This PDF file contains the front matter associated with SPIE Proceedings Volume 9907, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

We describe the current status of the Navy Precision Optical Interferometer (NPOI), including developments since the last SPIE meeting. The NPOI group has added stations as far as 250m from the array center and added numerous infrastructure improvements. Science programs include stellar diameters and limb darkening, binary orbits, Be star disks, exoplanet host stars, and progress toward high-resolution stellar surface imaging. Technical and infrastructure projects include on-sky demonstrations of baseline bootstrapping with six array elements and of the VISION beam combiner, control system updates, integration of the long delay lines, and updated firmware for the Classic beam combiner. Our plans to add up to four 1.8 m telescopes are no longer viable, but we have recently acquired separate funding for adding three 1 m AO-equipped telescopes and an infrared beam combiner to the array.

The CHARA Array, operated by Georgia State University, is located at Mount Wilson Observatory just north of Los Angeles in California. The CHARA consortium includes many groups, including LIESA in Paris, Observatoire de la Cote d’Azur, the University of Michigan, Sydney University, the Australian National University, the NASA Exoplanet Science Institute, and most recently the University of Exeter. The CHARA Array is a six-element optical/NIR interferometer, and for the time being at least, has the largest operational baselines in the world. In this paper we will give a brief introduction to the array infrastructure with a focus on our Adaptive Optics program, and then discuss current funding as well as opportunities of funding in the near future.

The Large Binocular Telescope Interferometer (LBTI) is a high spatial resolution instrument developed for coherent imaging and nulling interferometry using the 14.4 m baseline of the 2×8.4 m LBT. The unique telescope design, comprising of the dual apertures on a common elevation-azimuth mount, enables a broad use of observing modes. The full system is comprised of dual adaptive optics systems, a near-infrared phasing camera, a 1-5 μm camera (called LMIRCam), and an 8-13 μm camera (called NOMIC). The key program for LBTI is the Hunt for Observable Signatures of Terrestrial planetary Systems (HOSTS), a survey using nulling interferometry to constrain the typical brightness from exozodiacal dust around nearby stars. Additional observations focus on the detection and characterization of giant planets in the thermal infrared, high spatial resolution imaging of complex scenes such as Jupiter's moon, Io, planets forming in transition disks, and the structure of active Galactic Nuclei (AGN). Several instrumental upgrades are currently underway to improve and expand the capabilities of LBTI. These include: Improving the performance and limiting magnitude of the parallel adaptive optics systems; quadrupling the field of view of LMIRcam (increasing to 20"x20"); adding an integral field spectrometry mode; and implementing a new algorithm for path length correction that accounts for dispersion due to atmospheric water vapor. We present the current architecture and performance of LBTI, as well as an overview of the upgrades.

The Magdalena Ridge Observatory Interferometer (MROI) was the most ambitious infrared interferometric facility conceived of in 2003 when funding began. Today, despite having suffered some financial short-falls, it is still one of the most ambitious interferometric imaging facilities ever designed. With an innovative approach to attaining the original goal of fringe tracking to H = 14^th magnitude via completely redesigned mobile telescopes, and a unique approach to the beam train and delay lines, the MROI will be able to image faint and complex objects with milliarcsecond resolutions for a fraction of the cost of giant telescopes or space-based facilities. The design goals of MROI have been optimized for studying stellar astrophysical processes such as mass loss and mass transfer, the formation and evolution of YSOs and their disks, and the environs of nearby AGN.

The global needs for Space Situational Awareness (SSA) have moved to the forefront in many communities as Space becomes a more integral part of a national security portfolio. These needs drive imaging capabilities ultimately to a few tens of centimeter resolution at geosynchronous orbits. Any array capable of producing images on faint and complex geosynchronous objects in just a few hours will be outstanding not only as an astrophysical tool, but also for these types of SSA missions. With the recent infusion of new funding from the Air Force Research Lab (AFRL) in Albuquerque, NM, MROI will be able to attain first light, first fringes, and demonstrate bootstrapping with three telescopes by 2020.

MROI’s current status along with a sketch of our activities over the coming 5 years will be presented, as well as clear opportunities to collaborate on various aspects of the facility as it comes online. Further funding is actively being sought to accelerate the capability of the array for interferometric imaging on a short time-scale so as to achieve the original goals of this ambitious facility

The upgrade of the VLTI infrastructure for the 2nd generation instruments is now complete with the transformation of the laboratory, and installation of star separators on both the 1.8-m Auxiliary Telescopes (ATs) and the 8-m Unit Telescopes (UTs). The Gravity fringe tracker has had a full semester of commissioning on the ATs, and a first look at the UTs. The CIAO infrared wavefront sensor is about to demonstrate its performance relative to the visible wavefront sensor MACAO. First astrometric measurements on the ATs and astrometric qualification of the UTs are on-going. Now is a good time to revisit the performance roadmap for VLTI that was initiated in 2014, which aimed at coherently driving the developments of the interferometer, and especially its performance, in support to the new generation of instruments: Gravity and MATISSE.

MATISSE is the second-generation mid-infrared spectrograph and imager for the Very Large Telescope Interferometer (VLTI) at Paranal. This new interferometric instrument will allow significant advances by opening new avenues in various fundamental research fields: studying the planet-forming region of disks around young stellar objects, understanding the surface structures and mass loss phenomena affecting evolved stars, and probing the environments of black holes in active galactic nuclei. As a first breakthrough, MATISSE will enlarge the spectral domain of current optical interferometers by offering the L and M bands in addition to the N band. This will open a wide wavelength domain, ranging from 2.8 to 13 μm, exploring angular scales as small as 3 mas (L band) / 10 mas (N band). As a second breakthrough, MATISSE will allow mid-infrared imaging - closure-phase aperture-synthesis imaging - with up to four Unit Telescopes (UT) or Auxiliary Telescopes (AT) of the VLTI. Moreover, MATISSE will offer a spectral resolution range from R ∼ 30 to R ∼ 5000. Here, we present one of the main science objectives, the study of protoplanetary disks, that has driven the instrument design and motivated several VLTI upgrades (GRA4MAT and NAOMI). We introduce the physical concept of MATISSE including a description of the signal on the detectors and an evaluation of the expected performances. We also discuss the current status of the MATISSE instrument, which is entering its testing phase, and the foreseen schedule for the next two years that will lead to the first light at Paranal.

During the last decade, the first generation of beam combiners at the Very Large Telescope Interferometer has proved the importance of optical interferometry for high-angular resolution astrophysical studies in the nearand mid-infrared. With the advent of 4-beam combiners at the VLTI, the u - v coverage per pointing increases significantly, providing an opportunity to use reconstructed images as powerful scientific tools. Here, we present our ongoing studies to characterize the imaging capabilities of the Multi-AperTure mid-infrared SpectroScopic Experiment (MATISSE), a second-generation instrument for the Very Large Telescope Interferometer (VLTI). By providing simultaneous observations with 6 baselines and spectral resolutions up to R~5000. MATISSE will deliver, for the first time, thermal-IR interferometric data with enough u-v coverage and phase information for imaging. In this work, we report detailed image reconstruction studies carried out with the image reconstruction package SQUEEZE. For our studies, we use realistic simulated MATISSE data from radiative transfer simulations of a proto-planetary disk. In particular, we will discuss the role of the regularization function and of the initial brightness distribution. MATISSE will perform observations at three different mid-infrared bands: L, M and N. Hence, due to its large bandwidth, chromatic effects should be taken into account when image reconstruction is attempted. We also discuss the capabilities of SQUEEZE to perform multi-wavelength image reconstruction. Finally, we perform an analysis of the image quality and present our future line of research. The work here presented is being carried out within the Opticon FP7-2 joint research activity on interferometric imaging.

MATISSE (Multi AperTure mid-Infrared SpectroScopic Experiment) is the spectro-interferometer for the VLTI of the European Southern Observatory, operating in near and mid-infrared, and combining up to four beams from the unit or the auxiliary telescopes. MATISSE will offer new breakthroughs in the study of circumstellar environments by allowing the multispectral mapping of the material distribution, the gas and essentially the dust.

The instrument consists in a warm optical system (WOP) accepting four optical beams and relaying them after a dichroic splitting (for the L and M- and N- spectral bands) to cold optical benches (COB) located in two separate cryostats. The Observatoire de la Côte d’Azur is in charge of the WOP providing the spectral band separation, optical path equalization and modulation, pupil positioning, beam anamorphosis, beam commutation, and calibration. NOVA-ASTRON is in charge of the COB providing the functions of beam selection, reduction of thermal background emission, spatial filtering, pupil transfer, photometry and interferometry splitting, additional beam anamorphosis, spectral filtering, polarization selection, image dispersion, and image combination. The Max Planck Institut für Radio Astronomie is in charge of the operation and performance validation of the two detectors, a HAWAII-2RG from Teledyne for the L- and M- bands and a Raytheon AQUARIUS for the N-band. Both detectors are provided by ESO. The Max Planck Institut für Astronomie is in charge of the electronics and the cryostats for which the requirements on space limitations and vibration stability resulted on very specific and stringent decisions on the design.

The integration and test of the COB: the two cryogenic systems, including the cold benches and the detectors, have been conducted at MPIA in parallel with the integration of the WOP at OCA. At the end of 2014, the complete instrument was integrated at OCA. Following this integration, a period of interface and alignment between the COB and the WOP took place resulting in the first interference fringes in the L-band during summer 2015 and the first interference fringes in the N-ban in March 2016.

After a period of optimization of both the instrument reliability and the environmental working conditions, the test plan is presently being conducted in order to evaluate the complete performance of the instrument and its compliance with the high-level requirements. The present paper gives the first results of the alignment, integration and test phase of the MATISSE instrument.

Transition disks, protoplanetary disks with inner clearings, are promising objects in which to directly image forming planets. The high contrast imaging technique of non-redundant masking is well posed to detect planetary mass companions at several to tens of AU in nearby transition disks. We present non-redundant masking observations of the T Cha and LkCa 15 transition disks, both of which host posited sub-stellar mass companions. However, due to a loss of information intrinsic to the technique, observations of extended sources (e.g. scattered light from disks) can be misinterpreted as moving companions. We discuss tests to distinguish between these two scenarios, with applications to the T Cha and LkCa 15 observations. We argue that a static, forward-scattering disk can explain the T Cha data, while LkCa 15 is best explained by multiple orbiting companions.

Finding and maintaining an accurate cophasing solution for the large primary mirrors which comprise the coming generation of Extremely Large Telescopes has required a significant technological development effort that is still ongoing. Mirrors based on an assembly of a few large segments, such as the Giant Magellan Telescope (GMT – under construction) and the Large Binocular Telescope (LBT – operational) face a particular challenge: elements must be cophased across a gaps ranging from tens of centimeters to meters. Although it is widely believed that laser guide stars are not useful for this specific application, this paper advances a new concept that challenges this orthodoxy. By projecting a Fizeau interference pattern into the sky, and analyzing the form of the backscattered image, it is shown that at least in principle it is possible to cophase across arbitrary gaps.

