The Center for High Angular Resolution Astronomy (CHARA) Array is a six-element interferometer with baselines ranging from 34 to 331m. The Array has had many upgrades in recent years including new beam combiners: MYSTIC is a 6T combiner for K-band; SPICA is a 6T combiner for the visible R-band; and SILMARIL is a 3T combiner for high sensitivity in the H- and K-bands. A seventh, mobile telescope is now on site for use with fiber optics for beam transport. Observing time is available to the community through a program funded by NSF. The observing programs are solicited and peer-reviewed by NSF’s NOIRLab. Here we summarize the scientific work and the on-going technical advances of the CHARA Array.

SPICA (Stellar Parameters and Images with a Cophased Array) is a 6-telescope (6T) visible instrument for the CHARA Array (Center for High Angular Resolution in Astronomy) at Mount Wilson Observatory. It uses single mode fibers for feeding the interferometric spectrograph, which offers three different spectral resolutions: R=140, R=4000, and R=14000. CHARA/SPICA has been mainly designed for large programs (surveys) in the domain of stellar fundamental parameters but also permits fast imaging thanks to the 15 baselines and the large number of spectral channels (60 in low resolution mode). SPICA is made of the visible instrument SPICA-VIS and of a new H-band, 6T, ABCD combiner performing group delay and phase delay tracking. In this paper, we present the first light results of SPICA.

The Center for High Angular Resolution Astronomy (CHARA) Array currently consists of 6 telescopes at fixed positions, connected by vacuum pipes to the delay lines. The CHARA Michelson Array Pathfinder (CMAP) project includes two major components: 1) a mobile telescope that can be placed at a number of locations, and 2) a fiber optic relay system to transport light to the beam combining facility. The telescope will be equipped with a custom-built instrument bench with adaptive optics and fiber injection. The light will be transported by optical fibers to the existing CHARA delay lines. In this contribution, we present the progress on the various subsystems needed to integrate the new telescope and beam transport method into the existing CHARA environment. We will also describe our efforts to find internal fringes with the new fibers.

Silmaril is a new 3-telescope beam combiner at the CHARA Array. In this presentation, we present the current design of the instrument, its on-sky measured and theoretical best performance, and its future development. Its design is specifically made to push for sensitivity. It combines 3 beams of the CHARA Array, allowing closure phase measurement but limiting the loss in sensitivity. It limits the number of optical elements to the minimum needed for combining the 3 beams. Using a CRED ONE camera allows for sub-electron readout noise, which means that our sensitivity is limited by the thermal background. Long focal-length cylindrical mirrors limit the thermal background by using an f/20 cold stop in front of the camera. We use a low spectral resolution prism to allow fringe tracking without compromising the instrument’s sensitivity. Thanks to an ingenious edge filter design, we can observe both H- and K-band simultaneously, with a low thermal background on the H-band side of the detector. In the future, we intend to extend the instrument with a second set of 3 beams and add a Narcissus mirror to lower the thermal background and improve the sensitivity even more.

The Large Binocular Telescope Interferometer (LBTI) is a strategic instrument which combines the two 8.4m apertures of the LBT for sensitive, high-angular-resolution imaging and interferometric observations in the thermal infrared. Through its observing modes utilizing adaptive optics, Fizeau imaging, and nulling interferometry, the LBTI is in many respects the first ELT; it serves as a pioneer for upcoming ELTs in terms of both science and instrumentation. LBTI has completed a large survey for habitable-zone dust around main sequence stars, exploiting its angular resolution to obtain 100x better sensitivity than space-based photometric observations. Recently we have emphasized Fizeau imaging, supporting high-contrast and precision-astrometric observations. We obtained the first extragalactic and N band observations in this mode, demonstrating high-fidelity, high-sensitivity imaging on a 23m baseline. We are now pushing to image the first rocky planet in the habitable zone around a nearby Sun-like star. In this paper we present an overview of the LBTI’s design and capabilities as a 23m telescope. In particular, we focus on open loop Fizeau imaging, presenting the state of the art. We measure the stability of the Fizeau PSF, test frame selection criteria, and demonstrate PSF deconvolution. Finally, we outline future developments and synergies with current and upcoming facilities.

GLINT is a nulling interferometer downstream of the SCExAO extreme-adaptive-optics system at the Subaru Telescope (Hawaii, USA), and is a pathfinder instrument for high-contrast imaging of circumstellar environments with photonic technologies. GLINT is effectively a testbed for more stable, compact, and modular instruments for the era of ∼30m-class telescopes. GLINT is now undergoing an upgrade with a new photonic chip for more achromatic nulls, and for phase information to enable fringe tracking. Here we provide an overview of the motivations for the GLINT project and report on the design of the new chip, the on-site installation, and current status.

The Fiber Imager foR a Single Telescope (FIRST) is a visible spectro-interferometer (600-760 nm, R~400) installed on the Subaru telescope's extreme adaptive optics platform (SCExAO). It provides high-precision spatial coherence measurements with high angular resolution (~8 mas at 656 nm, ~1% coherence accuracy) by combining light from sub-apertures of the telescope pupil. We present two upgrades of the instrument towards H𝛼 detection of protoplanets. We report on the integration of a new 4000-resolution spectrograph and on the sensitivity of the instrument. We also present the characterisation of a high performance visible photonic integrated circuit prototype used for the interferometric combination of 5 sub-apertures.

We will present the status of the Navy Precision Optical Interferometer. We will discuss upgrades that occurred over the course of the last couple of years, their related scientific achievements, ongoing and future work. We will discuss the improvements done to the infrastructure of the site, the return to observations with 6 telescopes simultaneously and the results of these observations. We will discuss the deployment of new capabilities, such as an infrared beam combiner, siderostat controllers and a new angle tracker. We will also present the deployment of the Amon Hen hypertelescope experiment and changes done to the inner room in order to accommodate the use of both systems without the need of large rearrangements of the optics.

The VLTI continues to develop along the lines of its 2017 roadmap. A new 200m AT baseline is now available, pushing the VLTI angular resolution and imaging capability. The second generation instruments GRAVITY and MATISSE continue delivering outstanding results. The continued developments of the VLTI, especially with the UTs, are now happening under the umbrella of the GRAVITY+ project. Fringe tracking has significantly improved following an upgrade of the GRAVITY fringe tracker and the completion of GRAVITY for MATISSE. The wide off-axis capability of the interferometer, in science operation with GRAVITY wide, is now delivering key science results. The imminent installation of the new UT adaptive optics GPAO will soon improve the high-contrast and short wavelength performance on bright objects. The addition of laser guide stars in 2025 will do the same for much fainter targets.

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

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

ESO’s Very Large Telescope Interferometer has a history of record-breaking discoveries in astrophysics and significant advances in instrumentation. The next leap forward is its new visitor instrument, called Asgard. It comprises four natively collaborating instruments: HEIMDALLR, an instrument performing both fringe tracking and stellar interferometry simultaneously with the same optics, operating in the K band; Baldr, a Strehl optimizer in the H band; BIFROST, a spectroscopic combiner to study the formation processes and properties of stellar and planetary systems in the Y-J-H bands; and NOTT, a nulling interferometer dedicated to imaging nearby young planetary systems in the L band. The suite is in its integration phase in Europe and should be shipped to Paranal in 2025. In this article, we present details of the alignment and calibration unit, the observing modes, the integration plan, the software architecture, and the roadmap to completion of the project.

BIFROST is the short-wavelength, high-spectral resolution instrument in the Asgard Suite of VLTI visitor instruments. It will be optimized for spectral line studies in the Y, J, and H bands (1.05-1.75 μm) that include many strong lines & molecular features. In this presentation, we outline the BIFROST science drivers that have guided our design choices and map them against the operational modes that are being implemented. We give an overview about the status of the project and the milestones from the ongoing integration & testing phase in Exeter to shipping & commissioning on Paranal, scheduled for 2025 and 2026. We review the BIFROST subsystems and discuss how they interface with the broader Asgard Suite. Finally, we outline other BIFROST-related activities pursued by our group that are intended for implementation in BIFROST as part of future upgrades.

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.

The Magdalena Ridge Observatory Interferometer is an ambitious project to build a 10 telescope long-baseline optical/near-infrared in the mountains about a one-hour drive outside of Socorro, NM. The project is being led by New Mexico Institute of Mining and Technology and being built in cooperation with our primary collaborators at the University of Cambridge. We are currently funded via a cooperative agreement with the Air Force Research Lab in Albuquerque, NM to demonstrate imaging capabilities on geosynchronous objects. We have recently installed the second full beamline for the interferometer system and are working our way towards first fringes on an ~8m baseline later this year. In this manuscript, we report on the status of each of the subsystems, the installation progress and challenges to date, and on the ramp-up to measurements of first fringes. We also report on plans for early science and offer public shared-risk access with the facility in the near future.

The Magdalena Ridge Observatory Interferometer has been designed to deliver an unprecedented capability for model-independent imaging of faint astronomical targets. As a consequence, its design methodology has focused on optimizing the interferometric sensitivity of all of its opto-mechanical subsystems. We report here on initial testing of one of the MROI beam-trains, outlining the performance metrics utilized to characterize the elements of the optical train from the Unit Telescopes through to the MROI beam combiner tables, the tests performed on each subsystem, and how our results compare to the design error budget for the MROI. The impact of the tests on the initial sensitivity limit of the MROI are discussed.

In the early 1960’s a new stellar interferometer began to emerge in dusty outback NSW. The highly unorthodox design was the brainchild of Robert Hanbury Brown and Richard Twiss and was led from the University of Sydney. The technology represents the culmination of a pioneering series of experiments which were accepted by radio engineers, but which courted controversy when performed in the optical, generating skepticism (and worse) from some of the most eminent physicists of the day. These critics were eventually silenced by the overwhelming success of the Narrabri Stellar Intensity Interferometer which made outstanding contributions to fundamental stellar physics. The diameter and effective temperature scale for hot stars was based on data only matched more than a 60 years later. However NSII’s most profound legacy is as the foundational experiment in the field now known as Quantum Optics. Recent years have seen a resurgence of interest with several air ˇCherenkov arrays with large collectors, including the emerging CTA, building an intensity interferometer back-end. This review will draw lessons from the past and help inform prospects for the future from this enduring technique.

The ASTRI Stellar Intensity Interferometry Instrument (SI3) is a fast single photon counting instrument for performing intensity interferometry observations of bright stars with the ASTRI Mini-Array. SI3 is designed to perform accurate measurements of single photon arrival times (1ns) in a narrow optical bandwidth (1-8nm) centered at a wavelength in the range 420-500nm. The instrument will exploit the 36 simultaneous baselines over distances between 100m and 700m of the ASTRI Mini-Array to achieve angular resolutions below 100 microarcsec. 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. During 2023 SI3 underwent a significant redesign, with an optical fiber positioned on the focal plane to feed the detectors and electronics. Here we present this new baseline design of SI3, and the motivations behind this choice, including the possibility of future upgrades of the instrument with dedicated front-end electronics and channel multiplexing. We will also show the first results of the target selection procedure based on simulations. Stars with angular diameters of less than 500- 600 microarcseconds up to about magnitude 4.5 will be observable. Thanks to the 36 simultaneous baselines, accurate (up to ∼1%) angular measurements can be obtained with 10-30 hours of observations. This accuracy can rival with that obtained with other arrays of Cherenkov telescopes, despite the smaller collecting area of a single ASTRI telescope.

