The Very Large Telescope Interferometer (VLTI) is currently the best infrastructure for long-baseline interferometry in particular in terms of sensitivity and accessibility to the general user. MATISSE, installed at the VLTI focus since end of 2017, belongs to the second generation instruments. MATISSE, the Multi AperTure mid-Infrared SpectroScopic Experiment, for the first time accesses high resolution imaging over a wide spectral domain of the mid-infrared. The instrument is a spectro-interferometric imager in the atmospheric transmission windows called L, M, and N, from 2.8 to 13.0 microns, and combines four optical beams from the VLTI’s unit or auxiliary telescopes. The instrument utilises a multi-axial beam combination that delivers spectrally dispersed fringes. The signal provides the following quantities at several spectral resolutions: photometric flux, coherent flux, visibility, closure phase, wavelength differential visibility and phase, and aperture-synthesis imaging. MATISSE can operate as a stand alone instrument or with the GRA4MAT set-up employing the GRAVITY fringe tracking capabilities. The updated MATISSE performance are presented at the conference together with a selection of two front-line science topics explored since the start of the science operations in 2019. Finally we present the perspective and benefit of two technical improvements foreseen in the coming years: the MATISSE-Wide off-axis fringe tracking capability and new adaptive optics for the UTs in the context of the GRAVITY+ project.
VERMILION is a VLTI visitor instrument project intended to extend the sensitivity and the spectral coverage of Optical Long Baseline Interferometry (OLBIn). It is based on a new concept of Fringe Tracker (VERMILIONFT) combined with a J band spectro-interferometer (VERMILION-J). The Fringe Tracker is the Adaptive Optics module specific to OLBIn that measures and corrects in real time the Optical Path Difference (OPD) perturbations introduced by the atmosphere and the interferometer, by providing a sensitivity gain of 2 to 3 magnitudes over all other state of the art fringe trackers. The J band spectro-interferometer will provide all interferometric measurements as a function of wavelength. In addition to a possible synergy with MATISSE, VERMILION-J, by observing at high spectral resolution many strong lines in J (Paβ-γ, HeII, TiO and other metallic monoxides), will cover several scientific topics, e.g. Exoplanets, YSOs, Binaries, Active Hot, Evolved stars, Asteroseismology, and also AGNs.
Hierarchical Fringe Tracking (HFT) is a fringe tracking concept optimizing the sensitivity in optical long baseline by reducing to an absolute minimum the number of measurements used to correct the OPD fluctuations. By nature, the performances of an HFT do not decreases with the number of apertures of the interferometer and are set only by the flux delivered by the individual telescopes. This a critical feature for future interferometers with large number of apertures both for homodyne and heterodyne operation. Here we report the design and first optical bench tests of integrated optics HFT chips for a 4 telescopes interferometer such as the VLTI. These tests validate the HFT concept and confirm previous estimates that we could track accurately fringes on the VLTI up to nearly K~15.9 with the UTs and K~12.2 with the ATs with a J+H+K fringe tracker with one HFT chip per band. This is typically 2.5 magnitudes fainter than the best potential performance of the current ABCD fringe tracker in the K band. An active longitudinal and transverse chromatic dispersion correction allows the optimization of broad band fiber injections and instrumental contrast. We also present a preliminary evaluation of the potential of such a gain of sensitivity for the observations of AGNs with the VLTI.
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