Towards a Better Understanding of OPD Limitations for Higher Sensitivity and Contrast at the VLTI

Precise control of the optical path differences (OPD) in the Very Large Telescope Interferometer (VLTI) was critical for the characterization of the black hole at the center of our Galaxy - leading to the 2020 Nobel prize in physics. There is now significant effort to push these OPD limits even further, in-particular achieving 100nm OPD RMS on the 8m unit telescopes (UT's) to allow higher contrast and sensitivity at the VLTI. This work calculated the theoretical atmospheric OPD limit of the VLTI as 5nm and 15nm RMS, with current levels around 200nm and 100nm RMS for the UT and 1.8m auxillary telescopes (AT's) respectively, when using bright targets in good atmospheric conditions. We find experimental evidence for the $f^{-17/3}$ power law theoretically predicted from the effect of telescope filtering in the case of the ATs which is not currently observed for the UT's. Fitting a series of vibrating mirrors modelled as dampened harmonic oscillators, we were able to model the UT OPD PSD of the gravity fringe tracker to $<1nm/\sqrt{Hz}$ RMSE up to 100Hz, which could adequately explain a hidden $f^{-17/3}$ power law on the UTs. Vibration frequencies in the range of 60-90Hz and also 40-50Hz were found to generally dominate the closed loop OPD residuals of Gravity. Cross correlating accelerometer with Gravity data, it was found that strong contributions in the 40-50Hz range are coming from the M1-M3 mirrors, while a significant portion of power from the 60-100Hz contributions are likely coming from between the M4-M10. From the vibrating mirror model it was shown that achieving sub 100nm OPD RMS for particular baselines (that have OPD$\sim$200nm RMS) required removing nearly all vibration sources below 100Hz

The expected performance of nulling at the VLTI down to 5 mas

While VLTI offers the recombination of four 8-m telescopes with baselines of more than 100m, it has never hosted a dedicated high-contrast nulling beam-combiner. The SCIFY project aims to design, build and commission Hi- 5, the first nulling beam-combiner of the VLTI, optimized for the detection and characterization of young giant exoplanets near the snow line, with spectroscopy up to R=2000 in the L’ band. It will make use of advanced four-beam nulling combination schemes, like double-Bracewell and kernel-nulling implemented in a single-mode photonic device to produce differential nulled outputs with self-calibrating properties. In the wavelength range of interest, both instrumental errors and background noise are significant. In order to estimate the practical performance of these different configurations in the presence of instrumental errors and further optimize the instrumental design, we have developed SCIFYsim. SCIFYsim is an end-to-end simulator geared towards single- mode beam combiners with of a wide variety of instrumental errors, like optical path difference residuals from fringe tracking, wavefront error at the injection, longitudinal dispersion, chromaticity of the combiner chip, and more. In order to evaluate the performance of the combined spectral channels, we use statistical tests based on likelihood ratios, and account for the covariance in the data. In this paper, we present the expected performance of Hi-5 with a few examples and discuss the main technical limitations to reach the contrast required to image young giant exoplanets.SCIF

Asgard/NOTT: L-band nulling interferometry at the VLTI I. Simulating the expected high-contrast performance

Context: NOTT (formerly Hi-5) is a new high-contrast L' band (3.5-4.0 \textmu m) beam combiner for the VLTI with the ambitious goal to be sensitive to young giant exoplanets down to 5 mas separation around nearby stars. The performance of nulling interferometers in these wavelengths is affected both by fundamental noise from the background and by the contributions of instrumental noises. This motivates the development of end-to-end simulations to optimize these instruments. Aims: To enable the performance evaluation and inform the design of such instruments on the current and future infrastructures, taking into account the different sources of noise, and their correlation. Methods: SCIFYsim is an end-to-end simulator for single mode filtered beam combiners, with an emphasis on nulling interferometers. It is used to compute a covariance matrix of the errors. Statistical detection tests based on likelihood ratios are then used to compute compound detection limits for the instrument. Results: With the current assumptions on the performance of the wavefront correction systems, the errors are dominated by correlated instrumental errors down to stars of magnitude 6-7 in the L band, beyond which thermal background from the telescopes and relay system becomes dominant. Conclusions: SCIFYsim is suited to anticipate some of the challenges of design, tuning, operation and signal processing for integrated optics beam combiners. The detection limits found for this early version of NOTT simulation with the unit telescopes are compatible with detections at contrasts up to $10^5$ in the L band at separations of 5 to 80 mas around bright stars

