This PDF file contains the front matter associated with SPIE Proceedings Volume 12183 including the Title Page, Copyright information, and Table of Contents.

One of the main design considerations of the Large Binocular Telescope (LBT) was the goal to resolve the habitable zones (HZs) of the nearest stars at mid-infrared wavelengths around 10 μm. The LBT Interferometer (LBTI) makes use of the telescope’s two 8.4m mirrors on a common mount and their 22.7m edge-to-edge separation for sensitive, high-angular resolution observations at thermal-infrared wavelengths. In addition to adaptive optics imaging using the two mirrors separately, the instrument enables nulling and Fizeau imaging interferometry exploiting the full resolving power of the LBT. The LBTI team has successfully completed the Hunt for Observable Signatures of Terrestrial planetary Systems (HOSTS), for which we used nulling-interferometry to search for exozodiacal dust, and we are continuing the characterization of the detected systems. Here, we describe a new program to exploit the LBTI’s Fizeau imaging interferometric capabilities for a deep imaging search for low-mass, HZ planets around a small sample of particularly suitable, nearby stars. We also review the LBTI’s current status relevant to the proposed project to demonstrate the instrument is ready for such a large project.

The Center for High Angular Resolution Astronomy (CHARA) Array is a six-element interferometer with baselines ranging from 34 to 331 m. Three new beam combiners are entering operation: MYSTIC is a 6-telescope combiner for K-band; SPICA is a 6-telescope combiner for the visible R-band; and SILMARIL is a 3-telescope combiner for high sensitivity in H and K-bands. A seventh, portable telescope will use fiber optics for beam transport and will increase the baselines to 1 km. Observing time is available through a program funded by NSF. The programs are solicited and peer-reviewed by NSF’s National Optical-Infrared Astronomy Research Laboratory. The open community access has significantly expanded the range of astronomical investigations of stars and their environments. Here we summarize the scientific work and the on-going technical advances of the CHARA Array.

We have been progressing on our comprehensive program of improving high-resolution imaging at the Navy Precision Optical Interferometer (NPOI) hosted at Lowell Observatory’s Anderson Mesa site, for the purpose of spatially resolved observations of faint objects at scales down to less than 1 milliarcsecond. The ‘PALANTIR’ upgrade of NPOI has commenced with individually operating 1 meter PlaneWave PW1000 telescopes at the site, with integration of those telescopes into the array with interferometric operations expected in the near-term. These telescopes are housed in mobile domes for rapid relocation around the array, and are being augmented with adaptive optics. Another notable recent milestone has been the re-activation of full six-way on-sky operations with siderostat feeds during the summer of 2021. Additionally, our ‘NPOI Plus-Up’ plan will implement sweeping infrastructure updates, improving and streamlining its operations. Upcoming Plus-Up work taking place over the next few years includes expansion of the operating infrastructure to the array’s longest physical baselines at 432 meters, adding a near-infrared beam combiner, rehabilitation of the VISION visible combiner, modernization of the fast delay line control system, and implementation of the long delay lines in the framework of a beam train auto-aligner.

The Magdalena Ridge Observatory Interferometer has been conceived to be the most ambitious optical/near-infrared long-baseline imaging interferometer in the world today. We anticipate receiving the second telescope mount and enclosure and associated beamline infrastructure to enable us to attempt first fringes measurements early in 2023. Having reached this important milestone, we anticipate receiving the third copy of all beamline components about one year later and attempting closure phase measurements thereafter. We will present a status update and plans under the new Cooperative Agreement with AFRL for the next phases of the project.

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.

This conference presentation was prepared for the Optical and Infrared Interferometry and Imaging VIII conference at SPIE Astronomical Telescopes and Instrumentation, 2022.

With a possible angular resolution down to 0.1-0.2 millisecond of arc using the 330 m baselines and the access to the 600-900 nm spectral domain, the CHARA Array is ideally configured for focusing on precise and accurate fundamental parameters of stars. CHARA/SPICA (Stellar Parameters and Images with a Cophased Array) aims at performing a large survey of stars all over the Hertzsprung-Russell diagram. This survey will also study the effects of the different kinds of variability and surface structure on the reliability of the extracted fundamental parameters. New surface-brightness-colour relations will be extracted from this survey, for general purposes on distance determination and the characterization of faint stars. SPICA is made of a visible 6T fibered instrument and of a near-infrared fringe sensor. In this paper, we detail the science program and the main characteristics of SPICA-VIS. We present finally the initial performance obtained during the commissioning.

SPICA-FT is part of the CHARA/SPICA instrument which combines a visible 6T fibered instrument (SPICAVIS) with a H-band 6T fringe sensor. SPICA-FT is a pairwise ABCD integrated optics combiner. The chip is installed in the MIRC-X instrument. The MIRC-X spectrograph could be fed either by the classical 6T fibered combiner or by the SPICA-FT integrated optics combiner. SPICA-FT also integrates a dedicated fringe tracking software, called the opd-controller communicating with the main delay line through a dedicated channel. We present the design of the integrated optics chip, its implementation in MIRC-X and the software architecture of the group-delay and phase-delay control loops. The final integrated optics chip and the software have been fully characterized in the laboratory. First on-sky tests of the integrated optics combiner began in 2020. We continue the on-sky tests of the whole system (combiner + software) in Spring and Summer 2022. We present the main results, and we deduce the preliminary performance of SPICA-FT.

The Very Large Telescope Interferometer (VLTI) is currently the best infrastructure for long-baseline interferometry in particular in terms of sensitivity and accessibility to the general user. MATISSE, installed at the VLTI focus since end of 2017, belongs to the second generation instruments. MATISSE, the Multi AperTure mid-Infrared SpectroScopic Experiment, for the first time accesses high resolution imaging over a wide spectral domain of the mid-infrared. The instrument is a spectro-interferometric imager in the atmospheric transmission windows called L, M, and N, from 2.8 to 13.0 microns, and combines four optical beams from the VLTI’s unit or auxiliary telescopes. The instrument utilises a multi-axial beam combination that delivers spectrally dispersed fringes. The signal provides the following quantities at several spectral resolutions: photometric flux, coherent flux, visibility, closure phase, wavelength differential visibility and phase, and aperture-synthesis imaging. MATISSE can operate as a stand alone instrument or with the GRA4MAT set-up employing the GRAVITY fringe tracking capabilities. The updated MATISSE performance are presented at the conference together with a selection of two front-line science topics explored since the start of the science operations in 2019. Finally we present the perspective and benefit of two technical improvements foreseen in the coming years: the MATISSE-Wide off-axis fringe tracking capability and new adaptive optics for the UTs in the context of the GRAVITY+ project.

The Michigan Young STar Imager at CHARA (MYSTIC) is a K-band interferometric beam combining instrument funded by the United States National Science Foundation, designed primarily for imaging sub-au scale disk structures around nearby young stars and to probe the planet formation process. Installed at the CHARA array in July 2021, with baselines up to 331 meters, MYSTIC provides a maximum angular resolution of λ/2B ∼ 0.7 mas. The instrument injects phase corrected light from the array into inexpensive, single-mode, polarization maintaining silica fibers, which are then passed via a vacuum feedthrough into a cryogenic dewar operating at 220 K for imaging. MYSTIC utilizes a high frame rate, ultra-low read noise SAPHIRA detector, and implements two beam combiners: a 6-telescope image plane beam combiner, based on the MIRC-X design, for targets as faint as 7.7 Kmag, as well as a 4-telescope integrated optic beam-combiner mode using a spare chip leftover from the GRAVITY instrument. MYSTIC is co-phased with the MIRC-X (J+H band) instrument for simultaneous fringe-tracking and imaging, and shares its software suite with the latter to allow a single observer to operate both instruments. Herein, we present the instrument design, review its operational performance, present early commissioning science observations, and propose upgrades to the instrument that could improve its K-band sensitivity to 10th magnitude in the near future.