The Planet Formation Imager (PFI) Project is dedicated to defining a next-generation facility that can answer fundamental questions about how planets form, including detection of young giant exoplanets and their circumplanetary disks. The proposed expansive design for a 12-element array of 8m class telescopes with >1.2km baselines would indeed revolutionize our understanding of planet formation and is technically achievable, albeit at a high cost. It has been 10 years since this conceptual design process began and we give an overview of the status of the PFI project. We also review how a scaled back PFI with fewer large telescopes could answer a range of compelling science questions, including in planet formation and as well as totally different astrophysics areas. New opportunities make a space-based PFI more feasible now and we give a brief overview of new efforts that could also pave the way for the Large Interferometer for Exoplanets (LIFE) space mission.

MATISSE is the 2nd generation mid-infrared (3.0μm to 12.0μm) spectro-interferometric instrument of the Very Large Telescope Interferometer (VLTI). It was designed to deliver its advertised performance when supported by an external fringe tracker. This proceeding gives an historical account of how the fringe tracker of the GRAVITY instrument, another 2nd generation K-band spectro-astrometric instrument of VLTI, became this external fringe tracker. For a more technical and performance-oriented description of the GRAVITY for MATISSE project, Woillez, Petrov, et al. (2024) should be consulted.

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

We present the Wavefront Sensor units of the Gravity Plus Adaptive Optics (GPAO) system, which will equip all 8m class telescopes of the VLTI and is an instrumental part of the GRAVITY+ project. It includes two modules for each Wavefront Sensor unit: a Natural Guide Star sensor with high-order 40×40 Shack-Hartmann and a Laser Guide Star 30×30 sensor. The state-of-the-art AO correction will considerably improve the performance for interferometry, in particular high-contrast observations for NGS observations and all-sky coverage with LGS, which will be implemented for the first time on VLTI instruments. In the following, we give an overview of the Wavefront Sensor units system after completion of their integration and characterization.

The GRAVITY instrument has transformed the field of near-infrared interferometry and is redefining the limits of ground-based observations. In Galactic Center observations, this is shown by routinely achieving below 50 μas uncertainty on astrometric measurements within a 5-minute exposure and detecting stars fainter than 19th magnitude. Nevertheless, systematic effects are still limiting the instrument's performance. In this talk, I will introduce two observing modes to overcome these limitations: Pupil modulation to improve the astrometry and metrology attenuation to overcome SNR limitations. I will detail these two modes and show how significant the improvements are on examples of on-sky data.

Asgard/NOTT is an ERC-funded project hosted at KU Leuven and is part of a new visitor instrumental suite, called Asgard, under preparation for the Very Large Telescope Interferometer (VLTI). Leveraging nulling capabilities and the long VLTI baselines, it is optimized for high-contrast imaging of the snow line region around young nearby main-sequence stars. This will enable the characterization of the atmosphere of young giant exoplanets and warm/hot exozodiacal dust with spectroscopy in the L’-band (3.5-4.0μm). In this work, we present the first lab assembly of the instrument done at KU Leuven and the technical solutions to tackle the challenge of performing nulling in the mid-infrared despite the thermal background. The opto-mechanical design of the warm optics and the injection system for the photonic chip are described. The alignment procedure used to assemble the system is also presented. Finally, the first experimental results, including fringes and null measurements, are given and confirm the adequacy of the bench to test and optimize the Asgard/NOTT instrument.

As the high-contrast visitor instrument for VLTI, the NOTT aims to detect and characterize young exoplanets in the mid-infrared L’ band through nulling interferometry. This report outlines recent advancements in component fabrication via ultrafast laser inscription on GLS samples. Laboratory characterization, conducted on a 4-spectral interferometric testbench, is the first detailed mid-infrared characterization of such a photonic chip in its complete four-telescope nulling configuration, demonstrates favorable throughput, achromatic splitting ratios, and self-calibrated nulling performance within the 3.65-3.85 µm range. These achievements align with the specific requirements of NOTT, marking significant progress in meeting its specifications for high-precision observations.

Over the past two decades, photonics have been developed as technological solutions for astronomical instrumentation for, e.g., near-infrared spectroscopy and long baseline interferometry. With increasing instrument capabilities, large quantities of high precision optical components are required to guide, manipulate, and analyze the light from astronomical sources. Photonic integrated circuits (PICs) and fiber-based devices offer enormous potential for astronomical instrumentation, as they can reduce the amount of bulky free-space optics and pave the way for innovative solutions. Astrophotonic devices are particularly interesting for interferometry due to their compact design on the centimeter scale, even for a large number of telescope inputs. Already, astrophotonic components are integrated in high-end instruments at the VLTI and at the CHARA Array. Photonic beam combiners at wavelengths from visible to mid-infrared have been fabricated using lithographic techniques or ultrafast-laser inscription, with several components tested on-sky. This paper will provide a glimpse into the growing field of astrophotonics, its current status and potential for new technologies.

Coronagraphs allow for faint off-axis exoplanets to be observed, but are limited to angular separations greater than a few beam widths. Accessing closer-in separations would greatly increase the expected number of detectable planets, which scales inversely with the inner working angle. The Photonic Lantern Nuller (PLN) is an instrument concept designed to characterize exoplanets within a single beam-width of its host star, using a device called the mode-selective photonic lantern (MSPL), a photonic mode-converter that maps linearly polarized modes into individual single-mode outputs. The PLN leverages the spatial symmetry of an MSPL to create nulled ports, which cancel out on-axis starlight but allow off-axis exoplanet light to couple. The null-depths are limited by wavefront aberrations in the system as well as by imperfections in the lantern’s response. However, wavefront sensing and control can be used to improve the null-depths achievable. We extend the technique of Implicit Electric Field Conjugation, commonly used to create dark zones with coronagraphic instruments, to work with a PLN. We present results from simulations and from in-lab testbed experiments.

Spectroastrometry, which measures wavelength-dependent shifts in the center of light, is well-suited for studying objects whose morphology changes with wavelength at very high angular resolutions. Photonic lantern (PL)-fed spectrometers have potential to enable measurement of spectroastrometric signals because the relative intensities between the PL output SMFs contain spatial information on the input scene. In order to use PL output spectra for spectroastrometric measurements, it is important to understand the wavelength-dependent behaviors of PL outputs and develop methods to calibrate the effects of time-varying wavefront errors in ground-based observations. We present experimental characterizations of the 3-port PL on the SCExAO testbed at the Subaru Telescope. We develop spectral response models of the PL and verify the behaviors with lab experiments. We find sinusoidal behavior of astrometric sensitivity of the 3-port PL as a function of wavelength, as expected from numerical simulations. Furthermore, we compare experimental and numerically simulated coupling maps and discuss their potential use for offsetting pointing errors and for building PL spectral response models that could be used for further calibration strategies.

Nulling interferometry is a promising technology to enable prospecting for and characterising sub-stellar companions at extremely close separations and high contrasts. The most scientifically rewarding observations will require extremely well corrected wavefronts in order to deliver consistent, deep nulls that suppress the flux from the central star. We present a two stage system whose first stage deploys a hybrid mode-selective photonic lantern to optimally inject starlight into a single mode fibre. Starlight that does not couple into the primary science fibre is used to sense wavefront errors, including petaling modes that are typically invisible to other methods. The architecture also yields wavefront corrections that are free of non-common path errors. Repeated over multiple telescopes, our system then feeds a second-stage kernel nuller chip implemented as an operating mode of Bifrost in the Asgard instrument suite. This operating mode will enable a variety of science cases including constraining the entropy of formation of giant exoplanets, studying debris disk formation and surveying lensing events for the detection of black holes, all of which drive the requirements for the instrument. We illustrate candidate designs and present early simulations of the modules, finding that Seidr is a feasible means of capitalising a historical window of opportunity to further high contrast and high angular resolution imaging.

This conference presentation was prepared for SPIE Astronomical Telescopes + Instrumentation, 2024.

For Very Long Baseline Interferometry high-resolution imaging of exoplanets, an astrophotonic-based aperture synthesis concept is proposed for high-resolution direct imaging of exoplanets. A silicon photonic chip incorporates microheaters and optical phase shifters for precise control of delays and phase synchronization from widely separated receivers. A satellite laser guide star with a modulated optical signal serves as a global phase reference, enabling high-speed, high-stroke phase compensation and combining. The chip's design addresses challenges such as atmospheric turbulence and phase stability in optical frequencies. The study outlines the current proof-of-concept instrument status, measured performance, chip fabrication, and routes towards photonics-enabled exoplanet imaging.

Aperture masking interferometry (AMI) transforms a conventional telescope into an interferometric array via a pupil plane mask. By enforcing a known and often linear relationship between pupil-plane phase errors and Fourier phases, AMI allows for the calculation of “self-calibrating” Fourier observables. This enables imaging at moderate contrast down to and inside the classical telescope diffraction limit. AMI’s earliest applications resulted in sub-diffraction-limited angular resolution without adaptive optics. Subsequent observations coupled with powerful adaptive optics systems extended AMI’s reach to fainter targets and longer exposure times, opening doors to a wide range of studies of binaries, protoplanetary disks, and evolved massive stars. AMI has recently led to the first use of interferometry in space via the mask aboard the James Webb Space Telescope Near Infrared Imager and Slitless Spectrograph (JWST/NIRISS). I will give an overview of AMI’s technical and scientific progress over the years, beginning with ground-based studies and ending with a performance analysis of space-based AMI with JWST/NIRISS.

The Slicer Combined with an Array of Lenslets for Exoplanet Spectroscopy (SCALES) will be the first facility-class integral field spectrograph (IFS) to operate between 2-5 microns. Expected to see first light at W. M. Keck Observatory in 2025, SCALES will extend the parameter space of directly imaged exoplanets to those that are colder, and thus older. SCALES will perform high-contrast imaging of these objects and other targets including protoplanetary disks, Solar System objects, and supernovae. Interferometric techniques such as non-redundant aperture masking (NRM) have been demonstrated to improve spatial resolution at high contrasts. Aperture masking turns a telescope into an interferometer by blocking the pupil with an opaque mask with some number of circular holes. Here we present the final designs for the non-redundant masks that will be integrated into SCALES. We outline their design, manufacturing, characterization, and integration processes. We also present the injection and recovery of several planet and disk companion models into mock SCALES science frames to assess the performance of the selected designs.