Asgard/NOTT: L-band nulling interferometry at the VLTI. II. Warm optical design and injection system

Asgard/NOTT (previously Hi-5) is a European Research Council (ERC)-funded project hosted at KU Leuven and a new visitor instrument for the Very Large Telescope Interferometer (VLTI). Its primary goal is to image the snow line region around young stars using nulling interferometry in the L-band (3.5 to 4.0)$\mu$m, where the contrast between exoplanets and their host stars is advantageous. The breakthrough is the use of a photonic beam combiner, which only recently allowed the required theoretical raw contrast of $10^{-3}$ in this spectral range. Nulling interferometry observations of exoplanets also require a high degree of balancing between the four pupils of the VLTI in terms of intensity, phase, and polarization. The injection into the beam combiner and the requirements of nulling interferometry are driving the design of the warm optics and the injection system. The optical design up to the beam combiner is presented. It offers a technical solution to efficiently couple the light from the VLTI into the beam combiner. During the coupling, the objective is to limit throughput losses to 5% of the best expected efficiency for the injection. To achieve this, a list of different loss sources is considered with their respective impact on the injection efficiency. Solutions are also proposed to meet the requirements on beam balancing for intensity, phase, and polarization. The different properties of the design are listed, including the optics used, their alignment and tolerances, and their impact on the instrumental performances in terms of throughput and null depth. The performance evaluation gives an expected throughput loss of less than <6.4% of the best efficiency for the injection and a null depth of $\sim2.10^{-3}$, mainly from optical path delay errors outside the scope of this work.Comment: Accepted for publication in JATIS. 23 pages, 11 figures, 8 table

Technical requirements and optical design of the Hi-5 spectrometer

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

Large Interferometer For Exoplanets (LIFE): II. Signal simulation, signal extraction, and fundamental exoplanet parameters from single-epoch observations

peer reviewedContext. The Large Interferometer For Exoplanets (LIFE) initiative is developing the science and a technology road map for an ambitious space mission featuring a space-based mid-infrared (MIR) nulling interferometer in order to detect the thermal emission of hundreds of exoplanets and characterize their atmospheres. Aims. In order to quantify the science potential of such a mission, in particular in the context of technical trade-offs, an instrument simulator is required. In addition, signal extraction algorithms are needed to verify that exoplanet properties (e.g., angular separation and spectral flux) contained in simulated exoplanet data sets can be accurately retrieved. Methods. We present LIFEsim, a software tool developed for simulating observations of exoplanetary systems with an MIR space-based nulling interferometer. It includes astrophysical noise sources (i.e., stellar leakage and thermal emission from local zodiacal and exozodiacal dust) and offers the flexibility to include instrumental noise terms in the future. Here, we provide some first quantitative limits on instrumental effects that would allow the measurements to remain in the fundamental noise limited regime. We demonstrate updated signal extraction approaches to validating signal-to-noise ratio (S/N) estimates from the simulator. Monte Carlo simulations are used to generate a mock survey of nearby terrestrial exoplanets and determine to which accuracy fundamental planet properties can be retrieved. Results. LIFEsim provides an accessible way to predict the expected S/N of future observations as a function of various key instrument and target parameters. The S/Ns of the extracted spectra are photon noise dominated, as expected from our current simulations. Signals from multi-planet systems can be reliably extracted. From single-epoch observations in our mock survey of small (R < 1.5 REarth) planets orbiting within the habitable zones of their stars, we find that typical uncertainties in the estimated effective temperature of the exoplanets are ≲10%, for the exoplanet radius ≲20%, and for the separation from the host star ≲2%. Signal-to-noise-ratio values obtained in the signal extraction process deviate by less than 10% from purely photon-counting statistics-based S/Ns. Conclusions. LIFEsim has been sufficiently well validated so that it can be shared with a broader community interested in quantifying various exoplanet science cases that a future space-based MIR nulling interferometer could address. Reliable signal extraction algorithms exist, and our results underline the power of the MIR wavelength range for deriving fundamental exoplanet properties from single-epoch observations.Large Interferometer For Exoplanets (LIFE

