ESO’s Very Large Telescope Interferometer has a history of record-breaking discoveries in astrophysics and significant advances in instrumentation. The next leap forward is its new visitor instrument, called Asgard. It comprises four natively collaborating instruments: HEIMDALLR, an instrument performing both fringe tracking and stellar interferometry simultaneously with the same optics, operating in the K band; Baldr, a Strehl optimizer in the H band; BIFROST, a spectroscopic combiner to study the formation processes and properties of stellar and planetary systems in the Y-J-H bands; and NOTT, a nulling interferometer dedicated to imaging nearby young planetary systems in the L band. The suite is in its integration phase in Europe and should be shipped to Paranal in 2025. In this article, we present details of the alignment and calibration unit, the observing modes, the integration plan, the software architecture, and the roadmap to completion of the project.
MATISSE is the 2nd generation mid-infrared (3.0μm to 12.0μm) spectro-interferometric instrument of the Very Large Telescope Interferometer (VLTI). It was designed to deliver its advertised performance when supported by an external fringe tracker. This proceeding gives an historical account of how the fringe tracker of the GRAVITY instrument, another 2nd generation K-band spectro-astrometric instrument of VLTI, became this external fringe tracker. For a more technical and performance-oriented description of the GRAVITY for MATISSE project, Woillez, Petrov, et al. (2024) should be consulted.
NOTT (formerly Hi-5) is the L’-band (3.5-4.0μm) nulling interferometer of Asgard, an instrument suite in preparation for the VLTI visitor focus. The primary scientific objectives of NOTT include characterizing (i) young planetary systems near the snow line, a critical region for giant planet formation, and (ii) nearby mainsequence stars close to the habitable zone, with a focus on detecting exozodiacal dust that could obscure Earthlike planets. In 2023-2024, the final warm optics have been procured and assembled in a new laboratory at KU Leuven. First fringes and null measurements were obtained using a Gallium Lanthanum Sulfide (GLS) photonic chip that was also tested at cryogenic temperatures. In this paper, we present an overall update of the NOTT project with a particular focus on the cold mechanical design, the first results in the laboratory with the final NOTT warm optics, and the ongoing Asgard integration activities. We also report on other ongoing activities such as the characterization of the photonic chip (GLS, LiNbO3, SiO), the development of the exoplanet science case, the design of the dispersion control module, and the progress with the self-calibration data reduction software.
BIFROST is the short-wavelength, high-spectral resolution instrument in the Asgard Suite of VLTI visitor instruments. It will be optimized for spectral line studies in the Y, J, and H bands (1.05-1.75 μm) that include many strong lines & molecular features. In this presentation, we outline the BIFROST science drivers that have guided our design choices and map them against the operational modes that are being implemented. We give an overview about the status of the project and the milestones from the ongoing integration & testing phase in Exeter to shipping & commissioning on Paranal, scheduled for 2025 and 2026. We review the BIFROST subsystems and discuss how they interface with the broader Asgard Suite. Finally, we outline other BIFROST-related activities pursued by our group that are intended for implementation in BIFROST as part of future upgrades.
Very Large Telescope Interferometers (VLTI) are based on interferometry to combine the light collected by more than one telescopes (such as ESO’s telescopes combining light collected by four 8.2-metre Telescopes), enabling the observation of new phenomena, opening up new research areas. The light beams are brought together using a complex system of free space components based on pairwise combination utilising the ABCD scheme. Currently bulky free space optics, with complex and very voluminous setups (10 beam input results in 180 outputs), are too sensitive to operate in ambient, while require the path difference must be kept in sub-millimetre scale. Photonic Integrated Circuits (PIC) advantages of miniaturization, stability, and precise active phase control, make them good candidates to develop the beam combining circuits. Key elements towards realization of these circuits are power splitters, low-loss crossings and directional couplers, all operating in a wide range of wavelengths (600 nm – 820 nm). However, the splitting ratio of conventional directional couplers is very sensitive to wavelength, which limits the bandwidth and the transmission performance of the devices. In this paper, we present the design methodology on a low-loss, broadband, and large fabrication tolerance, bend directional coupler realised on Silicon Nitride integration platform. FDE simulation tool was employed for waveguide modes and coupled system supermodes calculation, 2D-FDTD for propagation simulations, while the results were verified via 3D-FDTD simulations. The proposed bend directional coupler enhances conventional couplers performance, achieving splitting ratio of +-10% around target splitting value for the whole 220 nm target wavelength range, for a footprint of 100 μm x 20 μm.