The enduring technique of aperture masking interferometry, now more than 150 years old, is still widely practised today for it opens a window of high angular resolution astronomy that remains difficult to access by any competing technology. However, the requirement to apodise the pupil into a non-redundant array dramatically limits the throughput, typically to 10% or less. This in turn has a dramatic impact on the sensitivity so that only bright classes of science have been targeted. This paper presents “Jewel masks”, a novel technology that leverages the gains in signal fidelity conferred by non-redundant Fizeau beam combination without the sensitivity penalty incurred by traditional aperture masks. Our approach fragments the pupil with a set of phase wedges, producing several sets of sparse-array tilings. After extensive searching, solutions were found where all individual sets are fully non-redundant. Each set is assigned a common phase wedge which diverts that pattern onto a defined region of the sensor. We describe transmissive and reflective implementations, as well as a case study of a VAMPIRES mask with realistic fabrication errors.

Whilst many algorithms exist for interferometric image reconstruction, there are not yet algorithms for polarimetric interferometric image reconstruction. The polarisation state of light contains critical information otherwise uncaptured by standard, unpolarised interferometry, and many major facilities are now looking towards fully leveraging this information to broaden the observational reach of new and existing instruments. Polarimetric image reconstruction has additional challenges compared to unpolarised image reconstruction, as reconstructions of polarised images (Stokes I, Q and U) are spatial maps of vector components. As such, they need to individually and collectively display physically realistic and mutually consistent scattering physics. Within the present work, we demonstrate that a two-stage machine learning framework (a convolutional neural network (CNN) + iterative fitting) can be used to successfully perform polarimetric image reconstruction, whilst satisfying these challenging regularisation requirements. Using a custom set of MCFOST radiative transfer models, we train a convolutional neural network to learn the mapping between polarised images and interferometric polarimetric observables. We then deploy an iterative fitting mechanism inspired by the Deep Image Prior, which iteratively improves the fit of polarimetric observables with cognisance of observational errors. In particular, the improvement provided by iterative fitting also results in the reconstruction of physically meaningful image structures that were missing from the original CNN image reconstruction. Our results suggest that this two-stage framework is a powerful tool for performing image reconstruction with complex regularisation constraints - in both polarimetric and non-polarimetric contexts. Here we briefly report our algorithm and initial results.

Protoplanetary disks are the best places for observing planetary embryos. Direct imaging coupled with interferometric techniques, such as non-redundant masking (NRM), can help us better understand gas giant accretion timescales and dynamical interactions by resolving protoplanetary disks that exhibit evidence of planet formation. By using NRM we can achieve angular resolution down to and within the diffraction limit, and image planet formation on solar system scales (down to ∼3-7AU for K and L band, respectively) given the distances to most young stars (∼150pc). We present progress on a NRM imaging survey designed to search for protoplanets embedded in protoplanetary disks. The goals of this survey are to detect and characterize protoplanets at solar system scales in a significant sample of protoplanetary disks and to characterize disk structure and dynamical interactions. From this survey, we can place constraints on the underlying protoplanet population and timescales under which giant gas planets form at spatial separations down to ∼3-7AU.

The James Webb Space Telescope Aperture Masking Interferometer provides NIRISS with its highest angular resolution imaging mode, an ultra-stable non-redundant masking Fizeau interferometer. Until recently, the precision of its interferometric visibilities has been limited to ~ 1% by systematic uncertainties in its optical state and detector noise properties. Using a data-driven calibration of AMI with a differentiable forwards model, this can be improved by more than an order of magnitude, uniquely enabling high angular resolution science not possible from the ground. We will discuss the pipeline and observing strategies required to achieve this, illustrated with science highlights enabled this way from the first two years of AMI data, and generalizations of this approach to kernel phase interferometry.

The Aperture Masking Interferometer (AMI) on board the James Webb Space Telescope (JWST) has a unique place in observational astronomy as the first imaging interferometer in space, promising highly-precise observations resistant to optical aberrations. While the optical system and Point-Spread Function (PSF) are very stable, the infrared detectors on board suffer from a series of non-linearities – primarily charge migration or the “brighter-fatter effect” that, while challenging for other observing modes, are ruinous to the visibility calibration of the AMI mode. Local nonlinear effects produced cannot be straightforwardly corrected in the Fourier domain. Efforts using the existing pipelines have delivered some improvements, but outcomes remain far from the theoretical photon-noise limit of the instrument. This manuscript presents initial work using a fundamentally different approach: the joint implementation of a differentiable physics model of the optics, and a machine-learned Effective Detector Model (EDM), using dLux. These are trained together end-to-end, by gradient descent using the full ensemble of point-source reference targets so far observed by AMI. We infer highly-precise metrology of the AMI and NIRISS optical systems, a preliminary EDM which restores commissioning data to near-ideal precision, and illustrate initial and final residual noise floors representing the present state of this ongoing project.

Images in optical long-baseline interferometry have seen a boost in the recent years thanks to new techniques and recipes invented by the community. These images are more and more used for science interpretation and not only illustration, and their fidelity has improved significantly, thanks mainly to the increase in the number of telescopes used in interferometers. The focus today is to improve their reliability and dynamic range. With this contest, we follow up the quest introduced in 2004 of comparing the state of the art image reconstruction software for long-baseline interferometry. This is done in a festive way in the form of an imaging contest, where the organizers propose simulated datasets of targets, whose brightness distributions are meant to be blindly retrieved using various algorithms by the contestants. A prize is offered to the winner of the contest. This year is not different from previous ones and we proposed to the contestants tools to compare their reconstructed images with original images. These tools are now distributed, together with example datasets and images, enabling further tests at home of any image reconstruction tool.

More than 5000 exoplanets have been discovered to date, yet the formation and early evolution of gas giant planets remains an unsolved puzzle. Taking advantage of the contrast gain obtained by combining adaptive optics with interferometric observations, we present a VLTI/GRAVITY survey of young, directly imaged gas giant planets to unveil their formation history. The observations provide astrometric data of unprecedented accuracy, being crucial for refining the planets’ orbital parameters and illuminating their dynamical histories. Repetitive observations of the exoplanets at medium spectral resolution (R ~ 500) provide a catalogue of K-band for a number of our targets, revealing molecular signatures from e.g., CO, H2O, CH4, and CO2. With the help of self-consistent atmosphere models and atmospheric retrievals, the physical parameters and the C/O ratio of the planets can be constrained, kick-starting the difficult process of linking planetary formation with measured atomic abundances. In the near future, the GRAVITY+ upgrade will enable the observation of even fainter and closer-in exoplanets.

We report progress on Project Prime (PRecision Interferometry with MIRC for Exoplanets) to detect exoplanets using precision closures using MIRC-X and MYSTIC at CHARA. Our investigations include modeling systematics caused by OPD drifts, differential dispersion, beamtrain birefringence, and flatfielding errors. Injection tests suggest we can recover hot Jupiter companions as faint at 1/5000 of the host star brightness with 4 nights of observing and we will present some results of our recent searches for the hot Jupiters. Our upper limits are starting to constrain current-generation Global Circulation Models (GCMs). We propose the addition of modest nulling (10:1) to today’s interferometers in order to vastly increase the ease of this work and to open up many more targets for detections.

Kernel phase interferometry is a data analysis technique that allows for the detection of moderate contrast asymmetries below λ/d in high-Strehl images. The technique is of particular interest within the area of planet formation, where the asymmetries around a young star can be from disk features or protoplanet candidates. Here we examine the performance achieved by a kernel phase interferometry program using the SCExAO/CHARIS integral field spectrograph on the Subaru telescope. We investigate the quality of the kernel phases as a function of the Strehl ratio of the observations. We also find that all but the highest quality observations are limited by random, as opposed to systematic errors. Finally we conduct a preliminary analysis of observations of V1247 Orionis, where we tentatively detect the presence of a previously identified companion candidate.

We present latest results and further development of the image reconstruction tool GRAVITY-RESOLVE (G^R), which is specifically designed for Galactic Center observations with the near-infrared phase-referencing VLTI instrument GRAVITY. We show deep images of the Galactic Center in recent years and movies in which the stellar motion around the central massive black hole SgrA* becomes apparent on yet unmatched scales. Moreover, we present a first result of the newly implemented mosaicing method in G^R to jointly reconstruct multiple datasets which have been separately recorded on sky.

Interferometry, essential in radio and infrared astronomy, faces a significant challenge: reconstructing images from sparsely sampled data. Current regularized minimization algorithms rely heavily on predefined priors and hyperparameters, leading to ambiguities and inaccuracies in the images. Here, we present a project to integrate Neural Networks into interferometric image reconstruction. By utilizing the principles of Compressed Sensing and generative Neural Networks, this approach can map infrared interferometric data to reconstruct images more accurately, reducing reliance on rigid priors. The adaptability of the Neural Network ensures that the reconstructions are more precise and less dependent on user input, which is a significant advancement over current methods that require extensive expertise. In this work, we present, as software demonstration, reconstructions obtained from the Event Horizon Telescope data of the black-hole shadow at the core of M87.

Nulling interferometry is a powerful observing technique to reach exoplanets and circumstellar dust at separations too small for direct imaging with single-dish telescopes and too large for indirect methods. With near-future instrumentation, it bears the potential to detect young, hot planets near the snow lines of their host stars. A future space mission could detect and characterize a large number of rocky, habitable-zone planets around nearby stars at thermal-infrared wavelengths. The null self-calibration is a method aiming at modelling the statistical distribution of the nulled signal. It has proven to be more sensitive and accurate than average-based data reduction methods in nulling interferometry. This statistical approach opens the possibility of designing a GPU-based Python package to reduce the data from any of these instruments, by simply providing the data and a simulator of the instrument. GRIP is a toolbox to reduce nulling and interferometric data based on the statistical self-calibration method. In this article, we present the main features of GRIP as well as applications on real data.

The newly installed Silmaril beam combiner at the CHARA array is designed to observe previously inaccessible faint targets, including Active Galactic Nuclei and T-Tauri Young Stellar Objects. Silmaril leverages cuttingedge optical design, low readout noise, and a high-speed C-RED1 camera to realize its sensitivity objectives. In this presentation, we offer a comprehensive overview of the instrument’s software, which manages critical functions, including camera data acquisition, fringe tracking, automatic instrument alignment, and observing interfaces, all aimed at optimizing on-sky data collection. Additionally, we offer an outline of the data reduction pipeline, responsible for converting raw instrument data products into the final OIFITS used by the standard interferometry modeling software. Finally, a thorough analysis of the camera and instrument characterization results will be presented, evaluating instrument performance in terms of sensitivity. The purpose of this paper is to provide a solid reference for studies based on Silmaril data.

The delivery of curated data from astronomical instruments has become a reality in many observatories. The European Southern Observatory (ESO) delivers science-ready data products for various instruments, ranging from imagers to integral field spectrographs. In the case of infrared long-baseline interferometry, scientists generally make their curated data available through the Optical Interferometry Database (OiDB) once it is published. We report on a project to create a curated data stream for the GRAVITY instrument at the Very Large Telescope Interferometer. We aim to transform the publicly available raw data in the ESO science archive into science-ready curated data.

