This PDF file contains the front matter associated with SPIE Proceedings Volume 11446, including the Title Page, Copyright Information, and Table of Contents.

Welcome Message: Optical and Infrared Interferometry and Imaging

On the Monday evening of December 13, 1920, the first interferometric measurement of a stellar angular diameter was made. This talk commemorates the centennial of that landmark of observational astrophysics. It resulted from the collaboration of Albert Michelson, America’s first science Nobel laureate, and Francis Pease, the brilliant engineer-turned astronomer who designed the Mount Wilson 100-inch telescope and much of its instrumentation. This paper is a brief overview of how that measurement came to be made with a focus on the founding pioneers of interferometry and the small group of Mount Wilson astronomers who, in the 1920s, launched optical interferometry on the trajectory toward today’s facilities employing multiple-telescope, optical/infrared interferometric arrays.

The CHARA Array is an optical/infrared interferometer that combines the light from six 1-meter telescopes. With baselines ranging from 34 to 331 meters, CHARA provides sub-milliarcsecond resolution to measure stellar diameters, image stellar surfaces, resolve close binary companions, and study circumstellar environments. In this paper, we present recent highlights from the CHARA Array, focusing on the implementation of adaptive optics, the ongoing development of next generation beam combiners, an update on the community access program, and a discussion about future developments.

Following the arrival of MATISSE, the second-generation of VLTI instrumentation is now complete and was simultaneously enhanced by a major facility upgrade including the NAOMI Adaptive Optics on the Auxiliary Telescopes. On the Unit Telescopes, significant efforts were also made to improve the injection stability into VLTI instruments. On top of GRAVITY's own evolution, its fringe tracker is now being used to allow coherent integrations on MATISSE (the so-called GRA4MAT project). Meanwhile, operations also evolved to be more flexible and make the most of an extended observing parameter space. In this context, we present an overview of the current VLTI performances. Finally, we will report on on-going improvements such as the extension of the longest baselines.

The Large Binocular Telescope Interferometer (LBTI) combines the light from the two 8.4 m primary mirrors of the LBT for interferometry and adaptive optics (AO) imaging. With two high performance, state-of-the-art AO systems and adaptive secondary mirrors, a cryogenic instrument, and an edge-to-edge baseline of 23 m, the LBTI is a unique instrument for sensitive, high-angular resolution and high-contrast thermal infrared observations. After the successful completion of the NASA-funded HOSTS nulling interferometry survey for exozodiacal dust, our team is now completing the commissioning and extending the capabilities of other observing modes, namely Fizeau imaging interferometry, spectro-interferometry, integral field spectroscopy, non-redundant aperture masking, and coronagraphy for general astronomical observations. In this paper we briefly review the design of the LBTI, summarize the results and performance of HOSTS, and describe the LBTI’s wider current and future capabilities.

We have been pursuing a comprehensive program of improving high-resolution imaging at the Navy Precision Optical Interferometer (NPOI) hosted at Lowell Observatory’s Anderson Mesa site, for the purpose of spatially resolved observations of faint objects at scales down to less than 1 milliarcsecond. This activity at NPOI is being implemented with two primary phases. First, the ‘PALANTIR’ upgrade of NPOI is augmenting the existing telescope array with three 1-meter PlaneWave PW1000 telescopes. These telescopes are housed in mobile domes for rapid relocation around the array, and are being augmented with adaptive optics. Second, we are implementing a ‘NPOI Plus-Up’ plan which is modernizing the array infrastructure and streamlining its operations. All of these activities are being carried out as our current operations are continuing.

The Magdalena Ridge Observatory Interferometer (MROI) is designed to operate 10 1.4m telescopes simultaneously, with baselines ranging from 7.8-347 m and limiting infrared fringe-tracking magnitudes of 14 – it is arguably the most ambitious optical/infrared imaging interferometer under construction today. In this paper we had intended to present an update of activities since the 2018 SPIE meeting as we approached a demonstration of first fringes with the facility originally anticipated for the fall of 2020. However, due to the global pandemic and a loss of funding for our project via AFRL, we have been unable to make the progress we intended. In this paper, we present results up through March, 2020 and a brief discussion of the path forward for the facility.

Imaging air Cherenkov telescope (IACT) arrays have long been viewed as potential observatories for performing stellar intensity interferometry (SII). We present results and the current status of an ongoing SII campaign using the four 12 m diameter VERITAS telescopes located at the Fred Lawrence Whipple Observatory. Each of the telescopes have been equipped with SII instrumentation, consisting of a removable SII camera and a continuous high speed data acquisition system. The correlation of digitized starlight intensities is performed off-line using FPGA hardware to calculate the squared visibilities. The system enables high angular resolution measurements with baselines ranging from approximately 81.5 to 173 m in the B photometric band. Stellar angular diameter measurements were successfully performed using all four VERITAS telescopes. These results show that IACTs can operate as SII observatories, demonstrating a technological pathway for a similar system with future IACT observatories with a hundred fold increase in the number of simultaneous baselines.

Long-baseline optical interferometry has provided a method with which stars are not only resolved, but directly imaged. Resolving features on stellar surfaces is becoming routine for observers using the Center for High Angular Resolution Astronomy (CHARA) Array or the Very Large Telescope Interferometer (VLTI), in particular. The CHARA Array presently has six 1-m telescopes at permanent locations with baselines ranging from 34-331 m.¹ Currently for imaging, the six CHARA Array telescopes are combined with the Michigan InfraRed Combiner-eXeter (MIRC-X; formerly the Michigan Infrared Combiner, MIRC).² Working in H- and J -bands, MIRC-X provides an angular resolution of θ_H ∼ 0.5 mas and θ_J ∼ 0.4 mas, respectively. Four telescopes at a time can be combined at VLTI, either the 8.2-m Unit Telescopes (UTs) or the 1.8-m Auxiliary Telescopes (ATs). The ATs can be moved to provide multiple configurations to increase the possible baselines (up to 130 m). The Precision Integrated-Optics Near-Infrared Imaging ExpeRiment (PIONIER),³ which operates in H-band (θ_H ∼ 1.3 mas), is the current beam combiner frequently used for imaging. In the following sections, the types of stars imaged by long-baseline optical interferometry will be discussed with significant astrophysical results highlighted.

The new generation of VLTI instruments (GRAVITY, MATISSE) aims to produce routinely interferometric images to uncover the morphological complexity of different objects at high angular resolution. Image recon- struction is, however, not a fully automated process. Here we focus on a specific science case, namely the complex circumbinary environments of a subset of evolved binaries, for which interferometric imaging provides the spatial resolution required to resolve the immediate circumbinary environment. Indeed, many binaries where the main star is in the post-asymptotic giant branch (post-AGB) phase are surrounded by circumbinary disks. Those disks were first inferred from the infrared excess produced by dust. Snapshot interferometric observations in the infrared confirmed disk-like morphology and revealed high spatial complexity of the emission that the use of geometrical models could not recover without being strongly biased. Arguably, the most convincing proof of the disk-like shape of the circumbinary environment came from the first interferometric image of such a system (IRAS08544-4431) using the PIONIER instrument at the VLTI. This image was obtained using the SPARCO image reconstruction approach that enables to subtract a model of a component of the image and reconstruct an image of its environment only. In the case of IRAS08544-4431, the model involved a binary and the image of the remaining signal revealed several unexpected features. Then, a second image revealed a different but also complex circumstellar morphology around HD101584 that was well studied by ALMA. To exploit the VLTI imaging capability to understand these targets, we started a large programme at the VLTI to image post-AGB binary systems using both PIONIER and GRAVITY instruments.