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.
Precise control of the optical path differences (OPD) in the Very Large Telescope Interferometer (VLTI) was critical for the characterization of the black hole at the center of our Galaxy - leading to the 2020 Nobel prize in physics. There is now significant effort to push these OPD limits even further, in-particular achieving 100nm OPD RMS on the 8m unit telescopes (UT’s) to allow higher contrast and sensitivity at the VLTI. This work calculated the theoretical atmospheric OPD limit of the VLTI as 5nm and 15nm RMS, with current levels around 200nm and 100nm RMS for the UT and 1.8m auxiliary telescopes (AT’s) respectively, when using bright targets in good atmospheric conditions. We find experimental evidence for the f−17/3 power law theoretically predicted from the effect of telescope filtering in the case of the ATs which is not currently observed for the UT’s. Fitting a series of vibrating mirrors modelled as dampened harmonic oscillators, we were able to model the UT OPD PSD of the gravity fringe tracker to <1nm/ √Hz RMSE up to 100Hz, which could adequately explain a hidden f−17/3 power law on the UTs. Vibration frequencies in the range of 60-90Hz and also 40-50Hz were found to generally dominate the closed loop OPD residuals of Gravity. Cross correlating accelerometer with Gravity data, it was found that strong contributions in the 40-50Hz range are coming from the M1-M3 mirrors, while a significant portion of power from the 60-100Hz contributions are likely coming from between the M4-M10. From the vibrating mirror model it was shown that achieving sub 100nm OPD RMS for particular baselines (that have OPD∼200nm RMS) required removing nearly all vibration sources below 100Hz.
ESO’s VLT interferometer (VLTI) is a general-user optical/infrared interferometric facility. Its operations scheme is fully integrated into the well-established scheme of all VLT instruments and profits enormously from this experience and the implemented infrastructure to offer a unique service to the community. Based on the greatly improved capabilities of the 2nd generation VLTI instruments and taking advantage of a further development of ESO’s Observation Handling Tools, we have evolved the VLTI operations scheme as well. We have offered to VLTI investigators the possibility to indicate baseline configurations in a more flexible way and have introduced nested scheduling containers to better formalize the observational strategy. We have prepared for dedicated support of different types of interferometric observations. For imaging observations specifically, we have introduced an improved workflow to fill the uv plane and to handle time-critical imaging.
Combining adaptive optics and interferometric observations results in a considerable contrast gain compared to single-telescope, extreme AO systems. Taking advantage of this, the ExoGRAVITY project is a survey of known young giant exoplanets located in the range of 0.1” to 2” from their stars. The observations provide astrometric data of unprecedented accuracy, being crucial for refining the orbital parameters of planets and illuminating their dynamical histories. Furthermore, GRAVITY will measure non-Keplerian perturbations due to planet-planet interactions in multi-planet systems and measure dynamical masses. Over time, repetitive observations of the exoplanets at medium resolution (R = 500) will provide a catalogue of K-band spectra of unprecedented quality, for a number of exoplanets. The K-band has the unique properties that it contains many molecular signatures (CO, H2O, CH4, CO2). This allows constraining precisely surface gravity, metallicity, and temperature, if used in conjunction with self-consistent models like Exo-REM. Further, we will use the parameter-retrieval algorithm petitRADTRANS to constrain the C/O ratio of the planets. Ultimately, we plan to produce the first C/O survey of exoplanets, kick-starting the difficult process of linking planetary formation with measured atomic abundances.
Instrumental polarization can have large effects on measurements with the VLTI, as it can alter measured polarization and introduce uncertainties. To understand these effects we measured and simulated the instrumental polarization of the VLTI and of GRAVITY. We are able to provide a calibration model for GRAVITY observations and quantify systematic uncertainties due to instrumental polarization. This work has shown to be crucial to measure the polarization of the galactic center black hole Sgr A* where we detect a swing in the polarization angle during flare events. While the analysis was done for GRAVITY, it also gives an important basis for the design of future near-infrared instruments at the VLTI.
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.
Several challenges will have to be faced by the staff at Paranal Observatory in order to be well prepared for a seamless integration of the ELT in the current VLT operations scheme. The Telescopes and Instruments Operator group (TIO) is already undergoing changes connected with some of the identified technological and operational needs for the ELT. This paper will have detailed information about the current training needs, group structural changes, the current activities using the adopted engineering-TIO [2] (eTIO) scheme and the staffing plan that will have to be applied in order to keep the centralized support of the biggest world infrastructure in astronomy at the time of the ELT, to handle daily science operations for seven different telescopes, the VLT interferometer and twenty-one scientific instruments in parallel.