The two MAGIC 17-m diameter Imaging Atmospheric Cherenkov Telescopes have been equipped to work also as an intensity interferometer with a deadtime-free, 4-channel, GPU-based, real-time correlator. Operating with baselines between ∼40 and 90 m the MAGIC interferometer is able to measure stellar diameters of 0.5 - 1 mas in the 400-440 nm wavelength range with a sensitivity roughly 10 times better than that achieved in the 1970’s by the Narrabri Stellar Intensity Interferometer. Besides, active mirror control allows to split the primary mirrors into sub-mirrors. This allows to make simultaneous calibration measurements of the zero-baseline correlation or to simultaneously collect six baselines below 17 m with almost arbitrary orientation, corresponding to angular scales of ∼1-50 mas. We plan to perform test observations adding the nearby Cherenkov Telescope Array (CTA) LST-1 23 m diameter telescope by next year. All three telescope pairs will be correlated simultaneously. Adding LST-1 is expected to increase the sensitivity by at least 1 mag and significantly improve the u-v plane coverage. If successful, the proposed correlator setup is scalable enough to be implemented to the full CTA arrays.

The VERITAS Imaging Air Cherenkov Telescope array (IACT) was augmented in 2019 with high-speed focal plane electronics to create a new Stellar Intensity Interferometry (SII) observational capability (VERITAS-SII, or VSII). VSII operates during bright moon periods, providing high angular resolution observations ( < 1 mas) in the B photometric band using idle telescope time. VSII has already demonstrated the ability to measure the diameters of two B stars at 416 nm (Bet CMa and Eps Ori) with < 5% accuracy using relatively short (5 hours) exposures.¹ The VSII instrumentation was recently improved to increase instrumental sensitivity and observational efficiency. This paper describes the upgraded VSII instrumentation and documents the ongoing improvements in VSII sensitivity. The report describes VSII’s progress in extending SII measurements to dimmer magnitude stars and improving the VSII angular diameter measurement resolution to better than 1%.

FIRSTv2 (Fibered Imager foR a Single Telescope version 2) is the upgrade of a post-AO spectro-interferometer (FIRST) that enables high contrast imaging and spectroscopy at spatial scales below the diffraction limit of a single telescope. It aims at using a photonic chip beam combiner, allowing the measurement of the complex visibility for every baseline independently thus improving the dynamic range. I will report on the first on-sky results obtained with several prototype chips integrated at the Subaru Telescope on unresolved and binary stars, for the first time in the visible.

The ASTRI Mini-Array is an International collaboration, led by the Italian National Institute for Astrophysics, that is constructing and operating an array of nine Imaging Atmospheric Cherenkov Telescopes to study gamma-ray sources at very high energy and perform optical stellar intensity interferometry (SII) observations. Angular resolutions below 100 microarcsec are achievable with stellar intensity interferometry, using telescopes separated by hundreds to thousands of meters baselines. At this level of resolution it turns out to be possible to reveal details on the surface and of the environment surrounding bright stars on the sky. The ASTRI Mini-Array will provide a suitable infrastructure for performing these measurements thanks to the capabilities offered by its 9 telescopes, which provide 36 simultaneous baselines over distances between 100 m and 700 m. After providing an overview of the scientific context and motivations for performing SII science with the ASTRI Mini-Array telescopes, we present the baseline design for the ASTRI Stellar Intensity Interferometry Instrument, a fast single photon counting instrument that will be mounted on the ASTRI telescopes and dedicated to performing SII observations of bright stars.

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.

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.

Hierarchical Fringe Tracking (HFT) is a fringe tracking concept optimizing the sensitivity in optical long baseline by reducing to an absolute minimum the number of measurements used to correct the OPD fluctuations. By nature, the performances of an HFT do not decreases with the number of apertures of the interferometer and are set only by the flux delivered by the individual telescopes. This a critical feature for future interferometers with large number of apertures both for homodyne and heterodyne operation. Here we report the design and first optical bench tests of integrated optics HFT chips for a 4 telescopes interferometer such as the VLTI. These tests validate the HFT concept and confirm previous estimates that we could track accurately fringes on the VLTI up to nearly K~15.9 with the UTs and K~12.2 with the ATs with a J+H+K fringe tracker with one HFT chip per band. This is typically 2.5 magnitudes fainter than the best potential performance of the current ABCD fringe tracker in the K band. An active longitudinal and transverse chromatic dispersion correction allows the optimization of broad band fiber injections and instrumental contrast. We also present a preliminary evaluation of the potential of such a gain of sensitivity for the observations of AGNs with the VLTI.

MIRC-X and MYSTIC are six telescope all-in-one beam combiners at the CHARA array, Mount Wilson Observatory, USA. These two instruments make simultaneous cophasing observations in J + H + K-band wavelengths and deliver λ/2B~0.5 mas angular resolution imaging utilizing the world's largest baseline lengths ranging from 30-330 m. MIRC-X and MYSTIC are operated in two observing modes based on the target spectrum and their over-resolved nature: (i) primary-secondary and (ii) combined mode. In the primary-secondary mode, we use one instrument as a fringe tracker and the other as a science combiner to enable high spectral resolution interferometry. Higher visibility fringes are selected depending on their baseline length and wavelengths for robust fringe tracking for over-resolved objects in the combined mode. We installed four accelerometers to study vibrations on the beam combiner table to improve fringe tracking. We here report the instrument concepts and the first on-sky science results.

Nulling interferometry is one of the most promising techniques for imaging exoplanets at solar system scales as it simultaneously meets the stringent requirements for contrast and angular resolution. The GLINT instrument, operating at Subaru telescope behind the SCExAO extreme adaptive optics system, has delivered significant advances in performance, paving the way for a science-ready instrument. Results from previous commissioning runs have confirmed that integrated optics and self-calibration methods yield a robust pathway to detecting exoplanets below the diffraction limit of the telescope. Furthermore, limitations to the current performance have been identified. The most critical are the compensation of remaining phase fluctuations and delivering deep achromatic nulls. Specifically, one technology that offers enormous promise to resolve these issues is the use of tricouplers to perform simultaneous nulling and fringe tracking, or wavefront characterization and correction free of non-common path aberrations. By adding a broadband π radian phase-shifter, fringe tracking and path length stabilization is performed around a deep achromatic null, providing excellent high contrast performance at small angular scales. In this paper, we present the commissioning of GLINT, its current challenges and describe the modelling of devices to overcome them. We discuss the expected performance of a nuller based on these principles of phase control and null depth as implemented within the GLINT instrument. We further present laboratory characterization of 3D-written tricouplers made using ultrafast laser inscription.

The Large Interferometer For Exoplanets (LIFE) is an envisioned nulling interferometry space mission to characterize the atmospheres of terrestrial exoplanets in the mid-infrared (MIR) wavelength range (∼4-18.5 μm.) The star-to-planet flux contrast for an Earth-twin exoplanet is ≈ 10⁷ at these wavelengths. Previous studies have shown that a “raw” null-depth of 10⁵ provided by the interferometer is sufficient as long as the residual starlight can be removed through signal modulation, phase-chopping and data post processing. Two main technological challenges for a nulling interferometer are instrument stability and sensitivity. Several test-benches were built for LIFE’s ancestral mission concepts DARWIN and TPF-I. Operating at ambient conditions, they demonstrated excellent stability and suppressed the artificial starlight by up to 10⁶ (depending on the spectral bandpass). However, instrument sensitivity/throughput for astronomical sources can not be characterized at background dominated ambient conditions. Cooling the instruments to cryogenic conditions reduces the thermal background and enables sensitivity driven instrument characterization. The Nulling Interferometer Cryogenic Experiment (NICE) is a single Bracewell nulling interferometer test-bench for LIFE. The ultimate aim of this test-bench is to attain a sensitivity level that demonstrates the feasibility of detecting an Earth-twin around a Sun-like star at 10 pc with a spectral bandwidth of 10% at 10 μm. The development of NICE is divided into two phases, the warm and cold phase. The warm phase focuses on the alignment of the optical components and maintaining their position and angular stability to achieve a null depth of 10⁻⁵ − 10⁻⁶ at 4 μm over several hours. In the cold phase, NICE will be cooled to 15 K to suppress the thermal background, and the throughput and sensitivity of the instrument will be characterized. This paper describes the development plan of NICE and presents the optical layout of the NICE warm phase. It also presents the preliminary null-depth reached by the NICE warm phase and the residual alignment errors in the system.

