GRAVITY is a state-of-the-art instrument for near-infrared astrometric interferometry that routinely achieves astrometric accuracy of 30-100 microarcsecond in its phase-referenced dual-field mode. However, its fundamental limit is not yet reached, and can still be improved by several factors. In this paper, we focus on the effect of systematics, and in particular the effect of non-common path aberrations between the science channel and the metrology signal in the GRAVITY Fiber Coupler. Through comprehensive laboratory measurements using a dedicated fiber coupler replica and phase-shifting interferometry at 1908 nm, we have characterized these high-order wavefront errors with nanometer precision in the laboratory. We characterized on-sky the effect of these high-order wavefront imperfections on narrow-angle astrometry, using observations of the binary system GJ65. As part of the GRAVITY+ project, high-precision mirrors designed for nanometer-level surface quality, bringing an order of magnitude improvement over existing GRAVITY injection optics, are currently in production and will be installed in the fiber coupler units by the end of 2027. This upgrade is expected to enable astrometric accuracy at the sub-10 microarcsecond level in dual-field mode with integration times of just a few minutes.

In the GRAVITY+ project, GRAVITY is presently undergoing a series of upgrades to enhance its performance, add wide field capability and thereby expand its sky coverage. Some aspects of these improvements have already been implemented and commissioned by the end of 2021, making them accessible to the community. The augmentation of sky coverage involves increasing the maximum angular separation between the celestial science object and the fringe tracking object from the previous 2 arcseconds (limited by the field of view of the VLTI) to 20 - 30 arcseconds (constrained by atmospheric conditions during observation). Phase 1 of GRAVITY+ Wide utilizes the earlier PRIMA Differential Delay Lines to compensate for the optical path length variation between the science and fringe tracking beams throughout an observation. In phase 2, we are upgrading the existing beam compressors (BC) to integrate optical path length difference compensation directly into the BC. This modification eliminates five optical reflections per beam, thereby enhancing the optical throughput of the VLTI-GRAVITY [GRAPHICS] system and the bandwidth of the vibrational control. We will present the implementation of phase 2 and share preliminary results from our testing activities for GRAVITY+ Wide.

The GRAVITY+ project consists of instrumental upgrades to the Very Large Telescope Interferometer (VLTI) for faint-science, high-contrast, milliarcsecond interferometric imaging. As an integral part of the GRAVITY+ Adaptive Optics (AO) architecture, the Wavefront Sensor (WFS) subsystem corrects image distortions caused by the turbulence of Earth's atmosphere. We present the opto-mechanical design of the WFS subsystem and the design strategies used to implement two payloads positioned diagonally opposite each other - Natural Guide Star (NGS) and Laser Guide Star (LGS) - within a single compact design structure. We discuss the implementation of relative motions of the two payloads covering their respective patrol fields and a nested motion within the LGS Payload covering the complete Sodium layer profile in the Earth's atmosphere.

Fringe stability and tracking are a determining aspect for the performance of current interferometric observations. While the theory predicts that the aperture of large telescopes such as the VLTI UT should yield smoothed-out piston perturbations that could be compensated using a slow fringe tracker running at a few tens of Hz, this is far from the current experimental reality. In practice, the optical path variations observed with the GRAVITY fringe tracker still contain high frequency components that limit the fringe-tracking exposure time and therefore its precision and limiting magnitude. Most of these perturbations seem to come from mechanical vibrations in the train of mirrors, leading to the instrument, and in particular from the mirrors of the telescope. With this work, and as part of the GRAVITY+ efforts, accelerometers were added to all the mirrors of the coude train, including the coming M8, to complement the existing instrumentation of M1, M2, and M3, and compensate in real-time the optical path using the main delay lines. We show how the existing architecture, while optimal for the first mirrors, is not suitable for the vibration content found in the new mirrors, and we opt instead for narrow-band filters based on phase-locked-loop filters (PLL). Thanks to this architecture, we were able to reclaim up to 50nm of OPD RMS from vibrations peaks between 40 and 200Hz. We also outline the avenues to push this approach further, through the upgrade of the deformable mirrors and the beam-compressor differential delay lines (BCDDL) as part of GRAVITY+, paving the way to obtaining better than 100nm RMS fringe tracking, even on faint targets.

