Discretized aperture mapping for wavefront sensing

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

Status and performance of the THD2 bench in multi-deformable mirror configuration

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 proceedin

Discretized aperture mapping with a micro-lenses array for interferometric direct imaging

Discretized Aperture Mapping (DAM) appears as an original filtering technique easy to play with existing adaptive optics (AO) systems. In its essential DAM operates as an optical passive filter removing part of the phase residuals in the wavefront without introducing any difficult-to-align component in the Fourier conjugate of the entrance pupil plane. DAM reveals as a new interferometric technique combined with spatial filtering allowing direct imaging over a narrow field of view (FOV). In fact, the entrance pupil of a single telescope is divided into many sub-pupils so that the residual phase in each sub-pupil is filtered up to the DAM cut-off frequency. DAM enables to smooth the small scale wavefront defects which correspond to high spatial frequencies in the pupil plane and to low angular frequencies in the image plane. Close to the AO Nyquist frequency, such pupil plane spatial frequencies are not well measured by the wavefront sensor (WFS) due to aliasing. Once bigger than the AO Nyquist frequency, they are no more measured by the WFS due to the fitting limit responsible for the narrow AO FOV. The corresponding image plane angular frequencies are not transmitted by DAM and are useless to image small FOVs, as stated by interferometry. That is why AO and DAM are complementary assuming that the DAM cut-off frequency is equal to the AO Nyquist frequency. Here we describe the imaging capabilities when DAM is placed downstream an AO system, over a convenient pupil which precedes the scientific detector. We show firstly that the imaging properties are preserved on a narrow FOV allowing direct imaging throughout interferometry. Then we show how the residual pupil plane spatial frequencies bigger than the AO Nyquist one are filtered out, as well as the residual halo in the image is dimmed

X-shooter, NACO, and AMBER observations of the LBV Pistol Star \footnote{Based on ESO runs 85.D-0182A, 085.D-0625AC}

We present multi-instruments and multi-wavelengths observations of the famous LBV star Pistol Star. These observations are part of a larger program about early O stars at different metallicities. The Pistol star has been claimed as the most massive star known, with 250 solar masses. We present the preliminary results based on X-Shooter spectra, as well as the observations performed with the VLTI-AMBER and the VLT-NACO adaptive optics. The X-shooter spectrograph allows to obtain simultaneously a spectrum from the UV to the K-band with a resolving power of $\sim$15000. The preliminary results obtained indicate that Pistol Star has similar properties of Eta Car, including shells of matter, but also the binarity. Other objects of the program, here briefly presented, were selected for their particular nature: early O stars with mass discrepancies between stellar evolution models and observations, discrepancies with the wind momentum luminosity relation.Comment: Poster at the 39th LIAC, submitted version of the proceeding

Fundamental gain in high-contrast imaging with the large binocular telescope interferometer

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

Sensitivity to differential piston and to adaptive optics errors with the Large Binocular Telescope Interferometer

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

High-angular resolution observations of the Pistol Star

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.Comment: poster at IAUS27

Luminous blue variables: An imaging perspective on their binarity and near environment

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

Imagerie Directe en Interférométrie Stellaire Optique:<br />Capacités d'Imagerie d'un Hypertélescope <br />& Densifieur de Pupille Fibré.