The optical turbulence profile is a key parameter in tomographic reconstruction. With interest in tomographic adaptive optics for the next generation of ELTs, turbulence profiling campaigns have produced large quantities of data for observing sites around the world. In order to be useful for Monte Carlo AO simulation, these large datasets must be reduced to a small number of profiles. There is commonly large variation in the structure of the turbulence, therefore statistics such as the median and interquartile range of each altitude bin become less representative as features in the profile are averaged out. Here we present the results of the use of a hierarchical clustering method to reduce the 2018A Stereo-SCIDAR dataset from ESO Paranal, consisting of over 10,000 turbulence profiles measured over 83 nights, to a small set of 18 that represent the most commonly observed profiles.
The GRAVITY instrument installed at VLTI uses differential fibered delay lines to spatially filter the incoming wavefronts and accurately control the optical path difference between the Fringe Tracker (FT) and Scientific Detector (SC) parts of the instrument. On top of the differential dispersion occurring in the air, the chromatic dispersion introduced by these fibers impacts the real time performances of the fringe tracker by generating a second-order chromatic phase shift. Moreover, differential dispersion also affects GRAVITY dual-feed measurements that require a length adjustment of both FT and SC fibers. In this contribution, we show how chromatic dispersion can be corrected both in the fringe tracker real-time control as well as in the astrometric data reduction.
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.
MATISSE (Multi AperTure mid-Infrared SpectroScopic Experiment) is the spectro-interferometer for the VLTI of the European Southern Observatory (ESO), operating in the L-, M- and N- spectral bands, and combining up to four beams from the unit or the auxiliary telescopes (UTs or ATs). MATISSE will offer new breakthroughs in the study of circumstellar environments by allowing the mapping of the material distribution, the gas and essentially the dust. The instrument consists in a warm optical system (WOP) accepting four beams from the VLTI and relaying them after a spectral splitting to cold optical benches (COB) located in two separate cryostats, one in L-M- band, and one in N-band. The test plan of the complete instrument has been conducted at the Observatoire de la Côte d’Azur in order to confirm the compliance of the performance with the high-level requirements. MATISSE has successfully passed the Preliminary Acceptance in Europe the 12th September 2017. Following this result, ESO gave approval for the instrument to be shipped to Paranal. The Alignment, Integration and Verification phase was conducted until end of February 2018, at the end of which first observations on sky have been performed to test the operations with the VLTI and to obtain first stellar light. The two first runs of the commissioning followed, respectively in March and in May 2018. It has the goal to optimize the MATISSE-VLTI communication, the acquisition procedures and the interface parameters. The observations were performed on bright L-M- and N- stars, with four ATs located on short baselines and UTs. The limit magnitudes will be deduced.
This paper reports on the performance of the instrument measured in laboratory (results of test plan in Nice and AIV in Paranal) in terms of spectral coverage, dispersion laws and spectral resolutions, and transfer function analysis: instrumental contrast, visibility accuracy, accuracy of the differential phase, of the closure-phase and of the differential visibility. It also provides results of the first tests on sky and the planning of the on-going commissioning.
The second version of the optical interferometry data exchange format (OIFITS 2) was published in January 2017. OIFITS 2 was intended to address the needs of future interferometric instruments, allow description of correlated measurement errors, and standardize header keywords for bookkeeping and data discovery. We report on a panel discussion held during the conference “Optical and Infrared Interferometry and Imaging VI” with the aims of raising awareness of OIFITS 2, identifying any difficulties in migrating to the new version, and planning for future development of OI data standards.
Sparse Aperture Masking (SAM) has recently been commissioned on SPHERE, the VLTs new adaptive optics high resolution imager. SAM extends the capabilities of SPHERE by providing high contrast measurements at and beyond the traditional diffraction limit. SAM can be used in conjunction with each of the SPHERE modules (IRDIS, IFS and ZIMPOL), allowing dual band imaging in the visible and near-infrared, near-infrared integral field spectroscopy, and polarized differential imaging in the visible and near-infrared. In this paper we report information relevant for observers as well as some commissioning observations.