The GRAVITY+ Project: Towards All-sky, Faint-Science, High-Contrast Near-Infrared Interferometry at the VLTI

The GRAVITY instrument has been revolutionary for near-infrared interferometry by pushing sensitivity and precision to previously unknown limits. With the upgrade of GRAVITY and the Very Large Telescope Interferometer (VLTI) in GRAVITY+, these limits will be pushed even further, with vastly improved sky coverage, as well as faint-science and high-contrast capabilities. This upgrade includes the implementation of wide-field off-axis fringe-tracking, new adaptive optics systems on all Unit Telescopes, and laser guide stars in an upgraded facility. GRAVITY+ will open up the sky to the measurement of black hole masses across cosmic time in hundreds of active galactic nuclei, use the faint stars in the Galactic centre to probe General Relativity, and enable the characterisation of dozens of young exoplanets to study their formation, bearing the promise of another scientific revolution to come at the VLTI.Comment: Published in the ESO Messenge

Robust observables for high-contrast detection at small angular separation

L'étude des exoplanètes est un domaine très actif. Malgré la nature indirecte de la plupart des détections, celles-ci nous fournissent déjà une quantité importante d'informations sur les propriétés de ces systèmes exoplanétaires: leur masse, taille et paramètres orbitaux. Pourtant, la détection directe de la lumière qu'elles émettent ou réfléchissent fournit des informations encore plus précieuses. Les instruments coronographiques conçus dans ce but sont généralement limités par la qualité de front d'onde, qui les empêchent notamment d'atteindre leurs performances théoriques. A l'inverse les techniques employant des observables qui sont robustes à ces aberrations sont limitées par le bruit de photon. Ces deux approches sont rendues incompatibles par la rupture de la relation de convolution qui lie l'objet à l'image. Les travaux décrits dans cette thèse explorent différentes avenues visant à contourner cette incompatibilité afin de combiner les performances des méthodes coronographiques et interférométriques, grâce à des transferts technologiques et au développement de nouvelles méthodes: Le développement des kernels différentiels angulaires (ADK) qui utilisent les principes de l'imagerie différentielle angulaire (ADI) dans le cadre des noyaux de phases et clôtures de phases. L'expérimentation de noyaux de phases avec une pupille apodisée. L'exploitation des noyaux de phases à partir d'images saturées. Le développement d'architectures exploitant les noyaux d'obscurité (kernel nulling) pour un nombre arbitraire de pupille, et notamment pour des architectures à grandes lignes de base. Ces travaux poussent les performances en contraste des observables robustes à faible séparation, et entament l'exploration des nouvelles voies du haut contraste robuste.The study of exoplanets has been a very active topic in the past years. Despite the indirect nature of most detections to date, those already provide us with a wealth of information on the properties of these exoplanetary systems, like their mass, size, and orbital elements. However, the direct detection of the light emitted or reflected by those planets provides some even more precious information. Coronagraphic instruments that would allow this are generally limited by the wavefront quality at their input, which prevents them from reaching their theoretical performance. Conversely, techniques employing observables designed to be robust to those aberrations are limited by photon noise. These two methods are made incompatible because coronagraphy breaks the convolution relationship betwen the object and its image. The works described in this thesis explore different avenues aiming to sidestep this incompatibility and to combine the performance of coronagraphic and interferometric methods though technology transfers as well as the development of new methods. The development of angular differential kernel phases (ADK) that make use the principles of angular differential imaging (ADI), transfering them for kernel and closure phases observables. The experimentation of kernel phases behind shaped pupil apodization masks. The exploitation kernel phases extracted from archival saturated images. The development of architectures for kernel nulling for an arbitrary number of apertures for Fizeau and long-baseline interferometry. These works push the contrast performance of robust observables at small angular separations and begin the exploration of new methods of robust high-contrast observations

Observables robustes pour la détection haut-contraste à faible séparation angulaire

