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The European Southern Observatory gives the opportunity to develop new third generation instruments for the Very Large Telescope Interferometer. Therefore, adaptive optics systems need to be upgraded, either for NAOMI on a 1.8-m auxiliary telescope or for MACAO on a 8-m unit telescope. It enables to access to new science cases such as active galactic nuclei with the GRAVITY+ project. We study here the requirements of such AO upgrade by increasing the number of sub-apertures of the wave-front sensor and the number of actuators of the deformable mirror. We evaluate the needs for a high-Strehl mode in the visible and near infrared wavelengths in various conditions of observation. We present numerical simulations to quantify the performance. We show that a moderate upgrade of NAOMI, and a significant upgrade of MACAO can enable both better dynamic range and sensitivity with the VLTI.

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

Direct imaging is crucial to increase our knowledge on extrasolar planetary systems. It can detect long orbits planets that are inaccessible by other methods and it allows the spectroscopic characterization of exoplanet’s athmospheres. During the past fewyears, several giant planetswere detected by direct imaging methods. Yet, as exoplanets are 103 to 1010 fainter than their host star in visible and near-infrared wavelengths, direct imaging requires extremely high contrast imaging techniques, especially to detect low-mass and mature exoplanets. Coronagraphs are used to reject the diffracted light of an observed star and obtain images of its circumstellar environment. Nevertheless, coronagraphs are efficient only if the wavefront is flat because aberrated wavefronts induce speckles in the focal plane which mask exoplanet images. Thus, wavefront sensors associated to deformable mirrors are mandatory to correct speckles by reducing aberrations. To test coronagraph techniques and focal plane wavefront sensors at very high contrast level, we developed the THD2 bench in the optical wavelengths. On the THD2 bench, we routinely reach 10􀀀8 raw contrast level inside the dark hole over broadbands but this level is not sufficient to detect low-mass exoplanets. At this level, it seems that many experimental factors can affect the contrast and understanding which one is limiting the final detection contrast will be useful to upgrade the THD2 bench and to develop the next generation of space-based instruments (LUVOIR, HabEx) aiming to reach 10 -10 contrast level. We started a complete study of the instrumental limitations of the THD2 bench, focusing on scattering which could add intensity on the detector or polarization effects and residual laboratory turbulences. In this paper, we present the methods used to estimate the amount of scattered light that reaches the final detector on the THD2 bench.

While radial velocity and transit techniques are efficient to probe exoplanets with short orbits, the study of long-orbit planets requires direct imaging and coronagraphic techniques. However, the coronagraph must deal with planets that are 10 4 to 10 10 fainter than their hosting star at a fraction of arcsecond, requiring efficient coronagraphs at short angular separation. Phase masks proved to be a good solution in monochromatic or limited spectral bandwidth but expansion to broadband requires complex phase achromatization. Solutions use photonic crystals, subwavelength grating or liquid crystal polymers but their manufacturing remains complex. An easier solution is to use photolithography and reactive ion etching and to optimize the azimuthal phase distribution like achieved in the six-level phase mask (SLPM) coronagraph (Hou et al. 2014). We present here the laboratory results of two SLPM coronagraphs enabling high-contrast imaging in wide-band. The SLPM is split in six sectors with three different depths producing three levels of optical path difference and yielding to uniform phase shifts of 0, π or 2π at the specified wavelength. Using six sectors instead of four sectors enables to mitigate the chromatic effects of the SLPM compared to the FQPM (Four-Quadrant Phase Mask) while keeping the manufacturing easy. Following theoretical developments achieved by University of Shanghai and based on our previous experience to fabricate FQPM components, we have manufactured SLPM components by reactive ion etching at Paris Observatory and we have tested it onto the THD2 facility at LESIA. The THD2 bench was built to study and compare high-contrast imaging techniques in the context of exoplanet imaging. The bench allows reducing the starlight below a 10 −8 contrast level in visible/near-infrared. In this paper, we show that the SLPM is easy to fabricate at low cost and is easy to implement with a unique focal plane mask and no need of pupil apodization. Detection of a planet can be achieved at small inner working angle down to 1 λ/D. The on-axis attenuation of the best SLPM component reaches 2 × 10 −5 at λ = 800 nm and is better than 10 −4 in intensity over a 10% spectral bandwidth. Along the diagonal transition, we show that the off-axis transmission is attenuated by less than 3% over a 10% bandwidth and will need to be calibrated. Any etching imperfections can affect the SLPM performance, by lowering the on-axis attenuation and by changing the optimal wavelength. Despite few nanometers of uncertainty for etching the depths, we show that this first component can provide a high-contrast attenuation in laboratory