The Large Interferometer For Exoplanets (LIFE) is a proposed space mission that enables the spectral characterization of the thermal emission of exoplanets in the solar neighborhood. The mission is designed to search for global atmospheric biosignatures on dozens of temperate terrestrial exoplanets and it will naturally investigate the diversity of other worlds. Here, we review the status of the mission concept, discuss the key mission parameters, and outline the trade-offs related to the mission’s architecture. In preparation for an upcoming concept study, we define a mission baseline based on a free-formation flying constellation of a double Bracewell nulling interferometer that consists of 4 collectors and a central beam-combiner spacecraft. The interferometric baselines are between 10–600m, and the estimated diameters of the collectors are at least 2m (but will depend on the total achievable instrument throughput). The spectral required wavelength range is 6–16μm (with a goal of 4–18.5μm), hence cryogenic temperatures are needed both for the collectors and the beam combiners. One of the key challenges is the required deep, stable, and broad-band nulling performance while maintaining a high system throughput for the planet signal. Among many ongoing or needed technology development activities, the demonstration of the measurement principle under cryogenic conditions is fundamentally important for LIFE.

Infrared nulling interferometry in space is the most promising technique for detecting possible signs of life on earth-like exoplanets, through the combination of ozone and other biomarkers. The Large Interferometer for Exoplanets (LIFE) mission extends on earlier work by Darwin and Terrestrial Planet Finder Interferometer (TPF-I) with a modern mission concept. In this paper, we describe why spatial filtering is critical in the context of both adaptive nulling and alternative approaches in order to achieve the ∼10⁷ contrast needed, and why a endlessly single mode photonics crystal waveguide fed by phase induced amplitude apodization optics is the currently preferred option for LIFE. Finally, we discuss the considered technological options and prototyping for production of such a waveguide spatial filter at different wavelengths, including both fibres and laser micromachining of infrared crystals.

The Large Interferometer For Exoplanets (LIFE) is a proposed space-based mid-infrared nulling interferometer featuring an array of formation-flying collectors and a central beam combiner. Its primary objective is the direct detection of dozens of temperate, terrestrial exoplanets and the investigation of their atmospheres to understand their composition and identify potential biosignatures. To get a realistic performance estimate of LIFE and derive technical requirements, a comprehensive understanding of all major noise sources impacting the mission performance is essential. Previous studies on the performance of LIFE have focused on fundamental noise from astrophysical sources and assumed the impact of instrumental noise to be non-dominant. Here, we report on our ongoing effort to explicitly model instrumental noise for LIFE. We consider two different methods: one providing a numerical solution by explicitly propagating the instability-induced errors in Monte Carlo simulations, and one providing an analytical solution using a second-order approximation of the leakage from instrumental instability noise. We give an overview of the two methods and argue in favor of the numerical method to support the efforts of the LIFE initiative in the ongoing concept phase, due to its flexibility for different observatory architectures, its fidelity in modeling the correlation of errors and fewer limitations concerning the parameter space of potential errors sources.

In recent years, there has been a renewed interest in technology development for space-based optical and infrared interferometry. One such pathfinder is Pyxis, a set of three autonomous robotic platforms designed to operate in the carpark of Mt Stromlo Observatory, Canberra, where it will simulate formation-flying while performing optical interferometry. In this paper, we will provide an update on the interferometer, detailing the initial results of the control subsystems. We will also share our future plans to begin space qualification and adaptation of Pyxis into a set of nano-satellites.

Context — The Large Interferometer for Exoplanets (LIFE) is a proposed space mission to characterise the atmosphere of terrestrial exoplanets, which is planned to operate in the mid-infrared wavelength region from 6μm to 16μm. A key requirement needed to study the feasibility of this mission is to demonstrate broadband nulling at cryogenic temperatures (15K), at flux levels comparable to the astronomical sources that LIFE will detect. The Nulling Interferometer Cryogenic Experiment (NICE) is a technology demonstrator built to fulfil this purpose.

Aim — The objective of NICE is to demonstrate a broadband null with a null depth of 10⁻⁵ and stability of 10⁻⁸ while maintaining a high system throughput, and consequently a high level of sensitivity, sufficient to detect an Earth twin at 10pc. We describe the optical requirements, the current progress of NICE in the warm phase, and future plans.

Methods — NICE is a Single-Bracewell nuller with closed loop optical path-length control, currently operating at ambient conditions. We use a 3.85μm laser with 150nm bandwidth to demonstrate achromatic nulling capability, and a narrowband (< 0.5nm bandwidth) 4.5μm laser to demonstrate stability.

Results — We achieve an achromatic null depth of 4.39 · 10⁻⁴ with a stability of σ = 5.02 · 10⁻⁴ over a duration of 60s without closed loop control, and a stabilised narrow-band null of 2.05 · 10⁻⁴ with σ = 9.36 · 10⁻⁵ over a duration of 120s.

Conclusions — NICE has both demonstrated achromatic operation and closed loop control to stabilise the null. However, the mean null depth and the null stability achieved do not yet meet the requirements, by a factor of 20 and 104 respectively. This will be improved in future iterations.

Terrestrial optical interferometers have produced the first resolved images of main-sequence stellar surfaces by using long co-phased baselines and correcting for atmospheric errors using fringe tracking and closure phases. These stellar surface images are helpful for understanding the formation, dynamical structure, and evolution of stars, and also for evaluating the potential habitability of exoplanetary systems. However, the astronomical targets suitable for ground based interferometric imaging remain limited due to restricted baseline diversity, atmospheric absorption in the ultraviolet and much of the infrared, and shot noise limits on the fringe tracker when observing faint objects through tropospheric turbulence. A spaceborne formation flying optical interferometer could potentially image dimmer targets with longer baselines, but must still contend with disturbances like propulsive stationkeeping and attitude control system noise. This work simulates on-orbit fringe tracking controller performance under representative disturbances for a small three-spacecraft Michelson interferometer, and evaluates the effects of imperfect fringe tracking on the measured visibilities. We investigate fringe tracker design options, including the choice of optical path delay estimator(s), number of spectral channels, and integration time. Assuming 10cm subapertures, we compute limiting target magnitudes and the maximum baseline beyond which noise dominates the interferometric visibility measurements. Based on this analysis, we identify design parameters for a cost effective demonstration mission that could complement ground-based stellar surface imaging capabilities.

NASA’s return to the Moon presents unparalleled opportunities to advance high-impact scientific capabilities. At the cutting edge of these possibilities are extremely high-resolution interferometric observations at visible and ultraviolet wavelengths. Such technology can resolve the surfaces of stars, explore the inner accretion disks of nascent stars and black holes, and eventually enable us to observe surface features and weather patterns on nearby exoplanets. We have been awarded Phase 1 support from NASA's Innovative Advanced Concepts (NIAC) program to explore the feasibility of constructing a high-resolution, long-baseline UV/optical imaging interferometer on the lunar surface, in conjunction with the Artemis Program. A 1996 study comparing interferometers on the Moon versus free-flyers in space concluded that, without pre-existing lunar infrastructure, free-flyers were preferable. However, with the advent of the Artemis Program, it is now crucial to revisit the potential of building lunar interferometers. Our objective is to conduct a study with the same level of rigor applied to large baseline, free-flying interferometers during the 2003-2005 NASA Vision Missions Studies. This preparation is essential for timely and effective utilization of the forthcoming lunar infrastructure. In this paper, we highlight the groundbreaking potential of a lunar surface-based interferometer. This concept study will be a huge step forward to larger arrays on both the moon and free-flying in space, over a wide variety of wavelengths and science topics. Our Phase 1 study began in April 2024, and here we present a concise overview of our vision and the progress made so far.

This work aims to present a complex mid-infrared (L-band : 3.4μm - 4.1μm) astrophotonic chip made in Lithium Niobate (LiNbO3), an electro-optic crystal, using Titanium diffused waveguides. The L-band presents several key characteristics interesting in astrophysics, notably for imaging and characterise young exo-planetary systems, as well as exo-zodiacal disks. With the increasing interest in exo-planetary science, new instruments and projects are focusing in the mid infrared, such as METIS (ground-based), NOTT (ground based), or LIFE (space-based). Combining such projects with photonics and on-chip beam combination will allow for more compact instruments, easing their integration on ground or, even more so, space based projects, hence the interest for improving the performances of photonic building blocks used for astrophysics.
Here, we are presenting building blocks such as Y-splitters, directional couplers, unbalanced beam splitters... that have been optimised for the L-band in Lithium Niobate. Although such blocks have already been developed in the mid-IR in this material, we are here using a different crystal orientation and newer design that are producing lower losses and birefringence. In particular, a 4-telescope mid-infrared combiner (linked to the NOTT project) was made in order to achieve nulling interferometry in the L-band. We show that we have relatively low loss waveguides, controlled photometric splitters (20/80 flux ratio), as well as functional couplers and beam splitting techniques. Furthermore, we will implement the electro-optic effect in this chip, in order to have internal modulation, and to be able to finely tune the fringes and improve the contrast, allowing for a step further into compact nulling interferometry.

Application of intensity interferometry in far-infrared and terahertz frequencies is discussed for high angular resolution observations. Interferometer technologies with heterodyne and double Fourier are compared to clarify the merit of intensity interferometry and challenges to aperture synthesis imaging. Photon bunching caused by thermal radiation can be used to determine the delay time in place of electromagnetic phase information, which may estimate the complex visibility for aperture synthesis imaging. For the fast measurement of photon bunching, superconducting tunnel junction detectors, such as SIS photon detectors, can be used. Laboratory demonstrator of intensity interferometry was developed using a 4-K pulse-tube cooler, ⁴He sorption fridges, and SIS photon detectors with fast readout electronics. Optical evaluation of the detector performance as well as developments of fast readout electronics are made toward intensity correlation and delay time measurements. Applications to long baseline terahertz interferometry in Antarctica and to space far-infrared interferometry are discussed.

While the angular resolution is the obvious driver for pushing instruments in the visible wavelength range, working with short wavelengths comes with challenges. In this presentation I will review the latest developments regarding photonic developments for the recombination of beams in the visible. Photonic devices constitute a promising avenue for the future instrumentation in interferometry, offering compact, stable and potentially complex architectures. Especially in the context of the spectro-interferometer FIRST installed on the SCExAO platform at the Subaru Telescope, we are exploring two different manufacturing technologies producing waveguides with low or high refractive index contrast. I will review the main challenges we are facing, regarding insertion and propagation losses, as well as polarization effects, and introduce current research developments towards visible design for active phase modulation devices, 3D-architectures and photonic lanterns.

Microarcsecond resolutions afforded by an optical-NIR array with kilometer-baselines would enable breakthrough science. However significant technology barriers exist in transporting weakly coherent photon states over these distances: primarily photon loss and phase errors. Quantum telescopy, using entangled states to link spatially separated apertures, offers a possible solution to the loss of photons. We report on an initiative launched by NSF NOIRLab in collaboration with the Center for Quantum Networks and Arizona Quantum Initiative at the University of Arizona, Tucson, to explore these concepts further. A brief description of the quantum concepts and a possible technology roadmap towards a quantum-enhanced very long baseline optical-NIR interferometric array is presented. An on-sky demonstration of measuring spatial coherence of photons with apertures linked through the simplest Gottesman protocol over short baselines and with limited phase fluctuations is envisaged as the first step.