With thousands of confirmed exoplanets, the era of discovery is giving way to that of characterization. Direct imaging is crucial, but extremely difficult due to high star-to-planet contrasts and high angular resolutions. Nulling interferometry, which suppresses contaminating starlight via destructive interference, aims to meet this challenge. A pathfinder of this technique is the GLINT nuller: a 6-baseline, spectrally dispersed pupil-remapping interferometer deployed at the Subaru telescope, in which a single photonic chip performs all the critical optical processes. We present the instrument, novel data processing based on self-calibrating methods, laboratory characterization and the latest on-sky results.

In this contribution we report on our work to increase the spectral range of the Michigan Infrared Combiner- eXeter (MIRC-X) instrument at the CHARA array to allow for dual H and J band interferometric observations. We comment on the key science drivers behind this project and the methods of characterisation and correction of instrumental birefringence and dispersion. In addition, we report on the first results from on-sky commissioning in November 2019.

In VIS/IR interferometry the presence of the turbulent atmosphere induces piston drifts, resulting in the optical path difference (OPD) between each telescope to fluctuate. Typically, this is compensated for by employing a fringe tracker. However, the fundamental need for a fringe tracker effectively limits the sensitivity and sky coverage. Previously, a novel method, the Piston Reconstruction Experiment (P-REx), showed how AO data can be used to estimate the piston drift over individual telescopes in order to stabilise the fringes over short timescales and extend the integration time of the fringe tracker. The principle idea behind this method is that the piston drift is simply the product of the wind velocity and the tip and tilt of the atmosphere. In this paper, new developments in the method for estimating the wind velocity will be presented, as well as comparisons between the P-REx OPD estimates obtained from the VLTI’s CIAO WFS data and GRAVITY fringe tracking data.

We report the ultrafast laser inscription (ULI) and characterization of 3 dB directional achromatic couplers for K-band between 2 and 2.4 μm. The couplers were fabricated in commercial Infrasil glass using 1030 nm femtosecond laser pulses. Straight waveguides inscribed using optimal fabrication parameters exhibit an average propagation loss of ∼1.21 dB over full range of K-band with a single-mode behavior for a length of 17 mm. Directional couplers with different interaction lengths and waveguide widths were fabricated and characterized. We demonstrate that 3 dB achromatic directional couplers for K-band can be fabricated using ULI. These results show that ULI can fabricate highquality couplers for future applications in astronomical interferometry. Our eventual aim is to develop a two-telescope K-band integrated optical beam combiner to replace JouFLU at CHARA.

MATISSE, the VLTI 2nd generation spectro-interferometric L, M and N bands imager, has been commissioned from March 2018 to March 2020. It is open to the General User since April 2019. A complete analysis of its performances is given in this paper for MATISSE standalone (with UTs and ATs) and for the GRAVITY for MATISSE (GRA4MAT) mode (with ATs) where the GRAVITY fringe tracker is used to stabilize the fringes in MATISSE and hence improve its sensitivity and spectral coverage at high spectral resolution. This paper presents the key operation parameters of MATISSE and decomposes its performances in fundamental precision per spectral channel for all measurements and in broad band calibration errors on the accuracy of visibility and closure phase. It is intended to give the user a full description of the different errors that must be considered and weighted in the model fitting and image reconstruction. The first image reconstructions achieved by MATISSE are discussed. The performances demonstrated here in the full very broad spectral domain of MATISSE open a very large domain of scientific applications that includes but strongly expands quantitatively and qualitatively the initial science program of the first generation instrument MIDI and, combined with GRAVITY, offers an extremely powerful tool to characterize the temperature and composition of dusty and molecular components of YSOs, AGNs and evolved stars.

The GRAVITY instrument has revolutionized optical/IR interferometry: fringe-tracking and phase-referencing allow for 30 micro-arcsecond astrometry in a dual beam mode, and for spectro-differential astrometry better than 10 micro-arcseconds. The control of systematic effects is essential to fully exploit this technological advancement. Among those systematics are static phase aberrations, introduced along the instrument's optical path, which in particular affect the inferred separation of two unresolved objects within the same FOV. Here, we present how the aberrations can be measured, characterized by low-order Zernike polynomials and, most importantly, how their impact on the astrometry is corrected. The resulting astrometry corrections are verified with calibration observations of a binary before we discuss how they affect GRAVITY's measurement of the galactic center distance.

MIRC-X is a six telescope beam combiner at the CHARA array that works in J and H wavelength bands and provides an angular resolution equivalent to a B=331m diameter telescope. The legacy MIRC combiner has delivered outstanding results in the fields of stellar astrophysics and binaries. However, we required higher sensitivity to make ambitious scientific measurements of faint targets such as young stellar objects, binary systems with exoplanets, and active galactic nuclei. For that purpose, MIRC-X is built and is offered to the community since mid-2017. MIRC-X has demonstrated up to two magnitudes of improved faint magnitude sensitivity with the best-case H <= 8. Here we present a review of the instrument and present early science results, and highlight some of our ongoing science programs.

Combining adaptive optics and interferometric observations results in a considerable contrast gain compared to single-telescope, extreme AO systems. Taking advantage of this, the ExoGRAVITY project is a survey of known young giant exoplanets located in the range of 0.1” to 2” from their stars. The observations provide astrometric data of unprecedented accuracy, being crucial for refining the orbital parameters of planets and illuminating their dynamical histories. Furthermore, GRAVITY will measure non-Keplerian perturbations due to planet-planet interactions in multi-planet systems and measure dynamical masses. Over time, repetitive observations of the exoplanets at medium resolution (R = 500) will provide a catalogue of K-band spectra of unprecedented quality, for a number of exoplanets. The K-band has the unique properties that it contains many molecular signatures (CO, H₂O, CH₄, CO₂). This allows constraining precisely surface gravity, metallicity, and temperature, if used in conjunction with self-consistent models like Exo-REM. Further, we will use the parameter-retrieval algorithm petitRADTRANS to constrain the C/O ratio of the planets. Ultimately, we plan to produce the first C/O survey of exoplanets, kick-starting the difficult process of linking planetary formation with measured atomic abundances.

The diversity of planetary systems discovered in recent years has reinforced the interest in the conditions of the formation of stars and their planetary systems. The new generation of imaging instruments at the VLTI (PIONIER, GRAVITY, MATISSE) and CHARA (MYSTIC, MIRC-X) provides not only the high spatial and spectral resolution necessary to study YSOs but also high astrometric precision and high sensitivity. This makes it possible to extend the study of stellar parameters, the dust distribution and composition in protoplanetary disks, and gas accretion and outflows to fainter objects. It also allows us to investigate the different stages of star and planet formation, from embedded objects over protoplanetary and debris disks to the orbits and atmospheres of planets. The use of arrays with four or even six telescopes provides more opportunities for image reconstruction, to understand complex structures. This paper is a review of recent results obtained on those topics.