The BIFROST instrument will be the first VLTI instrument optimised for high spectral resolution up to R=25,000 and operate between 1.05 and 1.7 μm. A key component of the instrument will be the spectrograph, where we require a high throughput over a broad bandwidth. In this contribution, we discuss the four planned spectral modes (R=50, R=1000, R=5000, and R=25,000), the key spectral windows that we need to cover, and the technology choices that we have considered. We present our plan to use Volume Phase Holographic Gratings (VPHGs) to achieve a high efficiency > 85%. We present our preliminary optical design and our strategies for wavelength calibration.

Optical interferometry is a powerful technique to achieve high angular resolution. However, its main issue is its lack of sensitivity, compared to other observation techniques. Efforts have been made in the previous decade to improve the sensitivity of optical interferometry, with instruments such as PIONIER and GRAVITY at VLTI, or MIRC-X and MYSTIC at CHARA. While those instruments pushed on sensitivity, their design focus was not the sensitivity but relative astrometric accuracy, imaging capability, or spectral resolution. Our goal is to build an instrument specifically designed to optimize for sensitivity. This meant focusing our design efforts on different parts of the instrument and investigating new technologies and techniques. First, we make use of the low noise C-RED One camera using e-APD technology and provided by First Light Imaging, already used in the improvement of sensitivity in recent new instruments. We forego the use of single-mode fibers but still favor an image plane design that offers more sensitivity than a pupil plane layout. We also use a minimum number of optical elements to maximize the throughput of the design, using a long focal length cylindrical mirror. We chose to limit our design to 3 beams, to have the capability to obtain closure phases, but not dilute the incoming flux in more beam combinations. We also use in our design an edge filter to have the capability to observe Hand K-band at the same time. We use a low spectral resolution, allowing for group delay fringe tracking but maximizing the SNR of the fringes for each spectral channel. All these elements will lead to a typical limiting magnitude between 10 and 11 in both H- and K-bands.

The Navy Precision Optical Interferometer (NPOI) is approaching 30 years of operational life. During that time it has carried out many observations of single and multiple star systems, and has seen upgrades to subsystems and expansion of capabilities. For the last couple of years there has been a major ongoing effort to expand the capabilities of the NPOI with new telescopes, new instruments, and completion of the bootstrapping and imaging capability that was part of the original NPOI design. As part of this upgrade we are also upgrading and replacing some electronic systems. Some technologies which were state-of-the art, e.g. VxWorks, have over the years been overtaken by inexpensive systems such as embedded microcontroller, consumer-grade compact system such as Raspberry Pi¹ and Arduino,² and inexpensive manufacture of simple and powerful custom PCBs. This makes it possible to incorporate remote controls of actuators which will be a major convenience compared to the existing system of manual controls.

The Magdalena Ridge Observatory Interferometer (MROI) will soon incorporate an Automated Alignment System (AAS) to help limit visibility losses due to beam misalignment to ~1%. This paper focuses on two key AAS components: (1) a dual-wavelength beacon at each unit telescope and (2) a detector for measuring the shear and tilt of beams of light arriving from the telescopes in the beam combining laboratory. We share initial results of acceptance tests for these components. Finally, we outline a plan for fully validating their performance against a list of derived requirements.

NASA’s latest and most ambitious flagship space observatory, the $10 Billion dollar James Webb Space Telescope (JWST) represents such a major step in capability that it is difficult to identify any area of astronomy that will not undergo a profound change following its successful December 2021 launch and subsequent deployment. The recovery of images of exoplanets and their environments is among the key scientific drivers for which the mission was built. In 2008 a dedicated interferometer, designed by the authors, was accepted by the JWST NIRISS Instrument Team based in Montreal, adding Aperture Masking Interferometry (AMI) to JWST’s suite of modes. Fabricated in Canada and tested by Honeywell and CSA as well as NASA, it is now one of JWST’s supported scientific modes. Here we provide a high level description of the mode, and the science themes that originally motivated it.

FIRST is a post Extreme Adaptive-Optics (ExAO) spectro-interferometer based on pupil remapping using single-mode fibers. Installed on the SCExAO platform at the Subaru Telescope, it operates in the Visible (600-800nm, R 400) and demonstrated companion detection below the telescope diffraction limit. As an interferometric device, FIRST is sensitive to phasing problems in the telescope pupil. This is particularly interesting to measure discontinuous aberrations, invisible to the ExAO sensors. Recent developments aimed to measure upstream aberrations directly from the same interferometric signal used for scientific data analysis. A key limitation to this new capability is the fiber thermal and mechanical instabilities, inducing up to 1 micron drift over a few seconds. We propose to use a metrology laser source, allowing to discriminate FIRST instrumental effects from all the upstream aberrations. We present the integration of this setup and the on-sky demonstration of the method.

The unique astrometric capability of GRAVITY has already resulted in a serie of transformational results, from the study of the Galactic Center to the characterization of exoplanets. Nonetheless, these breakthroughs have not yet reached the ultimate noise limits of interferometric astrometry, and are currently limited by the systematics of the instrument. As part of the GRAVITY+ project, a major goal is to keep pushing the performances down to the precision of 10-30µas. In this talk, we present the on-going analysis of the precision limits of GRAVITY astrometry, and the potential solutions envisioned to overcome its systematics.

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.

With the upgrade from GRAVITY to GRAVITY+ the instrument will evolve to an all-sky interferometer that can observe faint targets, such as high redshift AGN. Observing the faintest targets requires reducing the noise sources in GRAVITY as much as possible. The dominant noise source, especially in the blue part of the spectrum, is the backscattering of the metrology laser light onto the detector. To reduce this noise we introduce two new metrology modes. With a combination of small hardware changes and software adaptations, we can dim the metrology laser during the observation without losing the phase referencing. For single beam targets, we can even turn off the metrology laser for the maximum SNR on the detector. These changes lead to a SNR improvement of over a factor of two averaged over the whole spectrum and up to a factor of eight in the part of the spectrum currently dominated by laser noise.

Since 2019, GRAVITY has obtained unprecedented direct observations of exoplanets at high contrasts (down to 5 × 10⁻⁵) and small angular separations from the host star (down to 110 mas). To access deeper contrast (10⁻⁶) at smaller angular separations (less than 100 mas), we propose to dig a dark hole at the planet position. It relies on wavefront control using the adaptive optics deformable mirror to minimize the stellar flux injected in the GRAVITY interferometric combiner. We tested this technique on-sky on the instrument and concluded that the maximal contrast improvement it can achieve is ×4 on the current adaptive optics (NAOMI, MACAO). We also predict that the dark hole technique will bring a contrast improvement up to ×100 at less than 140 mas on the future GRAVITY+ adaptive optics of the Unit Telescopes.

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 2^nd 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.

