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.

One of the most ambitious goals of modern astronomy is to uncover signs of extraterrestrial biological activity, primarily achieved through spectroscopic analysis of light emitted by exoplanets to identify specific atmospheric molecules. Most exoplanets are indirectly identified through techniques like transit or Doppler shift of the host star's flux. Long-term surveys have yielded statistical insights into the occurrence rates of different planet types based on factors such as radius/mass, orbital period, and the spectral type of the host star. Initial estimates of terrestrial planets within the habitable zone have also emerged. However, the difficulty of detecting light from these exoplanets leaves much unknown about their nature, formation, and evolution. As the number of rocky exoplanets around nearby stars rises, questions about their atmospheric composition, evolutionary trajectory, and habitability increase. Direct measurement of an exoplanet's atmospheric composition through its spectral signature in the infrared can provide answers. Measuring the infrared spectrum of these planets poses significant challenges due to the star/planet contrast and very small angular separation from their host stars. Previous research showed that space-based telescopes are mandatory, and unless large primary mirrors (>30m in diameter) can be sent into space, interferometric techniques become essential. Combining light from distant telescopes with interferometric techniques allows access to information at minimal angular separation, operating within the diffraction limit of individual telescopes. Successful demonstrations of on-ground nulling interferometry open a new era for such space-based missions. They are vital to sidestep and tackle these scientific questions. We recently initiated a new study with the European Space Agency to explore the design parameters and the performances related to an interferometric concept based on a single spacecraft and sparse multiple sub-apertures. Launch constraints are linked to the use of an Ariane 6 launch vehicle. Our parametric study covers a range of 1-4 m for the diameter of the telescope and a 10-60 m baseline. The most promising concept working in the infrared range (3-20μm) will be highlighted. This study is conducted by TUDelft in cooperation with KULeuven, CSL/ULiège, and Amos with the support of the European Space Agency.

Context. NOTT (formerly Hi-5) is a new high-contrast L′ band (3.5–4.0 µm) beam combiner for the VLTI designed with an ambitious aim to be sensitive to young giant exoplanets down to 5 mas separation around nearby stars. The performance of nulling interferometers in these wavelengths is affected both by fundamental noise from the background and contributions of instrumental noise. This motivates the development of end-to-end simulations to optimize these instruments.

Aims. The aim of this study is to enable a performance evaluation of NOTT and inform the design of such instruments with current and future infrastructures in mind, taking into account the different sources of noise and their correlation.

Methods. SCIFYsim is an end-to-end simulator for single-mode-filtered beam combiners, with an emphasis on nulling interferometers. We use it to compute a covariance matrix of the errors. We then use statistical detection tests based on likelihood ratios to compute compound detection limits for the instrument.

Results. With the current assumptions as to the performance of the wavefront correction systems, the errors are dominated by correlated instrumental errors down to stars of magnitude 6–7 in the L band, beyond which thermal background from the telescopes and relay system becomes dominant.

Conclusions. SCIFYsim is suited to anticipating some of the challenges of design, tuning, operation, and signal processing for integrated-optics beam combiners. The detection limits found for this early version of NOTT simulation with the unit telescopes are compatible with detections at contrasts up to 105 in the L band at separations of 5–80 mas around bright stars.

Hi-5 is an ERC-funded project hosted at KU Leuven and a proposed visitor instrument for the VLTI. Its primary goal is to image the snow line region around young planetary systems using nulling interferometry in the L’ band, between 3.5 and 4.1 μm, where the contrast between exoplanets and their host stars is very advantageous. The breakthrough is the use of a photonic chip based beam combiner, which only recently allowed the required theoretical raw contrast of 10−3 in this spectral range. The VLTI long baseline interferometry enables to reach high angular resolution (4.2 mas at 3.8 μm wavelength with the Auxiliary Telescopes (ATs)), while high contrast detection is achieved using nulling interferometry. This polarisation requires a high degree of optical symmetry between the four pupils of the VLTI, only possible with precise phase, dispersion and intensity control systems. The instrument is currently in its design phase. In this paper, the warm optics design and the injection system up to the photonic chip are presented. The different properties of the design are presented including the optics used, the characteristics of the four beams and the current drawbacks. Particular attention is devoted to the optical alignment and the tolerance analysis in order to estimate the precision required for the alignment procedure and therefore to choose adapted optical mountings.