MATISSE is the mid-infrared second generation instrument of the VLTI. By sounding both the 3-5 microns and the 8-13 microns wavelength regions, MATISSE is sensitive to the gaz and dust in the vicinity of astrophysical objects. It is therefore possible to colocate the presence of gaz and dust and to better understand the link between the central sources and their environments. We will present in this talk the results of the science program of the MATISSE consortium, especially on the major science cases : stars, active galactic nucleai and, planet forming regions.

We present a new polarimetric mode for the MIRC-X 6-telescope beam combiner at CHARA. Utilizing the extensive u − v coverage afforded by CHARA this mode will be able to resolve and constrain scattered light in environs at milliarcsecond separations of target stars, a largely unexplored parameter space to-date in astronomy. Notably, this upgrade will allow for the investigation of the scattering properties of the inner dust wall at the sublimation radius of Herbig Ae/Be star disks, dust shells surrounding evolved stars, and gas-rich disks around Be stars. Our design adds a series of rotating half-wave plates, achromatic across J- and H-bands, and a polarizing beamsplitter into the MIRC-X beam path. In this work, we also preview on-sky observations, discussing ongoing work calibrating instrumental polarization effects in the CHARA beam path as well as upgrades to the MIRC-X data reduction pipeline.

Instrumental polarization can have large effects on measurements with the VLTI, as it can alter measured polarization and introduce uncertainties. To understand these effects we measured and simulated the instrumental polarization of the VLTI and of GRAVITY. We are able to provide a calibration model for GRAVITY observations and quantify systematic uncertainties due to instrumental polarization. This work has shown to be crucial to measure the polarization of the galactic center black hole Sgr A* where we detect a swing in the polarization angle during flare events. While the analysis was done for GRAVITY, it also gives an important basis for the design of future near-infrared instruments at the VLTI.

CHARA/SPICA (Stellar Parameters and Images with a Cophased Array) is currently being developed at Observatoire de la Cote d’Azur. It will be installed at the visible focus of the CHARA Array by the end of 2021. It has been designed to perform a large survey of fundamental stellar parameters with, in the possible cases, a detailed imaging of the surface or environment of stars. To reach the required precision and sensitivity, CHARA/SPICA combines a low spectral resolution mode R = 140 in the visible and single-mode fibers fed by the AO stages of CHARA. This setup generates additional needs before the interferometric combination: the compensation of atmospheric refraction and longitudinal dispersion, and the fringe stabilization. In this paper, we present the main features of the 6-telescopes fibered visible beam combiner (SPICA-VIS) together with the first laboratory and on-sky results of the fringe tracker (SPICA-FT). We describe also the new fringe-tracker simulator developed in parallel to SPICA-FT.

The study of supermassive black hole (SMBH) is one of the most confusing astronomical topics. However, little is known about the co-evolution of SMBH and galaxies, and also the formation process of SMBH. SMBH is considered to be a celestial body on the cosmic scale. As a probe, SMBH plays an irreplaceable key role in the study of the structure and evolution of the universe. The above-mentioned scientific researches requires 10 micro arcsecond astrometry precision. Based on the differential delay interferometry, we plan to develop a prototype long baseline optical interferometry facility to achieve high precision astrometry. The planned prototype facility includes three 500mm siderostats, and the detection waveband covers both near infrared and short wave infrared (J, H). The astrometry precision of the prototype facility is expected to be at the level of ten micro arcseconds.

We present the design and testing of FOURIER, the first generation science beam combiner for the MROI. FOURIER is a three-way, J, H and K band image plane combiner which is designed primarily for observations at faint limiting magnitudes. We outline the main science requirements and discuss how the design of FOURIER contributes to meeting these requirements. We present the first laboratory characterisation of the instrument including validation of the PSF profile, demonstration of high contrast fringes, and the spectral resolution of the instrument, all of which show promising results. We conclude by discussing the path to deployment of FOURIER at the MROI ahead of the first science observation at the array.

GRAVITY and the VLTI have transformed optical interferometry with ground-breaking results on the Galactic Center, Active Galactic Nuclei, and Exoplanets. With modest upgrades to GRAVITY+, the Paranal Observatory will open up the extragalactic sky for milli-arcsec resolution interferometric imaging, and give access to galactic targets as faint as 22 mag. GRAVITY+ will measure the black hole masses of active galactic nuclei across cosmic times, establish whether globular clusters harbor intermediate-mass black holes, and obtain high-quality exoplanet spectra and orbits. The straightforward upgrade consists of implementing off-axis fringe tracking, an improved throughput, and laser-guide star adaptive optics for all 8m telescopes and can be divided up into several phases. Most of the upgrades will be beneficial to all VLTI instruments. Here we present a few selected science cases and outline the project, which could be implemented within a few years.

The path toward a large imaging interferometer in the infrared, as proposed within the framework of the Planet Formation Imager initiative, represents an incredibly exciting and complex challenge of future infrared interferometry. In this context, heterodyne detection has been proposed as a potential alternative in order to recombine a large number of telescopes, with kilometric baselines, in a practical infrastructure, despite a poorer sensitivity compared to classical interferometry. Among the different building blocks necessary to an infrared heterodyne interferometer, the detection and correlation of wide-bandwidth signals remains a big obstacle, in particular to gain further in sensitivity. Here, we propose to address the problem of transport and correlation of wide- bandwidth signals over kilometric distances by presenting the concept of a photonic correlation dedicated to infrared heterodyne interferometry. We present the concept, the implementation and the experimental results for the correlation of two signals with a phase modulation implementation, its possible extrapolation to a larger number of telescopes and spectral channels, and an alternative correlation scheme based on amplitude modulation.

In less than a year, the James Webb Space Telescope (JWST) will inherit the mantle of being the world’s pre- eminent infrared observatory. JWST will carry with it an Aperture Masking Interferometer (AMI) as one of the supported operational modes of the Near-InfraRed Imager and Slitless Spectrograph (NIRISS) instrument. Aboard such a powerful platform, the AMI mode will deliver the most advanced and scientifically capable interferometer ever launched into space, exceeding anything that has gone before it by orders of magnitude in sensitivity. Here we present key aspects of the design and commissioning of this facility: data simulations (ami_sim), the extraction of interferometeric observables using two different approaches (IMPLANEIA and AMICAL), an updated view of AMI’s expected performance, and our reference star vetting programs.

Although discovery technologies are now populating exoplanet catalogs into the thousands, contemporary astronomy is poorly equipped to find the most compelling exoplanetary real-estate: earth-analog systems within our immediate solar neighbourhood. The TOLIMAN space telescope program aims to develop low-cost, agile mission concepts dedicated to astrometric detection of exoplanets within 10PC, and in particularly targeting the Alpha Cen system. It accomplishes this by deploying an innovative optical and signal encoding architecture that targets the most promising technique for this critical stellar sample: high precision astrometric monitoring. Two pathfinder missions, the first a cubesat slated for 2021 launch, and the second a 10cm space telescope under development at the University of Sydney. We will present an overview of the family of missions and the novel technologies underlying the signal detection strategy.