In recent years, speckle imaging has proven very useful for certain problems in single-aperture high-resolution imaging, including searching for faint stellar companions near exoplanet host stars and for satellite imaging. These developments have largely been the result of the availability of electron-multiplying CCD cameras, which allow for greater sensitivity and better photometric linearity when compared with other detectors that have comparable speed. This in turn has led to an increased use for speckle imaging at mid-sized and large telescopes. Some results of these efforts will be discussed, and the outlook for the future of speckle will be given.

We present plans for SRAO, the first Southern Robotic AO system. SRAO will use AO-assisted speckle imaging and Robo-AO-heritage high efficiency observing to confirm and characterize thousands of planet candidates produced by major new transit surveys like TESS, and is the first AO system to be capable of building a comprehensive several-thousand-target multiplicity survey at sub-AU scales across the main sequence. We will also describe results from Robo-AO, the first robotic LGS-AO system. Robo-AO has observed tens of thousands of Northern targets, often using a similar speckle or Lucky-Imaging assisted mode.

SRAO will be a moderate-order natural-guide-star adaptive optics system which uses an innovative photoncounting wavefront sensor and EMCCD speckle-imaging camera to guide on faint stars with the 4.1m SOAR telescope. The system will produce diffraction-limited imaging in the NIR on targets as faint as mν = 16. In AO-assisted speckle imaging mode the system will attain the 30-mas visible diffraction limit on targets at least as faint as mν = 17. The system will be the first Southern hemisphere robotic adaptive optics system, with overheads an order of magnitude smaller than comparable systems. Using Robo-AO's proven robotic AO software, SRAO will be capable of observing overheads on sub-minute scales, allowing the observation of at least 200 targets per night. SRAO will attain three times the angular resolution of the Palomar Robo-AO system in the visible.

Historically, two types of interferometer have been used to the study of solar system objects: coaxial and Fizeau. While coaxial interferometers are well-suited to a wide range of galactic and extra-galactic science cases, solar system science cases are, in most cases, better carried out with Fizeau imagers. Targets of interest in our solar system are often bright and compact, and the science cases for these objects often call for a complete, or nearly complete, image at high angular resolution. For both methods, multiple images must be taken at varying baselines to reconstruct an image. However, with the Fizeau technique that number is far fewer than it is for the aperture synthesis method employed by co-axial interferometers.

In our solar system, bodies rotate and their surfaces are sometimes changing over yearly, or even weekly, time scales. Thus, the need to be able to exploit the high angular resolution of an interferometer with only a handful of observations taken on a single night, as is the case for Fizeau interferometers, gives a key advantage to this technique.

The aperture of the Large Binocular Telescope (LBT), two 8.4 circular mirrors separated center-to-center by 14.4 meters, is optimal for supporting Fizeau interferometry. The first of two Fizeau imagers planned for LBT, the LBT Interferometer (LBTI),¹ saw first fringes in 2010 and has proven to be a valuable tool for solar system studies. Recent studies of Jupiters volcanic moon Io have yielded results that rely on the angular resolution provided by the full 23-meter baseline of LBT Future studies of the aurora at Jupiters poles and the shape and binarity of asteroids are planned.

While many solar system studies can be carried out on-axis (i.e., using the target of interest as the beacon for both adaptive optics correction and fringe tracking), studies such as Io-in-eclipse, full disk of Jupiter and Mars, and binarity of Kuiper belt objects, require off-axis observations (i.e., using one or more nearby guide-moons or stars for adaptive optics correction and fringe tracking). These studies can be plagued by anisoplanatism, or cone effect. LINC-NIRVANA (LN),2 the first multi-conjugate adaptive optics system (MCAO) on an 8-meter class telescope in the northern hemisphere, provides a solution to the ill-effects of anisoplanatism. One of the LN ground layer wave front sensors was tested on LBT during 2014.^3-5 Longer term, an upgrade planned for LN will establish its original role as the second LBT Fizeau imager. The full-disk study of several solar system bodies, most notably large and/or nearby bodies such as Jupiter and Mars which span tens of arcseconds, would be best studied with LN.

We will review the past accomplishments of Fizeau interferometry with LBTI, present plans for using that instrument for future solar system studies, and, lastly, explore the unique solar system studies that require the LN MCAO system combined with Fizeau interferometry.

Optical imaging with microarcsecond resolution will reveal details across and outside stellar surfaces but requires kilometer-scale interferometers, challenging to realize either on the ground or in space. Intensity interferometry, electronically connecting independent telescopes, has a noise budget that relates to the electronic time resolution, circumventing issues of atmospheric turbulence. Extents up to a few km are becoming realistic with arrays of optical air Cherenkov telescopes (primarily erected for gamma-ray studies), enabling an optical equivalent of radio interferometer arrays. Pioneered by Hanbury Brown and Twiss, digital versions of the technique have now been demonstrated, reconstructing diffraction-limited images from laboratory measurements over hundreds of optical baselines. This review outlines the method from its beginnings, describes current experiments, and sketches prospects for future observations.

Since a number of years our group is engaged in the design, construction and operation of instruments with very high time resolution in the optical band for applications to Quantum Astronomy and more conventional Astrophysics. Two instruments were built to perform photon counting with sub-nanosecond temporal accuracy. The first of the two, Aqueye+, is regularly mounted at the 1.8 m Copernicus telescope in Asiago, while the second one, Iqueye, was mounted at the ESO New Technology Telescope in Chile, and at the William Herschel Telescope and Telescopio Nazionale Galileo on the Roque (La Palma, Canary Islands). Both instruments deliver extraordinarily accurate results in optical pulsar timing. Recently, Iqueye was moved to Asiago to be mounted at the 1.2 m Galileo telescope to attempt, for the first time ever, experiments of optical intensity interferometry (à la Hanbury Brown and Twiss) on a baseline of a few kilometers, together with the Copernicus telescope. This application was one of the original goals for the development of our instrumentation. To carry out these measurements, we are experimenting a new way of coupling the instruments to the telescopes, by means of moderate-aperture, low-optical-attenuation multi-mode optical fibers with a double-clad design. Fibers are housed in dedicated optical interfaces attached to the focus of another instrument of the 1.8 m telescope (Aqueye+) or to the Nasmyth focus of the 1.2 m telescope (Iqueye). This soft-mount solution has the advantage to facilitate the mounting of the photon counters, to keep them under controlled temperature and humidity conditions (reducing potential systematics related to varying ambient conditions), and to mitigate scheduling requirements. Here we will describe the first successful implementation of the Asiago intensity interferometer and future plans for improving it.

We describe the performance of detector modules containing silicon single photon avalanche photodiodes (SPADs) and superconducting nanowire single photon detectors (SNSPDs) to be used for intensity interferometry. The SPADs are mounted in fiber-coupled and free-space coupled packages. The SNSPDs are mounted in a small liquid helium cryostat coupled to single mode fiber optic cables which pass through a hermetic feed-through. The detectors are read out with microwave amplifiers and FPGA-based coincidence electronics. We present progress on measurements of intensity correlations from incoherent sources including gas-discharge lamps and stars with these detectors. From the measured laboratory performance of the correlation system, we estimate the sensitivity to intensity correlations from stars using commercial telescopes and larger existing research telescopes.

The mercury-manganese (HgMn) stars are a class of peculiar main-sequence late-type B stars. Their members show a wide variety of abundance anomalies with both depletions (e.g., He) and enhancements (Hg, Mn) and tend to be slow rotators relative to their normal analogs. More than two thirds of the HgMn stars are known to belong to spectroscopic binaries with a preference of orbital periods ranging from 3 to 20 days.¹ Interferometric orbits were already measured for the HgMn binaries Φ Herculis,² X Lupi,³ and α Andromedae.⁴ Here we report on a program to study the binarity of HgMn stars with the PIONIER near-infrared interferometer at the VLTI on Cerro Paranal, Chile. Among 40 stars, companions were found for 11 of them, and the data allowed the determination of the orbital elements of 41 Eridani, with a period of just 5 days and a semi-major axis of under 2 mas.

The "torus" is the central element of the most popular theory unifying various classes of AGNs, but it is usually described as "putative" because it has not been imaged yet. Since it is too small to be resolved with single-dish telescopes, one can only make indirect assumptions about its structure using models. Using infrared interferometry, however, we were able to resolve the circum-nuclear dust distributions for several nearby AGNs and achieved constraints on some further two dozen sources. We discovered circum-nuclear dust on parsec scales in all sources and, in two nearby sources, were able to dissect this dust into two distinct components. The compact component, a very thin disk, appears to be connected to the maser disk and the extended one, which is responsible for most of the mid-IR flux, is oriented perpendicularly to the circum-nuclear gas disks. What may come as a surprise when having in mind the standard unification cartoon actually connects well to observations on larger scales. Optically thin dust in the polar region, perhaps driven by a disk wind, could solve both the scale height problem of the torus and explain the missing anisotropy in the mid-IR − X-ray relation.

For over two decades, astronomers have considered the possibilities for interferometry in space. The first of these missions was the Space Interferometry Mission (SIM), but that was followed by missions for studying exoplanets (e.g Terrestrial Planet Finder, Darwin), and then far-infrared interferometers (e.g. the Space Infrared Interferometric Telescope, the Far-Infrared Interferometer). Unfortunately, following the cancellation of SIM, the future for space-based interferometry has been in doubt, and the interferometric community needs to reevaluate the path forward. While interferometers have strong potential for scientific discovery, there are technological developments still needed, and continued maturation of techniques is important for advocacy to the broader astronomical community. We review the status of several concepts for space-based interferometry, and look for possible synergies between missions oriented towards different science goals.

We present the optics of Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII) as it gets ready for launch. BETTII is an 8-meter baseline far-infrared (30-90 μm) interferometer mission with capabilities of spatially resolved spectroscopy aimed at studying star formation and galaxy evolution. The instrument collects light from its two arms, makes them interfere, divides them into two science channels (30-50 μm and 60-90 μm), and focuses them onto the detectors. It also separates out the NIR light (1-2.5 μm) and uses it for tip-tilt corrections of the telescope pointing. Currently, all the optical elements have been fabricated, heat treated, coated appropriately and are mounted on their respective assemblies. We are presenting the optical design challenges for such a balloon borne spatio- spectral interferometer, and discuss how they have been mitigated. The warm and cold delay lines are an important part of this optics train. The warm delay line corrects for path length differences between the left and the right arm due to balloon pendulation, while the cold delay line is aimed at introducing a systematic path length difference, thereby generating our interferograms from where we can derive information about the spectra. The details of their design and the results of the testing of these opto-mechanical parts are also discussed. The sensitivities of different optical elements on the interferograms produced have been determined with the help of simulations using FRED software package. Accordingly, an alignment plan is drawn up which makes use of a laser tracker, a CMM, theodolites and a LUPI interferometer.

