’Azur of Nice and the INSU-CNRS in Paris, the Max Planck Institut fur Astronomie of Heidelberg; the University of Leiden and the NOVA-ASTRON Institute of Dwingeloo, the Max Planck Institut fur Radioastronomie of Bonn, the Institut fur Theoretische Physik und Astrophysik of Kiel, the Vienna University and the Konkoly Observatory.

Context. Phase-mask coronagraphy is advantageous in terms of inner working angle and discovery space. It is however still plagued by drawbacks such as sensitivity to tip-tilt errors and chromatism. A nulling stellar coronagraph based on the adaptive phase-mask concept using polarization interferometry is presented in this paper. Aims. Our concept aims at dynamically and achromatically optimizing the nulling efficiency of the coronagraph, making it more immune to fast low-order aberrations (tip-tilt errors, focus, ...). Methods. We performed numerical simulations to demonstrate the value of the proposed method. The active control system will correct for the detrimental effects of image instabilities on the destructive interference. The mask adaptability both in size, phase and amplitude also compensates for manufacturing errors of the mask itself, and potentially for chromatic effects. Liquid-crystal properties are used to provide variable transmission of an annulus around the phase mask, but also to achieve the achromatic {\pi} phase shift in the core of the PSF by rotating the polarization by 180 degrees. Results. We developed a new concept and showed its practical advantages using numerical simulations. This new adaptive implementation of the phase-mask coronagraph could advantageously be used on current and next-generation adaptive optics systems, enabling small inner working angles without compromising contrast., Comment: 7 pages, 6 figures

Context. A new four-telescope interferometric instrument called PIONIER has recently been installed at VLTI. It provides improved imaging capabilities together with high precision. Aims. We search for low-mass companions around a few bright stars using different strategies, and determine the dynamic range currently reachable with PIONIER. Methods. Our method is based on the closure phase, which is the most robust interferometric quantity when searching for faint companions. We computed the chi^2 goodness of fit for a series of binary star models at different positions and with various flux ratios. The resulting chi^2 cube was used to identify the best-fit binary model and evaluate its significance, or to determine upper limits on the companion flux in case of non detections. Results. No companion is found around Fomalhaut, tau Cet and Regulus. The median upper limits at 3 sigma on the companion flux ratio are respectively of 2.3e-3 (in 4 h), 3.5e-3 (in 3 h) and 5.4e-3 (in 1.5 h) on the search region extending from 5 to 100 mas. Our observations confirm that the previously detected near-infrared excess emissions around Fomalhaut and tau Cet are not related to a low-mass companion, and instead come from an extended source such as an exozodiacal disk. In the case of del Aqr, in 30 min of observation, we obtain the first direct detection of a previously known companion, at an angular distance of about 40 mas and with a flux ratio of 2.05e-2 \pm 0.16e-2. Due to the limited u,v plane coverage, its position can, however, not be unambiguously determined. Conclusions. After only a few months of operation, PIONIER has already achieved one of the best dynamic ranges world-wide for multi-aperture interferometers. A dynamic range up to about 1:500 is demonstrated, but significant improvements are still required to reach the ultimate goal of directly detecting hot giant extrasolar planets., 11 pages, 6 figures, accepted for publication in A&A

PIONIER (Precision Integrated Optics Near-infrared Imaging ExpeRiment) is a visitor instrument [1] that performs the interferometric beam combination in the near infrared of the light delivered by the 4 main telescopes (8m) or auxiliary telescopes (1.8m) of the VLTI on the ESO Paranal site (Chile).

The fringe sensor unit (FSU) is the central element of the phase referenced imaging and micro-arcsecond astrometry (PRIMA) dual-feed facility for the Very Large Telescope interferometer (VLTI). It has been installed at the Paranal observatory in August 2008 and is undergoing commissioning and preparation for science operation. Commissioning observations began shortly after installation and first results include the demonstration of spatially encoded fringe sensing and the increase in VLTI limiting magnitude for fringe tracking. However, difficulties have been encountered because the FSU does not incorporate real-time photometric correction and its fringe encoding depends on polarisation. These factors affect the control signals, especially their linearity, and can disturb the tracking control loop. To account for this, additional calibration and characterisation efforts are required. We outline the instrument concept and give an overview of the commissioning results obtained so far. We describe the effects of photometric variations and beam-train polarisation on the instrument operation and propose possible solutions. Finally, we update on the current status in view of the start of astrometric science operation with PRIMA., 12 pages, 11 figures, SPIE 2010 conference proceedings

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.