The extension of infrared interferometry to an array with a large number of telescopes and kilometric baselines such as the Planet Formation Imager represents an exciting but formidable challenge. Such an infrastructure will require major technological developments, with several key aspects still to be solved on a mid and long term horizon. Mid-infrared heterodyne interferometry is considered as one potential technology despite its well documented lower sensitivity but its stronger scalability and lower hard infrastructure requirements. Exploring pathfinder instruments is a way to increase the maturity of interferometric technologies. In this study we propose to use the 8 VLTI telescopes (Unit and Auxiliary) as a coherent array using infrared heterodyne interferometry by exploiting the potential of state of the art technology in the field of high bandwidth detectors, laser frequency combs, fiber links and innovative photonics correlator. We analyze the sensitivity of an eight beam combining heterodyne instrument called V8 and present a possible sub-system breakdown. By comparing its performances with the ones advertised by ESO for MATISSE we conclude that V8, despite its lower sensitivity, has an interesting science potential since it allows to trade a higher limiting magnitude with an incomparable better mapping capability. As such it should be a formidable tool to explore evolved stars complex mass-loss processes. Moreover, it should allow the interferometry community to explore pathways for future long-baseline arrays, combining or not, direct and heterodyne interferometry.

In optical and IR wavelengths, typical astronomical sources have only one or a few photons arriving at a time, meaning the light is really a quantum object. This makes interferometry over longer baselines a major challenge, requiring a truly quantum interference measurement between distant elements in the array rather than local measurements which can be combined later. Luckily, our ability to manipulate and preserve quantum states has taken major strides forward in the last few decades with the rise of quantum information technologies. I will explain how entangled quantum states and a technology called "quantum repeaters" can make possible telescope arrays with much longer baselines than existing facilities, and discuss progress towards making such quantum telescopes a reality.

The impact photonic technologies have had, and continue to have, on astronomical interferometry are considerable and wide-ranging. Microscopic waveguide structures that coherently rout starlight are combined with other photonic components into `photonic chips', which can perform a myriad of functions useful to interferometry, such as beam combination, routing, and active phase control. Here we will highlight the exploits of existing photonic technologies active and in development on telescopes around the world, the many state of the art prototype devices in various stages of on-sky testing, and some exciting novel photonic technologies just beyond the horizon.

It has been recently suggested that optical interferometers may not require a phase-stable optical link between the stations if instead sources of quantum-mechanically entangled pairs could be provided to them, enabling extra- long baselines and benefiting numerous topics in astrophysics and cosmology. We developed a new variation of this idea, proposing that two photons from different sources could be interfered at two decoupled stations, requiring only a slow classical information link between them. We show that this approach could allow high- precision measurements of the relative astrometry of the two sources, with a basic calculation giving angular precision of 10 µas in a few hours’ observation of two bright stars. We also give requirements on the instrument for these observations, in particular on its temporal and spectral resolution. Finally, we discuss possible technologies for the instrument implementation and first proof-of-principle experiments.

Single-mode integrated optics is an increasingly employed technology in interferometric instruments to reach high-accuracy visibility and closure phases measurements. Here, we address the progress achieved in the area of mid-infrared integrated optics beam combiners beyond 2.5 µm fabricated in MIR chalcogenide glasses by ultrafast laser inscription. Thanks to the synergistic cooperation between the Universities of Cologne and Macquarie, we fabricated optimized asymmetric and multimode interference beam combiners and tested them around 4 µm. In contrast to our earlier works, we also focused on the high-contrast capabilities of these devices and measured levels of null depth. This may become relevant for future projects such as Hi5/VLTI or the space mission LIFE.

Vortex Fiber Nulling (VFN) is an interferometric method for suppressing starlight to detect and spectroscopically characterize exoplanets. It relies on a vortex phase mask and single-mode fiber to reject starlight while simultaneously coupling up to 20% of the planet light at separations of ⪅ 1λ/D, thereby enabling spectroscopic characterization of a large population of RV and transit-detected planets, among others, that are inaccessible to conventional coronagraphs. VFN has been demonstrated in the lab at visible wavelengths and here we present the latest results of these experiments. This includes polychromatic nulls of 5 10⁻⁴ in 10% bandwidth light centered around 790 nm. An upgraded testbed has been designed and is being built in the lab now; we also present a status update on that work here. Finally, we present preliminary K-band (2 micron) fiber nulling results with the infrared mask that will be used on-sky as part of a VFN mode for the Keck Planet Imager and Characterizer Instrument in 2021.

Optical and infrared interferometry from the ground has had major successes but will soon meet fundamental limitations. Adaptive optics and creative fringe tracking solutions has the potential to greatly increase the reach of interferometry from the ground, but the factor of 10⁷ in thermal infrared background means that space interferometry with small telescopes is a more cost-effective solution at long wavelengths. Additionally, important spectral windows for high angular resolution astrophysics are only open from space. I will briefly outline the limitations of interferometry from the ground, and make the case for space. In addition to reviewing the historical cases of low fringe visibility (complex) imaging, precision astrometry, high contrast mid-infrared interferometry and far infrared interferometry, I will make the case for wavelength-differential and polarimetric interferometry at visible and ultraviolet wavelengths. A major long baseline interferometry mission will require higher technological readiness levels of key technologies, including interspacecraft metrology, fringe tracking in a space environment and high contrast nulling. For some mission concepts, relatively little new subsystem development is required and tolerances are surprisingly loose. I will argue that in order to fast-track the inevitability of space interferometry, some key subsystem development, whole mission simulation and a pathfinder mission are all required in the near future.

We describe a new concept for future interferometric observations. Our laboratory experiment simulates an interferometer with two telescopes observing through different volumes of atmospheric turbulence. We simulate both vertical and horizontal propagation through the atmosphere; the latter mimics free-space beam propagation without the need for vacuum pipes or fiber optics. Practically, we simulate the effects of atmospheric turbulence using numerically calculated Kolmogorov phase screens injected onto a spatial light modulator. We correct this distorted wavefront using an adaptive optics system to determine the range of turbulence conditions over which we can detect fringes from the two telescopes. Our experiment lays the groundwork for investigating the potential of having movable telescopes in which light propagates from the telescopes to the beam combiner through free space.

MAGIC is a system of two 17m diameter Imaging Atmospheric Cherenkov Telescopes (IACTs) situated on the observatory of the Roque de los Muchachos on the island of La Palma, 2200m asl. MAGIC is currently in operation for the observation of VHE gamma-ray sources. Here we report the installation of a system to perform intensity interferometry observations with these telescopes. We investigated the feasibility of this technique for these telescopes to be used for high resolution observation of stellar objects. The optics and electronics of MAGIC are designed for single photoelectron resolution and a sub-nano second timing precision and they are ideally suited for optical interferometry observations. During easter 2019 we have installed a simple setup using the central PMT pixels in the camera to test the working principle that MAGIC can be used for intensity interferometry observations. We demonstrate MAGIC is able to reach resolutions of about 0.7 mas with a baseline of 85m between the telescopes. Thanks to the large mirror area of MAGIC of about 239 m 2 we achieve a sensitivity of roughly 10 times better than that achieved in the 1970's with the Narrabri interferometer. Currently, a professional readout with GPU realtime analysis has been developed that will be used in the near future for observations.

In long-baseline interferometry, over the last few decades integrated optics beam combiners have become at- tractive technological solutions for new-generation instruments operating at infrared wavelengths. We have investigated different architectures of discrete beam combiners (DBC), which are 2D lattice arrangement of channel waveguides that can be fabricated by exploiting the 3D capability of the ultrafast laser inscription (ULI) fabrication techniques. Here, we present the first interferometric on-sky results of an integrated optics beam combiner based on a coherent pupil remapper and 4 input/23 output zig-zag based DBC, both written monolith- ically in a single borosilicate glass. We show the preliminary results of visibility amplitudes and closure phases obtained from the Vega star by using the previously calibrated transfer matrix of the device.