The Wide-field Imaging Interferometry Testbed (WIIT) was developed at NASA’s Goddard Space Flight Center to demonstrate and explore the practical limitations inherent in wide field-of-view “double Fourier” (spatio-spectral) interferometry. The testbed delivers high-quality interferometric data and is capable of observing spatially and spectrally complex hyperspectral test scenes. Although WIIT operates at visible wavelengths, by design the data are representative of those from a space-based far-infrared observatory. We used WIIT to observe a calibrated, independently characterized test scene of modest spatial and spectral complexity, and an astronomically realistic test scene of much greater spatial and spectral complexity. This paper describes the experimental setup, summarizes the performance of the testbed, and presents representative data.

Circumstellar disks around young stars are the birthsites of planets. It is thus fundamental to study the disks in which they form, their structure and the physical conditions therein. The first astronomical unit is of great interest because this is where the terrestrial-planets form and the angular momentum is controlled via massloss through winds/jets. With its milli-arcsecond resolution, optical interferometry is the only technic able to spatially resolve the first few astronomical units of the disk. In this review, we will present a broad overview of studies of young stellar objects with interferometry, and discuss prospects for the future.

The characterization of exozodiacal light emission is both important for the understanding of planetary systems evolution and for the preparation of future space missions aiming to characterize low mass planets in the habitable zone of nearby main sequence stars. The Large Binocular Telescope Interferometer (LBTI) exozodi survey aims at providing a ten-fold improvement over current state of the art, measuring dust emission levels down to a typical accuracy of ~12 zodis per star, for a representative ensemble of ~30+ high priority targets. Such measurements promise to yield a final accuracy of about 2 zodis on the median exozodi level of the targets sample. Reaching a 1 σ measurement uncertainty of 12 zodis per star corresponds to measuring interferometric cancellation (“null”) levels, i.e visibilities at the few 100 ppm uncertainty level. We discuss here the challenges posed by making such high accuracy mid-infrared visibility measurements from the ground and present the methodology we developed for achieving current best levels of 500 ppm or so. We also discuss current limitations and plans for enhanced exozodi observations over the next few years at LBTI.

We present a method to recontruct temperature map directly from spectro-interferometric data. It uses a sparse coding method to describe each pixel as a blackbody. Results are shown on the Herbig Be HD98922. Aside from recovering the mean environment temperature of 1654K in agreement with the photometry, this technique allows us to study the temperature distribution in the first astronomical units around the star.

We present the second version of the OI Exchange Format (OIFITS2), the standard for exchanging calibrated data from optical (visible/infrared) interferometers. This new version provides definitions of several new tables addressing the needs of future interferometric instruments such as GRAVITY and MATISSE, optional data columns for a more rigorous description of measurement errors and their correlations and various new header keywords for bookkeeping and data discovery. In this paper, we outline the new features of OIFITS2 and describe their anticipated usage. Some considerations not present in the normative text are also discussed.

ASPRO2 is a complete observation preparation tool developed and maintained by the JMMC that allows to prepare interferometric observations with the VLTI or other interferometers (CHARA, SUSI, NPOI). Available since 2010, ASPRO2 is regularly updated to provide new features, enhancements and to follow the instrumental changes for each ESO and CHARA Call For Proposals.

As the new 2nd generation VLTI instruments GRAVITY and MATISSE, will be soon available to the community, ASPRO2 is evolving to support them. For example, the noise modelling has been improved for the MATISSE instrument which covers new L+M bands by including the thermal noise contribution and atmospheric transmission. Moreover, the OIFITS simulator has been rewritten to generate correctly correlated quantities for the interferometric observables (VIS2, VIS, T3) and will support OIFITS-2 soon. It is already supporting the commissioning of the GRAVITY instrument through its capability to create Observing Blocks compatible with the ESO P2PP tool.

One can process images acquired by single telescopes using adaptve optics (AO) in a manner similar to data usually acquired with a non-redundant aperture mask. Because it relies on a linearization of the phase transfer equation that describes the value of the phase in the Fourier-plane, the kernel- and eigen-phase analysis of images corrected by adaptive-optics is intrinsically limited to a regime of aberration where the rms error is less than the wavelength. This paper outlines the properties of an extension of the Fourier-phase analysis method, here applied to a set of spectrally dispersed data, similar to those produced by an intergral field spectrograph. Given sufficient wavelength coverage, the original baseline mapping model can be used to produce novel spectrally dispersed kernel-phase information for any aberration regime.

NASA has funded a project called the Hunt for Observable Signatures of Terrestrial Systems (HOSTS) to survey nearby solar type stars to determine the amount of warm zodiacal dust in their habitable zones. The goal is not only to determine the luminosity distribution function but also to know which individual stars have the least amount of zodiacal dust. It is important to have this information for future missions that directly image exoplanets as this dust is the main source of astrophysical noise for them. The HOSTS project utilizes the Large Binocular Telescope Interferometer (LBTI), which consists of two 8.4-m apertures separated by a 14.4-m baseline on Mt. Graham, Arizona. The LBTI operates in a nulling mode in the mid-infrared spectral window (8-13 μm), in which light from the two telescopes is coherently combined with a 180 degree phase shift between them, producing a dark fringe at the location of the target star. In doing so the starlight is greatly reduced, increasing the contrast, analogous to a coronagraph operating at shorter wavelengths. The LBTI is a unique instrument, having only three warm reflections before the starlight reaches cold mirrors, giving it the best photometric sensitivity of any interferometer operating in the mid-infrared. It also has a superb Adaptive Optics (AO) system giving it Strehl ratios greater than 98% at 10 μm. In 2014 into early 2015 LBTI was undergoing commissioning. The HOSTS project team passed its Operational Readiness Review (ORR) in April 2015. The team recently published papers on the target sample, modeling of the nulled disk images, and initial results such as the detection of warm dust around η Corvi. Recently a paper was published on the data pipeline and on-sky performance. An additional paper is in preparation on β Leo. We will discuss the scientific and programmatic context for the LBTI project, and we will report recent progress, new results, and plans for the science verification phase that started in February 2016, and for the survey.

Resolving the spatial structure of transient events provides insights into their physical nature and origin. Recent observations using long baseline optical/infrared interferometry have revealed the size, shape, and angular expansion of bright novae within a few days after their outbursts. This has implications for understanding the timescale for the development of asymmetric features in novae ejecta. Additionally, combining spectroscopic measurements of the expansion velocity with the angular expansion rate provides a way to measure a geometric distance to the nova. In this paper, I provide a review of interferometric observations of novae, with a focus on recent results on the expansion and spatial structure of nova V339 Del in 2013. I also discuss other promising applications of interferometry to transient sources, such as measuring the image size and centroid displacements to measure planetary masses in gravitational microlensing events. Given the timescales of transient events, it is critical for interferometric arrays to respond rapidly to targets of opportunity in order to optimize the instrumental sensitivity and baselines required to resolve the source while its brightness and size change over time.

New disruptive technologies are now emerging for detectors dedicated to interferometry. The detectors needed for this kind of applications need antonymic characteristics: the detector noise must be very low, especially when the signal is dispersed but at the same time must also sample the fast temporal characteristics of the signal. This paper describes the new fast low noise technologies that have been recently developed for interferometry and adaptive optics. The first technology is the Avalanche PhotoDiode (APD) infrared arrays made of HgCdTe. In this paper are presented the two programs that have been developed in that field: the Selex Saphira 320x256 [1] and the 320x255 RAPID detectors developed by Sofradir/CEA LETI in France [2], [3], [4]. Status of these two programs and future developments are presented. Sub-electron noise can now be achieved in the infrared using this technology. The exceptional characteristics of HgCdTe APDs are due to a nearly exclusive impaction ionization of the electrons, and this is why these devices have been called "electrons avalanche photodiodes" or e-APDs. These characteristics have inspired a large effort in developing focal plan arrays using HgCdTe APDs for low photon number applications such as active imaging in gated mode (2D) and/or with direct time of flight detection (3D imaging) and, more recently, passive imaging for infrared wave front correction and fringe tracking in astronomical observations. In addition, a commercial camera solution called C-RED, based on Selex Saphira and commercialized by First Light Imaging [5], is presented here. Some groups are also working with instruments in the visible. In that case, another disruptive technology is showing outstanding performances: the Electron Multiplying CCDs (EMCCD) developed mainly by e2v technologies in UK. The OCAM2 camera, commercialized by First Light Imaging [5], uses the 240x240 EMMCD from e2v and is successfully implemented on the VEGA instrument on the CHARA interferometer (US) by the Lagrange laboratory from Observatoire de la Cote d'Azur. By operating the detector at gain 1000, the readout noise is as low as 0.1 e and data can be analyzed with a better contrast in photon counting mode.

The advent of low-dark-current eAPD arrays in the near infrared ushers in the possibility for photon-counting, high quantum efficiency detectors at these wavelengths. Such detectors would revolutionise the sensitivity of interferometry because near-infrared wavelengths are at the "sweet spot" between the corrupting effects of atmospheric seeing at shorter wavelengths and thermal noise at longer wavelengths. We report on laboratory experiments with cooled Selex Saphira detectors aimed at demonstrating photon-counting performance with these devices by exploiting enhanced avalanche gain and multiple non-destructive readouts. We explain the optimum modes for employing these detectors in interferometry.

As sky surveys continue to document an increasing number of transient celestial phenomena, an intriguing subset of objects are emerging that show variations in brightness, interpreted as the transit of a circumstellar disk in front of a companion star in a binary system. The brightest member of this class is the F0 supergiant star plus disk binary, epsilon Aurigae, along with more than a dozen new candidates sharing similarities. Better-known cases include EE Cep, BM Ori and KH15D. Characteristics of all of these are discussed in terms of their suitability for interferometric study. Next generation interferometric imaging offers the potential to detect disk structures that are driven by dynamical forces, chemical transitions and thermal gradients. These include observable effects of tidal spiral density waves, dust and planetessimal formation/evolution in disks, and orbital phase-dependent heating of the disk by the external companion star.

We review the potential of Astrophotonics, a relatively young field at the interface between photonics and astronomical instrumentation, for spectro-interferometry. We review some fundamental aspects of photonic science that drove the emergence of astrophotonics, and highlight the achievements in observational astrophysics. We analyze the prospects for further technological development also considering the potential synergies with other fields of physics (e.g. non-linear optics in condensed matter physics). We also stress the central role of fiber optics in routing and transporting light, delivering complex filters, or interfacing instruments and telescopes, more specifically in the context of a growing usage of adaptive optics.