International audience; New space missions dedicated to exoplanet imaging will rely on coronagraphs to address the high contrast between the stars and their environments. In order to avoid the image of planets to be lost in post-coronagraphic starlight residuals, high precision wavefront sensing and control is a key element to these missions. We present recent results of simultaneous post-coronagraphic phase and amplitude sensing obtained on the THD bench using the coronagraphic phase diversity. We also present results of simulation studies on the non-linear dark hole technique to assess the main limitations of this technique. Finally, we present a first experimental validation of its principle and corroborate expectations on its speed of convergence. These results suggest that the non-linear dark hole is a good candidate for wave-front control for future space-based exoplanet imaging missions, where fast techniques to produce deep dark holes are of paramount importance.

The architecture of exoplanetary systems is relatively well known inward to 1 AU thanks to indirect techniques, which have allowed characterization of thousands of exoplanet orbits, masses and sometimes radii. The next step is the characterization of exoplanet atmospheres at long period, which requires direct imaging capability. While the characterization of a handful of young giant planets is feasible with dedicated instruments like SPHERE/VLT, GPI/Gemini, SCExAO/Subaru and soon with the coronagraphic capabilities aboard JWST, the spectroscopic study of mature giant planets and lower mass planets (Neptune-like, Super Earths) requires the achievement of better coronagraphic performance. While space-based coronagraph on WFIRST-AFTA might start this study at low spectroscopic resolution, dedicated projects on large space telescope and on the ELT will be required for a more complete spectroscopic study of these faint planets. To prepare these future instruments, we developed a high contrast imaging bench called THD, then THD2 for the upgraded version using multi-DM configuration. The THD2 bench is designed to test and compare coronagraphs as well as focal plane wavefront sensors and wavefront control techniques. It can simulate the beam provided by a space telescope and soon the first stage of adaptive optics behind a ground-based telescope. In this article, we describe in details the THD2 bench and give the results of a recent comparison study of the chromatic behavior for several coronagraph on the THD2., Comment: 9 pages, 5 figures, 1 Table, AO4ELT 2017 conference proceeding

The six-level phase mask (SLPM) can be used in a focal plane as an efficient coronagraph [Opt. Express 22, 1884 (2014)]. It has several advantages: high-contrast imaging in broadband with small inner working angle; easy fabrication at low cost by photolithography and reactive ion etching processes; easy implementation with no need of pupil apodization. We present in this paper the first laboratory results demonstrating the high performance of a SLPM with an unobscured pupil. The on-axis attenuation reaches 2 × 10−5 at λ = 800 nm and is better than 10−4 over a 10% spectral bandwidth and better than 10−3 over a 20% bandwidth. Finally, the detection of a planet can be achieved down to 1 λ/D.

Context. Luminous blue variables (LBVs) are rare massive stars with very high luminosity. They are characterized by strong photo-metric and spectroscopic variability related to transient eruptions. The mechanisms at the origin of these eruptions is not well known. In addition, their formation is still problematic and the presence of a companion could help to explain how they form. Aims. This article presents a study of seven LBVs (about 20% of the known Galactic population), some Wolf-Rayet stars, and massive binaries. We probe the environments that surround these massive stars with near-, mid-, and far-infrared images, investigating potential nebula/shells and the companion stars. Methods. To investigate large spatial scales, we used seeing-limited and near diffraction-limited adaptive optics images to obtain a differential diagnostic on the presence of circumstellar matter and to determine their extent. From those images, we also looked for the presence of binary companions on a wide orbit. Once a companion was detected, its gravitational binding to the central star was tested. Tests include the chance projection probability, the proper motion estimates with multi-epoch observations, flux ratio, and star separations. Results. We find that two out of seven of LBVs may have a wide orbit companion. Most of the LBVs display a large circumstellar envelope or several shells. In particular, HD168625, known for its rings, possesses several shells with possibly a large cold shell at the edge of which the rings are formed. For the first time, we have directly imaged the companion of LBV stars., Astronomy and Astrophysics, EDP Sciences, 2016

