In this talk I will present the concept of an instrument, called V8, capable of combining
all eight VLTI telescope coherently. Unlike all existing instruments the concept of
V8 is based on mid-infrared heterodyne technologies. These have the potential to
considerably simplify the infrastructure of a future large interferometric
facility dedicated to planet formation studies such as the Planet Formation
Imager. I will explain  1) how progresses in many areas of photonics have changed
the field since the historical experiment of C. Townes and his team at ISI;
2) present our current laboratory experiments and finally 3) describe our
roadmap to reach the technogical maturity requested for V8. I will finally point
out how such research and development programs can benefit both direct
heterodyne interferometry.

The STELLIM interferometer aims at providing fast and reliable imaging of the surface of cool evolved stars by combining the light of 13 telescopes in the visible. It is designed to take advantage of the existing VLTI infrastructure. Optical fibres carry the light from the telescopes to the VLTI delay line tunnels where the geometrical optical path lengths are compensated. Using a mirror switchyard, several possible combinations of 7 telescopes are selected in turn and their light is recombined on a dedicated table in the VLTI laboratory. One single night of observations is enough to reconstruct a reliable interferometric snapshot image. STELLIM aims at delivering stellar surface images at a high temporal cadence. It will deliver a complete spatio-temporal characterization of surface and circumstellar environment structures and will allow to monitor how matter is injected into the interstellar medium.

The Fibered Imager foR a Single Telescope (FIRST) is a post-Adaptive Optics (AO) spectro-interferometer that combines pupil remapping and spatial filtering by single-mode fibers techniques. Its unique design enables spectral measurements beyond the diffraction limit of the telescope, a regime that is out of reach for conventional AO imaging systems. The pupil of the telescope is divided into sub-pupils thanks to a micro-lens array, which couples the light into single-mode fibers, removing speckle noise. Fibers rearrange the pupil in a non-redundant configuration, and fringes are spectrally dispersed, covering the 600-900nm range.

Currently, FIRST has 18 fibered inputs, and is installed on the Subaru/SCExAO high contrast imaging platform. Thanks to the wavefront stability delivered by the xAO setup, FIRST can acquire long exposures. Therefore, it can operate on significantly fainter sources than previously possible with aperture masking techniques, currently reaching a magnitude limit of 6 in R band. To push FIRST sensitivity further, a photonic device is currently under development for the sub-pupils interferometric recombination. Its laboratory characterization and performance are more thoroughly described in Kevin Barjot's SF2A proposed talk. The instrument will next be upgraded with a higher spectral resolution spectrograph (from 300 to 2000) in order to perform H alpha imaging. This new mode will open new exciting target possibilities, in particular the detection and characterization of accreting protoplanets, that are bright in the Halpha line. The angular resolution capability of FIRST will enable to probe the 1-2 AU region around the closest young stars, where the distribution of young giant planets is expected to be maximal. In this talk, we present the FIRST instrument integrated on the Subaru telescope and the upgrade of the spectrograph design, as well as the scientific goal of studying exoplanet's formation through H alpha imaging of protoplanets.

GRAVITY/VLTI have transformed optical interferometry with groundbreaking results on the Galactic Center (see Nobel prize in Physics 2020), active galactic nuclei, and exoplanets. Through its transformative upgrades (off-axis fringe-tracking, extreme adaptive optics and laser guide stars for the 4 UTs), GRAVITY+ will open up the extragalactic sky for interferometric imaging, give access to targets as faint as K = 22 mag, and provide unprecedented contrast in the vicinity of bright stars. This presentation will detail the link between the scientific objectives of GRAVITY+ and the implementation of the new capabilities at VLTI, recall the planning, and discuss the main challenges. The presentation will conclude with further perspectives open-up by the deployment of GRAVITY+ at the VLTI. 

En tant que service du pôle thématique national JMMC, SUV (Service aux Utilisateurs du VLTI) fournit une assistance complète à tout utilisateur, nouveau ou plus expérimenté, des instruments du VLTI, en particulier GRAVITY et MATISSE. SUV est coordonné par l'Observatoire de la Côte d'Azur et inclut trois noeuds locaux à Paris (Observatoire de Paris), Grenoble (IPAG), et Lyon (Observatoire de Lyon). Je présenterai le fonctionnement du service et les différents types d'assistance proposées, ainsi que le statut actuel et les perspectives de développement

I will shortly present the status of CHARA/SPICA. It is composed of an infrared fringe tracker included into the MIRCx instrument and of a visible spectrograph. It has been designed to answer to the question of the fundamental parameters with application for exoplanet host stars, coupling high angular resolution and asteroseismology, calibrating surface brightness colour relations, and last but not least to understand the possible bias in all these methods due to the stellar activity.

The Center for High Angular Resolution Astronomy (CHARA), located in Mount Wilson, California, is an interferometric array of six telescopes feeding instruments observing from 0.55 µm to 2.45 µm.

Since the beginning, it has been equipped with a Longitudinal Dispersion Compensator (LDC) necessary for the instruments working at low spectral resolution (R=20) in the near-infrared. However, the transmission of this LDC above 1.9 µm is not optimal and the dispersion compensation is not optimal for observing simultaneously in the whole band from 0.55 to 2.45 µm at low spectral resolution.

The SPICA-FT fringe-tracker working in the H-band inside the MIRCx instrument has been installed in 2020 at CHARA and is being commissioned and optimized in 2021. It will enable long exposures on the different instruments of CHARA. With the aim of multiplying the data collection on spatial frequencies and spectral distribution of the observed objects, we are replacing the current LDC with a more transmissive and performant LDC configuration that will enable simultaneous observations at low spectral resolution with the new generation of instruments CHARA/SPICA in R-band (0.6 – 0.9 µm) and MYSTIC in K-band (1.95 – 2.45 µm) assisted with SPICA-FT in H-band (1.45 – 1.75 µm) and recording also in J-band (1.15 – 1.3 µm).

This new configuration is a step further to a high coverage of the (u,v) plane of faint objects with the stellar interferometer CHARA. I will present the design and expected performance of the new LDC configuration.

We propose a new approach, based on the Hanbury Brown and Twiss intensity interferometry, to transform a Cherenkov telescope to its equivalent optical telescope. We show that, based on the use of photonics components borrowed from quantum-optical applications, we can recover spatial details of the observed source down to the diffraction limit of the Cherenkov telescope, set by its diameter at the mean wavelength of observation. For this, we propose to apply aperture synthesis techniques from pairwise and triple correlation of sub-pupil intensities, in order to reconstruct the image of a celestial source from its Fourier moduli and phase information, despite atmospheric  turbulence. We examine the sensitivity of the method, i.e. limiting magnitude, and its implementation on existing or future high energy arrays of Cherenkov telescopes. We show that despite its poor optical quality compared to extremely large optical telescopes under construction, a Cherenkov telescope can provide diffraction limited imaging of celestial sources, in particular at the visible, down to violet wavelengths.