This paper presents an analysis of the predicted performance of Microwave Kinetic Inductance Detectors (MKIDS) in interferometric instruments. MKIDS are energy sensitive photon counting detectors which can be designed to operate over a wide range of photon energies, and have excellent timing capabilities. Several Optical and near-IR (OIR) MKID instruments have already been used on-sky, and there are ongoing developments in the US and Europe to improve their capabilities for a wide range of science cases. For interferometric instruments the benefits are clear, with no read out noise or dark current, and time resolution of a microsecond, they can be used for both fringe tracking and science detectors. The inherent energy resolution allows for a range of potential improvements to instruments, from rejection of stray light and cosmic rays, to improved wavelength coverage, to simplified optical designs with few, or no, moving elements.

Here we demonstrate how diffraction limited polarized flux imaging at single filled-aperture telescope can be implemented using an instrument combining properties of a speckle interferometer and dual—beam polarimeter. By processing of a series of short-exposure, seeing limited images of an object obtained at orthogonal polarizations simultaneously the ratio of its visibilities at orthogonal polarizations is estimated. Two such differential polarimetric visibilities can be defined: for Stokes Q and U; both the amplitude and argument of them are being estimated. Assuming that the object under study is dominated by unpolarized unresolved source, e.g. star, the image of polarized circumstellar envelope can be recovered from the described ratio of visibilities. With wpeckle polarimeter, an instrument implementing DSP at 2.5-m SAI MSU telescope, differential polarimetric visibility can be measured with precision of 2.2 10⁻³ for R_c = 6 star in 150 sec of accumulation. This precision is significantly higher than precision of visibility measurement using conventional speckle interferometry in the same conditions. Using laboratory measurements we show that in current design differential polarization aberrations of instrument and telescope do not limit the precision of instrument for targets fainter than R_c = 6. The latter is mainly defined by photon noise and detector noise.

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

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

Interferometry can reach high resolution not only with separated apertures, but also across a single telescope aperture. Indeed, as shown by the Palomar Fiber Nuller (PFN), cross-aperture nulling interferometry can detect companions at much smaller angular offsets from stars than coronagraphy on the same telescope can reach. In particular, the PFN was able to detect the faint secondary companion of the spectroscopic binary η Peg at roughly a third of the telescope’s diffraction beamwidth, with null depth accuracies of several 10^-4, while also demonstrating the use of baseline rotation to detect companions, as originally envisioned for space-based nullers. Cross-aperture nulling observations can thus resolve separations well inside the central core of a telescope’s diffraction beam. Here we summarize the lessons learned from the PFN’s demonstration experiments, as well as from larger mid-infrared nullers, pointing toward potential performance improvements that could enable a variety of nulling-based exoplanet and dust observations interior to the coronagraphic regime on larger telescopes. Two interesting new developments are the grating nuller, in which both beam combination and phase shifting are provided by the same optical element, and the vortex fiber nuller, which straddles the coronagraphic and separated-subaperture interferometric regimes.

The Southern Connecticut Stellar Interferometer (SCSI) is an intensity interferometer that is designed to use correlated photon arrival times to determine the geometry of stars. Originally a low-cost, two-telescope instrument that used a 1-pixel single-photon avalanche diode (SPAD) detector at the focal plane of each telescope to record photon events, it is now being upgraded to include a third telescope. This will allow for the simultaneous detection of the photon correlation at three baselines, and thus the ability to map out the two-dimensional geometry of the source much more efficiently than with the two-telescope arrangement. Recent papers in the literature suggest that it may be possible to derive phase information in the Fourier domain from such triple correlations for the brightest stars, potentially giving SCSI an imaging capability. Prior to investigating this possibility, steps must be taken to maximize the observing efficiency of the SCSI. We present here our latest efforts in achieving better pointing, tracking, and collimation with our telescopes, and we discuss our first modeling results of the three-telescope situation in order to understand under what conditions the upgraded SCSI could retrieve imaging information.

The problem of inference in optical interferometry, i.e. turning the on-sky data into meaningful astrophysics, is a difficult ill-posed problem. But in the last two decades, several exciting developments have taken place and novel algorithms have arisen; in imaging: multi-wavelength imaging, dynamical imaging, imaging on spheroids, and production of error bars on images; in model-fitting: new bootstrapping techniques and Bayesian model selection for model-fitting. Both the characterization of the data (likelihood) and of our expectation of the solution (regularization) have improved. Buzzword-sounding techniques such as Compressed Sensing, Machine Learning, ADMM, and GPU Computing are now finding practical applications. The recent algorithmic work by the Event Horizon Telescope team has also sparked interest in optical interferometry. This paper covers these topics in an attempt to predict what the future holds for inference in our field.

Interferometry is a high resolution technique that enables us to study physical processes at the smallest spatial scales that we can probe with our telescopes. In the infrared and in (sub-)millimetric Very-Long-Baseline Interferometry, the technique is restricted to sparse arrays with only a few telescopes or antennas. While imaging would the most intuitive way to interpret interferometric data, recovering images from sparse arrays is an ill-posed" problem and Fourier inversion techniques are restricted. In this work, we explore a novel imaging scheme based on Compressed Sensing to recover interferometric images. For this purpose, simulated data from the Aperture Masking mode of the James Webb Space Telescope are presented. Our results suggest that reliable interferometric images can be recovered using this technique. In particular, we highlights the recovery of the source structure with high-contrast and low-level residuals.

Coronagraphs are powerful tools to probe the direct neighborhood of stars at very high contrasts but their vulnerability to wavefront errors however makes them less efficient for observations at angular separations smaller than two or three resolution elements. In this regime, observables robust to instrumental phase noise, like the closure- and kernel-phase extracted from non-coronagraphic images, have proven capable of effectively picking up moderate contrast features down to the formal diffraction limit. Direct kernel-phase analysis of coronagraphic images is unfortunately not possible in theory. The focal plane mask used in a coronagraph indeed destroys the shift-invariance properties that give meaning to the analysis of their Fourier transform. We nevertheless investigate how techniques initially developed in the context of coronagraphic observations can be applied with kernel-phase to boost the contrast detection limits. Firstly, inspired by Angular Differential Imaging, we devised a similar method called Angular Differential Kernel to remove static biases from our measurements which are a limiting factor for reaching high contrasts. We present a recent comparative on-sky analysis of its performance using the SCExAO instrument at the Subaru Telescope. Secondly, we show how pupil plane apodization masks can be used to locally decrease the photon noise in the images, and how their effect translates into the kernel-phase observables, therefore improving the capability of kernel-phase to detect faint companions around nearby stars.

In this paper we report the simulation of an achromatised LiNbO₃ phase modulator with a range of 300nm centered around 670nm by using a two stage cascaded Mach-Zehnder controlled by independent electric field on each stage to overcome the chromatic dispersion brought by the Pockels effect. Development of a two stages electro-optic phase modulator test setup and first proofs of phase compensation between the stages will be presented.