Integrated optics (IO) has proven to be a competitive solution for beam combination in the context of astronomical interferometry (e.g. GRAVITY at the VLTI). However, conventional silica-based lithographic IO is limited to wavelengths shorter than 2.2μm. We report in this paper the progress on our attempt to extend the operation of IO to longer wavelengths. Previous work has demonstrated the suitability of chalcogenide devices in the MID-IR in the N band and monochromatically at 3.39 μm. Here, we continue this effort with the manufacturing of new laser written GLS IO as beam combiners designed for the astronomical L band and characterized interferometrically at 3.39 μm. In the era of multi-telescope interferometers, we present a promising solution to strengthen the potential of IO for new wavelength ranges.

The ALOHA research program aims to propose a breakthrough generation of instrument for high resolution imaging in astronomy. This fully innovative concept results from our unique skills with a simultaneous competence in nonlinear optics and high resolution imaging with telescope arrays. Acting like a mixer in a radio receiver, the nonlinear process (sum frequency generation) shifts the infrared radiations emitted by the observed astrophysical source to a visible spectral domain. This way, the light beam is more easily processed by mature optical devices and detectors. The compatibility of the nonlinear process with the spatial coherence analysis has been successfully tested through preliminary in lab experiments. Now it’s time to apply this technique in a real astronomical environment. First on-sky results have been observed during the last missions at the CHARA Array.

The late evolutionary stages of stellar evolution are a key ingredient for our understanding in many fields of astrophysics, including stellar evolution and the enrichment of the interstellar medium (ISM) via stellar yields. Already the first interferometric campaigns identified evolved stars as the primary targets because of their extended and partially optically thin atmospheres, and the brightness in the infrared. Interferometric studies spanning different wavelength ranges, from visual to mid-infrared, have greatly increased our knowledge of the complex atmospheres of these objects where different dynamic processes are at play. In less than two decades this technique went from measuring simple diameters to produce the first images of stellar surfaces. By scanning the extended atmospheres we constrained theoretical models, learnt about molecular stratification, dust formation, and stellar winds, and there is still a lot to be done. In this contribution I will review the recent results that optical/infrared interferometry has made on our current understanding of cool evolved stars. The presentation will focus on asymptotic giant branch stars, and red supergiants. I will discuss the challenges of image reconstruction, and highlight how this field of research will benefit from the synergy of the current interferometric instrument(s) with the second generation VLTI facilities GRAVITY and MATISSE. Finally I will conclude with a short introspection on applications of a visible interferometer and of the the Planet Formation Imager (PFI) to the field of evolved stars.

Image reconstruction is slowly maturing to reach a phase where solutions to most interferometric imaging problems are starting to emerge. This paper gives a brief overview of current state-of-the-art techniques in the field of optical interferometry imaging, and attempts to extrapolate the current research efforts to the foreseeable future.

Image reconstruction in optical interferometry has gained considerable importance for astrophysical studies during the last decade. This has been mainly due to improvements in the imaging capabilities of existing interferometers and the expectation of new facilities in the coming years. However, despite the advances made so far, image synthesis in optical interferometry is still an open field of research. Since 2004, the community has organized a biennial contest to formally test the different methods and algorithms for image reconstruction. In 2016, we celebrated the 7th edition of the "Interferometric Imaging Beauty Contest". This initiative represented an open call to participate in the reconstruction of a selected set of simulated targets with a wavelength-dependent morphology as they could be observed by the 2nd generation of VLTI instruments. This contest represents a unique opportunity to benchmark, in a systematic way, the current advances and limitations in the field, as well as to discuss possible future approaches. In this contribution, we summarize: (a) the rules of the 2016 contest; (b) the different data sets used and the selection procedure; (c) the methods and results obtained by each one of the participants; and (d) the metric used to select the best reconstructed images. Finally, we named Karl-Heinz Hofmann and the group of the Max-Planck-Institut fur Radioastronomie as winners of this edition of the contest.

Many image reconstruction algorithms have been proposed to cope with optical/IR interferometric data. A first difficulty is to deal with the sparsity of the data and the uneven u-v coverage. This is usually overcome by imposing a priori constraints to the sought image. The reconstruction then amounts to solving an optimization problem where fidelity to the data and to the priors must both be satisfied to a chosen level. Due to the type of interferometric measurements, the image restored by the current algorithms depends on some initial guess and on the optimization strategy. Even though the resulting image is often satisfactory, there are no guarantees that the best image is obtained given the data and the constraints. This is a strong defect which may badly impact the interpretation of interferometric observations by astronomers. In this paper, we consider using stochastic methods such as simulated annealing to solve for the problem of image reconstruction with sparse Fourier amplitudes and unknown Fourier phases. This preliminary study is a first step toward image reconstruction with partial Fourier phase information.

The full scientific potential of the VLTI with its second generation instruments MATISSE and GRAVITY require fringe tracking up to magnitudes K>14 with the UTs and K>10 with the ATs. The GRAVITY fringe tracker (FT) will be limited to K~10.5 with UTs and K~7.5 with ATs, for fundamental conceptual reasons: the flux of each telescope is distributed among 3 cophasing pairs and then among 5 spectral channels for coherencing. To overcome this limit we propose a new FT concept, called Hierarchical Fringe Tracker (HFT) that cophase pairs of apertures with all the flux from two apertures and only one spectral channel. When the pair is cophased, most of the flux is transmitted as if it was produced by an unique single mode beam and then used to cophase pairs of pairs and then pairs of groups. At the deeper level, the flux is used in an optimized dispersed fringe device for coherencing. On the VLTI such a system allows a gain of about 3 magnitudes over the GRAVITY FT. On interferometers with more apertures such as CHARA (6 telescopes) or a future Planet Formation Imager (12 to 20 telescopes), the HFT would be even more decisive, as its performance does not decrease with the number of apertures. It would allow building a PFI reaching a coherent magnitude H~10 with 16 apertures with diameters smaller than 2 m. We present the HFT concept, the first steps of its feasibility demonstration from computer simulations and the optical design of a 4 telescopes HFT prototype.

The Large Binocular Telescope Interferometer uses a near-infrared camera to measure the optical path length variations between the two AO-corrected apertures and provide high-angular resolution observations for all its science channels (1.5-13 microns). There is however a wavelength dependent component to the atmospheric turbulence, which can introduce optical path length errors when observing at a wavelength different from that of the fringe sensing camera. Water vapor in particular is highly dispersive and its effect must be taken into account for high-precision infrared interferometric observations as described previously for VLTI/MIDI or the Keck Interferometer Nuller. In this paper, we describe the new sensing approach that has been developed at the LBT to measure and monitor the optical path length fluctuations due to dry air and water vapor separately. After reviewing the current performance of the system for dry air seeing compensation, we present simultaneous H-, K-, and N-band observations that illustrate the feasibility of our feedforward approach to stabilize the path length fluctuations seen by the LBTI nuller.

In the coming year, the CHARA 1-meter telescopes will be equipped with Adaptive Optics (AO) systems. This improvement opens the possibility to apply, in the visible domain, the principle of spatial filtering with single mode fibers well demonstrated in the near-infrared. It will clearly open new astrophysical fields by taking benefit of an improved sensitivity and state-of-the-art precision and accuracy on interferometric observables. A demonstrator called FRIEND (Fibered and spectrally Resolved Interferometric Experiment - New Design) has been developed. FRIEND combines the beams coming from 3 telescopes after injection in single mode optical fibers and provides photometric channels as well as some spectral capabilities for characterization purposes. It operates around the R spectral band (from 600nm to 750nm) and uses the fast and sensitive analog detector OCAM2. On sky tests at the focus of the CHARA interferometer have been performed during the last year to get the optimal DIT or an estimation of the stability of the instrumental visibility. Complementary lab tests have permitted to characterize the birefringence of the fibers, and the characteristics of the detector. In this paper, we present the results of these tests.

Hypertelescopes are large optical interferometric arrays, employing many small mirrors and a miniature pupildensifier before the focal camera, expected to produce direct images of celestial sources at high resolution. Their peculiar imaging properties, initially explored through analytical derivations, had been verified with simulations before testing a full-size testbed instrument. We describe several architectures and optical design solutions and present recent progress made on the Ubaye hypertelescope experiment. Arecibo-like versions with a fixed spherical primary meta-mirror, or an active aspheric one, have a suspended focal beam combiner equipped for pupil-drift accommodation, with a field-mosaic arrangement for observing multiple sources such as exoplanetary systems, globular clusters or active galactic nuclei. We have developed a cable suspension and drive system with tracking accuracy reaching a millimeter at 100m above ground.

The Planet Formation Imager (PFI) project aims to provide a strong scientific vision for ground-based optical astronomy beyond the upcoming generation of Extremely Large Telescopes. We make the case that a breakthrough in angular resolution imaging capabilities is required in order to unravel the processes involved in planet formation. PFI will be optimised to provide a complete census of the protoplanet population at all stellocentric radii and over the age range from 0.1 to ~100 Myr. Within this age period, planetary systems undergo dramatic changes and the final architecture of planetary systems is determined. Our goal is to study the planetary birth on the natural spatial scale where the material is assembled, which is the "Hill Sphere" of the forming planet, and to characterise the protoplanetary cores by measuring their masses and physical properties. Our science working group has investigated the observational characteristics of these young protoplanets as well as the migration mechanisms that might alter the system architecture. We simulated the imprints that the planets leave in the disk and study how PFI could revolutionise areas ranging from exoplanet to extragalactic science. In this contribution we outline the key science drivers of PFI and discuss the requirements that will guide the technology choices, the site selection, and potential science/technology tradeoffs.

The Planet Formation Imager (PFI) project aims to image the period of planet assembly directly, resolving structures as small as a giant planet's Hill sphere. These images will be required in order to determine the key mechanisms for planet formation at the time when processes of grain growth, protoplanet assembly, magnetic fields, disk/planet dynamical interactions and complex radiative transfer all interact - making some planetary systems habitable and others inhospitable. We will present the overall vision for the PFI concept, focusing on the key technologies and requirements that are needed to achieve the science goals. Based on these key requirements, we will define a cost envelope range for the design and highlight where the largest uncertainties lie at this conceptual stage.

The Planet Formation Imager initiative aims at developing the next generation large scale facility for imaging astronomical optical interferometry operating in the mid-infrared. Here we report on the progress of the Planet Formation Imager Technical Working Group on the beam-combination instruments. We will discuss various available options for the science and fringe-tracker beam combination instruments, ranging from direct imaging, to non-redundant fiber arrays, to integrated optics solutions. Besides considering basic characteristics of the schemes, we will investigate the maturity of the available technological platforms at near- and mid-infrared wavelengths.