This paper reviews the recent laboratory results we have obtained on the demonstration of a cophasing algorithm based on the chromatic phase diversity method. The SIRIUS testbed was initially dedicated to the demonstration of the direct imaging capabilities of arrays of telescope. We have developed and numerically modeled a piston sensor based on the chromatic dependance of the spectral density phase. This method allows a global cophasing of the array over a capture range of many wavelengths aiming at improving the robustness of the method.

Since April 2011, realtime fringe tracking data are recorded simultaneously with data from the VLTI/AMBER interferometric beam combiner. Not only this offers possibilities to post-process AMBER reduced data to obtain more accurate interferometric quantities, it also allows to estimate the performance of the fringe tracking a function of the conditions of seeing, coherence time, flux, etc. First we propose to define fringe tracking performance metrics in the AMBER context, in particular as a function of AMBER's integration time. The main idea is to determine the optimal exposure time for AMBER: short exposures are dominated by readout noise and fringes in long exposures are completely smeared out. Then we present this performance metrics correlated with Paranal local ASM (Ambient Site Monitor) measurements, such as seeing, coherence time or wind speed for example. Finally, we also present some preliminary results of attempts to model and predict fringe tracking performances, using Artificial Neural Networks., SPIE conference, Optical and Infrared Interferometry III

We illustrate here two applications of spectro-interferometry to the study of velocity fields at the surface of stars: pulsation and rotation. Stellar pulsation has been resolved spectroscopically for a long time, and interferometry has resolved stellar diameters variations due to pulsation. Combining the two provides unique insights to the study of Cepheids, in particular regarding the structure of the photosphere or investigating the infamous projection factor which biases distances measured by the Baade–Wesselink method. On the other hand, resolving the surface velocity field of rotating stars offers a unique opportunity to potentially study differential rotation in other cases than for the Sun. We also present the model we have implemented recently, as well as two applications to VLTI/AMBER Data: the pulsation of Cepheids and the rotation of intermediate mass main sequence stars.

Dense Aperture Masking (DAM) is a new interferometric technique allowing high-angular resolution over a narrow field of view (FOV) imaged by the present class of mono-pupil telescopes equipped with adaptive optics (AO). DAM is realized by a suited afocal double lenslet array (BIGRE), remapping the entrance aperture (telescope pupil) into coherent sub-apertures (sub-pupils), and adopted as sub-pupils spatial filter and re-imager.We focus our attention on the point spread function (PSF) properties of DAM, highlighting those related to spatial sampling and filtering of the frequency coverage of the entrance pupil.We stress why the high spatial frequency sampling of the pupil and the low spatial frequency filtering of the sub-pupils are consistent with both a mono-pupil (telescope) and an array of sub-pupils (hypertelescope). We explain how DAM provides high Strehl and high-angular resolution images, first by filtering the low frequencies, which in turn are not so well corrected with a standard AO, second by preserving an object-image convolution relation over a narrow FOV. Finally, we make a comparison with the imaging properties of a telescope and a hypertelescope with the aim to show the complementary of DAM with other techniques adopted in high-contrast imaging.

Dense Aperture Masking (DAM) is a high-contrast imaging technique which enhances the capabilities of the current direct imaging instrumentation, mainly to detect low bright companions at small separation from their parent star. DAM benefits from the experience achieved with the integral field unit installed on SPHERE spectrograph at the VLT (BIGRE, Antichi et al. 2009) with a very similar optical design. More in detail, it is obtained by exploiting the BIGRE integral field unit - composed of two consecutive micro-lens arrays - to subdivide the telescope pupil in many sub-pupils, preserving their relative positio n and providing the same amount of spatial f iltering to each one. We present here results of a system study we pursued for a proficient implementation of BIGRE-DAM. We focus on the case of an 8 m class telescope coupled with the instrument NACO at the VLT. We detail on how the optical design and the related mechanical implementati on of a DAM unit could be successfully achieved within NACO thanks to a wise optimization of the BIGRE micro-lenses array, adopted as sub-pupils re-imager instead of integral field unit. Diffraction limit is achieved by optical design and good apochromatic performan ces are proven for a narrow-b and filter around 2.18 micron. Keywords: imaging interferometry, integr al field unit, optical design