In this thesis work, we exploit the unique capabilities of long baseline interferometry to fill two gaps in exoplanet parameter space: 1) the discovery of new planets around stars more massive than the Sun (Project ARMADA), and 2) the characterization of known planets that are extremely close to their host star (Project PRIME). Current detection methods struggle to find exoplanets around hot (A/B-type) stars. We are pushing the astrometric limits of ground-based optical interferometers to carry out a survey of sub-arcsecond A/B-type binary systems with ARMADA. We are achieving astrometric precision at the few tens of micro-arcsecond level in short observations at CHARA/MIRC-X and VLTI/GRAVITY. This incredible precision allows us to probe the au-regime for giant planets orbiting individual stars of the binary system. We present the status of our survey, including our newly implemented etalon wavelength calibration method at CHARA, detection of new stellar mass companions, and non-detection limits down to a few Jupiter masses in some cases. With Project PRIME, we show that ground-based optical interferometry can be used to measure the orbit-dependent spectra of close-in “hot Jupiter”-type exoplanets with precision closure phases. Detecting the infrared spectra of such planets allows us to place useful constraints on atmosphere circulation models. We perform injection tests with MIRC-X and MYSTIC at CHARA for the hot Jupiter exoplanet Ups And b to show that we are reaching down to a contrast of 2e-4. The promise of both these methods demonstrate that optical interferometers are a valuable tool for probing unique regimes of exoplanet science.

The Very Large Telescope Interferometer is one of the most proficient observatories in the world for high angular resolution. Since its first observations, it has hosted several interferometric instruments operating in various bandwidths in the infrared. As a result, the VLTI yields countless discoveries and technological breakthroughs. We introduce to the VLTI the new concept of Asgard: an instrumental suite including four natively collaborating instruments: BIFROST, a stellar interferometer dedicated to the study of the formation of multiple systems; Hi- 5, a nulling interferometer dedicated to imaging young nearby planetary systems in the M band; HEIMDALLR, an all-in-one instrument performing both fringe tracking and stellar interferometry with the same optics; Baldr, a fibre-injection optimiser. These instruments share common goals and technologies. Thus, the idea of this suite is to make the instruments interoperable and complementary to deliver unprecedented sensitivity and accuracy from J to M bands. The interoperability of the Asgard instruments and their integration in the VLTI are the main challenges of this project. In this paper, we introduce the overall optical design of the Asgard suite, the different modules, and the main challenges ahead.

Long-baseline interferometry and high-resolution spectroscopy are two examples of areas that have benefited from astrophotonics devices, but the application range is expanding to other subareas and other wavelength ranges. The VLTI has been one of the pioneering astronomical infrastructure to exploit the potential of astrophotonics instrumentation for high-angular resolution interferometric observations, whereas new opportunities will arise in the context of the future ELTs. In this contribution, I review the current state of the art regarding the interplay between photonic-based solutions and astronomical instrumentation and highlight the growth of the field, as well as its recognition in recent strategy surveys such as the Decadal. I will explain the benefits of different technological platforms making use of photolithography or laser-writing techniques. I will review the most recent results in the field covering simulations, laboratory characterization and on-sky prototyping. Astrophotonics may have a unique role to play in the forthcoming era of new ground-based astronomical facilities, and possibly in the field of space science.

The recombination of a large number of telescopes in an imaging array represents a major long-term challenge of infrared astronomy, which motivates active instrumental research. In the mid-infrared, as initiated in the early 1980s by the group of C.H. Townes at Berkeley, heterodyne interferometry offers a potential path in complement to classical interferometry, being scalable more easily to a large number of telescopes and by relaxing the requirement on a complex infrastructure, but at the cost of a sensitivity penalty. In this review, we present the current status of heterodyne interferometry and its prospects in light of recent technological developments in this technique. We start by recalling the basic working principles of heterodyne interferometry and the sensitivity budget of this technique. We then present the impact of current technological developments — detectors, local oscillators, correlators, phase synchronization — on the building blocks of a heterodyne interferometer. In the last part, we focus on the interest of developing pathfinders of imaging interferometric instruments, and the synergies with classical interferometry. In particular, the dimensioning of pathfinder instruments highlights the trade-off between angular resolution and sensitivity in the design of large imaging interferometers.

We present a characterization bench of a complete photonics correlation scheme for mid-infrared heterodyne interferometry. The bench can handle very high bandwidths RF signals generated by the heterodyne beating of celestial light with a local oscillator on future generation mid-infrared detectors. The bench is composed of a first two-beam stage allowing the mixing of the "science" source with the local oscillator and a second photonics correlation stage made with telecom components. We present the first experimental proof of concept. Two possible photonics correlation concepts including a patented double loop correlation are introduced.

We report the ultrafast laser inscription (ULI) of a 2-telescope integrated optic (IO) beam combiner for K-band interferometry in commercial Infrasil glass. The ULI setup used for this work is based on a 1030 nm femtosecond laser which is paired with a spatial-light-modulator (SLM). The SLM controls the numerical aperture of the focused beam used to write waveguides in the substrate. The optimum ULI parameters were found to inscribe straight single-mode waveguides exhibiting an insertion loss of 1.1 ± 0.1 dB for a 17 mm long chip over the entire K-band. To develop optimal directional couplers, we focused our efforts on investigating the effect of varying the core-to-core separation and the effect of detuning the waveguide parameters in the coupler. By doing so, we have identified fabrication parameters that are suitable for the fabrication of a beam combiner integrating an achromatic 3 dB directional coupler and two photometric taps with a splitting ratio of 80:20. These results demonstrate the capability of the ULI fabrication technique to inscribe efficient achromatic directional couplers in the K-band range. A final fabrication step will involve simple assembly of the beam combiner with input/output fibers in preparation for on-sky testing at the CHARA array planned for July 2022.

One of the major challenges of mid-infrared astronomical heterodyne interferometry is its sensitivity limitations. Detectors capable of handling several 10 GHz bandwidths have been identified as key building blocks of future instruments. Intersubband detectors based on heterostructures have recently demonstrated their ability to provide such performances. In this work we characterize a Quantum Well Infrared Photodetector in terms of noise, dynamic range and bandwidth in a non-interferometric heterodyne set-up. We discuss the possibility to use them on astronomical systems to measure the beating between the local oscillator and the astronomical signal.

A small-footprint 4-telescope photonic beam combiner is at the heart of the Hi-5 instrument, the high-contrast VLTI visitor instrument focusing on the detection and characterization of young exoplanets in the mid-infrared L’ band. Hi-5 implements the technique of nulling interferometry to efficiently suppress the strong stellar radiation of the central source and enhance the detection of the nearby faint planetary signal. Based on the “Double Bracewell” architecture, the photonic nulling beam combiner is designed around three cascaded achromatic directional couplers with 50/50 coupling ratios. This allows the nulled signals of the first two couplers to be cross-combined with a third central combiner, which produces two conjugated asymmetric transmission maps projected onto the sky. Each individual telescope beam passes first through a side-step to suppress uncoupled stray-light. The corresponding flux is then sampled by an asymmetric Y-junction to provide a simultaneous photometric channel for the estimation of the self-calibrated nulls. We report here on the prototyping phase of the Hi-5 4-telescope photonic beam combiner that is manufactured by ultrafast laser inscription in a Gallium-Lanthanum-Sulphide (GLS) glass substrate, which exhibits high transparency in the L’ band of interest. Using our 2-beam spectro-interferometric lab bench, we measure the throughput of the beam combiners, the chromatic and broadband coupling ratios in the 3.6-3.9 μm range for the couplers and the Y-junctions, as well as the broadband interferometric properties of these 4-telescope mid-infrared photonic beam combiners.

Linear-mode, avalanche photodiode arrays (LmAPDs) based on bandgap-engineered HgCdTe, grown by Metal Organic Vapour Phase Epitaxy (MOVPE) are used in low-flux applications, where the signal-to-noise ratio would otherwise be very low. The LmAPD mesa-device architecture provides 100% fill factor, low crosstalk and minimal interpixel capacitance. Saphira (320×256/24 μm) devices operating at an avalanche gain of 50-100 and temperature of 80-90 K are deployed in 12 telescopes as wavefront sensors and notably control the four 8.2 m telescopes in the Very Large Telescope (VLT) interferometer. Some of these devices have operated for many years at full gain. Applications now split into three main categories. Firstly, those with intrinsically weak infrared sources that need moderate avalanche gain but very low dark current – 1k×1k/15 μm and 2k×2k/15 μm arrays are currently in development to service this requirement. Secondly, future adaptive optics (AO) systems, associated with 30 m class telescopes, require larger arrays and frame-rates over 2000 frames/s. A 512×512/24 μm device, specifically for pyramid wavefront sensors, is currently under development for the Extremely Large Telescope (ELT). The third category covers high speed (GHz) APDs mainly for free-space optical communications and LIDAR. This paper provides an update on the technology and status of the developments.