The Planet Formation Imager (PFI) Project has formed a Technical Working Group (TWG) to explore possible facility architectures to meet the primary PFI science goal of imaging planet formation in situ in nearby starforming regions. The goals of being sensitive to dust emission on solar system scales and resolving the Hill-sphere around forming giant planets can best be accomplished through sub-milliarcsecond imaging in the thermal infrared. Exploiting the 8-13 micron atmospheric window, a ground-based long-baseline interferometer with approximately 20 apertures including 10km baselines will have the necessary resolution to image structure down 0.1 milliarcseconds (0.014 AU) for T Tauri disks in Taurus. Even with large telescopes, this array will not have the sensitivity to directly track fringes in the mid-infrared for our prime targets and a fringe tracking system will be necessary in the near-infrared. While a heterodyne architecture using modern mid-IR laser comb technology remains a competitive option (especially for the intriguing 24 and 40μm atmospheric windows), the prioritization of 3-5μm observations of CO/H2O vibrotational levels by the PFI-Science Working Group (SWG) pushes the TWG to require vacuum pipe beam transport with potentially cooled optics. We present here a preliminary study of simulated L- and N-band PFI observations of a realistic 4-planet disk simulation, finding 21x2.5m PFI can easily detect the accreting protoplanets in both L and N-band but can see non-accreting planets only in L band. We also find that even an ambitious PFI will lack sufficient surface brightness sensitivity to image details of the fainter emission from dust structures beyond 5 AU, unless directly illuminated or heated by local energy sources. That said, the utility of PFI at N-band is highly dependent on the stage of planet formation in the disk and we require additional systematic studies in conjunction with the PFI-SWG to better understand the science capabilities of PFI, including the potential to resolve protoplanetary disks in emission lines to measure planet masses using position-velocity diagrams. We advocate for a specific technology road map in order to reduce the current cost driver (telescopes) and to validate high accuracy fringe tracking and high dynamic range imaging at L, M band. In conclusion, no technology show-stoppers have been identified for PFI to date, however there is high potential for breakthroughs in medium-aperture (4-m class) telescopes architecture that could reduce the cost of PFI by a factor of 2 or more.

PIONIER is a four beams combiner instrument developed by the Astrophysics Laboratory of Grenoble (LAOG, now IPAG). This instrument arrived at the ESO Paranal Interferometer in 2010 as a visitor instrument and was supposed to be decommissioned with the arrival of the second generation instruments GRAVITY and MATISSE. However, the success of PIONIER induced the needs to keep it available for the scientific community inside the packed environment of the VLTI. This paper presents the technical solutions that were applied to place the instrument in mezzanine without impacting its performance and with the constraint of reducing its operational workload.

Numerical simulations for the Large Binocular Telescope Interferometer have shown a fundamental gain in contrast when using two 8m adaptive optics telescopes instead of one, assuming a high Strehl and a cophasing mode. The global gain is improved by a factor 2 in contrast by using the long exposures and by a factor of 10 in contrast by using the short exposures. Indeed, fringes are still present in the short exposure, contrary to the long exposure where the fringes are blurred. Thus, there is some gain in grouping some short exposures with high gain G. This makes the LBTI well suitable for the Angular Differential Imaging technique. A planet will be alternatively located in the dark fringes (G ≈ 10 to 100) and/or in the dark rings (G ≈ 4 to 20). A rotation of 15° is sufficient to pass through at least one gain zone. The LBTI can provide in the visible wavelengths not only high angular resolution (≈ 6:5mas at 750nm) and high sensitivity (by a factor 4), but also a gain in contrast (by a factor 10 to 100) compared to the stand-alone adaptive optics used on each LBT aperture.

On-sky adaptive optics wavefront screens have been used and random optical path fluctuations - differential pistons - have been included in numerical simulations for the Large Binocular Telescope Interferometer. We characterize the Point Spread Function (PSF) and the Optical Transfer Function (OTF) by computing respectively the interferometric Strehl and the visibility criteria. We study the contribution of the wavefront disturbance induced by each adaptive optics system and by the optical path difference between the arms of the LBTI. To provide an image of quality (Strehl above 70%) suitable with standard science cases , the requirements for a LBTI mode in the visible wavelengths (750nm) must be at least an adaptive optics wavefront RMS fluctuations below λ/18≈40nm (Strehl above 90%) provided by each adaptive optics system, and a differential piston RMS fluctuations below λ/8≈100nm in the overall LBTI system. The adaptive optics wavefront errors - mainly the differential tip-tilt - appear to be more critical than the differential piston.

The construction of a new prototype visible-light intensity interferometer for use in stellar astronomy is described. The instrument is located in New Haven, Connecticut, at Southern Connecticut State University, but key components of the system are also portable and have been taken to existing research-class telescopes to maximize sensitivity and baseline. The interferometer is currently a two-station instrument, but it is easily expandable to several stations for simultaneous measurement using multiple baselines. The design features single photon avalanche diode (SPAD) arrays, which increase the throughput and signal-to-noise ratio of the instrument. Predicted system performance and preliminary observations will be discussed.

As a part of regular operations, the Navy Precision Optical Interferometer (NPOI) uses Narrow Angle Trackers (NAT) for atmospheric tip-tilt correction. This correction is done using a quad cell array for each station, and is based on the error signals measured by these arrays. We compiled NPOI NAT jitter information for the period of 2005 to 2014. Here we investigate the correlation of the NAT jitter between different NPOI stations, and determine a correction for shot-noise induced jitter. We present initial results from the correlation between NAT jitter and quasi simultaneous seeing measurements done with the Lowell Observatory 31" telescope, separated by 500 m. We also discuss some limitations of this technique and future improvements.

ESO is undertaking a large upgrade of the infrastructure on Cerro Paranal in order to integrate the 2nd generation of interferometric instruments Gravity and MATISSE, and increase its performance. This upgrade started mid 2014 with the construction of a service station for the Auxiliary Telescopes and will end with the implementation of the adaptive optics system for the Auxiliary telescope (NAOMI) in 2018. This upgrade has an impact on the infrastructure of the VLTI, as well as its sub-systems and scientific instruments.

The New Adaptive Optics Module for Interferometry (NAOMI) will be developed for and installed at the 1.8-metre Auxiliary Telescopes (ATs) at ESO Paranal. The goal of the project is to equip all four ATs with a low-order Shack– Hartmann adaptive optics system operating in the visible. By improving the wavefront quality delivered by the ATs for guide stars brighter than R = 13 mag, NAOMI will make the existing interferometer performance less dependent on the seeing conditions. Fed with higher and more stable Strehl, the fringe tracker(s) will achieve the fringe stability necessary to reach the full performance of the second-generation instruments GRAVITY and MATISSE.

The new VLTI (Very Large Telescope Interferometer) 1 instrument GRAVITY^{5, 22, 23} is equipped with a fringe tracker¹⁶ able to stabilize the K-band fringes on six baselines at the same time. It has been designed to achieve a performance for average seeing conditions of a residual OPD (Optical Path Difference) lower than ~300 nm with objects brighter than K = 10. The control loop implementing the tracking is composed of a four stage real time computer system compromising: a sensor where the detector pixels are read in and the OPD and GD (Group Delay) are calculated; a controller receiving the computed sensor quantities and producing commands for the piezo actuators; a concentrator which combines both the OPD commands with the real time tip/tilt corrections offloading them to the piezo actuator; and finally a Kalman¹⁵ parameter estimator. This last stage is used to monitor current measurements over a window of few seconds and estimate new values for the main Kalman¹⁵ control loop parameters. The hardware and software implementation of this design runs asynchronously and communicates the four computers for data transfer via the Reflective Memory Network³. With the purpose of improving the performance of the GRAVITY^{5, 23} fringe tracking^{16, 22} control loop, a deviation from the standard asynchronous communication mechanism has been proposed and implemented. This new scheme operates the four independent real time computers involved in the tracking loop synchronously using the Reflective Memory Interrupts² as the coordination signal. This synchronous mechanism had the effect of reducing the total pure delay of the loop from ~3.5 [ms] to ~2.0 [ms] which then translates on a better stabilization of the fringes as the bandwidth of the system is substantially improved. This paper will explain in detail the real time architecture of the fringe tracker in both is synchronous and synchronous implementation. The achieved improvements on reducing the delay via this mechanism will be quantified.

The VLTI instrument GRAVITY combines the beams from four telescopes and provides phase-referenced imaging as well as precision-astrometry of order 10 μas by observing two celestial objects in dual-field mode. Their angular separation can be determined from their differential OPD (dOPD) when the internal dOPDs in the interferometer are known. Here, we present the general overview of the novel metrology system which performs these measurements. The metrology consists of a three-beam laser system and a homodyne detection scheme for three-beam interference using phase-shifting interferometry in combination with lock-in amplifiers. Via this approach the metrology system measures dOPDs on a nanometer-level.

We present in this paper the general formalism and data processing steps used in the MATISSE data reduction software, as it has been developed by the MATISSE consortium. The MATISSE instrument is the mid-infrared new generation interferometric instrument of the Very Large Telescope Interferometer (VLTI). It is a 2-in-1 instrument with 2 cryostats and 2 detectors: one 2k × 2k Rockwell Hawaii 2RG detector for L&M-bands, and one 1k × 1k Raytheon Aquarius detector for N-band, both read at high framerates, up to 30 frames per second. MATISSE is undergoing its first tests in laboratory today.

The interferometric concept named ALOHA (Astronomical Light Optical Hybrid Analysis) offers an alternative for high resolution imaging in the mid-infrared domain by shifting the astronomical light to shorter wavelength where optical guided components from telecommunications are available and efficient. A prototype with two arms converting a signal from 1.55 μm to 630 nm is used to validate the concept in laboratory and on-sky. Thanks to collaboration with the CHARA team, photometric tests were achieved with a single arm of the interferometer and have allowed to predict instrument performance in its interferometric configuration in order to obtain first fringes in H band.

The New Classic instrument was built as a electronics and computer upgrade to the existing Classic beam combiner at the Navy Precision Optical Interferometer (NPOI). The classic beam combiner is able to record 32 of 96 available channels and has a data throughput limitation which results in a low duty cycle. Additionally the computing power of the Classic system limited the amount of fringe tracking that was possible. The New Classic system implements a high-throughput data acquisition system which is capable of recording all 96 channels continuously. It also has a modern high-speed computer for data management and data processing. The computer is sufficiently powerful to implement more sophisticated fringe-tracking algorithms than the Classic system, including multi-baseline bootstrapping. In this paper we described the New Classic hardware and software, including the fringe-tracking algorithm, performance, and the user interface. We also show some initial results from the first 5-station, 4-baseline bootstrapping carried out in January 2015.

GRAVITY acquisition camera implements four optical functions to track multiple beams of Very Large Telescope Interferometer (VLTI): a) pupil tracker: a 2×2 lenslet images four pupil reference lasers mounted on the spiders of telescope secondary mirror; b) field tracker: images science object; c) pupil imager: reimages telescope pupil; d) aberration tracker: images a Shack-Hartmann. The estimation of beam stabilization parameters from the acquisition camera detector image is carried out, for every 0.7 s, with a dedicated data reduction software. The measured parameters are used in: a) alignment of GRAVITY with the VLTI; b) active pupil and field stabilization; c) defocus correction and engineering purposes. The instrument is now successfully operational on-sky in closed loop. The relevant data reduction and on-sky characterization results are reported.

