Optical interferometry

The large two interferometric networks currently in operation, VLTI and CHARA, exploit the first generation scientific instruments, AMBER/MIDI and VEGA, for which the Lagrange Laboratory has a leading role. On the VLTI, the second-generation instruments are currently under test and validation: MATISSE (a polychromatic imager for which Lagrange is PI and which is expected to be open to the community by the end of 2018) and GRAVITY, as well as the general infrastructure (GRAV4MAT, NAOMI). On CHARA, work on the definition and development of the adaptive optics and the visible and near-infrared instrumentation is in progress. These developments will allow us to substantially extend the studies that are led in the stellar and planetologic fields, and to access the extragalactic field through the study of Active Galactic Nuclei (AGNs). Our team is involved in this context, in particular in the fields of fringe tracking  and visible instrumentation.

 

The hierarchical fringe tracker

The full VLTI exploitation requires an substantial extension of its limit of sensitivity. For conceptual reasons, the currently best fringe tracker, namely GRAVITY, limits the VLTI sensitivity to K~11 (with the UTs). To go beyond this limit and gain 3 to 4 magnitudes, we study a concept called the hierarchical fringe tracker. Such a concept will allow us to obtain detailed images of disks around young solar-mass stars (T Tauri) or dust tores of the brightest AGNs, and to resolve the BLR of tens of quasars, converting them as cosmological probes to provide masses and direct measurement of distances up to z~3. The main goal of the HFT project is to provide a new generation fringe tracker for the VLTI by 2020-2021 to reach K~14 sensitivity.

Visible instrumentation

In the framework of CHARA, we prepare the development of a new visible instrumentation. These research works are threefolds:

• the development of an prototype instrument (FRIEND - test concepts/technologies)
• the definition of scientific cases
• a structured VIS6T project - visible instrument enabling the recombination of 6 telescopes that are equipped with adaptive optics.

This instrument will be a machine of fundamental stellar parameters and a high-resolution spectro-imager. Our goal is to obtain the first light in 2019-20. In parallel, we will study a third-generation visible instrument of the VLTI facility, in relation with the development of the VLTI facility itself (introduction of additional ATs in the network).

 

Finally, we will pursue the study of the next generation optical interferometers and focal instruments, as well as the hypertelescopes or the new generation of intensity interferometers. In this context, we will contribute to the management and study of the Planet Formation Imager (PFI) project, by being in charge of the coordination of the studies on the cophasing and the visible/near-infrared architecture. PFI will be an imaging network of at least 20 telescopes on kilometric baselines with the aim to discover telluric planets in the habitable zone of stars to understand the general mechanisms of planetary formation.