One of the largest limitations faced by nulling interferometers is their extreme sensitivity to any residual optical path differences between the incoming telescope beams induced by the atmosphere, even after adaptive optics or fringe-tracker correction. Here, we present laboratory characterization of active phase-actuators built into a photonic chip containing nulling interferometers in the H-band. Utilizing the thermo-optic effect to actively control the phase delay in the waveguides, we present their performance in terms of accuracy, chromaticity, response time, and stability. We also measure a response time of less than 1ms, and a reproducibility to less than 0.5 degrees of phase (2 nm of optical path). We show a closed-loop demonstration of on-chip piston correction by using the flux measured on multiple ‘bright’ outputs of a Multimode Interference coupler based nuller to servo upstream phase actuators.

Kernel-nulling is a beam combination architecture for a nulling interferometer optimized for high-contrast companion detection at high angular resolutions. This novel approach is able to suppress the photon noise from stellar light by coherent destructive interference, similar to a classical nuller, and in addition produces a pair of raw nulled outputs that possess the same response to instrumental/environmental perturbations while still producing distinct response to off-axis light. The difference between a pair of raw outputs will result in a kernel-null observable which is robust to piston induced phase error terms to second order, removing the dominant source of noise in classical nulling. We present initial laboratory characterization results of a four-input kernel-nuller photonic device at near-infrared wavelengths. The necessary routing, beam splitting, and recombination is performed within a planar silicon nitride (SiN) photonic chip fabricated using photo-lithography. The four input beams are recombined to create one bright output and three nulled outputs. The linear combination of a pair of these nulled outputs creates one kernel-null. This device is designed to explore the possible circuitry for the VIKiNG visitor instrument being commissioned for the VLTI. We also present simulation results of how kernel-nulling can be used at the VLTI for the astrophysical detection of high-contrast companions.

An accurate metrology system is required to stabilize the differential path lengths in the Nulling Interferometry Cryogenic Experiment (nice) to within 0.45nm peak-to-peak to achieve broadband mid-infrared nulls with long exposure times, which are required for potential future space missions, such as the Large Interferometer for Exoplanets (life) mission, that aim to directly image and characterize temperate terrestrial exoplanets. For this purpose, a differential heterodyne laser distance metrology is developed to enable differential path length measurements that are stable over multiple days with sub-nanometer accuracy at a bandwidth of 1 kHz. The system aims to solve several challenges that arise in the context of NICE, such as the need for long-term stability, the high intensity attenuation through the NICE beam path, and the requirement that the metrology be able to deliver low-latency feedback for closed-loop operation to compensate vibrations and drifts of the nulling testbed. The metrology uses a 633nm HeNe laser and operates at ambient temperature and pressure with a beat frequency of 10 kHz, which is generated by acousto-optic modulators. To improve long-term stability, the compact optical layout is optimized for low susceptibility to temperature variations. Over periods of 2 s, the intrinsic instability of the metrology is ≈ 80pm RMS when sampling at 10 kHz, and it is stable to within ≈ 0.5nm peak-to-peak for 2 hours. When correcting the distance measurements for the temperature of the metrology board, it is stable to within ≈ 1nm peak-to-peak for 14 hours. The metrology fulfils the stability and bandwidth requirements for nice for a duration of at least 2 hours. To achieve stability over even longer time periods, the metrology will later be placed in a temperature-controlled vacuum environment.

Optical interferometry from space is arguably the most exciting prospect for high angular resolution astrophysics; including the analysis of exoplanet atmospheres. This was highlighted in the recent ESA Voyage 2050 plan, which pointed out the exciting potential of this technology, but also indicated the critical need for technological demonstrators. Here we present the Pyxis interferometer; a ground-based pathfinder for a CubeSat space interferometer, currently being built at Mt Stromlo Observatory. We outline its technological and scientific potential as the only visible wavelength interferometer in the Southern Hemisphere, and the optical systems designed to provide CubeSat compatible metrology for formation flying.

The Pyxis interferometer is a ground-based pathfinder for an optical space interferometer being built at Mt Stromlo Observatory. In this second paper, we discuss the system breakdown of the interferometer and its mechanical design. We outline the interferometer’s three robotic platforms, with two telescopic collectors and one central beam combiner. Each collector utilises a full aluminium, diamond-turned telescope, designed to remove thermal distortions for future space applications. We also explain the chosen control system for the interferometer and how it will be used to ensure the linear and angular control requirements, including sub-milli-radian angular control of each robot.

Astrophysical goals articulated in the recent Astronomy Decadal Review demand significant strides in high sensitivity, high spatial resolution observing. Current ground-based interferometric observatories have validated and demonstrated synthetic aperture observation, but are fundamentally limited in sensitivity by the Earth’s atmosphere. Lowell Observatory and Redwire Space, Inc., have recently concluded NASA SBIR Phase II work in developing ‘Optimast’ interferometry technologies which provide us with a toolkit for mission development. Optimast research included demonstrations of a prototype beam combiner, outboard feed optics tracking, and in-situ boom manufacturing for structurally connected free fliers. Our mission’s roadmap includes affordable small demonstrators such as free fliers for flexible at-will observing of the entire sky, and lunar surface ‘suitcase’ concepts for rapid, simple implementation of sensitive, high resolution facilities.

The mirrors of astronomical interferometers need to be aligned within a fraction of a wavelength relative to one another. This would be especially challenging for optical instruments with mirrors separated by hundreds of meters flying in Earth’s orbit. However, in this work, we show that this alignment can be achieved by means of: (i) flying the mirror cluster in a particular orbital configuration; (ii) closing a coarse positioning loop using GNSS (Global Navigation Satellite System); and (iii) closing a fine wavefront-control loop using light from a laser guide star. The orbital configuration is designed to keep the mirrors passively pointing at the target star (up to a small orbital perturbation) while the interferometer cluster is orbiting and changing its baseline. The laser guide star would be flying in the same orbit but in the opposite direction. In medium- or high-Earth orbit, the interferometer would be able to observe a star for several hours per orbit. In this work, we analyzed the performance of an optical space interferometer consisting of nine 20 cm mirrors mounted on CubeSats and flying 3 km apart (together with a combiner and a laser guide star small satellite). This configuration supports a resolution of 0.04 milliarcseconds - an order of magnitude better than current ground-based interferometers. We estimate the performance of this system imaging stellar surfaces assuming perfect wavefront estimation and control.

Image reconstruction in optical interferometry has become an important asset for astrophysical studies during the last decades. This has been mainly due to improvements in the imaging capabilities of existing interferometers such as the second generation of beam combiners at the Very Large Telescope Interferometer; or the expected facilities like the Sparse Aperture Masking mode of the James Webb Space Telescope. Since 2004, the community has organized a biennial contest to formally test the different methods and algorithms for image reconstruction. In 2022, we celebrated the 9th edition of the ”Optical Interferometry Imaging Contest”. This initiative represents an open call for the different scientific groups to present their advances in the field of interferometric image reconstruction with sparse infrared arrays. This contest represents a unique opportunity to benchmark, in a systematic way, the current advances and limitations in the field, as well as to discuss possible future approaches. In this work, we summarize: (a) the rules of the 2022 contest; (b) the different data sets used and the selection procedure; (c) the methods and results obtained by each one of the participants; and (d) the metrics used to select the best reconstructed images. Finally, we named John Young as winner of this edition of the contest and Jacques Kluska as winner of an honorific mention for his participation in the contest.