The Astronomical Multi-BEam Recombiner (AMBER), has been operational at the Very Large Telescope Interferometer (VLTI) for many years. We present here some of the constant improvements we have been providing while still operating the instrument, with a heavy load of visitor and service observing programs, most of the nights of the year. In particular, we present here improvements regarding the spectral calibration and correction of the atmospheric loss in squared visibility due to path difference jitter, allowing the instrument to achieve greater precision.

High angular resolution images obtained with a hypertelescope can strongly constrain the radiative-hydrodynamics simulations of red supergiant (RSG) stars, in terms of intensity contrast, granulation size and temporal variations of the convective motions that are visible on their surface. The characterization of the convective pattern in RSGs is crucial to solve the mass-loss mechanism which contributes heavily to the chemical enrichment of the Galaxy. We show here how the astrophysical objectives and the array configuration are highly dependent to design a hypertelescope. For a given field of view and a given resolution, there is a trade-off between the array geometry and the number of required telescopes to optimize either the (u,v) coverage (to recover the intensity distribution) or the dynamic range (to recover the intensity contrast). To obtain direct snapshot images of Betelgeuse with a hypertelescope, a regular and uniform layout of telescopes is the best array configuration to recover the intensity contrast and the distribution of both large and small granulation cells, but it requires a huge number of telescopes (several hundreds or thousands). An annular configuration allows a reasonable number of telescopes (lower than one hundred) to recover the spatial structures but it provides a low-contrast image. Concerning the design of a pupil densifier to combine all the beams, the photometric fluctuations are not critical Delta photometry < 50%) contrary to the residual piston requirements (OPD

Algol and Comptage de Photons Nouvelle Génération (CPNG) are new generation photon counting cameras developed for high angular resolution in the visible by means of optical aperture synthesis and speckle interferometry and for photon noise limited fast imaging of biological targets. They are intensified CCDs. They have been built to benefit from improvements in photonic commercial components, sensitivity, and personal computer workstations processing power. We present how we achieve optimal performances (sensitivity and spatiotemporal resolution) by the combination of proper optical and electronics design, and real-time elaborated data processing. The number of pixels is 532 x 516 and 1024(2) read at a frame rate of 262 and 100 Hz for CPNG and Algol, respectively. The dark current is very low: 5.5 x 10(-4) e(-) .pixel(-1). s(-1). The saturation flux is approximately 7 photon events /pixel/s. Quantum efficiencies reach up to 36% and 26% in the visible with the GaAsP photocathodes and in the red with the GaAs ones, respectively, thanks to the sensitivity of the photocathodes and to the photon centroiding algorithm; they are likely the highest values reported for intensified CCDs.

Long baseline interferometry is now a mature technique in the optical domain. Current interferometers are however highly limited in number of sub apertures and concepts are being developed for future generations of very large optical arrays and especially with the goal of direct imaging. In this paper, we study the effects of introducing single-mode fibers in direct imaging optical interferometers. We show how the flexibility of optical fibers is well adapted to the pupil densification scheme. We study the effects of the truncation of the gaussian beams in the imaging process, either in the Fizeau mode or in the densified pupil mode or in the densified image mode. Finally, in the pupil densification configuration, we identify an optimum of the diaphragm width. This optimum maximizes the on-axis irradiance and corresponds to a trade-off between the loss of transmission and the efficiency of the densification.

First results of near-IR adaptive optics (AO)-assisted imaging, interferometry, and spectroscopy of this Luminous Blue Variable (LBV) are presented. They suggest that the Pistol Star is at least double. If the association is physical, it would reinforce questions concerning the importance of multiplicity for the formation and evolution of extremely massive stars., poster at IAUS272