There have been several versions of optical wavelength astronomical imaging interferometers over the years, with the preferred form being Michelson direct detection (though Hanbury-Brown-Twiss is currently in revival). Even though it is prevalent in radio astronomy, using a common reference (e.g., a laser) is known to have a poor signal-to-noise ratio at visible wavelengths as the shot noise introduced by the reference overwhelms the considerably weaker signal collected by the telescopes. In 2012, a team of quantum physicists (Gottesman, Jennewein and Croke (GJC)) proposed a novel method for using a common optical reference that would abate the shot-noise issue: a path-entangled single-photon reference (i.e., a single photon that is split on a beam splitter). Transported to the various telescopes using a quantum network to overcome loss, the distributed single photon is then interfered with the optical field collected by the telescopes. Previously, we successfully demonstrated a proof-of-principle table-top experiment that implements the GJC protocol where we recovered the spatial autocorrelation of quasi-thermal double-slit sources in a single spectral-temporal mode where the single photon was produced by heralded parametric down conversion. Using quantum optics theory, we modeled our system and found good agreement allowing us to extend our model, and compare and contrast with similarly weak, non-single-photon reference sources (e.g., coherent states). Using the knowledge gained from this experiment, we document the plausibility of an on-sky measurement of the sun utilizing a similar phase reference.

Thermal light such as blackbody radiation including starlight has been known to exhibit photon bunching behaviour. This is the characteristic property of thermal photons to propagate closer together than as described by random Poissonian timing statistics. Although first theorised in the 1960s, attempts to directly probe the temporal coherence of starlight has remained challenging due to the very short timescales required. This work aims to address that by using narrowband spectral filtering to increase the timescale sufficiently for a direct measurement. We present calibration tests using laboratory light sources towards an observatory measurement.

The optical/infrared interferometry community is a vibrant, active community. Nonetheless, the interferometric technique is not without its challenges. The VLTI Expertise Centers were created precisely to fulfill this shortcoming in optical/infrared interferometry expertise and to maximize the impact of VLTI instrumentation. In this talk I will address the various activities in which we try to engage and stimulate the optical/infrared interferometry community through user support and training, as well as activities aimed at wider audiences, namely complementary observing communities, the general public, stakeholders and policy makers.

The Big Fringe Telescope (BFT) is a facility concept under development for a next-generation, kilometer-scale optical interferometer. Observations over the past two decades from routinely operational facilities such as CHARA and VLTI have produced groundbreaking scientific results, reflecting the mature state of the techniques in optical interferometry. However, routine imaging of bright main sequence stars remains a surprisingly unexplored scientific realm. Additionally, the three-plus decade old technology infrastructure of these facilities leads to high operations & maintenance costs, and limits performance. We are developing the BFT, based upon robust, modern, commercially-available, automated technologies with low capital construction and O&M costs, in support of kilometer-scale optical interferometers that will open the door to regular ‘snapshot’ imaging of main sequence stars. Focusing on extreme angular resolution for bright objects leads to substantial reductions in expected costs through use of COTS elements and simplified infrastructure.

The European Interferometry Initiative (EII; https://european-interferometry.eu/) is an open association of Institutes from 15 European countries collaborating on the exploitation and development of optical/infrared long baseline interferometry. Since its formation in the early 2000s the EII has fostered the development of interferometry in Europe and worldwide through programmes that develop the technology for making interferometric observations, support existing users of interferometry facilities, and encourage new generations of users to learn about and exploit interferometry for science observations. We discuss the successes and lessons learned in the delivery of the programmes initiated and managed by the EII, including the development of new hardware, software and techniques, the initiation of the VLTI Expertise Centres to support users, and training and networking through the VLTI summer schools and the Fizeau exchange programme. We discuss possible future programmes to further support and develop the interferometry community.

Astronomy at far-infrared (far-IR) wavelengths is essential to our understanding of the evolution of the cosmos, from the star formation history of galaxies to how the ice distribution affects the formation of extrasolar planetary systems. The Hubble Space Telescope, James Webb Space Telescope, and the Atacama Large Millimeter Array have already produced ground-breaking astronomical observations with high angular resolution spanning the visible to sub-millimetre wavelength regimes. However, this presents a gap in the far-IR, from roughly 30−400μm, where ground-based observations are largely intractable due to the opacity of Earth’s atmosphere. Indeed, no telescope, observatory, or interferometry array has ever achieved sub-arcsecond angular resolution over this wavelength range. A space-based solution is needed. However, a space-based far-IR telescope capable of subarcsecond angular resolution and high sensitivity, at a cost comparable to the largest space missions to date, presents unique physical, practical, and engineering challenges. In this paper, we envisage what a far-IR Great Observatory class mission might look like in the context of the already-studied Origins Space Telescope (OST) and the Space Infrared Interferometric Telescope (SPIRIT). We begin with a historical reflection of far-IR missions, including OST and the recommendations by the Astro2020 Decadal Survey for a de-scoped mission. We use this to motivate the recommendation of a space-based interferometer as a reasonable path towards sub-arcsecond angular resolution at far-IR wavelengths. Using the SPIRIT mission concept as inspiration, we consider multiple point designs for a two element, structurally connected spatial-spectral space-based far-IR interferometer to understand the implications on achieved angular resolution and estimate total mission cost in context of the Decadal Survey recommended far-IR Great Observatory cost cap. This paper illustrates the unique capabilities only possible through a space-based far-IR double Fourier interferometry mission capable of sub-arcsecond resolution.

The CHARA Array has added a 7th telescope to extend the existing 6 telescope array. The CHARA Michelson Array Pathfinder (CMAP) includes a 1m Planewave RC Telescope mounted in a custom designed mobile trailer and pier system. The telescope and trailer can be placed at multiple locations around the Mount Wilson Observatory site; each site consisting of a flat concrete pad with a novel pier design. Optical fibers will connect each site to the CHARA optical delay and combiner lab. This enables new short baselines of ∼17m for imaging the surfaces of large stars and new long baselines on the order of ∼600m for resolving small stars. There are two sites developed at the array for this telescope. In the future, there are plans to expand the array to greater than 1 km maximum baselines. These baselines will be used in conjunction with the existing 15 baselines that range from 34 to 331m. Moving such a telescope around the observatory presents some unique challenges. The telescope can make use of the same optical delay lines and beam combiners as the other CHARA Array telescopes.

The goal of the CHara ARray Integrated Optics Testbench (CHARIOT) is to establish a fully characterized (nulling) interferometry setup for on-sky tests of novel astrophotonic 2D or 3D beam combiners for the interferometry community worldwide. CHARIOT is planned for four telescope beams covering the J-, H-, and K-bands with plug-and-play fiber interfaces. Verifying novel astrophotonics on-sky with CHARIOT will enable the development of components and advances in instruments in many fields, including nulling and spectro-interferometry.

MIRC-X and MYSTIC are six-telescope near-infrared beam (1.08-2.38μm) combiners at the CHARA Array on Mt Wilson CA, USA. Ever since the commissioning of MIRC-X (J and H bands) in 2018 and MYSTIC (K bands) in 2021, they have been the most popular and over-subscribed instruments at the array. Observers have been able to image stellar objects with sensitivity down to 8.1mag in H and 7.8mag in K-band under the very best conditions. In 2022 MYSTIC was upgraded with a new ABCD mode using the VLTI/GRAVITY 4-beam integrated optics chip, with the goal of improving the sensitivity and calibration. The ABCD mode has been used to observe more than 20T Tauri stars; however, the data pipeline is still being developed. Alongside software upgrades, we detail planned upgrades to both instruments in this paper. The main upgrades are: 1) Adding a motorized filter wheel to MIRC-X along with new high spectral resolution modes 2) Updating MIRC-X optics to allow for simultaneous 6T J+H observations 3) Removing the warm window between the spectrograph and the warm optics in MYSTIC 4) Adding a 6T ABCD mode to MIRC-X in collaboration with CHARA/SPICA 5) Updating the MIRC-X CRED-ONE camera funded by Prof. Kraus from U. Exeter 6) Carrying out science verification of the MIRC-X polarization mode 7) Developing new software for ABCD-mode data reduction and more efficient calibration routines. We expect these upgrades to not only improve the observing experience, but also increase the sensitivity by 0.4mag in J+H-bands, and 1mag in K-band.

Directly imaging habitable-zone exoplanets and obtaining their spectra is a central goal in exoplanet science, but is extremely challenging due to the high contrasts and small angular separations which must be achieved. Nulling interferometry is a solution which suppresses on-axis starlight through destructive interference such that the star is ‘nulled out’. The Guided Light Interferometric Nulling Technology Instrument (GLINT) at the 8.2-meter Subaru telescope performs nulling interferometry using waveguides and couplers within a photonic chip, and will provide generalisable insights that are applicable to future nullers. However, to produce high starlight suppression and deeper nulls, correcting seeing-induced wavefront error is of paramount importance. Here we present the design for the real-time control loop of the GLINT instrument which performs fringe tracking and active fringe modulation to correct wavefront errors. To allow for user interaction, the control loop is encapsulated within a graphical user interface, enabling manual, automatic, and simulated functionality, and it utilises the MILK framework for high speed data acquisition and control.

A photonic integrated circuit (PIC) is a lightweight, compact alternative to bulk optics. The Fibered Imager foR a Single Telescope (FIRST) instrument, is a spectro-interferometer performing pupil remapping and designed to operate in the 600 to 800nm visible wavelength range. It is installed on the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument’s platform at the Subaru telescope. In the integrated optical version of FIRST (FIRST PIC), the interferometric combination of the beams occurs by the pairwise combination of five sub-apertures to achieve 20 baselines. This paper introduces a PIC design with novel components for splitting and pairwise coupling the light for FIRST PIC, fabricated and packaged by LioniX International. A high index difference between the waveguide fundamental mode and cladding material was selected to allow compact circuits with prospects of increasing the number of combined sub-apertures with designs of much greater complexity. The high confinement waveguides were simulated to produce approximately 50% injection loss, a tapering system in both height and width to a low confinement waveguide at the PIC interface ameliorated the loss. The optimised throughput prediction is estimated at 80%. Standard couplers and splitters are expected to have high losses due to fabrication tolerances, and due to the high confinement, standard couplers’ performance is highly dependent on the wavelength. Presented here are novel component designs to replace them. Tapered directional couplers, which maintain an acceptable coupling ratio over the entire bandwidth, and tri-couplers, with higher throughput than conventional splitters but high intolerance to fabrication issues, were designed and fabricated for this purpose. The designs and individual experimental verification will be discussed, focusing on polarization and wavelength response. The full combination of components was used to create a five sub-aperture combiner for FIRST. Cross-talk between crossed waveguides was measured independently for the two polarisations. The cross-coupling over the full bandwidth was measured as low as 2% in one polarization, without active subtraction for scattered light to confirm the light in the ports was guided from the cross-coupling, and as low as 8% in the other. Additional work, and potential refinement of the photonic chip components, are required to differentiate the scattered light from the cross-talk to ensure it is reduced in both polarisations.