The Optical interferometry DataBase (OiDB) aims at facilitating the access to science-ready data provided by various existing or decommissioned interferometers. The first version of OiDB has been released in June 2015. Today it contains more than 5000 OIFITS datafiles including the full collection of PIONIER data since 2011. All these reduced data are made publicly available and easily downloadable from OiDB. After presenting the characteristics of OiDB, we analyse how the community made use of it during this first year of operation and how we will improve it.
The idea behind FIRST (Fibered Imager foR a Single Telescope) is to use single-mode fibers to combine multiple apertures in a pupil plane as such as to synthesize a bigger aperture. The advantages with respect to a pure imager are i) relaxed tolerance on the pointing and cophasing, ii) higher accuracy in phase measurement, and iii) availability of compact, precise, and active single-mode optics like Lithium Niobate. The latter point being a huge asset in the context of a space mission. One of the problems of DARWIN or SIM-like projects was the difficulty to find low cost pathfinders missions. But the fact that Lithium Niobate optic is small and compact makes it easy to test through small nanosats missions. Moreover, they are commonly used in the telecom industry, and have already been tested on communication satellites. The idea of the FIRST-S demonstrator is to spatialize a 3U CubeSat with a Lithium Niobate nulling interferometer. The technical challenges of the project are: star tracking, beam combination, and nulling capabilities. The optical baseline of the interferometer would be 30 cm, giving a 2.2AU spatial resolution at distance of 10 pc. The scientific objective of this mission would be to study the visible emission of exozodiacal light in the habitable zone around the closest stars.
We report on a database service that allows users to query calibrated optical interferometry data (OIFITS format) as well as regularly-updated observation logs obtained with a wide range of interferometric instruments. It widely uses Virtual Observatory tools to increase diffusion and operability. In this contribution, we present the characteristics and functionalities of the first global optical interferometry archive service.
Differential chromatic dispersion in single-mode optical fibres leads to a loss of contrast of the white light fringe. For the GRAVITY instrument, this aspect is critical since it limits the fringe tracking performance. We present a real-time algorithm that compensates for differential dispersion due to varying fibre lengths using prior calibration of the optical fibres. This correction is limited by the accuracy to which the fibres stretch is known. We show how this affects the SNR on the white light fringe for different scenarios and we estimate how this phenomenon might eventually impact the astrometric accuracy of GRAVITY observations.
GRAVITY is an adaptive optics assisted Beam Combiner for the second generation VLTI instrumentation. The
instrument will provide high-precision narrow-angle astrometry and phase-referenced interferometric imaging in the
astronomical K-band for faint objects. We describe the wide range of science that will be tackled with this instrument,
highlighting the unique capabilities of the VLTI in combination with GRAVITY. The most prominent goal is to observe
highly relativistic motions of matter close to the event horizon of Sgr A*, the massive black hole at center of the Milky
Way. We present the preliminary design that fulfils the requirements that follow from the key science drivers: It includes
an integrated optics, 4-telescope, dual feed beam combiner operated in a cryogenic vessel; near-infrared wavefrontsensing
adaptive optics; fringe-tracking on secondary sources within the field of view of the VLTI and a novel metrology
concept. Simulations show that 10 μas astrometry within few minutes is feasible for a source with a magnitude of
mK = 15 like Sgr A*, given the availability of suitable phase reference sources (mK = 10). Using the same setup, imaging of mK = 18 stellar sources in the interferometric field of view is possible, assuming a full night of observations and the corresponding UV coverage of the VLTI.
We present the second-generation VLTI instrument GRAVITY, which currently is in the preliminary design phase.
GRAVITY is specifically designed to observe highly relativistic motions of matter close to the event horizon of Sgr A*,
the massive black hole at center of the Milky Way. We have identified the key design features needed to achieve this
goal and present the resulting instrument concept. It includes an integrated optics, 4-telescope, dual feed beam combiner
operated in a cryogenic vessel; near infrared wavefront sensing adaptive optics; fringe tracking on secondary sources
within the field of view of the VLTI and a novel metrology concept. Simulations show that the planned design matches
the scientific needs; in particular that 10µas astrometry is feasible for a source with a magnitude of K=15 like Sgr A*,
given the availability of suitable phase reference sources.
Two of the three instruments proposed to ESO for the second generation instrumentation of the VLTI would
use integrated optics for beam combination. Several design are studied, including co-axial and multi-axial
recombination. An extensive quantity of combiners are therefore under test in our laboratories. We will present
the various components, and the method used to validate and compare the different combiners. Finally, we will
discuss the performances and their implication for both VSI and Gravity VLTI instruments.
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