Exoplanetary biosignatures, molecular compounds which indicate a likelihood of extraterrestrial life, can be detected by highly sensitive spectroscopy of starlight which passes through the atmospheres of exoplanets towards the Earth. Such sensitive measurements can only be accomplished with the next generation of telescopes, leading to a corresponding increase in cost and complexity spectrometers. Integrated astrophotonic instruments are well-suited to address these challenges through their low-cost fabrication and compact geometries. We propose and characterize an integrated photonic gas sensor which detects the correlation between the near-infrared quasi-periodic vibronic absorption line spectrum of a gas and a silicon waveguide ring resonator transmittance comb. This technique enables lock-in amplification detection for real-time detection of faint biosignatures for reduced observation timescales and rapid exoplanetary atmosphere surveys using highly compact instrumentation.

With the advent of interferometric instruments with 4 telescopes at the VLTI and 6 telescopes at CHARA, the scientific possibility arose to routinely obtain milli-arcsecond scale images of the observed targets. Such an image reconstruction process is typically performed in a Bayesian framework where the function to minimize is made of two terms: the data likelihood and the Bayesian prior. This prior should be based on our prior knowledge of the observed source. Up to now, this prior was chosen from a set of generic and arbitrary functions, such as total variation for example. Here, we present an image reconstruction framework using generative adversarial networks where the Bayesian prior is defined using state-of- the-art radiative transfer models of the targeted objects. We validate this new image reconstruction algorithm on synthetic data with added noise. The generated images display a drastic reduction of artefacts and allow a more straightforward astrophysical interpretation. The results can be seen as a first illustration of how neural networks can provide significant improvements to the image reconstruction post processing of a variety of astrophysical sources.

During the past years, the VLTI-instrument GRAVITY has made spectacular discoveries with phase-referenced interferometric imaging with milliarcsecond resolution and ten microarcsecond astrometry. Here, we report on the upgrade of the GRAVITY science spectrometer with two new grisms in October 2019, increasing the instrument throughput by a factor > 2. This improvement was made possible by using a high refractive index Germanium substrate, which reduces the grism and groove angles, and by successfully applying an anti-reflection coating to the ruled surface to overcome Fresnel losses. We present the design, manufacturing, and laboratory testing of the new grisms, as well as the results from the re-commissioning on sky.

The CHARA Array is the longest baseline optical interferometer in the world. Operated with natural seeing, it has delivered landmark sub-milliarcsecond results in the areas of stellar imaging, binaries, and stellar diameters. However, to achieve ambitious observations of faint targets such as young stellar objects and active galactic nuclei, higher sensitivity is required. For that purpose, adaptive optics are developed to correct atmospheric turbulence and non-common path aberrations between each telescope and the beam combiner lab. This paper describes the AO software and its integration into the CHARA system. We also report initial on-sky tests that demonstrate an increase of scientific throughput by sensitivity gain and by extending useful observing time in worse seeing conditions. Our 6 telescopes and 12 AO systems with tens of critical alignments and control loops pose challenges in operation. We describe our methods enabling a single scientist to operate the entire system.

FIRST (Fibered Imager foR a Single Telescope instrument) is a post-AO instrument that enables high contrast imaging and spectroscopy at spatial scales below the diffraction limit. FIRST achieves sensitivity and accuracy by a unique combination of sparse aperture masking, spatial filtering by single-mode fibers and cross-dispersion in the visible. The telescope pupil is divided into sub-pupils by an array of microlenses, coupling the light into single-mode fibers. The output of the fibers are rearranged in a non redundant configuration, allowing the measurement of the complex visibility for every baseline over the 600-900 nm spectral range. A first version of this instrument is currently integrated to the Subaru Extreme AO bench (SCExAO). This paper focuses on the on-going instrument upgrades and testings, which aim at increasing the instrument’s stability and sensitivity, thus improving the dynamic range. FIRSTv2’s interferometric scheme is based on a photonic chip beam combiner. We report on the laboratory characterization of two different types of 5-input beam combiner with enhanced throughput. The interferometric recombination of each pair of sub-pupils is encoded on a single output. Thus, to sample the fringes we implemented a temporal phase modulation by pistoning the segmented mirrors of a Micro-ElectroMechanical System (MEMS). By coupling high angular resolution and spectral resolution in the visible, FIRST offers unique capabilities in the context of the detection and spectral characterization of close companions, especially on 30m-class telescopes.

FIRST (Fibered Imager foR a Single Telescope instrument) is an instrument that enables high contrast imaging and spectroscopy, thanks to a unique combination of sparse aperture masking, spatial filtering by single-mode waveguides and cross-dispersion in the visible. In order to increase the instrument’s stability and sensitivity, we proposed a new series of photonic beam combiners. The idea is to achieve phase modulation inside an optical chip and get rid of external delay lines, and improve the transmission by using novel techniques that will allow for beam combination in 3D, avoiding planar X-crossings and large bending radii observed in planar integrated optics instruments, between first and last inputs to combine, when the inputs separation is large (i.e. in 9 telescopes beam combiners). In a previous paper [4] we presented first prototypes of beam combiners for FIRST/SUBARU 9T. Planar 2D concepts were studied, but transmission was low due to the high number of crossings and the sharp bending angles needed to achieve beam combination within the length of the wafer. In this paper we will present our recent results on improved designs concerning: A) A hybrid Lithium-Niobate active beam splitter and phase modulator (9T, 1x8), coupled to a passive glass beam combiner (72x36, by pairs). B) A full passive device (5T splitter+beam combiner) and C) a narrow 5T splitter + phase modulator based on lithium niobate, to reduce the bending losses and optimize the overall transmission once coupled to the passive combiner. A comparative analysis of different performances will be presented.

The first unit telescope of Ridge Observatory Interferometer is integrated on the array and starlight has been observed in the Beam Combining Area for the first time. From the telescope, the beam travels in vacuum over a path of >50m, including a beam relay system and delay line. This feat was made possible by a prototype version of the Automated Alignment System that we are developing for minimising fringe visibility loss due to misalignment. We present results of on-site validation of UTLIS, a reference light source at the unit telescope acting as a proxy for starlight, and BEASST, a Shack-Hartmann sensor that simultaneously detects beam angle and position.

FIRST, the Fibered Imager foR a Single Telescope, is a spectro-imager using single-mode fibers for pupil remap- ping, allowing measurements beyond the telescope diffraction limit. Integrated on the Subaru Coronagraphic Extreme Adaptive Optics instrument at the Subaru Telescope, it benefits from a very stable visible light wave- front allowing to acquire long exposure and operate on significantly fainter sources than previously possible. On-sky results demonstrated the ability of the instrument to detect stellar companions separated 43mas in the case of the Capella binary system. A similar approach on an extremely large telescope would offer unique scientific opportunities for companion detection and characterization at very high angular resolution.