MATISSE (Multi AperTure mid-Infrared SpectroScopic Experiment) is the next generation spectro-interferometer at the European Southern Observatory VLTI operating in the spectral bands L, M and N, and combining four beams from the unit and auxiliary telescopes. MATISSE is now fully integrated at the Observatoire de la Cˆote d’Azur in Nice (France), and has entered very recently its testing phase in laboratory. This paper summarizes the equations describing the MATISSE signal and the associated sources of noise. The specifications and the expected performances of the instrument are then evaluated taking into account the current characteristics of the instrument and the VLTI infrastructure, including transmission and contrast degradation budgets. In addition, we present the different MATISSE simulation tools that will be made available to the future users.

The next generation of fringe tracker (FT) is intended to allow continuous fringe observation and to improve significantly the sensitivity of the interferometer. A promising control approach is presented to cope with contradictory requirements. The FT system must be accurate and stable, which implies high frequency sampling of the optical path differences introduced by the atmosphere and the interferometer vibrations. It must also be as sensitive as possible, which needs to minimize the sampling frequency. The optimum between these concurrent requirements must be maintained through atmospheric and instrument conditions that change very rapidly. We consider a discrete time feedback system where the controller design is based on the frequency domain method. Performance is considered through the use of the H_∞ norm. This approach provides the best tradeoff between the largest sampling time and the validity of the discrete time feedback system. The effectiveness of the presented approach is illustrated through dedicated simulations involving a realistic case study.

Imaging with optical interferometers requires fringe measurements on baseline long enough to resolve the target. These long baselines typically have low fringe contrast. Phasing them requires fringe tracking on shorter baselines which typically have greater fringe contrast and combining the fringe-tracking signals on the short baselines to phase the long baselines in a baseline bootstrapping configuration. On long resolving baselines coherent integration also becomes necessary in order to shorten the integration time. This paper addresses both the baseline bootstrapping and the coherent integration. The Navy Precision Optical Interferometer (NPOI) is laid out in a way which permits long-baseline phasing from shorter baselines in a multi-baseline scheme. The New Classic instrument for NPOI was designed specifically to implement the multi-baseline bootstrapping capability and multi-baseline observations can now be carried out routinely at the NPOI. This paper provides details about the bootstrapping scheme at NPOI and shows some initial results. We also discuss the bootstrapping error budget, describe our new Bayesian coherent integration algorithm and compare its performance to theory.

Name for Person Card: Observatoire de la Côte d'Azur First Light Imaging’ C-RED One infrared camera is capable of capturing up to 3500 full frames per second with a sub-electron readout noise and very low background. This breakthrough has been made possible thanks to the use of an e- APD infrared focal plane array which is a real disruptive technology in imagery. C-RED One is an autonomous system with an integrated cooling system and a vacuum regeneration system. It operates its sensor with a wide variety of read out techniques and processes video on-board thanks to an FPGA. We will show its performances and expose its main features. The project leading to this application has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement N° 673944.

GRAVITY is a second generation instrument for the VLT Interferometer, designed for high-precision narrow-angle astrometry and phase-referenced interferometric imaging in the K-band. It will combine the AO corrected beams of the four VLT telescopes. In total, the GRAVITY instrument uses five eAPD detectors four for the infrared wavefront sensors of each telescope and one for the fringe tracker. In addition two Hawaii2RG arrays are installed, one for the acquisition camera and one for the spectrometer. The SAPHIRA eAPD array is a newly developed near-infrared detector with sub-electron noise performance at frame rates > 1Kfps. For all seven detectors the ESO common controller, NGC, is used. This paper presents an overview and comparison of GRAVITY detector systems and their final performances at the telescope

We introduce a general technique for frequency stability characterization of Fabry-Perot etalons that are being explored for astronomical spectrograph calibration. In our approach a frequency-stabilized laser frequency comb is employed as a reference for a scanning CW laser measurement of the temperature sensitivity of a fiber Fabry-Perot interferometer (FFP). For an in-house constructed, actively stabilized FFP, we observe the thermal sensitivity of a resonance mode at 1319 nm of ∼7.4 GHz C⁻¹, which corresponds to a fractional thermal sensitivity of ∼3.2 × 10⁻⁵ C⁻¹. We compare these results to a simple model and discuss further the materials construction and stabilization of the FFP. Our measurement technique is one step toward a broad characterization of Fabry-Perot instruments, and this FFP in particular is currently being investigated as a wavelength calibration source in precision radial velocity spectroscopy to discover terrestrial-mass exoplanets.

In this communication we present the first experimental results obtained on the Crossed-cubes nuller (CCN), that is a new type of Achromatic phase shifter (APS) based on a pair of crossed beamsplitter cubes. We review the general principle of the CCN, now restricted to two interferometric outputs for achieving better performance, and describe the experimental apparatus developed in our laboratory. It is cheap, compact, and easy to align. The results demonstrate a high extinction rate in monochromatic light and confirm that the device is insensitive to its polarization state. Finally, the first lessons from the experiment are summarized and discussed in view of future space missions searching for extrasolar planets located in the habitable zone, either based on a coronagraphic telescope or a sparse-aperture nulling interferometer.

In the temporal study of faint, extended sources at high resolving power, Spatial Heterodyne Spectrometer (SHS) can offer significant advantages about conventional dispersive grating spectrometers. We describe here a four-year continuous progress in Mt. Hamilton, Lick Observatory, toward development of a prototype reflective Spacial Heterodyne Spectrometer, Khayyam, instrument-telescope configuration to combine all of the capabilities necessary to obtain high resolving power visible band spectra of diffuse targets from small aperture on-axis telescopes where significant observing time can be obtained. We will discuss the design considerations going into this new system, installation, testing of the interferometer-telescope combination, the technical challenges and procedures moving forward.

In the framework of the European FP7-FISICA (Far Infrared Space Interferometer Critical Assessment) program, we developed a miniaturized version of the hyper-telescope to demonstrate multi-aperture interferometry on ground. This setup would be ultimately integrated into a CubeSat platform, therefore providing the first real demonstrator of a multi aperture Fizeau interferometer in space. In this paper, we describe the optical design of the ground testbed and the data processing pipeline implemented to reconstruct the object image from interferometric data. As a scientific application, we measured the Sun diameter by fitting a limb-darkening model to our data. Finally, we present the design of a CubeSat platform carrying this miniature Fizeau interferometer, which could be used to monitor the Sun diameter over a long in-orbit period.

We present concept and first experimental lab results for a low-cost near-infrared heterodyne interferometer based on commercial 1.55μm fiber components with relative phase-stabilization between both telescopes. After a demonstration with 14”-telescopes, the concept should be upgradable to larger numbers of mid- or large-class telescopes. Given that the employed fiber phase stabilization scheme should enable the operation of long baselines, we discuss the applicability of this concept for long-baseline, high telescope number systems (scalability of the concept) and mid-infrared wavelengths. This could finally result in contributions to the design of the large infrared Planet Formation Imager which is being proposed currently.

A wide array of general astrophysics studies including detecting and characterizing habitable exoplanets could be enabled by a future large segmented telescope with sensitivity in the UV, optical, and infrared bands. When paired with a starshade or coronagraph, such an observatory could enable direct imaging and detailed spectroscopic observations of nearby Earth-like habitable zone planets. Over the past several years, a laboratory-based Visible Nulling Coronagraph (VNC) has evolved to reach requisite contrasts over a ~ 1 nm bandwidth at narrow source angle separation using a segmented deformable mirror in one arm of a Mach-Zehnder layout. More recent efforts targeted broadband performance following the addition of two sets of half-wave Fresnel rhomb achromatic phase shifters (APS) with the goal of reaching 10^-9 contrast, at a separation of 2λ/D, using a 40 nm (6%) bandwidth single mode fiber source. Here we present updates on the VNC broadband nulling effort, including approaches to addressing system contrast limitations.

Two new instruments are currently being built for the Gemini-North and WIYN telescopes. They are based on the existing DSSI (Differential Speckle Survey Instrument), but the new dual-channel instruments will have both speckle and "wide-field" imaging capabilities. Nearly identical copies of the instrument will be installed as a public access permanent loan at the Gemini-N and WIYN telescopes. Many exoplanet targets will come from the NASA K2 and TESS missions. The faint limiting magnitude, for speckle observations, will remain around 16 to 17th magnitude depending on observing conditions, while wide-field, high speed imaging should be able to go to 21+. For Gemini, the instrument will be remotely operable from either the mid-level facility at Hale Pohaku or the remote operations base in Hilo.

Sparse Aperture Masking (SAM) has recently been commissioned on SPHERE, the VLTs new adaptive optics high resolution imager. SAM extends the capabilities of SPHERE by providing high contrast measurements at and beyond the traditional diffraction limit. SAM can be used in conjunction with each of the SPHERE modules (IRDIS, IFS and ZIMPOL), allowing dual band imaging in the visible and near-infrared, near-infrared integral field spectroscopy, and polarized differential imaging in the visible and near-infrared. In this paper we report information relevant for observers as well as some commissioning observations.

DAM (Discretized Aperture Mapping) is an original optical concept able to improve the performance in high angular resolution and high contrast imaging by the present class of large telescopes equipped with adaptive optics. By discretizing the entrance pupil of a large telescope into an array of many coherent sub-apertures, DAM provides unique imaging and filtering properties by means of spatial filtering and interferometric techniques. DAM can be achieved by means of single-mode fibers, integrated optic waveguides, pinholes, or simply with an innovative BIGRE optical device. BIGRE is formed of an afocal double micro-lenses array. In addition to the pupil discretization process by spatial filtering, BIGRE can also provide two other optical processes: the pupil densification or the pupil dilution. DAD (Discretized Aperture Densification) increase the sub-aperture sizes and is suitable to a hypertelescope, whereas DADI (Discretized Aperture Dilution Interferometry) reduces the sub-aperture sizes and turns a large telescope into a Fizeau interferometer. This paper deals with the first in-lab experiment at visible wavelength of BIGRE devices for the three configurations above. We study the point spread function (PSF) when observing a point-like object located either on-axis or at various off-axis positions across the field of view. Both interferometric and diffractive effects are described. The experimental measurements are in good agreement with the BIGRE theory. It results that BIGRE fulfils the requirements to carry out spatially filtered pupil discretization (DAM), with possible densification (DAD) or dilution (DADI).