The ELT Phasing and Diagnostic Station (PDS), is a multi-purpose optomechanical system providing metrology tools to phase the segmented primary mirror of the ELT and hosting the sensors required to verify AO-assisted diffraction limited image quality at the ELT. The purpose of the PDS in the context of the ELT lifecycle is twofold. On one side, during the AIV phase of the ELT, the PDS will be the fundamental tool enabling commissioning of the telescope. On the other hand, during operation, the PDS will provide the essential metrology means to monitor performance and detect and isolate potential failures within the observatory. The project, which is one of the most important internal development endeavors at ESO, passed PDR in mid-2021 and underwent an optical final design review in late 2021, where challenges associated to the schedule were identified and more time was given to optimize the design. The project underwent a restructuring in early 2022 before starting its final design phase which has been successfully completed in 2023. In the same period all procurements concerning critical long lead items have been launched. The present contribution first introduces the project in the context of the ELT construction programme, outlining the project structure and the project management tools employed for planning and progress monitoring. Subsequently, the main system engineering processes used within the project will be described. Finally, we report on the main technical results obtained during the final design phase and the plans for the assembly, integration and test of the system.

The Very Large Telescope Interferometer (VLTI) is a wonderful infrastructure for long-baseline interferometry. MATISSE, the Multi AperTure mid-Infrared SpectroScopic Experiment, installed at the VLTI focus, accesses high resolution imaging over a wide spectral domain of the mid-infrared. The instrument is a spectro-interferometric imager operating in the L, M, and N transmission windows and combining four optical beams from the VLTI's unit or auxiliary telescopes. We propose at the SPIE conference to advertise the use of the MATISSE instrument. We will illustrate the instrument capabilities through astrophysical results recently achieved (the focus on the astrophysical results is not reported in the article). We also show what are the expected future infrastructure optimizations and instrument adaptations (off-axis tracking, frame of GRAVITY+) that will permit to push the sensitivities and accuracies for the astrophysical programs in the context of the JWST.

The GRAVITY instrument has been revolutionary for near-infrared interferometry by pushing sensitivity and precision to previously unknown limits. With the upgrade of GRAVITY and the Very Large Telescope Interferometer (VLTI) in GRAVITY+, these limits will be pushed even further, with vastly improved sky coverage, as well as faint-science and high-contrast capabilities. This upgrade includes the implementation of wide-field off-axis fringe-tracking, new adaptive optics systems on all Unit Telescopes, and laser guide stars in an upgraded facility. GRAVITY+ will open up the sky to the measurement of black hole masses across cosmic time in hundreds of active galactic nuclei, use the faint stars in the Galactic centre to probe General Relativity, and enable the characterisation of dozens of young exoplanets to study their formation, bearing the promise of another scientific revolution to come at the VLTI.

As part of the GRAVITY^+ project, the near-infrared beam combiner GRAVITY and the VLTI are currently undergoing a series of significant upgrades to further improve the performance and sky coverage. The instrumental changes will be transformational, and for instance uniquely position GRAVITY to observe the broad line region of hundreds of Active Galactic Nuclei (AGN) at a redshift of two and higher. The increased sky coverage is achieved by enlarging the maximum angular separation between the celestial science object (SC) and the off-axis fringe tracking (FT) star from currently 2 arcseconds (arcsec) up to unprecedented 30 arcsec, limited by the atmospheric conditions. This was successfully demonstrated at the VLTI for the first time.

After the pause imposed by the pandemic, VLTI resumed science operations and restarted technical activities aiming to close commissionings of different modes. While the community develops projects of visiting instruments, the VLTI infrastructure is about to be significantly upgraded with new visible AO and laser guide star systems by the GRAVITY+ project. VLTI operations also evolve, in particular to support imaging programmes, but also towards a more automated and integrated model. In this context, we will present a review of current capabilities, ongoing activities and future plans for the VLTI.