The Quad-camera Wavefront-sensing Six-channel Speckle Interferometer (QWSSI) is a new speckle imaging instrument available on the 4.3-m Lowell Discovery Telescope (LDT). QWSSI is built to efficiently make use of collected photons and available detector area. The instrument images on a single Electron Multiplying CCD (EMCCD) at four wavelengths in the optical (577, 658, 808, and 880nm) with 40nm bandpasses. Longward of 1µm, two imaging wavelengths in the NIR are collected at 1150 and 1570nm on two InGaAs cameras with 50nm bandpasses. All remaining non-imaging visible light is then sent into a wavefront EMCCD. All cameras are operated synchronously via concurrent triggering from a timing module. With the simultaneous wavefront sensing, QWSSI characterizes atmospheric aberrations in the wavefront for each speckle frame. This results in additional data that can be utilized during post-processing, enabling advanced techniques such as Multi-Frame Blind Deconvolution. The design philosophy was optimized for an inexpensive, rapid build; virtually all parts were commercial-off-the-shelf (COTS), and custom parts were fabricated or 3D printed on-site. QWSSI’s unique build and capabilities represent a new frontier in civilian high-resolution speckle imaging.

Time has come for the VLTI to prepare its jump into the visible domain, revealing many physical processes occuring in stars, planets and galaxies. Visible interferometry has a superior angular resolution than its IR siblings with the same baselines. Science with a visible VLTI ranges from evolved stars mass loss, fundamental parameters of main sequence stars, surface-brightness relation refinement, and the progenitors of gravitational waves ... These critical science cases are becoming feasible thanks to recent instrumental improvements: better detectors, adaptive optics, fringe trackers, use of optical fibers and integrated optics. The appeal for visible light at VLTI is now stronger than ever because IR has already been explored substantially at ESO. This presentation further addresses possible path forward toward a visible instrument with scientific-driven specifications. We will discuss the benefits of a survey-type observing program to allow it to operate at its full potential.

We propose the use of Laguerre-Gaussian (LG) mode sorters to spatially filter and analyse light from a telescope. An LG mode sorter spatially decomposes an incoming beam into a Cartesian grid of identical Gaussian diffraction-limited spots. Each spot contains a particular LG spatial component of the original beam. These individual LG components, which are now independently accessible as spatially separated Gaussian spots could be analysed and processed in various ways. For example, circularly symmetric components could be removed, in a function similar to a vortex coronagraph. Individual LG spatial components could also be selectively interfered and spectrally decomposed.

A space interferometer could reach a sensitivity and angular resolution which is unattainable on Earth due to the distortion and absorption of the atmosphere. It would enable many unique science cases, including the direct imaging and characterisation of temperate terrestrial exoplanets. This ambitious vision relies on the formation flying of individual spacecrafts, and the demonstration of precision metrology measuring positions in to better than 1mm in at least 2 dimensions, and velocities in the range of nm/s. These significant technical challenges are one of the main reasons progress in space interferometry has been seriously hampered in the two last decades. To overcome this obstacle, we propose a novel metrology concept operating in two steps. The coarse positioning of the array elements is achieved through commercially demonstrated components, such as GPS, wide angle cameras and time-of-flight sensors. For the critical fine metrology, multiple longitudinal mode Fabry-Perot lasers in a central spacecraft are split and retro-reflected off each telescope bearing spacecraft. The reflected beams are then coherently combined in the central spacecraft and the resulting fringes are spectrally dispersed. In this manner, the phase difference is measured at the different Fabry-Perot wavelengths, allowing the unambiguous differential position measurements over a couple of mm capture range. We present the concept together with a prototype system in the laboratory.

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

We present science cases and instrument design considerations for a short-wavelength (Y/J-band), high spectral dispersion (R=25,000) instrument that could be implemented at the VLTI visitor focus. The instrument will unlock powerful venues for studying accretion and mass-loss processes at the early/late stages of stellar evolution, for detecting accreting protoplanets around young stars, and for probing the spin-orbit alignment in directly-imaged planetary systems and multiple star systems. In conjuncture with infrastructure improvements on the VLTI unit telescopes, the instrument will also enable off-axis spectroscopy of exoplanets in the 0.1-2” separation range, enabling high-SNR, high spectral resolution follow-up of exoplanets detected with ELT/JWST. We discuss technical implementations, including on fringe tracking and fast laboratory low-order adaptive optics. We outline a pathway for implementation and stress possible synergies with other instruments in the proposed ASGARD suite.

Future goals for astrophysics at the frontiers of high spatial resolution demand synthetic apertures significantly larger than the current or upcoming generations of single-aperture space observatories. Space-based interferometry enabled by in-space manufacturing delivers cost-effective observation of faint objects at unprecedented levels of angular resolution. Using the Made In Space (MIS) Optimast technology, a simple, two aperture Structurally Connected Interferometer (SCI) can be produced via in-space Additive Manufacturing (AM) technology. Optimast enables the manufacturing and deployment of small interferometric apertures at the tips of two large primary trusses unconstrained by launch loads or volume restrictions that meet science requirements for high angular resolutions, in the milliarcsecond regime. The LightBeam mission concept enabled by this capability has a wide range of ground-breaking science capabilities, including: measuring the sizes, shapes, and surface morphology of single asteroids, and binary asteroid orbits; imaging of young stellar object disk sculpting in the terrestrial planet regime; and probing the inner regions of active galactic nuclei. Space-based deployment of an interferometer unconstrained by the turbulent atmosphere will increase the sensitivity reach of such an instrument relative to its ground-based counterparts by a factor of 1,000 or more.

We will present on the status of a new NUV (3080 Å) miniature SHS instrument for detecting the hydroxyl OD/OH isotope ratio remotely in diffused and faint targets such as cometary tails, comae, and planetary exospheres. Developing the SHS instrument has a wide significance to Astrophysics and Planetary science. SHS provides integrated spectra at high R, over a wide FOV in compact designs that offer the ability to make key science measurements for a variety of planetary targets. Many planetary objects that produce UV emissions are angularly extended low brightness targets, i.e., solar wind charge exchange emission, solar system planetary atmospheres and exospheres, mapping atomic emission, comets, and the interplanetary medium. SHS could be implemented on a dedicated SmallSat or ISS that can sit and stare at its target for a long duration of time that cannot be done from the ground or on big missions.

A space-based mid-infrared nulling interferometer is one of the most promising concepts to achieve one of the long-term goals of exoplanet science - the characterization of many terrestrial planets and the assessment of their potential habitability. In preparation of a potential future mission, we are continuing the efforts of previous mission concepts and their associated nulling testbeds that operated at ambient conditions. While they successfully proved that the required deep null can be achieved and stabilised over many hours, we are building a cryogenically cooled nulling interferometer testbed working in the potential mission wavelength range (4 - 18 µm) and with comparable sensitivity in order to demonstrate the measurement concept at fluxes similar to the astronomical targets. Many new challenges arise from this goal such as the need to optimize the setup for throughput and symmetry in order to avoid performance losses when exposed to unpolarized and broadband light. In this paper we present first concepts and theoretical results of a fully symmetric setup.

For the telescope number and size of the PFI proposal (20 x 8m), the Q-band (around 25 μm) was shown to be an optimal complement to submm- and optical interferometry, to detect substructures in proto-planetary disks. Here, broadband medium-resolution heterodyne would be an appropriate option, considering recent experimental findings on a quantum-limit heterodyne detection (HD) scheme. Under high thermal background and the very small beam filling factors in interferometry, HD was shown to be more sensitive than direct detection for the same bandwidth for wavelengths larger 3-8 micron, depending on the conditions. A new class of heterodyne mixers based on graphene could extend the instantaneous IF-bandwidth up to hundred GHz, for which photonic correlators may be developed since a resolution of R=30000 would be sufficient at 10μm. Such large IF-bandwidths could then even be multiplied in “dispersed heterodyne receivers” using optical frequency combs and photonic techniques derived from the DWDM technology.