The study of exoplanets has been a very active topic in the past years. Despite the indirect nature of most detections to date, those already provide us with a wealth of information on the properties of these exoplanetary systems, like their mass, size, and orbital elements. However, the direct detection of the light emitted or reflected by those planets provides some even more precious information. Coronagraphic instruments that would allow this are generally limited by the wavefront quality at their input, which prevents them from reaching their theoretical performance. Conversely, techniques employing observables designed to be robust to those aberrations are limited by photon noise. These two methods are made incompatible because coronagraphy breaks the convolution relationship betwen the object and its image. The works described in this thesis explore different avenues aiming to sidestep this incompatibility and to combine the performance of coronagraphic and interferometric methods though technology transfers as well as the development of new methods. The development of angular differential kernel phases (ADK) that make use the principles of angular differential imaging (ADI), transfering them for kernel and closure phases observables. The experimentation of kernel phases behind shaped pupil apodization masks. The exploitation kernel phases extracted from archival saturated images. The development of architectures for kernel nulling for an arbitrary number of apertures for Fizeau and long-baseline interferometry. These works push the contrast performance of robust observables at small angular separations and begin the exploration of new methods of robust high-contrast observations.L'étude des exoplanètes est un domaine très actif. Malgré la nature indirecte de la plupart des détections, celles-ci nous fournissent déjà une quantité importante d'informations sur les propriétés de ces systèmes exoplanétaires: leur masse, taille et paramètres orbitaux. Pourtant, la détection directe de la lumière qu'elles émettent ou réfléchissent fournit des informations encore plus précieuses. Les instruments coronographiques conçus dans ce but sont généralement limités par la qualité de front d'onde, qui les empêchent notamment d'atteindre leurs performances théoriques. A l'inverse les techniques employant des observables qui sont robustes à ces aberrations sont limitées par le bruit de photon. Ces deux approches sont rendues incompatibles par la rupture de la relation de convolution qui lie l'objet à l'image. Les travaux décrits dans cette thèse explorent différentes avenues visant à contourner cette incompatibilité afin de combiner les performances des méthodes coronographiques et interférométriques, grâce à des transferts technologiques et au développement de nouvelles méthodes: Le développement des kernels différentiels angulaires (ADK) qui utilisent les principes de l'imagerie différentielle angulaire (ADI) dans le cadre des noyaux de phases et clôtures de phases. L'expérimentation de noyaux de phases avec une pupille apodisée. L'exploitation des noyaux de phases à partir d'images saturées. Le développement d'architectures exploitant les noyaux d'obscurité (kernel nulling) pour un nombre arbitraire de pupille, et notamment pour des architectures à grandes lignes de base. Ces travaux poussent les performances en contraste des observables robustes à faible séparation, et entament l'exploration des nouvelles voies du haut contraste robuste

Kernel nullers for an arbitrary number of apertures

International audienceContext. The use of interferometric nulling for the direct detection of extrasolar planets is in part limited by the extreme sensitivity of the instrumental response to tiny optical path differences between apertures. The recently proposed kernel-nuller architecture attempts to alleviate this effect with an all-in-one combiner design that enables the production of observables inherently robust to residual optical path differences (≪λ).Aims. To date, a unique kernel-nuller design has been proposed ad hoc for a four-beam combiner. We examine the properties of this original design and generalize them for an arbitrary number of apertures.Methods. We introduce a convenient graphical representation of the complex combiner matrices that model the kernel nuller and highlight the symmetry properties that enable the formation of kernel nulls. The analytical description of the nulled outputs we provide demonstrates the properties of a kernel nuller.Results. Our description helps outline a systematic way to build a kernel nuller for an arbitrary number of apertures. The designs for three- and six-input combiners are presented along with the original four-input concept. The combiner grows in complexity with the square of the number of apertures. While one can mitigate this complexity by multiplexing nullers working independently over a smaller number of sub-apertures, an all-in-one kernel nuller recombining a large number of apertures appears as the most efficient way to characterize a high-contrast complex astrophysical scene.Conclusions. Kernel nullers can be designed for an arbitrary number of apertures that produce observable quantities robust to residual perturbations. The designs we recommend are lossless and take full advantage of all the available interferometric baselines. They are complete, result in as many kernel nulls as the theoretically expected number of closure-phases, and are optimized to require the smallest possible number of outputs