In interferometry, the quality of the reconstructed image depends on the algorithm used and its parameters, and users often need to compare the results of several algorithms to disentangle artifacts from actual features of the astrophysical object. Such comparisons can rapidly become cumbersome, as these software packages are very different. OImaging is a graphical interface intended to be a common frontend to image reconstruction software packages. With OImaging, the user can now perform multiple reconstructions within a single interface. From a given dataset, OImaging allows benchmarking of different image reconstruction algorithms and assessment of the reliability of the image reconstruction process. To that end, OImaging uses the IMAGE-OI OIFITS extension proposed to standardize communication with image reconstruction algorithms.

The GRAVITY near-IR interferometer allows to observe the Galactic Center at very high angular resolution and to precisely track stellar orbits around Sagittarius A*. In the context of testing General Relativity close to the supermassive black hole, deep imaging is essential to search for faint stars on potentially short orbits. Here, we present G^R , a new imaging tool specifically designed for Galactic Center observations with GRAVITY. The algorithm is based on a Bayesian interpretation of the imaging problem, formulated in the framework of Information Field Theory and builds upon existing work from radio-interferometric imaging. Its application to GRAVITY observations from 2021 yields the deepest images of the Galactic Center on scales of a few milli-arcseconds, to date. Apart from detecting the known stars S38, S42, S60 and S63 within the central 100 mas around Sagittarius A*, the discovery of S300, a faint star moving at high angular velocity, demonstrates the capability of our met

Photonic technologies have enabled a new generation of nulling interferometers such as the GLINT instrument, potentially capable of imaging exoplanets and circumstellar structure at extreme contrast ratios by suppressing contaminating starlight, and paving the way to the characterisation of habitable planet atmospheres. But even with cutting edge photonic nulling instruments, the achievable starlight suppression (null-depth) is only as good as the instrument’s wavefront control, and its accuracy is only as good as the instrument’s calibration. Here we present a new approach wherein outputs from non-science channels of a photonic nulling chip are used as a precise null-depth calibration method, and can also be used in realtime for fringe tracking. This is achieved by using a deep neural network to learn the true in-situ complex transfer function of the instrument, and then predict the instrumental leakage contribution (at millisecond timescales) for the science (nulled) outputs, enabling accurate calibration. In this method, this pseudo-realtime approach is used instead of the statistical methods used in other techniques (such as numerical self calibration, or NSC), and also resolves the severe effect of read-noise seen when NSC is used with some detector types.

Interferometry delivers the highest angular resolution. It is being used extensively in radio astronomy and, since about a decade, it is becoming an important player in infrared astronomy. However, infrared interferometry is restricted to sparse arrays and no full-phase information is recovered. While imaging is arguably the most intuitive way to analyze interferometric data, recovering images from sparsely sampled visibilities is an “ill-posed" problem. The current algorithms work under the framework of using regularized minimization techniques. These algorithms strongly depend on the priors and hyperparameters pre-defined. This gives rise to ambiguities and artifacts in the interpretation of the images and limits their accuracy/precision as well as their signal-to-noise ratio if the priors/regularizers are not well-defined. Also, it means that imaging is the domain of a handful of highly experienced astronomers, thus keeping the interferometric community small. CASSINI-AUTOMAP aims at disrupting this situation by creating a novel framework for interferometric image reconstruction. This project is based on the exploitation of the compressibility of a signal (following the principles of theory of Compressed Sensing) with a novel optimization scheme supported by Neural Networks. In particular, we focus our efforts in designing a Neural Network with adaptive activation functions to find an optimal mapping system between the infrared interferometric data and the reconstructed images. The online adaptability of the Neural Network frees us from having to rely on strong priors, making the reconstructions more accurate and less dependent on users' inputs. Our preliminary network architecture has been tested with Sparse Aperture Masking (SAM) data taken with the infrared camera NACO at the Very Large Telescope and it demonstrates the potential and reliability of the algorithm by recovering the interferometric observables. Future improvements on the software aims at analyzing data from instruments like GRAVITY at the Very Large Telescope Interferometer or the Sparse Aperture Masking mode of the James Webb Space Telescope.

Kernel phase interferometry (KPI) is a data processing technique that allows for the detection of assymetries arising from companions of disks in high-Strehl AO-corrected images, close to or beyond the limit of traditional approaches. We show that this technique can be applied to the CHARIS IFS (with SCExAO), by recovering a known binary HD 44927 with CHARIS’s high resolution K-band mode. Currently, KPI is limited by residual systematic errors in the kernel phases. We show that these errors can be mitigated during calibration, by taking advantage of the information available in adjacent wavelength bands. This spectral differential imaging (SDI) calibration method is a promising avenue towards achieving photon noise limited kernel phase observations.

Kernel phase is a method to interpret stellar point source images by considering their formation as the analytical result of an interferometric process. Using Fourier formalism, this method allows for observing planetary companions around nearby stars at separations down to half a telescope resolution element, typically 20 mas for a 8m class telescope in H band. The Kernel-phase analysis has so far been mainly focused on working with a single monochromatic light image, recently providing theoretical contrast detection limits down to 10⁻⁴ at 200 mas with JWST/NIRISS in the mid-infrared by using hypothesis testing theory. In this communication, we propose to extend this approach to data cubes provided by integral field spectrographs (IFS) on ground-based telescopes with adaptive optics to enhance the detection of planetary companions and explore the spectral characterization of their atmosphere by making use of the Kernel-phase multi-spectral information. Using ground-based IFS data cube with a spectral resolution R=20, we explore different statistical tests based on kernel phases at three wavelengths to estimate the detection limits for planetary companions. Our tests are first conducted with synthetic data before extending their use to real images from ground-based exoplanet imagers such as Subaru/SCExAO and VLT/SPHERE in the near future. Future applications to multi-wavelength data from space telescopes are also discussed for the observation of planetary companions with JWST.

Despite image reconstruction becoming more widespread when interpreting OIFITS Data, model fitting in u,v space often remains the best way to interpret data, either because of the sparsity of the data, or because a quantitative measurement needs to be done. PMOIRED, is a flexible Python library to visualize, manipulate and model OIFITS data using simple geometric models. The strength of PMOIRED resides in its capability to combine linearly various simple components to create complex scenes, while linking, constraining, and adding priors to fitted parameters. The code also enables grid search to find global minima, as well as data resampling to better evaluate uncertainties. In addition to analytical functions, arbitrary radial profiles, azimuthal variations or sparse wavelet modelling of spectra are implemented.

RHAPSODY is an intensity profile reconstruction code built to handle 2D centro-symmetric structures using interferometric data. RHAPSODY has been built to provide the community with a code requiring less parameters than classical image reconstruction code. This has as consequence leading to less solution non-uniqueness problems, a better convergence, and a better dynamic range assuming a centro-symmetric source at all wavelengths. RHAPSODY main steps are organized as following: In the first place, at each wavelengths, the code build a unique 1D structure made of concentric discrete uniform rings. Then, it applies the Hankel transform to reconstruct the equivalent visibility profile. Next, a change of coordinate is used on the 1D visibility profile to simulate the inclination and the rotation of the structure in the 2D Fourier plane. After a fitting process on the interferometric observations based on a χ² and Bayesian method, RHAPSODY apply an inverse Fourier transform and reconstruct the equivalent structure in the 2D image plane. According to preliminary tests made on RHAPSODY, the code is able to well reproduce 1D and 2D centrosymmetric structures up to a dynamic range of 0.5%.

This session is to recall the career and contributions to our community of Matt Wilson, who tragically passed away prematurely.

The VLTI Expertise Centres were created to fulfill a shortcoming in optical interferometry expertise and to maximize the impact of the VLTI instrumentation. We here present the various activities in which the VLTI Expertise Centres are engaged, with a particular focus on providing user support and expanding the VLTI community.