We present in this proceeding the results of the test phase of the GRAVITY+ adaptive optics. This extreme AO will enable both high-dynamic range observations of faint companions (including exoplanets) thanks to a 40×40 sub-apertures wavefront control, and sensitive observations (including AGNs) thanks to the addition of a laser guide star to each UT of the VLT. This leap forward is made thanks to a mostly automated setup of the AO, including calibration of the NCPAs, that we tested in Europe on the UT+atmosphere simulator we built in Nice. We managed to reproduce in laboratory the expected performances of all the modes of the AO, including under non-optimal atmospheric or telescope alignment conditions, giving us the green light to proceed with the Assembly, Integration and Verification phase in Paranal.

The BIFROST instrument is poised to revolutionize high-spectral resolution interferometry at the VLTI by extending the accessible wavelength range down to 1.0μm, encompassing the Y, J, and H bands. In this paper, we discuss the optical design for BIFROST’s pre-injection optics which correct for birefringence effects and longitudinal dispersion between the different beam lines. We present the optomechanical design for the light injection module that compresses the beams and injects them into single-mode fibres, while maintaining the full field-of-view of the VLTI auxiliary or unit telescopes. Our fibre switching module will allow for the injection of light into photonic devices optimised for different wavebands or applications. Finally, we outline our integration and alignment strategy and present the first characterisation results obtained in the optics lab at the University of Exeter.

BIFROST is one of the upcoming visitor instruments within the ASGARD array. BIFROST will be deployed at the Very Large Telescope Interferometer (”VLTI”) and is currently under development at University of Exeter. We present the instrumentation design and implementation of a dual-field light source for the calibration and design of the BIFROST instrument within Exeter ”Vanaheim”. The system consists of a beam expander and splitter with associated optics to create an on- and off-axis beam simultaneously. An approach to calibrate the angle of the off-axis beam using an open loop controller is discussed. This report will discuss the OpticStudio-Zemax design and the initial implementation of the system at Exeter.

The BIFROST instrument is part of the Asgard Suite of VLTI visitor instruments. It is an interferometric beam combination instrument optimised for high-resolution spectroscopy. Due to the similarities with the MIRCX instrument at CHARA the BIFROST control software framework uses the MIRCX software as a starting point. We make modifications to the MIRCX software to account of the differences between BIFROST and MIRCX, such as the choice of actuators, and differences to the optical design. Many of the changes are also required to take account of the requirements of VLTI, including the use of an ESO-compliant gateway computer between the Asgard network and VLTI. This includes implementing a system of observing templates that complement the user interaction GUIs in use at MIRCX.

The 100m-baseline optical interferometer in China is now under construction. It consists of three 600mm telescopes and forms a maximum baseline of 100 meter. The three telescopes are placed in domes with an auxiliary room used for the adaptive optics and the dual field optics. The central optical room are used to place the delay lines and the beam combiners . The dome and the central optical room are connected by vacuum pipeline. It is hoped to compensate the opd with a residual of 100nm @ H band. The goal of imaging resolution is about 1.7mas when observation in H band. The main science goals of the telescope array are to achieve high precision astrometry, and to image the bright stars with high resolution.

BIFROST is a new Y+J and H band beam combiner for the VLTI, and part of the Asgard suite of visitor instruments. BIFROST will unlock a new parameter space at the VLTI by including the astronomical J band and high spectral resolution (up to R ≈ 25,000). It will also have the ability to simultaneously observe on- and off-axis targets. BIFROST’s beam combiner will be an integrated optics chip, fed by single mode optical fibers. BIFROST therefore requires a light injection module to couple the starlight from free space in the VLTI laboratory into the single mode fibers. The light injection module of BIFROST is also responsible for redirecting the starlight towards the fiber couplers; removing the optical path difference between the beams; co-phasing BIFROST with the rest of the Asgard suite; splitting the light off for the off-axis field; selecting the pointing of the off-axis field; optimizing the injection into the fibers; co-phasing the on- and off-axis light; supporting the passage of the full two arcsecond diameter field of view and providing sufficient space for additional BIFROST pre-injection optics. In this contribution we detail the novel design of the BIFROST light injection module, highlighting how it achieves this functionality using as few optics as possible. We also present Zemax Opticstudio tolerancing analysis, demonstrating the feasibility of building this design in the laboratory.

A new generation of optical intensity interferometers are emerging in recent years taking advantage of the existing infrastructure of Imaging Atmospheric Cherenkov Telescopes (IACTs). The MAGIC SII (Stellar Intensity Interferometer) in La Palma, Spain, has been operating since its first successful measurements in 2019 and its current design allows it to operate regularly. The current setup is ready to follow up on bright optical transients, as changing from regular gamma-ray observations to SII mode can be done in a matter of minutes. A paper studying the system performance, first measurements and future upgrades has been recently published. MAGIC SII’s first scientific results are the measurement of the angular size of 22 stars, 13 of which with no previous measurements in the B band. More recently the Large Sized Telescope prototype from the Cherenkov Telescope Array Observatory (CTAOLST1) has been upgraded to operate together with MAGIC as a SII, leading to its first correlation measurements at the beginning of 2024. MAGIC+CTAO-LST1 SII will be further upgraded by adding the remaining CTAOLSTs at the north site to the system (which are foreseen to be built by the end of 2025). MAGIC+CTAO-LST1 SII shows a feasible technical solution to extend SII to the whole CTAO.

In commissioning the second telescope and first fringe observations at Magdalena Ridge Observatory Interferometer (MROI) the Free-space Optical multi-apertUre combineR for IntERferometry (FOURIER) will be used for fringe tracking during early commissioning while awaiting the readiness of the Infrared Coherencing Neares Neighbor (ICoNN) Tracker. FOURIER is an image plane beam combiner which can accommodate three beams and has wavelength coverage from approximately 1μm to 2.5μm in approximately 10² wavelength channels, resulting in a spectral resolution of approximately 10². We have chosen to use a group-delay fringe-tracking approach and have implemented a configurable search-and-track algorithm. We have also created an operator interface for controlling the fringe tracker and displaying status information and data. A simulator that sends simulated data to the fringe tracker and accepts delay corrections from the fringe tracker can be used to test the fringe tracker under different conditions. We will describe the algorithms and their software implementation, as well as the user interface. Then we will present results of simulations showing expected fringe-tracking performance.

The Magdalena Ridge Observatory Interferometer (MROI) Beam Relay System (BRS) comprises a network of airevacuated pipes and relay stations, consisting of a pier and vacuum can containing a relay mirror, shear alignment sensors, and control electronics. Located at precise points along the arms of the interferometer array, the BRS piers contain remotely controllable mirrors which can be precisely adjusted to direct light from the adjacent unit telescope down the beamline and into the Beam Combining Facility (BCF), where interference fringes are made. Changing the array configuration is a planned function of interferometer operation, but is time consuming and complicated, as it will involve moving mirror assemblies between the vacuum cans (VC). The Vacuum Can Hub (VCH) is a network Modbus message processor and instrumentation hub that connects the Vacuum Can (VC) instrumentation to the MROI power and communication infrastructure via a single Power over Ethernet (PoE) access point. This greatly simplifies and speeds up array reconfiguration. In this paper we shall discuss the MROI Automated Alignment System (AAS), which is tasked with ensuring precise alignment of beamlines connecting the UTs with the BCF, and its role as supervisor of the VCH. We also discuss the BRS components interfaced by the VCH: first, the VC 1-wire temperature sensor network, whose data is used by the AAS for driving fine adjustments of the BRS relay mirrors via the AAS’s feed-forward open-loop thermal mechanical model. Second, twin shear sensors used for coarse beam alignment, each consisting of custom designed 10 × 10 pixel photodiode arrays, whose electronics and software allow direct access by the AAS by using the VCH’s message routing capabilities. The VCH’s ability to translate and relay Modbus messages between the network and serial domain allow high flexibility in defining the quantity and types of BRS hardware that can be installed in VCs.

The Beam Relay System at the Magdalena Ridge Observatory Interferometer, exposed to outdoor environmental conditions, includes 6-inch mirrors mounted on aluminum frames and steel platforms, equipped with piezoelectric motors and a laser/camera alignment system. This subsystem faces challenges with misalignments that disrupt observations, addressed by a proposed correction strategy. The system uses temperature sensor data around mirrors to predict and correct misalignments as a feedforward control system through calibrated motors, and incorporates a periodic closed-loop control system using light source and camera. Advanced predictive models refined over time using temperature, shear, and tilt data, aim to maintain beam stability within interferometric tolerances, ensuring optimal performance.

Beam misalignment causes visibility loss in fringe measurements made by long-baseline optical interferometers. An Automated Alignment System (AAS) has been designed for the Magdalena Ridge Observatory Interferometer (MROI) to keep the visibility loss associated with misalignment under ∼1%. Production versions of collimated reference light sources and precision beam alignment sensors for the AAS have recently been integrated into the first beamline of the MROI. This paper describes the lessons learned during their installation and provides results from their site acceptance tests.

The direct detection of exoplanets and circumstellar disks is currently limited by a combination of high contrast and small angular separation. At the scale of single telescopes, these limitations are fought with coronagraphs, which remove the diffracted light from the central source. To obtain similar benefits with interferometry, one must employ specialized beam-combiners called interferometric nullers. Nullers discard the on-axis light and part of the astrophysical information to optimize the recording of light present in the dark fringe of the central source, which may contain light from circumstellar sources of interest. Asgard/NOTT will deploy an advanced beam-combination scheme offering favorable instrumental noise characteristics when the inputs are phased appropriately, although this tuning will require a specific strategy to overcome the resulting degeneracy. Furthermore, this must bring the phase of the incoming light to a good accuracy across the usable spectrum. Since the fringe-tracker operates at different wavelengths, it can only sense part of the offending errors, and we discuss the measurement of these errors with the science detector. NOTT operates in the L band and suffers from various effects such as water vapor, which has already been experienced with N-band nullers (Keck Interferometer Nuller, Large Binocular Telescope Interferometer). This effect can be corrected with prisms forming a variable thickness of glass and an adjustment of air optical path. Moreover, observations in the L band suffer from an additional and important chromatic effect due to longitudinal atmospheric dispersion coming from a resonance of carbon dioxide at 4.3μm that is impractical to correct with glass plates because of its non-linear wavelength dependency. To compensate for this effect efficiently, a novel type of compensation device will be deployed leveraging a gas cell of variable length at ambient pressure. After reviewing the impact of water vapor and CO₂, we present the design of this atmospheric dispersion compensation device for Asgard/NOTT and describe a strategy to maintain this tuning on-sky.

BIFROST, an upcoming instrument for the VLTI, is part of the Asgard Suite, a VLTI visitor instrument. It comprises two spectrograph arms that are optimised for wavelength range of 1- 1.75μm (fed by a fiber that is placed on-axis for fringe tracking/monitoring) and 1-1.3μm (fed by a fiber that can positioned either on-axis or off-axis to observe a faint target) wavelength range, respectively. Volume phase holographic gratings (VPHGs) are employed to achieve high spectral resolving power up to 25,000 and a throughput above 75% for all dispersing elements. In this contribution, we discuss the optical and optomechanical design of the spectrographs, as well as a new relay optics design that minimizes the thermal background, with a factor 4 reduction in thermal background compared to the non-relay optics design. We will also present the first lab results obtained with the YJH spectrograph.