Photonic integrated circuits are established as the technique of choice for a number of astronomical processing functions due to their compactness, high level of integration, low losses, and stability. Temperature control, mechanical vibration and acoustic noise become controllable for such a device enabling much more complex processing than can realistically be considered with bulk optics. To date the benefits have mainly been at wavelengths around 1550 nm but in the important Mid-Infrared region, standard photonic chips absorb light strongly. Chalcogenide glasses are well known for their transparency to beyond 10000 nm, and the first results from coupler devices intended for use in an interferometric nuller for exoplanetary observation in the Mid-Infrared L’ band (3800-4200 nm) are presented here showing that suitable performance can be obtained both theoretically and experimentally for the first fabricated devices operating at 4000 nm.

In this work we propose a new geometry of discrete beam combiners (DBC) for spectrally-resolved stellar interferometry which overcomes limitations of previous designs. The new beam combiner is based on an array of coupled waveguides arranged in zig-zag pattern. It has been numerically optimized for the combination of 4 telescopes and engineered to operate in the L-band. We manufactured a first sample by direct laser writing in Gallium Lanthanum Sulfide glass, a highly transmissive material in the mid-infrared (550 nm to 10 μm). Initial near-field characterization of the fabricated sample at a wavelength of 3.4 μm are encouraging, but highlighted the necessity of a better control of the polarization dispersion of individual waveguides, as well as induced stresses from manufacturing process.

In the aim of access the high angular resolution for mid infrared observations, our team propose to include non linear processes on each arm of an interferometer. This project called ALOHA is now adapted for the L band detection, specially at 3.39 μm. Our team has previously published the first contrast measured in laboratory with such an up-conversion interferometer. The fringe contrast we measured was closed to the theoretical maximum at 100%. In a second step, we investigated the stability of the instrument over several months. The residual drifts are mainly due to the non real-time photometry monitoring.

We conduct an extensive numerical study to single out the best performing rectangular array of evanescently coupled waveguides (discrete beam combiner) that can be used as an integrated optic beam combiner for 6-telescopes at once. We found that the performance of a discrete beam combiner only depends on the conditioning of the Visibility to Pixel Matrix (V2PM) describing it. However, we found that the condition number of V2PM pertaining to different beam combiner architectures cannot be compared. We further report on the possible input waveguide configuration of an 8-telescope discrete beam combiner featuring 8x8 or 9x9 waveguides.

The accurate characterization of the field at the output of the optical fibres is of relevance for precision spectroscopy in astronomy. The modal effects of the fibre translate to the illumination of the pupil in the spectrograph and impact on the resulting point spread function (PSF). A Model is presented that is based on the Eigenmode Expansion Method (EEM) that calculates the output field from a given fibre for different manipulations of the input field. The fibre design and modes calculation are done via the commercially available Rsoft-FemSIM software. We developed a Python script to apply the EEM. Results are shown for different configuration parameters, such as spatial and angular displacements of the input field, spot size and propagation length variations, different transverse fibre geometries and different wavelengths. This work is part of the phase A study of the fibre system for MOSAIC, a proposed multi-object spectrograph for the European Extremely Large Telescope (ELT-MOS).

We investigate the use of photonic lanterns in a fibre beam-combiner. One of the problems of fibre combiners is that the single-mode fibres coupling is affected by the atmospheric seeing. This effect could be potentially mitigated using a "photonic lantern". A photonic lantern converts the multi-mode input at the focal plane of a telescope to N single mode outputs. In presence of atmospheric seeing the light of the telescope will couple with one or more of the single-mode outputs unlike the current interferometers where coupling can be sporadic in bad seeing conditions. In a stellar interferometer each telescope focal plane could be coupled with a photonic lantern. After beam combination each single-mode outputs of the lantern will interfere with all the others. N single mode outputs from each lantern will make N non independent closure phases that can be averaged. Using this method the noise on the closure phase on independent baseline triangles could in principle be reduced by the square root of N.

Fringe tracking at longer wavelengths is advantageous for its larger Fried parameter (R₀) and longer coherence time (τ₀). The fringe trackers which are currently available at the VLTi (Finito, FSU, Gravity, etc.) tracks fringes at the near infrared wavelengths (H and K bands). In our work we try to explore the possibilities to track near and mid- infrared fringes using GLS based laser written integrated optics beam combiners. We simulate the atmospheric optical path difference (OPD) using Kolmogorov/Von-Karman atmospheric turbulence statistics. We also include the measured the piston noise generated due to the instrumental vibrations. Using the resulting OPD time series we can estimate the sensitivity of the fringe tracker at the L band.

Integrated optic devices are nowadays achieving extremely good performances in the field of astronomical interferometry, as shown by PIONIER or GRAVITY silica/silicon-based instruments, already installed at VLTI. In order to address other wavelengths, increase the number of apertures to be combined and eventually ensure on-chip phase modulation, we are working on a novel generation of beam combiners, based on the hybridization of glass waveguides, that can ensure very sharp bend radius, high confinement and low propagation losses, together with lithium niobate phase modulators and channel waveguides that can achieve on-chip, fast (<100kHz) phase modulation. The work presented here has been realized in collaboration with our technological partners TeemPhotonics for glass waveguides and iXBlue-PSD for lithium niobate phase modulators. We will present our results on a hybrid glass/niobate (passive/active) beam combiner that has been developed in the context of FIRST/SUBARU 9T beam combiner. The combiner is structured in three parts: a) the first stage (passive glass) achieves beam splitting from one input to eight outputs, and that for nine input fibers coming from the sub-apertures of the Subaru telescope; b) the second stage consists on a 72 channel waveguides lithium niobate phase modulator in a push-pull configuration that allows to modify on-chip the relative phase between the 36 pairs of waveguides; c) a final recombination system of Y-junctions (passive glass) that allows to obtain combination of each input to every other one. The aim of this presentation is to discuss different issues of the combiners, such as transmission, birefringence, half-wave voltage modulation and spectral range.

Direct laser writing is a powerful technique for the development of astrophotonic devices, namely by allowing 3D structuring of waveguides and structures. One of the main interests is the possibility to avoid in-plane crossings of waveguides that can induce losses and crosstalk in future multi-telescope beam combiners. We will present our results in 3D three telescope beam combiners in the near infrared, that allow for phase closure studies. Besides, laser writing can be used to inscribe a grating over long distances along the waveguide direction. This can be used as an on-chip diffraction grating or as a way to sample a stationary wave that can be obtained in the waveguide. Thus, integrated optics spectrometers based on the SWIFTS concept (stationary wave integrated Fourier transform spectrometer) have been realized and characterized in the near and mid infrared using commercial chalcogenide glasses. Finally, we will also present our results on laser writing on electro-optic materials, that allow to obtain waveguides and beam combiners that can be phase-modulated using electrodes. We have focused our work on two well-known materials: Lithium Niobate, that allows for TM waveguides and has a high electro-optical coefficient, and BGO, that has a lower coefficient, but presents the advantage of being isotropic, guiding both TE and TM polarizations identically.

In August 2014 we performed technical observations at the VLTI with the AMBER and PIONIER beam combiners to measure the interferometric field of view (FOV). As targets we included binaries with component separations between 100 and 300 mas, for which orbits and/or interferometric speckle measurements are available from the Washington Double Star databases or from the literature. The analysis included effects such as bandwidth and time smearing of the interferograms, and photometric attenuation due to the seeing and image quality based on a new formalism of the ESO Exposure Time Calculators. We also consulted the literature for results of interferometric surveys such as the SMASH survey.1 to estimate the effective FOV for these instruments. Based on our analysis, we conclude that emission outside a FOV diameter of 160 mas will be significantly suppressed if not completely invisible. These results provide important information as to the size of the source structure to be included when modeling interferometric data obtained with these instruments.

We have recently shown that a posteriori co-phasing of multi-spectral interferograms was possible.¹ In this contribution, we extend our approach so that it can be applied to actual data as provided by Amber ² or Matisse instruments. The main advantage of the proposed post-processing technique is that it requires no modifications of the instruments and yields interferometric observables with higher SNR and much fewer unknowns (in particular for the Fourier phase) than conventional measurements. In order to perform the co-phasing of a complete sequence of interferograms, we jointly estimate a global phase template and the frame dependent optical path errors due to the turbulence. We show that this strategy is effective for very low SNR data. We assess the effectiveness of our method on simulated and actual AMBER data. We also compare the lowest SNR that can be achieved to the theoretical bounds and estimate the gain in sensitivity compared to usual interferometric data.

For sensitive infra-red long-baseline interferometry, it is crucial to control the differential piston between the apertures. Classically this is achieved with a fringe tracker which measures the movement of the interferometric fringes. In this paper, we describe a new method to reconstruct the piston variation introduced by atmospheric turbulence with real-time data from adaptive optics wave-front sensing. Concurrently, the dominant wind speed vector can also be retrieved. The method is analyzed in simulation for atmospheric turbulence of various strength, and wind vectors varying with layer altitude. The results from the simulations show that this method could help to reliably retrieve the piston variation and wind speed from wavefront sensor data. The method is related to concepts of predictive control AO algorithms and reconstruction of the point spread function.

An effective aperture with several tens or more kilometers is needed to resolve exoplanets. A hypertelescope consists of multiple elemental telescopes like an interferometric array. Light beams from the elemental telescopes are collected and densified and used to form a snap-shot image. Thus formed image, however, does not exhibit high quality features, because the spatial frequency sampling is not dense enough to image properly exoplanets. Some kind of image restoration should be implemented to reveal the surface features of exoplanets. We conduct the image restoration and show the results and the effectiveness of the image restoration through computer simulations.

Image reconstruction algorithms for wide-field spatio-spectral interferometry require knowledge of registration parameters associated with low-resolution image measurements at various baseline orientations, such that the images can be registered to within the fine resolution of the final desired image. We have developed an image registration procedure that combines a nonlinear optimization algorithm with the sub-pixel precision of chirp z-transform resampling, particularly for rotation and translation, of bandlimited images with non-radially symmetric aberrations. We show the accuracy of this image registration technique on simulated images that have a complexity comparable to scenes observed experimentally with NASA’s wide-field imaging interferometry testbed. Registration to within a tenth of a pixel for translation and within three arcminutes for rotation is demonstrated at the largest simulated noise levels.