We present science cases and instrument design considerations for the BIFROST instrument that will open the short-wavelength (Y/J/H-band), high spectral dispersion (up to R=25,000) window for the VLT Interferometer. BIFROST will be part of the Asgard Suite of instruments and unlock powerful venues for studying accretion & mass-loss processes at the early/late stages of stellar evolution, for detecting accreting protoplanets around young stars, and for probing the spin-orbit alignment in directly-imaged planetary systems and multiple star systems. Our survey on GAIA binaries aims to provide masses and precision ages for a thousand stars, providing a legacy data set for improving stellar evolutionary models as well as for Galactic Archaeology. BIFROST will enable off-axis spectroscopy of exoplanets in the 0.025-1" separation range, enabling high-SNR, high spectral resolution follow-up of exoplanets detected with ELT and JWST. We give an update on the status of the project, outline our key technology choices, and discuss synergies with other instruments in the proposed Asgard Suite of instruments.

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.

BIFROST will be a short-wavelength (λ = 1.0 - 1.7 μm) beam combiner for the VLT Interferometer, combining both high spatial (λ/2B = 0.8 mas) and spectral (up to R = 25,000) resolution. It will be part of the Asgard Suite of visitor instruments. The new window of high spectral resolution, short wavelength observations brings with it new challenges. Here we outline the instrumental design of BIFROST, highlighting which beam combiner subsystems are required and why. This is followed by a comparison All-In-One (AIO) beam combination scheme and an Integrated Optics (IO) scheme with ABCD modulation both in terms of expected sensitivity and the practical implementation of each system.

VERMILION is a VLTI visitor instrument project intended to extend the sensitivity and the spectral coverage of Optical Long Baseline Interferometry (OLBIn). It is based on a new concept of Fringe Tracker (VERMILIONFT) combined with a J band spectro-interferometer (VERMILION-J). The Fringe Tracker is the Adaptive Optics module specific to OLBIn that measures and corrects in real time the Optical Path Difference (OPD) perturbations introduced by the atmosphere and the interferometer, by providing a sensitivity gain of 2 to 3 magnitudes over all other state of the art fringe trackers. The J band spectro-interferometer will provide all interferometric measurements as a function of wavelength. In addition to a possible synergy with MATISSE, VERMILION-J, by observing at high spectral resolution many strong lines in J (Pa_β-γ, HeII, TiO and other metallic monoxides), will cover several scientific topics, e.g. Exoplanets, YSOs, Binaries, Active Hot, Evolved stars, Asteroseismology, and also AGNs.

We propose a method to transform a Cherenkov telescope, primarily designed for gamma-ray astronomy, into an imaging telescope at the diffraction limit of an equivalent astronomical telescope. The method can be applied onto existing and planned Cherenkov telescopes, both taken as a single dish or a set of such telescopes configured for aperture synthesis operating as intensity interferometers. We examine the sensitivity of our method by performing extensive numerical simulations including two and three point correlations, for amplitude and closure phase quantities permitting image reconstruction of stellar surfaces with subtle structures in the range of milli-arc-second resolutions. As a case of study we apply our method to a single 20m class Cherenkov telescope with the perspective of its generalization to the diluted Cherenkov Telescope Array (CTA) under construction in North and South hemispheres.

We present the testbench aimed at integrating the GRAVITY+ adaptive optics GPAO. It consists of two independent elements, one reproducing the Coudé focus of the telescope, including the telescope deformable mirror mount (with its surface facing down), and one reproducing the Coudé room opto-mechanical environment, including a downwards-propagating beam, and the telescope mechanical interfaces in order to fit in the new GPAO wavefront sensor. We discuss in this paper the design of this bench and the solutions we adopted to keep the cost low, keep the design compact (allowing it to be fully contained in a 20 sqm clean room), and align the bench independently from the adaptive optics. We also discuss the features we have set in this bench.

Hi-5 is a proposed L' band high-contrast nulling interferometric instrument for the visitor focus of the Very Large Telescope Interferometer (VLTI). As a part of the ERC consolidator project called SCIFY (Self-Calibrated Interferometry For exoplanet spectroscopY), the instrument aims to achieve sufficient dynamic range and angular resolution to directly image and characterize the snow line of young extra-solar planetary systems. The spectrometer is based on a dispersive grism and is located downstream of an integrated optics beam-combiner. To reach the contrast and sensitivity specifications, the outputs of the I/O chip must be sufficiently separated and properly sampled on the Hawaii-2RG detector. This has many implications for the photonic chip and spectrometer design. We present these technical requirements, trade-off studies, and phase-A of the optical design of the Hi-5 spectrometer in this paper. For both science and contract-driven reasons, the instrument design currently features three different spectroscopic modes (R=20, 400, and 2000). Designs and efficiency estimates for the grisms are also presented as well as the strategy to separate the two polarization states.

The vibrations affecting the telescopes and downstream optics largely contribute to the optical path length error in interferometry and constitute an important limitation for fringe tracking sensitivity and high-contrast imaging. At the VLTI-UT, vibrations are measured and compensated with the MANHATTAN-II system. This system currently measures the acceleration in the first three mirrors of the Coud´e train with piezo-electric accelerometers and sends the compensation error to the delay lines. The GRAVITY+ program and the next generation of the VLTI planet-finding instruments require the current Optical Path Distance (OPD) residual of approximately 200 nm rms in the UTs to be smaller than 100 nm, and consequently an extension of the MANHATTAN-II system is envisaged to all the Coud´e train mirrors that are known to be affected by high-frequency vibrations. In this work, we present the planned hardware upgrade of the Manhattan system and an analysis of the self-intrinsic noise of MANHATTAN-II. Piezo accelerometer noise depends on parasitic electric charges, generated for example from long cables. The noise theoretical model is experimentally tested with a dummy accelerometer reproducing the equivalent circuit, and by a three-sensors coherence analysis. We find that the intrinsic-noise for M2-UT1 is of the order of 2 μg/Hz^0.5 in the amplifier bandwidth 1 Hz – 25 kHz. Accounting for all sensors in every mirrors in the extended configuration, the Manhattan-II self-intrinsic noise corresponds to an overall equivalent OPD rms of 166 nm for the shortest possible cable lengths configuration, before the fringe tracker. We suggest that sensors with sub- μg/Hz^0.5 noise could be necessary to approach the ultimate limit of the atmospheric piston calculated from a Kolmogorov model.

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.

This conference presentation was prepared for the Optical and Infrared Interferometry and Imaging VIII conference at SPIE Astronomical Telescopes + Instrumentation, 2022.

The ASTRI Mini-Array is an International collaboration led by the Italian National Institute for Astrophysics (INAF), that will operate nine telescopes to perform Cherenkov and optical stellar intensity interferometry (SII) observations. At the focal plane of these telescopes we are planning to install a stellar intensity interferometry instrument. Here we present the final selected design, based on Silicon Photomultipliers (SiPMs) detectors matching the telescope point spread function together with a dedicated front end electronics.

FOURIER is the first-generation science beam combiner for the MROI. It is a three-way, J, H and K band image plane combiner. The FOURIER design emphasises low visibility losses and high optical throughput and is designed around a low-noise SAPHIRA detector. Based on laboratory measurements of its throughput and visibility losses, FOURIER is expected to reach limiting magnitudes of 12.3, 13.2 and 11.7 in the J, H and K bands, respectively, within 5 minutes of incoherent integration assuming 0.7′′ seeing and a detector read noise of 0.3 electrons. As FOURIER observes as red as the K band, the detector and most of its optics are placed within a liquid nitrogen cryostat. We present the design of FOURIER’s cryostat, as well as laboratory tests of the instrument’s cryogenic performance. We also report room temperature characterisation of the optics. Finally, we discuss the path forward from the current status of the instrument to first fringes in 2023.