A new space-borne intensity interferometer is discussed in this article. By the virtue of the statistical correlations concerning the fluctuations of photon differential flux due to a large variety of transient astrophysical radiation. Intensity interferometry is able to be used as a promising imaging technique, which is not being restricted by diffraction in optical systems to achieve µas angular resolution. In this contribution, we present a design of telescope array via spacecraft formation flying. An individual telescope consists of an off-axis three-mirror system, Silicon Photomultipliers (SiPM) detectors and electronic trigger systems. The challenge is of high order mutual intensity interfering between sparse apertures under u-v spatial frequency coverage in principle. We optimized the optical performance of SiC mirrors and SiPM light concentrators for intensity interferometry.

We propose to vary the shape of the sub-apertures both in cases of aperture masking and for segmented space telescopes. Most sub-apertures assume the shape of circles, rectangles and hexagons, but it is possible to use other shapes such as ellipses. Compared to polygons, ellipses reduce diffraction spokes in the PSF. They can have different axis ratios, and more importantly be placed at different rotation angles. This allows their easier identification in the focal plane for tip-tilt measurement. In the pupil auto-correlation (uv) plane it also allows the unique separation of sub-aperture pairs for piston measurements. Optimisation of separation angles lead to rough values of 67, 84, 113 and 96 degrees, for a range of ellipticity values.

We describe the process of analysing a beam using spatial state tomography, a generalization of Stokes polarimetry to higher-dimensions. The process consists of measuring the intensity of spatial components in various spatial bases to construct a generalized Stokes vector and its corresponding density matrix. Just as for polarization, this matrix can describe coherent, partially coherent and completely incoherent states of light incoming from a telescope. As applied to the spatial properties of a beam, this density matrix quantifies the spatial amplitude and phase of each spatial state of which the beam is composed, giving the correlated wavefront phase information.

Stationary Wave Integrated Fourier Transform Spectrometers (SWIFTS) are based on the sampling of a stationary wave using nano-sampling centres on the surface of a channel waveguide. Single nanogroove sampling centres above the waveguide surface will radiate the sampled signal with wide angular distribution, which is not compatible with the buried detection area of infrared detectors, resulting in crosstalk between pixels. An implementation of multiple diffraction nano-grooves (antenna) for each sampling position is proposed as an alternative solution to improve directivity towards the detector pixel by narrowing the scattering angle of the extracted light. Its efficiency is demonstrated from both simulated and measured far field radiative patterns exhibiting a promising method to be used for future integrated IR-SWIFTS. The implementation of the antennas will allow for a high resolution spectrometer in Infra-Red (here 1550nm) with no crosstalk problem (ref. [1]). These antennas, combined with the technology used (direct laser writing) will provide a robust, low-cost efficient tool that can be implemented as a 3D-3T spectro-interferometer (multi telescope beam-combiner), useful for astrophysics applications, such as phase closure studies.

The current work presents a fiber coupling tip-tilt controller developed for a three-telescope experimental prototype of an Astronomical Fiber-Based Near-Infrared Heterodyne Interferometer. It is based on a commercial magneto-mechanical compact-disk laser-beam actuator on which the fiber-ferrule is mounted. The actuator is driven by a two-axis controller electronics board which was developed by us based on digital processing in a dsPIC33EP device with analog periphery, which reads the quad-photodiode signals amplified by 109, and drives the actuator with two high-current outputs. While this realizes the very fine and relatively fast (up to 100 Hz) fiber-position control in the telescope focus, as a basis to this, a relatively coarse and slow auto-guiding is given by an amateur guiding camera. During first optical bench testing we obtained an average coupled power increase of up to 50% under certain perturbations.

Active phase control is a vital component to any interferometry system. On a simple photonic device this can often be achieved using bulk optics before the chip, but for complicated systems active phase control on-chip is a vital component of the photonic design. One method of active phase control is using the thermo-optic effect. Using a chalcogenide waveguides, chromium heaters actively change the refractive index of the glass, this changes the optical path length of the light. This paper shows that chromium deposited above arms of a Mach-Zehnder interferometer will be able to produce multiple pi phase shifts at a rate of approximately 40 mW per π phase shift. Hence a chalcogenide based platform is suitable for a complicated photonic device like a Kernel-Nulling interferometer.

Recent results for the cross-correlation signal of a newly proposed balanced correlation receiver at 1.5 μm pointed towards a possible bypassing of the standard quantum limit for the receiver noise-temperature hν⁄k in cross-correlation by a factor of 4-6. The only radiation source strong enough for a clear hot-cold measurement was a heavily attenuated fiber-coupled superluminant LED (SLED), because a multi-mode fiber-coupled thermal halogen lamp was difficult to control in polarization due to its weakness when coupled to a single-mode fiber. This peculiarity left some doubts regarding a possible “strange” quantum-mechanical behavior of the signal light from the SLED. Here we want to present the concept for more convincing measurements using a true thermal signal source.

Spectroscopic measurements often require increased observing time to achieve a desired SNR. Multiplexed measurements of faint signals can improve the SNR in a more efficient manner. The Hadamard transform multiplexing technique offers the highest improvement in SNR of reconstructed signals. A Hadamard Transform Spectral Imager (HTSI) implements the technique through 2D spatial multiplexing to measure spatially resolved spectra with a gain in SNR. This can be implemented on existing and proposed Digital Micromirror Device (DMD) based multi-object spectrometers (MOS) as an HTSI observing mode. The HTSI mode enables the instrument to perform integral field spectroscopy with a gain in SNR. In this work we present preliminary results from modeling the performance of a DMD based HTSI for various sources of noise, signal fluctuations, and observation targets. We also present a plan to implement the technique with the SOAR Adaptive-Module Optical Spectrograph (SAMOS) instrument, a DMD based MOS.

DAM (Discretized Aperture Mapping) is an original filtering device able to improve the performance in high-angular resolution and high-contrast imaging by the present class of large telescopes equipped with adaptive optics (Patru et al. 2011, 2014, 2015). DAM is a high-spatial frequency filter able to remove the problematic phase errors produced by the small scale defects in the wavefront. Various effects are related to high-order aberrations (ie the high-spatial frequency content) which are neither seen by any wavefront sensor (WFS) nor corrected by any adaptive optics (AO) and is thus transmitted up to the final detector. In particular, any wavefront sensor, due to its finite sub-apertures size, is fundamentally limited by the well-known aliasing effect, where high-spatial frequencies are seen as spurious low frequencies. DAM can be used as an anti-aliasing filter in order to improve both the accuracy of the WFS measurements and the stability of the AO compensation.

The isopistonic angle is the angle over which the atmospheric piston can be considered constant. It is an important parameter for ground based optical interferometry, particularly in the context of phasing using guide stars. This angle is analogous to the isoplanatic angle, which is the angle over which an adaptive optics correction may be considered valid for a single aperture. While the isopistonic angle is well understood theoretically, there have been relatively few measurements of it as compared to the isoplanatic angle. We have used masked aperture interferometry on binary stars of various angular separations at the USNO Flagstaff astrometry telescope to measure the isopistonic angle. We report on our measurement methodology, data processing, and experimental results.