The STELLIM interferometer aims at providing fast and reliable imaging of the surface of cool evolved stars by combining the light of 13 telescopes in the visible. It is designed to take advantage of the existing VLTI infrastructure. Optical fibers carry the light from the telescopes to the VLTI delay line tunnels where the geometrical optical path lengths are compensated. Using a mirror switchyard, several possible combinations of 7 telescopes are selected in turn and their light is recombined on a dedicated table in the VLTI laboratory. One single night of observations is enough to reconstruct a reliable interferometric snapshot image. STELLIM aims at producing stellar surface images at a high temporal cadence. It will deliver a complete spatio-temporal characterization of surface and circumstellar environment structures of bright evolved stars, which is pivotal to understanding the way those stars lose their mass in the interstellar environment and their evolution into supernovae. The mission of STELLIM is to push the limits of the description of stellar evolution and stellar activity with milli-arcsecond resolution images. Moreover, as an interferometric imaging platform recombining 10+ apertures, it also paves the way for future astronomical facilities. In this contribution, we will present the design and the steps we took to progress towards a STELLIM demonstrator.

We present the current status of the I2C stellar intensity interferometer used towards high angular resolution observations of stars in visible wavelengths. In these proceedings, we present recent technical improvements to the instrument, and share results from ongoing campaigns using arrays of small diameter optical telescopes. A tip-tilt adaptive optics unit was integrated into the optical system to stabilize light injection into an optical fiber. The setup was successfully tested with several facilities on the Calern Plateau site of the Observatoire de la Côte d’Azur. These include one of the 1m diameter telescopes of the C2PU observatory, a portable 1m diameter telescope, and also the 1.5m MéO telescope. To better constrain on-sky measurements, the spectral transmission of instrument was characterized in the laboratory using a high resolution spectrograph. The system was also tested with two of the auxiliary telescopes of the VLTI resulting in successful temporal and spatial correlation measurements of three stars.

Context:Optical interferometry is at a key development stage. ESO's VLTI has established a stable, robust infrastructure for long-baseline interferometry for general astronomical observers. The present second-generation instruments offer a wide wavelength coverage and improved performance. Their sensitivity and measurement accuracy lead to data and images of high reliability. Aims:We have developed MATISSE, the Multi AperTure mid-Infrared SpectroScopic Experiment, to access high resolution imaging in a wide spectral domain and explore topics such: stellar activity and mass loss; planet formation and evolution in the gas and dust disks around young stars; accretion processes around super massive black holes in AGN. Methods:The instrument is a spectro-interferometric imager covering three atmospheric bands (L,M,N) from 2.8 to 13.0 mu, combining four optical beams from the VLTI's telscopes. Its concept, related observing procedure, data reduction and calibration approach are the product of 30 years of instrumental research. The instrument utilizes a multi-axial beam combination that delivers spectrally dispersed fringes. The signal provides the following quantities at several spectral resolutions: photometric flux, coherent fluxes, visibilities, closure phases, wavelength differential visibilities and phases, and aperture-synthesis imaging. Results:We provide an overview of the physical principle of the instrument and its functionalities, the characteristics of the delivered signal, a description of the observing modes and of their performance limits. An ensemble of data and reconstructed images are illustrating the first acquired key observations. Conclusion:The instrument has been in operation at Cerro Paranal, ESO, Chile since 2018, and has been open for science use by the international community since April 2019. The first scientific results are being published now.

We present the successful demonstration of world's first large-separation ~30" off-axis fringe tracking with four telescopes in October 2019. With this technique we increase the sky-coverage for optical interferometry by orders of magnitude compared to current technology. Following the early work at the Palomar Testbed Interferometer, the first demonstration of off-axis fringe tracking at the Keck Interferometer and with PRIMA at the ESO Very Large Telescope Interferometer, and the breakthrough with the GRAVITY Galactic Center observations, we enhanced the VLTI infrastructure for GRAVITY to take advantage of the PRIMA Star separators and Differential Delay Lines for off-axis fringe tracking. In our presentation we give an introduction to the subject, present the enhancements of the VLTI, and present our results from the first on-sky operation in October 2019, with observations of the Orion Trapezium Cluster, a field brown dwarf, and a high redshift quasar.

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.