Field of View (FoV) and contrast limitations of stellar interferometers have been the scope of numerous publications for more than thirty years. Recently, this topic regained some interest since long-baseline terrestrial interferometers or space borne nulling interferometers are envisioned for detecting and characterizing extra-solar planets orbiting in the habitable zone of their parent star. This goal supposes to achieving sufficient contrast ratio in the high angular frequency domain, thus on the whole interferometer FoV. In this paper are reviewed some of the contrast and FoV limiting factors, including spectral bandwidth, flux mismatches, fringe tracking, telescope image quality, atmosphere seeing, optical conjugation mismatch of the telescopes pupils, influence of anamorphous optics, pupil aberrations, signal-to-noise ratio and deviations with respect to “the golden rule of imaging interferometers”. Finally, a tentative classification of all these factors is provided.

The Hi-5 instrument, a proposed high-contrast L' band (3.5-4.0 μm) nulling interferometer for the visitor focus of the Very Large Telescope Interferometer (VLTI), will characterize young extra-solar planetary systems and exozodiacal dust around nearby main-sequence stars. Thanks to VLTI's angular resolution (λ=B = 5 mas for the longest UT baseline), it will fill the gap between young giant exoplanets discovered by ongoing single-aperture direct imaging surveys and exoplanet populations discovered by radial velocity surveys. In this paper, we investigate the exoplanet detection yield of Hi-5. First, we present the latest catalog of stars identified as members of young stellar associations within 150 pc of the Sun thanks to the BANYAN algorithm and other searches for young moving group members. Realistic exoplanet populations are then generated around these stars and processed with the SCIFYsim tool, the end-to-end simulator for the Hi-5 instrument. Then, two formation models are used to estimate the giant planet's luminosity. The first is the New Generation Planetary Population Synthesis (NGPPS), also known as the Bern model, and the second is a statistical model based on gravitational instability (hot-start model - AMES-Dusty model). We show that Hi-5 is insensitive to cold-start planets but can detect giant hot-start planets. With ATs, more than 40 planets could be detected assuming 20 nights of observations. With its unique capabilities, Hi-5 is also able to constrain in mass the observed systems. Hi-5 is sensitive to planets with a mass > 2 M_jup around the snow line.

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.

The Navy Precision Optical Interferometer (NPOI) has been in operation since 1996, building a substantial data archive. This provided the opportunity to determine how many data points are needed for a single star’s angular diameter fit to become stable as more data points were included. In an iterative process, we calculated the diameters for an ever-increasing number of data points for 31 stars. We found that at approximately 1,000 data points, the scatter in the diameter fits fell below 2%. Assuming a 3-telescope triangle and using 15 channels across a range of wavelengths, 1,000 data points equates to 22 to 25 bracketed observations, which can usually be accomplished in 2 to 4 nights. This will be a useful rule-of-thumb when planning observations and gathering data on single, symmetrical stars.

The differential polarization visibilities R_Q and R_U of an object are the ratios of its visibilities corresponding to orthogonal polarizations, the interferometric analogs to Stokes Q and U intensity images. The measurement of differential polarization visibilitites can be used e.g. for constraining inner parts of circumstellar envelopes of young or evolved stars at the diffraction limited resolution of the feeding telescope. Here we demonstrate the estimation of both amplitude and phase of R_Q and R_U from data obtained using SCExAO VAMPIRES through the full pupil of the 8-m Subaru telescope using the Differential Speckle Polarimetry technique. The correction for biases arising due to instrumental polarization effects is discussed. The accuracy of R_Q, R_U measurement with VAMPIRES is limited by imperfect knowledge of instrumental polarization and amounts to 5 x 10^-3.

Interferometric techniques such as aperture masking have the potential to enhance spatial resolution capabilities when imaging moderate-contrast sources with small angular size, such as close-in exoplanets and circumstellar disks around distant young stars. The Slicer Combined with an Array of Lenslets for Exoplanet Spectroscopy (SCALES) instrument, currently under development, is a lenslet integral field spectrograph that will enable the W. M. Keck Observatory to carry out high-contrast direct imaging of exoplanets between 2 and 5 microns. We explore the potential benefit of aperture masking to SCALES by testing the contrast achievable by several mask designs. The scalessim software package was used to simulate observations at wavelength bins in the M, L, and K bands, with optical path difference (OPD) maps used to simulate realistic Keck adaptive optics performance. Noise from astrophysical and instrumental sources was also applied to simulated signals. Mask designs were assessed based on depth of the generated contrast curves.

We present here the latest results obtained with the C-RED One camera developed by First Light Imaging for fast ultralow noise infrared applications. This camera uses the Leonardo Saphira e-APD 320x256 infrared sensor in an autonomous cryogenic environment with a low vibration pulse tube and with embedded readout electronics system. Some recent improvements were made to the camera. The first important one concerns the total noise of the camera. Limited to 1.75 μm wavelength cut-off with proper cold filters, looking at a blackbody at room temperature and f/4 beam aperture, we now measure total noise down to 0.6 e at gain 50 in CDS mode 1720 FPS, dividing previous noise figure by a factor 2. The total camera background of 30-400 e/s is now achieved with a factor 3 of background reduction, the camera also looking at a room temperature blackbody with an F/4 beam aperture. Image bias oscillations, due to electronics grounding scheme, were carefully analyzed and removed. Focal plane detector vibrations transmitted by the pulse tube cooling machine were also analyzed, damped and measured down to 0.3 μm RMS, reducing focal plane vibrations by a factor 3. In addition, a vacuum getter of higher capacity is now used to offer camera operation without camera pumping during months. We also delivered C-RED One cameras with K band limitation instead of our classical H band configuration. The camera can deal with room temperature observation in K band by limiting the beam aperture to f/20. The camera main characteristics are detailed: pulse tube cooling at 80K with limited vibrations, permanent vacuum solution, ultra-low latency Cameralink full data interface, safety management of the camera by firmware, online firmware update, ambient liquid cooling and reduced weight of 20 kg. C-RED one is the only modern infrared camera offering such level of performances in terms of reliability, noise and speed.

Observations using interferometers provide sensitivity to features of images on angular scales much smaller than any single telescope, on the order of Δθ ∼ λ/b where b is the interferometric baseline. Present-day optical interferometers are essentially classical, interfering single photons with themselves. However, there is a new wave of interest in interferometry using multiple photons, whose mechanisms are inherently quantum mechanical, which offer the prospects of increased baselines and finer resolutions among other advantages. We will discuss recent ideas and results for quantum-assisted interferometry using the resource of entangled pairs, and specifically a two-photon amplitude technique aimed at improved precision in astrometry.

We recently performed tests of the discrete beam combiner (DBC) through an on-sky experiment using a 4-input pupil remappers-based integrated optics device. Here, we report on the lessons learned, as well as visibilities and closure phase results for our stellar target, Vega. Through complementary simulations, we analyze how the residual phase errors, input power imbalance at the waveguides, slow environmental changes, and different photon levels affect the performance of the DBC. This is an important aspect to improve future on-sky calibration strategies for this type of beam combiner, in particular when combining a large number of apertures.

The JWST Early Release Science Program 1386: “High Contrast Imaging of Exoplanets and Exoplanetary Systems includes a NIRISS aperture masking interferometry (AMI) component to demonstrate achievable contrast in a deep observation. As part of this program, we have developed SAMpy: data reduction tools tailored for NIRISS AMI, which are also flexible enough for arbitrary masking setups (e.g. LBT/LMIRCam, Keck/NIRC2, VLT/SPHERE). SAMpy takes a Fourier-plane approach to processing AMI data, which we describe here. We detail the individual pipeline steps, including image pre-processing, calculation and calibration of Fourier observables, OIFITS file generation, and model fitting. We also demonstrate SAMpy on NIRISS commissioning observations of the AB Dor system.

Much research has been done to show the possibilities of using long transport fibers in optical interferometry. The CHARA Michelson Array Pathfinder will extend the spatial coverage of the CHARA Array by adding a mobile 1-meter telescope connected by optical fibers. The pathfinder will operate in H-band and will explore baselines up to approximately 1 km, giving an angular resolution of 0.2 mas. The new telescope will be placed at short baselines to image the surfaces of large stars and at long baselines to resolve small stars. Here we describe the project and our progress on various subsystems.