IRBis is an image reconstruction method for optical/infrared long-baseline interferometry. IRBis can reconstruct images from (a) measured visibilities and closure phases, or from (b) measured complex visibilities (i.e. the Fourier phases and visibilities). The applied optimization routine ASA CG is based on conjugate gradients. The method allows the user to implement different regularizers, as for example, maximum entropy, smoothness, total variation, etc., and apply residual ratios as an additional metric for goodness-of-fit. In addition, IRBis allows the user to change the following reconstruction parameters: (a) FOV of the area to be reconstructed, (b) the size of the pixel-grid used, (c) size of a binary mask in image space allowing reconstructed intensities < 0 within the binary mask only, (d) the strength of the regularization, etc. The two main reconstruction parameters are the size of the binary mask in image space (c) and the strength of the regularization (d). Several values of these two parameters are tested within the algorithm. The quality of the different reconstructions obtained is roughly estimated by evaluation of the differences between the measured data and the reconstructed image (using the reduced χ² values and the residual ratios). The best-quality reconstruction and a few reconstructions sorted according to their quality are provided to the user as resulting reconstructions. We describe the theory of IRBis and will present several applications to simulated interferometric data and data of real astronomical objects: (a) We have investigated image reconstruction experiments of MATISSE target candidates by computer simulations. We have modeled gaps in a disk of a young stellar object and have simulated interferometric data (squared visibilities and closure phases) with a signal-to-noise ratio as expected for MATISSE observations. We have performed image reconstruction experiments with this model for different flux levels of the target and different amount of observing time, that is, with different uv coverages. As expected, the quality of the reconstructions clearly depends on the flux of the source and the completeness of the uv coverage. (b) We also discuss reconstructions of the Luminous Blue Variable η Carinae obtained from AMBER observations in the high spectral resolution mode in the K band. The images were reconstruction (1) using the closure phases and (2) using the absolute phases derived from the measured wavelength-differential phases and the closure phase reconstruction in the continuum.

Interferometry currently provides the only practicable way to image satellites in Geosynchronous Earth Orbit (GEO) with sub-meter spatial resolution. The Magdalena Ridge Observatory Interferometer (MROI) is being funded by the US Air Force Research Laboratory to demonstrate the 9.5 magnitude sensitivity (at 2.2 μm wavelength) and baseline-bootstrapping capability that will be needed to realize a useful turn-key GEO imaging capability. This program will utilize the central three telescopes of the MROI and will aim to validate routine acquisition of fringe data on faint well-resolved targets. In parallel with this effort, the University of Cambridge are investigating the spatial resolution and imaging fidelity that can be achieved with different numbers of array elements. We present preliminary simulations of snapshot GEO satellite imaging with the MROI. Our results indicate that faithful imaging of the main satellite components can be obtained with as few as 7 unit telescopes, and that increasing the number of telescopes to 10 improves the effective spatial resolution from 0.75 meter to 0.5 meter and enables imaging of more complex targets.

This paper presents an extension of the spatio-spectral (“3D”) image reconstruction algorithm called PAINTER (Polychromatic opticAl INTErferometric Reconstruction software). The algorithm is able to solve large scale problems and relies on an iterative process, which alternates estimation of polychromatic images and of complex visibilities. The complex visibilities are not only estimated from squared moduli and closure phases, but also from differential phases, which helps to constrain the polychromatic reconstruction. Alternative methods to construct the specific differential phases used in PAINTER are proposed. Simulations on synthetic data illustrate the specificities of the proposed methods.

OPTICON currently supports a Joint Research Activity (JRA) dedicated to providing easy to use image reconstruction algorithms for optical/IR interferometric data. This JRA aims to provide state-of-the-art image reconstruction methods with a common interface and comprehensive documentation to the community. These tools will provide the capability to compare the results of using different settings and algorithms in a consistent and unified way. The JRA is also providing tutorials and sample datasets to introduce the principles of image reconstruction and illustrate how to use the software products. We describe the design of the imaging tools, in particular the interface between the graphical user interface and the image reconstruction algorithms, and summarise the current status of their implementation.

The study of binary stars is critical to apprehend many of the most interesting classes of stars. Moreover, quite often, the study of stars in binary systems is our only mean to constrain stellar properties, such as masses and radii. Unfortunately, a great fraction of the most interesting binaries are so compact that they can only be apprehended by high-resolution techniques, mostly by interferometry. I present some results highlighting the use of interferometry in the study of binary stars, from finding companions and deriving orbits, determining the mass and radius of stars, to studying mass transfer in symbiotic stars, and tackling luminous blue variables. In particular, I show how interferometric studies using the PIONIER instrument have allowed us to confirm a dichotomy within symbiotic stars, obtain masses of stars with a precision better than 1%, and help us find a new η Carinae-like system. I will also illustrate the benefits for the study of binary stars one would get from upgrading the VLT Interferometer so as to be able to observe in the visible range.

MATISSE represents a great opportunity to image the environment around massive and evolved stars. This will allow one to put constraints on the circumstellar structure, on the mass ejection of dust and its reorganization, and on the dust-nature and formation processes. MATISSE measurements will often be pivotal for the understanding of large multiwavelength datasets on the same targets collected through many high-angular resolution facilities at ESO like sub-millimeter interferometry (ALMA), near-infrared adaptive optics (NACO, SPHERE), interferometry (PIONIER, GRAVITY), spectroscopy (CRIRES), and mid-infrared imaging (VISIR). Among main sequence and evolved stars, several cases of interest have been identified that we describe in this paper.

Some carbon-rich Wolf-Rayet stars are permanent dust producers, as seen by their infrared excess. In famous targets like WR104, the dust is found in the form of a pinwheel nebula around the central source, providing an indirect evidence of binarity. WR104 has been studied in details with the Keck, and more recently with the VLTI by our team. We present here images obtained with the SPHERE instrument and modelling based on AMBER data. First results show that the pinwheel appears to be diluted by diffuse emission. Moreover, a minimum distance between the central binary and the dust-formation zone appears necessary to reproduce both the AMBER and SPHERE data.

We present an overview of the scientific potential of MATISSE, the Multi Aperture mid-Infrared SpectroScopic Experiment for the Very Large Telescope Interferometer. For this purpose we outline selected case studies from various areas, such as star and planet formation, active galactic nuclei, evolved stars, extrasolar planets, and solar system minor bodies and discuss strategies for the planning and analysis of future MATISSE observations. Moreover, the importance of MATISSE observations in the context of complementary high-angular resolution observations at near-infrared and submillimeter/millimeter wavelengths is highlighted.

We plan to measure the angular diameters of a sample of Penn State-Torun Planet Search (PTPS) giant exoplanet host star candidates using the Navy Precision Optical Interferometer. The radii of evolved giant stars obtained using spectroscopy are usually ill-defined because of the method’s indirect nature and evolutionary model dependency. The star’s radius is a critical parameter used to calculate luminosity and mass, which are often not well known for giant stars. Therefore, this problem also affects the orbital period, mass, and surface temperature of the planet. Our interferometric observations will significantly decrease the errors for these parameters. We present preliminary results from NPOI observations of six stars in the PTPS sample.

The Optical interferometry DataBase (OiDB) aims at facilitating the access to science-ready data provided by various existing or decommissioned interferometers. The first version of OiDB has been released in June 2015. Today it contains more than 5000 OIFITS datafiles including the full collection of PIONIER data since 2011. All these reduced data are made publicly available and easily downloadable from OiDB. After presenting the characteristics of OiDB, we analyse how the community made use of it during this first year of operation and how we will improve it.

The Planet Formation Imager (PFI) is a project for a very large optical interferometer intended to obtain images of the planet formation process at scales as small as the Hill sphere of giant exoplanets. Its main science instruments will work in the thermal infrared but it will be cophased in the near infrared, where it requires also some capacity for scientific imaging. PFI imaging and resolution specifications imply an array of 12 to 20 apertures and baselines up to a few kilometers cophased at near infrared coherent magnitudes as large as 10. This paper discusses various cophasing architectures and the corresponding minimum diameter of individual apertures, which is the dominant element of PFI cost estimates. From a global analysis of the possible combinations of pairwise fringe sensors, we show that conventional approaches used in current interferometers imply the use of prohibitively large telescopes and we indicate the innovative strategies that would allow building PFI with affordable apertures smaller than 2 m in diameter. The approach with the best potential appears to be Hierarchical Fringe Tracking based on "two beams spatial filters" that cophase pairs of neighboring telescopes with all the efficiency of a two telescopes fringe tracker and transmit most of the flux as if it was produced by an unique single mode aperture to cophase pairs of pairs and then pairs of groups of apertures. We consider also the adaptation to PFI of more conventional approaches such as a combination of GRAVITY like fringe trackers or single or multiple chains of 2T fringe trackers.

The Planet Formation Imager (PFI) is a future kilometric-baseline infrared interferometer to image the complex physical processes of planet formation. Technologies that could be used to transport starlight to a central beam-combining laboratory in PFI include free-space propagation in air or vacuum, and optical fibres. This paper addresses the design and cost issues associated with free-space propagation in vacuum pipes. The signal losses due to diffraction over long differential paths are evaluated, and conceptual beam transport designs employing pupil management to ameliorate these losses are presented and discussed.

The Wide-field Imaging Interferometry Testbed (WIIT) is a double Fourier (DF) interferometer operating at optical wavelengths, and provides data that are highly representative of those from a space-based far-infrared interferometer like SPIRIT. This testbed has been used to measure both a geometrically simple test scene and an astronomically representative test scene. Here we present the simulation of recent WIIT measurements using FIInS (the Far-infrared Interferometer Instrument Simulator), the main goal of which is to simulate both the input and the output of a DFM system. FIInS has been modified to perform calculations at optical wavelengths and to include an extended field of view due to the presence of a detector array.

The main goal of the Astrometric Gravitation Probe mission is the verification of General Relativity and competing gravitation theories by precise astrometric determination of light deflection, and of orbital parameters of selected Solar System objects. The key element is the coherent combination of a set of 92 circular entrance apertures, each feeding an elementary inverted occulter similar to the one developed for Solar Orbiter/METIS.¹ This provides coronagraphic functions over a relevant field of view, in which all stars are observed for astrometric purposes with the full resolution of a 1 m diameter telescope. The telescope primary mirror acts as a beam combiner, feeding the 92 pupils, through the internal optics, toward a single focal plane. The primary mirror is characterized by 92 output apertures, sized according to the entrance pupil and telescope geometry, in order to dump the solar disk light beyond the instrument. The astronomical objects are much fainter than the solar disk, which is angularly close to the inner field of view of the telescope. The stray light as generated by the diffraction of the solar disk at the edges of the 92 apertures defines the limiting magnitude of observable stars. In particular, the stray light due to the diffraction from the pupil apertures is scattered by the telescope optics and follows the same optical path of the astronomical objects; it is a contribution that cannot be eliminated and must therefore be carefully evaluated. This paper describes the preliminary evaluation of this stray light contribution.

This paper describes the current opto-mechanical design of AGP, a mission designed for astrometric verification of General Relativity (GR) and competing gravitation theories by means of precise determination of light deflection on field stars, and of orbital parameters of selected Solar System objects. The optical concept includes a planar rear-view mirror for simultaneous imaging on the CCD mosaic detector of fields of view also from the direction opposite to the Sun, affected by negligible deflection, for the sake of real time calibration. The precision of astrometric measurements on individual stars will be of order of 1 mas, over two fields separated by few degrees around the Sun and observed simultaneously. We describe the optical design characteristics, with particular reference to manufacturing and tolerancing aspects, evidencing the preservation of very good imaging performance over the range of expected operating conditions.