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

Since its introduction at the Very Large Telescope Interferometer (VLTI), the GRAVITY instrument has emerged as a key player in interferometry. Recognizing its substantial contributions, the European Southern Observatory and the GRAVITY consortium have embarked on an initiative to enhance the instrument's functions. This initiative, named GRAVITY+, aims to broaden the instrument's utility for the international astronomical community and foster new areas of research. GRAVITY+ incorporate advanced features such as a novel laser Adaptive Optics system and an improved fringe tracker to augment interferometric observation sky coverage. This paper details the development of a new Germanium (Ge) prism. This new technical capability of GRAVITY+ will serve to substantially increase the spectral resolution of the instrument to approximately R~15000. The design process, along with the scientific rationale underpinning this advancement, are thoroughly examined in this study.

Hierarchical Fringe Trackers (HFT) maximize the sensitivity and accuracy of fringe tracking. Their performances are independent from the number of apertures. They cophase pairs of telescopes, then pairs of pairs and so on. We report the key results of mathematical analysis, design, manufacturing, optical tests and simulated performances of 4 telescopes HFT chips for the VLTI and 6 to 8 telescopes HFT chips for CHARA or a VLTI extension. An end-to-end simulation with realistic input piston and flux, based on the experimental characterization of the signals on the test bench, validates the servo loop and state machine architectures and supports the performance analysis, confirming the expected performance gain of about 3 magnitudes (with a limiting magnitude K>16 on the VLTI with UTs) and the fact that the performances do not decrease with the number of apertures. The performance gain is based on the combination of the HFT architecture with a very broad band HFT covering the 1.1 to 2.2μm domain with 3 to 5 HFT chips working in broad sub bands in J, H and K. Analysis of fringe jumps and losses at the sensitivity limit show that an HFT manages then more efficiently than the standard pairwise architecture. The impact of HFT characteristics on AGN science programs for optical interferometers is illustrated, showing that this architecture is the key for fascinating applications including direct distance measurements of AGNs accurate enough to contribute to the Hubble tension problem.

We discuss a new generation fringe tracker (FT) that implements a Hierarchical Fringe Tracker (HFT) architecture with a very broad band near infrared spectral coverage from 1.1 to 2.2μm in the J, H and K bands. The goal is to approach the absolute maximum fringe tracking sensitivity in optical long baseline interferometry, first on the VLTI, and to show that an HFT has performances independent from the number of apertures, a key characteristic for larger interferometers from CHARA to the VLTI with more UTs or combining all UTs and ATs to future very large interferometers. This paper describes the development in progress of an end-to-end simulator of such a system based on our first laboratory tests of prototype HFT. This simulator already allowed us to define a new optimization for the integrated optics HFT chips, to discuss a set of operating parameters for our new generation fringe tracker and to confirm that it is applicable to an indefinite number of apertures and should approach or even exceed a limiting sensitivity on the VLTI of K~16, which is a gain of at least 3 magnitudes over the expected performance of the current GRAVITY FT in the context of the ongoing GRAVITY+ VLTI upgrade.

We are developing a “dual-aperture fiber nuller” (DAFN) as a technology to bridge the gap in observation of exoplanets with orbital separations between 1-10s of AU. Such an instrument interferometrically achieves an on-axis (starlight) null while off-axis light (planet light) is transmitted to a high-resolution spectrograph. The performance of the DAFN is competitive among only a few existing technologies such as the vortex fiber nuller. Furthermore, it has the cost-effective advantage of improving angular resolution by expanding the interferometric baseline rather than increasing aperture size. We present a monochromatic demonstration of this technology’s angular resolution (< 1 λ/D) and sensitivity to starlight suppression in the lab. The DAFN technology can potentially be deployed to preexisting interferometric frameworks such as the Large Binocular Telescope Interferometer or the Very Large Telescope Interferometer. It can also benefit prospective space-based exoplanet direct imaging missions, e.g. LIFE, as well as ground-based ELT searches for terrestrial planets in the habitable zone.

Hydrogen Hα (656.2nm) and HeI 1083nm are the spectral lines most commonly used for diagnosing the solar chromosphere. We aim to investigate the scientific advantage in combining these lines in imaging spectroscopic observation using a narrowband tunable filter. Simultaneous imaging in two wavelengths is possible by splitting the light with a polarizing beam splitter attached at the exit of the filter and using two cameras. We have constructed a setup that enables spectroscopic imaging in Hα and HeI 1083nm lines almost simultaneously, with a time required for switching the wavelength of about 0.1 second. The full width at half maximum of the transmission in Hα is 0.25A, while, with an additional stage of the Lyot filter, it is 0.367A in HeI 1083nm. In this paper, we describe the overview of the system and demonstrate its observing capability by presenting some examples of observations. We also discuss the advantage of combining these two lines in view of their responses to the physical condition as inferred from a non-LTE calculation.

Detecting exoplanets around host stars and characterizing the physical properties of these planets requires unprecedented high-resolution and high-contrast imaging. Achieving this using current large-aperture telescopes and optical interferometers faces several key challenges. A new alternative to conventional interferometry and huge full apertures (>8m) is the novel hybrid optical telescope (HOT) design, which consists of phase-distributed aperture arrays. The HOT architecture follows an image-plane interferometric setup by placing apertures constructed from lightweight optics on a ring configuration. The interferometric design of HOT can leverage PSF engineering techniques that can locally create contrast levels up to 1e-7 in the image. We will present results from the analysis of WFS methods for a HOT, including Photon Lanterns, and report on the level of wavefront correction possible for a magnitude range of targets. Based on the WFS analysis, we will report on potential capabilities for achieving quantum-limited super-resolution when imaging faint exo-planets near a host star.

Starlight suppression techniques for High-Contrast Imaging (HCI) are crucial to achieving the demanding contrast ratios and inner working angles required for detecting and characterizing exoplanets with a wide range of masses and separations. The advent of photonic technologies provides new opportunities to control the amplitude and phase characteristics of light, with the potential to enhance and control starlight suppression. Here, we present a focal plane optical-fiber-based nulling interferometer working with commercially available components for amplitude and phase modulation. The instrument implements single-mode fiber-coupled elements: a MEMS variable optical attenuator (VOA) matches the on-axis and off-axis starlight amplitude, while a piezoelectric-driven fiber stretcher modifies the optical path difference between the channels to achieve the π phase shift condition for destructive interference. We show preliminary lab results using a narrowband light source working at 632nm and discuss future opportunities for testing on-sky with the Astrophotonics Advancement Platform at Lick Observatory (APALO) at the Shane 3-m Telescope.

Co-phasing technique is used to detect and compensate the fluctuation of optical path difference (OPD) between subtelescopes of long baseline optical interferometers caused by atmospheric turbulence. At present, the sensitivity of cophasing technology is limited, which cannot meet the observation needs for more and darker objects such as Broad Line Region of Active Galactic Nucleus and Quasars. A kind of chromatic phase diversity method (CPD) used to obtain OPD by extracting phase and unwrapping phase difference from the optical transfer function of multi-aperture interferometry was proposed in 2014. Compared to other traditional co-phasing or fringe tracking methods, this method is of some significant advantages such as less power loss, larger capture range of piston error and synchronous sensing for all apertures. In the past, the method was verified by bulk optics or integrated photonics chip only combining beams. Wavelength Separation was still accomplished by many dichroic beamsplitters and fold mirrors which would increase light loss. We present a new compact solution that Fizeau multi-axial beam combination and multiple spectral channels can be merged on single photonic chip. Our works including Verifying of CPD algorithm, model simulation and design of the chip on H-band will be introduced respectively.

Exoplanets can potentially be observed very close to stars using single-mode cross-aperture nulling interferometry, an optical-fiber based approach in which starlight rejection occurs as a result of a coupling mismatch between an antisymmetric stellar input field and a symmetric fiber mode. The input stellar point-spread function is first modified by passage through an appropriate upstream pupil-plane phase mask, such as a phase-knife mask, which provides a p-radian phase step, or equivalently, a relative field reversal, across a pupil bisector. Using a phase mask with a “crossed” halfwave plate structure to produce the desired relative field reversal, a narrowband null depth of 2.2 × 10^-5 has been demonstrated in the laboratory, a rejection level that is sufficient for the detection of Hot Jupiters with large ground-based telescopes. Moreover, as is shown here, phase masks with slightly more complex spatial patterns can in theory lower the stellar leakage due to finite stellar diameters into the 10^-9 to 10^-10 range needed for terrestrial exoplanet observations. Because nulling performance improves with wavelength, near-infrared cross-aperture nulling may thus be able to serve as a long-wavelength complement to visible wavelength coronagraphy on future exoplanet missions such as the Habitable Worlds Observatory.

Speckle polarimeter (SPP) is a facility instrument of the 2.5-m telescope of the Caucasian Mountain Observatory of SAI MSU. By design it is a combination of a speckle interferometer and a dual–beam polarimeter. In 2022 we performed a major upgrade of the instrument. New version of the instrument features Hamamatsu ORCA-Quest qCMOS C15550-20UP, having subelectron readout noise, as a main detector, as opposed to EMCCD Andor iXon 897 used in previous version. Optical distortions present in the instrument are considered as they directly affect the accuracy of the speckle interferometric astrometric measurements of binary stars. We identified the Atmospheric Dispersion Compensator (ADC) as the main source of distortions which are not constant and depend on the rotational angles of ADCs prisms. Distortions are estimated using internal calibration light source and multiple binary stars measurements. Method for their correction is developed. Flux ratio estimates are subject to CMOS-specific negative factors: spatially correlated noise and flux–dependent pixel–to–pixel sensitivity difference. We suggest ways to mitigate these factors. The use of speckle transfer function measured using a reference star further improves flux ratio estimation performance. We discuss the precision of the estimates of position angle, separation and flux ratio of binary stars.

The multi-national James Webb Space Telescope (JWST) enables several new technologies, one of which is the first space-based infrared interferometer, the Aperture Masking Interferometry (AMI) mode of the Near Infrared Imager and Slitless Spectrograph (NIRISS). AMI is a niche but powerful tool for high resolution imaging of a variety of moderate- to high-contrast astronomical sources. The non-redundant mask (NRM) in the entrance pupil enables detection of structure below the classical Rayleigh diffraction limit, well inside the inner working angle of JWST’s coronagraphs. This explores a parameter space largely inaccessible to existing ground- and other space-based observatories. Early science observations leveraged the capabilities of this unique mode to observe dusty Wolf-Rayet binaries, spatially resolved solar system objects, massive exoplanet systems, and protoplanetary disks. The high quality of this space-based data demonstrated the need for improved analysis methods. We describe approaches to extracting interferometric observables, as well as pre- and post-extraction data cleaning routines we made available to the user community. We also discuss insights and unique challenges that were revealed during the commissioning, early calibration, and first science cycles of this promising observing mode: mitigation strategies for instrumental effects, lessons learned for optimizing observation configuration, and plans for ongoing calibration efforts. Knowledge gained from commissioning and calibration data – which are always non-proprietary – provide valuable insight into the capabilities and limitations of this mode, highlight areas that need improvement, and lay the groundwork for furthering JWST’s scientific objectives.