In the next future, the optical stellar interferometers are going to provide real images, by increasing the number of telescopes and by cophasing the beams. These conditions are requiered to have sufficient resolution elements (resel) in the image and to observe the low bright objects. If both conditions are achieved, direct imaging becomes more interesting than Fourier synthesis imaging. From then on, it is time to study the future large array using the hypertelescope mode, which optimizes the image properties. An hypertelescope provides snapshot images with a significant gain in sensitivity, without inducing any loss of the useful field of view. Indeed, it has been shown that the direct imaging capabilities of a sparse aperture are determined by the geometry of the array only, whatever the beam combination scheme. The pupil densification allows to equalize the Direct Imaging Field (DIF) with the real usable field of view offered by the baselines of the interferometer.Numerical simulations (HYPERTEL) have been performed to study the direct imaging properties. For that, criteria are defined to characterize the image pattern. It is shown that the choice of the geometry of the array is a trade-off between the resolution, the dynamic, the field of view and the astrophysical objectives. A regular and non-redondant pattern of the input pupil optimizes the dynamic, the contrast and the fidelity of the densified image, but decreases the useful field of view. A spotted star, with a low contrast, requiere dynamic, whereas a large field is suitable for the multiple stars.A pupil densifier using monomode optical fibres in the visible wavelength (SIRIUS) has been developed at the Observatoire de la Côte d'Azur. The effects of introducing single-mode fibres in direct imaging optical interferometers have been studied with numerical simulations. We identify an optimum to define properly the output densified pupil. First densified images have been obtained in laboratory. Spatial filtering greatly enhances the quality and the stability of the densified image, but mainly decrease partially the sensitivity of the signal. Atmospheric perturbations are converted into differential photometric fluctuations, which are easier to calibrate. These photometric fluctuations have few influence on the image quality, which simplify the image deconvolution and the beams cophasing. Finally, the flexibility of the optical fibres is well adapted to carry the beams from the entrance to the exit pupil with the appropriate rearrangement of the sub-apertures, which is convenient for next generation of interferometers, such as VLTI, CHARA, NPOI, MROI or OHANA.Les interféromètres stellaires optiques sont en passe de devenir de véritables imageurs. Pour cela, il faut disposer d'un grand nombre de télescopes pour augmenter le nombre d'éléments de résolution (resel) dans l'image, et il faut cophaser activement les faisceaux pour observer des objets peu brillants en pose longue. Si ces deux conditions sont remplies, il devient plus intéressant de travailler en mode Imagerie Directe qu'en mode Synthèse de Fourier. Dès lors, une prospective est menée sur les futurs grands réseaux en mode hypertélescope, qui optimise les propriétés de l'image. En effet, un hypertélescope fournit une image directe instantanée, avec un fort gain en sensibilité sans perte de champ utile. Il a été démontré que le champ utile d'un interféromètre dilué est imposé par la géométrie du réseau, indépendamment du mode de recombinaison. Le fait de densifier la pupille optimise l'image en ajustant le champ d'imagerie direct avec le champ réellement exploitable par l'interféromètre.Un programme de simulation (HYPERTEL) étudie les propriétés d'une image directe à partir d'un ensemble de critères d'imagerie qualitatifs. Il est montré que le choix de la configuration du réseau est un compromis entre la résolution, la dynamique, le champ et l'objectif astrophysique. Un pavage régulier et non redondant des ouvertures améliore à la fois la dynamique, le contraste et la fidélité de l'image, mais minimise le champ d'imagerie. Les étoiles multiples requièrent un champ d'imagerie suffisant, tandis que les surfaces stellaires faiblement contrastées exigent de la dynamique.Un nouveau concept de densifieur de pupille à fibres optiques monomodes dans le visible (SIRIUS) a été développé au laboratoire optique de Grasse de l'Observatoire de la Côte d'Azur. Des études préliminaires sur l'influence des fibres dans le processus d'imagerie ont mis en évidence un optimum pour redéfinir la pupille de sortie du densifieur. Les premières images de SIRIUS ont montré que la densification monomodale améliore la qualité et la stabilité de l'image d'un hypertélescope, moyennant une perte de flux global. Le filtrage spatial des fibres monomodes convertit les perturbations atmosphériques en fluctuations photométriques plus faciles à étalonner. Ces fluctuations photométriques affectent peu la qualité de l'image densifiée, ce qui permet de simplifier la déconvolution de l'image et le cophasage des faisceaux. Enfin, la flexibilité des fibres permet une reconfiguration entrée/sortie plus aisée de la pupille, ce qui convient bien aux nouveaux interféromètres comme le VLTI, CHARA, NPOI, ou encore MROI et OHANA