Hi-5 is the L’-band (3.5-4.0 μm) high-contrast imager of Asgard, an instrument suite in preparation for the visitor focus of the VLTI. The system is optimized for high-contrast and high-sensitivity imaging within the diffraction limit of a single UT/AT telescope. It is designed as a double-Bracewell nulling instrument producing spectrally-dispersed (R=20, 400, or 2000) complementary nulling outputs and simultaneous photometric outputs for self-calibration purposes. In this paper, we present an update of the project with a particular focus on the overall architecture, opto-mechanical design of the warm and cold optics, injection system, and development of the photonic beam combiner. The key science projects are to survey (i) nearby young planetary systems near the snow line, where most giant planets are expected to be formed, and (2) nearby main sequence stars near the habitable zone where exozodiacal dust that may hinder the detection of Earth-like planets. We present an update of the expected instrumental performance based on full end-to-end simulations using the new GRAVITY+ specifications of the VLTI and the latest planet formation models.

Exoplanet science is one of the most thriving fields of modern astrophysics. A major goal is the atmospheric characterization of dozens of small, terrestrial exoplanets in order to search for signatures in their atmospheres that indicate biological activity, assess their ability to provide conditions for life as we know it, and investigate their expected atmospheric diversity. None of the currently adopted projects or missions, from ground or in space, can address these goals. In this White Paper, submitted to ESA in response to the Voyage 2050 Call, we argue that a large space-based mission designed to detect and investigate thermal emission spectra of terrestrial exoplanets in the mid-infrared wavelength range provides unique scientific potential to address these goals and surpasses the capabilities of other approaches. While NASA might be focusing on large missions that aim to detect terrestrial planets in reflected light, ESA has the opportunity to take leadership and spearhead the development of a large mid-infrared exoplanet mission within the scope of the “Voyage 2050” long-term plan establishing Europe at the forefront of exoplanet science for decades to come. Given the ambitious science goals of such a mission, additional international partners might be interested in participating and contributing to a roadmap that, in the long run, leads to a successful implementation. A new, dedicated development program funded by ESA to help reduce development and implementation cost and further push some of the required key technologies would be a first important step in this direction. Ultimately, a large mid-infrared exoplanet imaging mission will be needed to help answer one of humankind’s most fundamental questions: “How unique is our Earth?”

One of the main goals of exoplanet science is to characterize the atmosphere of rocky exoplanets in the habitable zone of nearby stars. A space-based nulling interferometry, observing in the mid-infrared (3-20 μm), is considered to be one of the most promising solutions to tackle this observing challenge. The LIFE project, a free-flying space-based mid-infrared nulling interferometer, would have this capability. However, several key technologies need to be demonstrated before launching such an ambitious mission. A small space-based mission can be considered as a useful prerequisite. In this paper, we consider three small satellite architectures, two CubeSats, and a PROBA-like satellite. Based on a Bracewell architecture and without free-flying, these monolithic satellites can demonstrate some key components like the null capability and its stability on real targets. The achromatic phase shifter needs also to be demonstrated in space. Based on the scientific capabilities and exoplanet detection yield of these architectures, optical constraints are derived (pointing stability, and optical path difference correction). Orbital simulations, exploring a range of classical orbits for such a satellite, are also discussed.

Space-based nulling interferometry is one of the most promising solutions to spectrally characterize the atmosphere of rocky exoplanets in the mid-infrared (3 to 20 μm). It provides both high angular resolution and starlight mitigation. This observing capability depends on several technologies. A CubeSat (up to 20 kg) or a medium satellite (up to a few hundreds of kg), using a Bracewell architecture on a single spacecraft could be an adequate technological precursor to a larger, flagship mission. Beyond technical challenges, the scientific return of such a small-scale mission needs to be assessed. We explore the exoplanet science cases for various missions (several satellite configurations and sizes). Based on physical parameters (diameter and wavelength) and thanks to a state-of-the-art planet population synthesis tool, the performance and the possible exoplanet detection yield of these configurations are presented. Without considering platform stability constraints, a CubeSat (baseline of b 1 m and pupils diameter of D 0.1 m) could detect 7 Jovian exoplanets, a small satellite (b 5 m / D 0.25 m) 120 exoplanets, whereas a medium satellite (b 12.5 m / D 0.5 m) could detect 1/4250 exoplanets including 51 rocky planets within 20 pc. To complete our study, an analysis of the platform stability constraints (tip/tilt and optical path difference) is performed. Exoplanet studies impose very stringent requirements on both tip/tilt and OPD control.

Nulling interferometry is considered as one of the most promising solutions to spectrally characterize rocky exoplanets in the habitable zone of nearby stars. It provides both high angular resolution and starlight mitigation. It requires however several technologies that need to be demonstrated before a large interferometry space-based mission flies. A small-sat mission is a good technological precursor. Based on a Bracewell architecture, this unique satellite can demonstrate some key components (null capability, fiber injection, achromatic phase shifter). Scientific capabilities of such a mission are presented. An exoplanet detection yield is derived, and we show that the detection of exoplanets around nearby stars is feasible.