Aperture masking is a technique to transform a filled-aperture telescope into an interferometer. One reason masks help boost sensitivity is that they facilitate the measurement of closure phases. Closure phases are insensitive to differential piston in the wavefront of light captured by each subaperture, so are a precise observable encoding the structure of the observed object. Spatial structure in the wavefront of light over each subaperture biases visibilities and closure phases. All extant aperture masking data sets show residual closure phase dispersion after calibration larger than estimates based on photon-noise alone, suggesting time variable substructure in the wavefront is playing a role in limiting closure phase precision. We are developing a technique harnessing the capabilities of holographic aperture masks to multiplex subapertures to provide for simultaneous focal-plane wavefront sensing of each subaperture. This device can be used to measure the spatial structure of the wavefront, facilitating self-calibrating closure phases. We will present the results of simulations demonstrating the concept and describe a prototype instrument design.

The majority of the 5600+ confirmed exoplanets orbit stars that will eventually become white dwarfs (WDs). Very few planets have been discovered around WDs, and only two planet candidates have been directly imaged, which leaves gaps in our knowledge of how planetary systems evolve as stars evolve. Directly imaging planets orbiting WDs poses challenges, as WDs are very faint and the limitations of adaptive optics cause poor ground-based image quality, making it difficult to detect planets at tight angular separations. While space observatories can offer better image quality, there is a limit to detecting planets at solar system scales due to their smaller telescope diameters. The data processing technique of kernel phase interferometry (KPI) can overcome these challenges, boosting angular resolution by a factor of a few to several. Here we present a KPI analysis of archival Hubble Space Telescope imaging of seven WDs in the Hyades cluster, which we use to search for planets that have survived the death of their host star. Applying KPI to this dataset has resulted in the highest resolution infrared direct imaging planet search around WDs to date. This will provide new constraints on post main sequence planetary evolution.

It has been more than 20 years since optical-interferometric facilities first started producing spectro-interferometric data, and yet the number of general user tools able to handle this type of data properly is limited. Moreover, they often either focus on simple generic cases (e.g., simple grey geometrical models defined with analytical functions) or very specific objects (e.g., binaries, kinematics in emissions lines of circumstellar disks, stellar photospheres). To overcome these limitations, we have developped oimodeler, a python-based optical-interferometric data modelling software allowing one to build complex models from various type of components ranging from simple geometrical to external outputs of radiative transfer models. The software allows one to add chromaticity (continuum, line and bands) and time-dependence easily. The code is object-oriented to be more flexible and easily expandable. Here we present its concept, implementation and some examples of fitting of real data. The code is available at https://github.com/oimodeler/oimodeler.

The Magdalena Ridge Observatory Interferometer (MROI) and its first science beam combiner the Free-space Optical multi-apertUre combineR for IntERferometry (FOURIER) are undergoing the first phase of construction near Magdalena, New Mexico. MROI and FOURIER are designed for unprecedented sensitivity to enable imaging of faint astronomical targets. FOURIER provides highly sensitive simultaneous interferometric observations in the J, H, and K bands. In preparation for first fringes with FOURIER, we are developing a data reduction pipeline to produce high quality science ready data products adhering to the OIFITS2 data standard.

Accurate piston error detection and closed-loop control are one of the key technologies to ensure the imaging quality of the interferometric imaging telescope. In this paper, we proposed a piston error detection and control scheme based on three computers and multithreading,which has been successfully applied to a four 0.1-m apertures interferometric telescope. This scheme adopts a kind of fringe contrast measurement and climbing method to achieve closed-loop control. The results implied that the fringe contrast can be raised through piston closed-loop correction. Compared with a single telescope with 0.1-m aperture, we can get a 2.63x improvement in resolution for the new interferometric telescope with four 0.1-m apertures. It is proved that the feasibility and effectiveness of this scheme. We will further carry out astronomical observation experiments and improve the piston error detection and control scheme, in order to provide technical guarantees for the implementation of interferometric imaging telescopes.

Scattered light in Advanced LIGO disrupts gravitational wave signals, hindering detections. It causes low-frequency noise (20-40Hz) that interferes with the frequency band where gravitational detections occur (10-100Hz). Identifying this noise as arches in the time-frequency plane is a critical tool to tracking the scattered light surface. Gravitational waves, predicted by Einstein's General Theory of Relativity, reveal hidden cosmic phenomena. LIGO's upgrades expanded its detection range to 150 Mpc for binary neutron star detections, further increasing the need for detector characterization. To further increase sensitivity, we investigate scattering surfaces with the intent to mitigate scattered light effects. We have created an algorithm used with GravitySpy's machine learning tool to filter noise and study arch properties. Observation Run 4 (O4) data shows that, scattered light occurrences decreased at LIGO Livingston Observatory (LLO), but their characteristics changed. The algorithm accurately calculates arch velocity and frequency in over 90% of cases. Future research will apply it to LIGO Hanford (LHO) data, and we plan to compare the FFT method with the HHT method.

Astronomical space interferometers have the potential to achieve milliarcsecond resolution via formation-flying collectors hundreds of meters apart. The collectors’ role is to transfer the starlight beams to a combiner that coherently interferes them. One challenge is controlling optical distances within a fraction of a wavelength while maneuvering the spacecraft to maintain their relative position. Since measuring relative position is much easier than controlling it, we propose long compact delay lines that significantly relax formation flying requirements. We present a proof-of-concept demonstrating an optical 4-m free-space delay in the lab. The delay line utilizes four high-reflectance mirrors in a configuration that fits within a 10cm x 20cm footprint suitable for a CubeSat. We also describe a visible-laser metrology approach that controls the optical path across the 4-m range. The delay line and metrology system would be part of the combiner spacecraft. Such an arrangement will not only relax the relative positioning requirements but also enable a two-spacecraft (total) interferometer that would make a technology demonstration mission more feasible in the near future.

Formation flying interferometry can increase the distance between satellites compared to ground-based interferometers. However, as the baseline length increases, longer observing time is required to fill the U-V plane. Here, focusing on the continuity of observation wavelengths from space, we develop a method to reconstruct spectra of diffuse objects only by rotating the baseline without changing the baseline length. The proposed method enables future space interferometry, such as the Large Interferometer for Exoplanets (LIFE), to spatially resolve distant objects and obtain their spectra.

Drones provide a versatile platform for remote sensing and atmospheric studies. However, strict payload mass limits and intense vibrations have proven obstacles to adoption for astronomy. We present a concept for system-level testing of a long-baseline CubeSat space interferometer using drones, taking advantage of their cm-level xyz station-keeping, 6-dof freedom of movement, large operational environment, access to guide stars for end-to-end testing of optical train and control algorithms, and comparable mass and power requirements. We have purchased two different drone platforms (Aurelia X6 Pro, Freefly Alta X) and present characterization studies of vibrations, flight stability, GPS positioning precision, and more. We also describe our progress in sub-system development, including inter-drone laser metrology, realtime gimbal control, and LED beacon tracking. Lastly, we explore whether custom-built drone-borne telescopes could be used for interferometry of bright objects over km-level baselines using vibration-isolation platforms and a small fast delay for fringe-tracking.

Far-infrared (far-IR) astronomical observations with sub-arcsecond angular resolution and high spectral resolution require a space-based interferometer observatory with baselines of at least tens of meters in length. The European-funded Far Infrared Space Interferometer Critical Assessment (FISICA) studied Far Infrared Interferometer (FIRI) in detail, and developed software simulation tools (FIInS and PyFIInS) for modeling a FIRI-like interferometer and simulating the hyperspectral output datacubes. Here we present on-going work expanding upon the foundations of FIInS and pyFIInS towards an end-to-end simulation software suite. The software tools presented in this work provide a framework with which to study double Fourier interferometry in the far-IR and allow the astronomical community further exploration of the unique capabilities of such instrumentation.

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 1-4m for the diameter of the telescope and a 10-60m 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.

The Far-Infrared (FIR; 25−350μm) band remains relatively unexplored in astronomy despite its importance for studying the formation and evolution of planets, stars, and galaxies. One factor which limits FIR observing capabilities is the impractically large single aperture telescopes that would be required to achieve the sub-arcsecond angular resolution that has been obtained in the optical and radio bands. A Double-Fourier Interferometer (DFI) has been proposed, which combines both a spatial interferometer and a Fourier Transform Spectrometer (FTS). Such an instrument, however, is lacking in experimental validation. This work contributes to DFI development by demonstrating the technique for a simple, spectrally uniform source.

We present our progress in stabilising the warm precursor of the Nulling Interferometry Cryogenic Experiment (NICE), a laboratory testbed demonstrating key mid-infrared nulling technologies for the Large Interferometer for Exoplanets (LIFE). To fulfil the preliminary requirements of NICE, the optical path length difference (OPD) between the beams has to be controlled to within 0.8nm RMS, while beam pointing and shear have to be controlled to within ≈1μm and ≈1μrad RMS, respectively. A heterodyne laser metrology system was implemented to monitor OPD and shear in NICE, with pointing measurements following soon. The metrology beams follow the science beams with minimal non-common paths, and modulation techniques enable simultaneous measurements of all critical paths through the testbed at 1kHz analog bandwidth. We use quadrant photodiodes and a small number of components in a compact layout, simplifying future cryogenic or space-based implementations. The noise of the metrology measured in a mock-up is < 0.8nm RMS in OPD, and < 85nm RMS in beam position, with crosstalk between the beams lower than the sensor noise, despite performing multiple measurements on the same detector with overlapping metrology beams. In the warm precursor of NICE, we achieve closed-loop OPD control to within 14nm RMS, which enables an improvement in null depth and stability by a factor of ≈10, reaching a 2 ⋅ 10⁻⁴ average null over a two-minute period. The metrology and control systems thus provide both an accurate diagnostic and characterisation tool for NICE, as well as a means to achieve deeper and more stable nulls. The main limitation is a 48Hz vibration from mechanical resonances in the setup, which we will mitigate with an improved mechanical design.

Spectro-interferometry is a powerful technique to study astrophysical objects. All modern interferometric instruments offer at least a limited spectral resolution allowing to probe geometry as the function of the wavelength. Some even offer high-enough resolutions to resolve narrow spectral features allowing to constrain physical, chemical and dynamical properties. However, due to detectors size, current instruments are either limited in term of resolution (up to 4000 for VLTI/GRAVITY to cover the full K-band) or bandwidth (20-50nm). To overcome this limitation, we started investigating the possibility of multi-order Echelle-spectro-interferometry. In such scheme, the full spectrum is divided into multiple orders dispersed perpendicularly, taking advantage of the square size of modern detectors. We are currently building a first visible prototype, allowing us to simultaneously observe the full R & I photometric bands (600-900nm) with a resolution of R=20000. We describe in this paper the instrumental concept, simulations, our first in-lab results and some driving science programs in the visible and near-infrared.