High-contrast optical stellar interferometry generally refers to instruments able to detect circumstellar emission at least a few hundred times fainter than the host star at high-angular resolution (typically within a few λ/D). While such contrast levels have been enabled by classical modal-filtered interferometric instruments such as VLTI/PIONIER, CHARA/FLUOR, and CHARA/MIRC the development of instruments able to filter out the stellar light has significantly pushed this limit, either by nulling interferometry for on-axis observations (e.g., PFN, LBTI, GLINT) or by off-axis classical interferometry with VLTI/GRAVITY. Achieving such high contrast levels at small angular separation was made possible thanks to significant developments in technology (e.g., adaptive optics, integrated optics), data acquisition (e.g., fringe tracking, phase chopping), and data reduction techniques (e.g., nulling self-calibration). In this paper, we review the current status of high-contrast optical stellar interferometry and present its key scientific results. We then present ongoing activities to improve current ground-based interferometric facilities for high-contrast imaging (e.g., Hi-5/VIKING/BIFROST of the ASGARD instrument suite, GRAVITY+) and the scientific milestones that they would be able to achieve. Finally, we discuss the long-term future of high-contrast stellar interferometry and, in particular, ambitious science cases that would be enabled by space interferometry (e.g., LIFE, space-PFI) and large-scale ground-based projects (PFI).

Proxima b is our nearest potentially rocky exoplanet and represents a formidable opportunity for exoplanet science and possibly astrobiology. With an angular separation of only 35 mas (or 0.05 AU) from its host star, Proxima b is however hardly observable with current imaging telescopes and future space-based coronagraphs. One way to separate the photons of the planet from those of its host star is to use an interferometer that can easily resolve such spatial scales. In addition, its proximity to Earth and its favorable contrast ratio compared with its host M dwarf (approximately 10^-5 at 10 microns) makes it an ideal target for a space-based nulling interferometer with relatively small apertures. In this paper, we present the motivation for observing this planet in the mid-infrared (5-20 microns) and the corresponding technological challenges. Then, we describe the concept of a space-based infrared interferometer with relatively small (<1m in diameter) apertures that can measure key details of Proxima b, such as its size, temperature, climate structure, as well as the presence of important atmospheric molecules such as H₂O, CO₂, O₃, and CH₄. Finally, we illustrate the concept by showing realistic observations using synthetic spectra of Proxima b computed with coupled climate chemistry models.

The development of small space-based platforms for nulling interferometric observations could be the pathfinder of a new era in exoplanetology. While planetary transit and radial velocity are the most productive ways to detect exoplanets, such techniques are indirect detections. For deeper characterization of exoplanets, direct detection techniques should be developed. By injecting direct light coming from exoplanets into spectrometers, we could study their chemical composition, search for biosignatures, and possibly infer the presence of life. The low number of photons to be gathered from the planets, high contrast with the star and small angular resolution are the major difficulties for a direct detection. However, nulling interferometry seems to be a solution to tackle these challenges. By combining the light of two or more telescopes, we would considerably increase the angular resolution, and thus could potentially lead to the detection of Earth-size rocky exoplanets around Solar-type stars. Moreover, with a I€-phase shift between the two interferometer arms, the starlight is reduced which allows the detection of much fainter objects around the star. In this paper it will be presented the development of a new mission based on nulling interferometry and dedicated to the Alpha Centauri system. As our nearest stellar system, it is a prime target to investigate for the research of new worlds. Monte-Carlo simulations about potential exoplanet yield of such an interferometer will be described, for different assumptions such as the detection wavelength and telescope size. Single-mode fibers and integrated optics will also be investigated for this mission. This could lead to low-cost type missions with a high potential of scientific return.

We propose a new family of achromatic phase shifters for infrared nulling interferometry. These key optical components can be seen as optimized Fresnel rhombs, using the total internal reflection phenomenon, modulated or not. The total internal reflection indeed comes with a phase shift between the polarization components of the incident light. We propose a solution to implement this vectorial phase shift between interferometer arms to provide the destructive interference process needed to disentangle highly contrasted objects from one another. We also show that, modulating the index transition at the total internal reflection interface allows compensating for the intrinsic material dispersion in order to make the subsequent phase shift achromatic over especially broad bands. The modulation can be induced by a thin film of a well-chosen material or a subwavelength grating whose structural parameters are thoroughly optimized. We present results from theoretical simulations together with preliminary fabrication outcomes and measurements for a prototype in Zinc Selenide.