The Very Large Telescope Interferometer

The Very Large Telescope Interferometer (VLTI) consists in the coherent combination of the four VLT Unit Telescopes or the four moveable 1.8m Auxiliary Telescopes. The VLTI provides milli-arcsec angular resolution at low and intermediate (R=5000) spectral resolution at near and mid-infrared wavelengths.

General Description

The four 8.2-m Unit Telescopes (UTs) and the four 1.8-m Auxiliary Telescopes (ATs) are the light collecting elements of the VLTI. The UTs are set on fixed locations while the ATs can be relocated on more than 10 different stations. VLTI instruments all recombine the light from four telescopes simultaneously. After the light beams have passed through a complex system of mirrors and the light paths have been equalized by the delay line system, the light re-combination is performed by the PIONIER and GRAVITY instruments in the near infrared and by MATISSE in the mid-infrared part of the spectrum.

Due to its unique characteristics, the VLTI has become a very attractive means for scientific research on various objects like young pre-main sequence stars and their protoplanetary disks, post-main sequence mass-losing stars, binary objects and their orbits, solar system asteroids, and extragalactic objects such as active galactic nuclei.


Operational Instruments

  • MATISSE: 4-way beam combiner from L, M, and N infrared bands.
  • GRAVITY: 4-way beam combiner in K band, with spectroscpic, fringe tracking and astrometric capabilities.
  • PIONIER: The 4-way beam combiner in H band using the visitor focus of VLTI.

Decommissioned Instruments

  • AMBER: The Near-Infrared (J, H, K; 1-2.5 mm) 3-way beam combiner. Spectal resolution R=30 (low resolution), 1500 (medium r.), 12000 (high r.).
  • MIDI: The Mid-Infrared (8-13 mm) 2-way beam combiner Spectral resolution R=30 (prism), R=230 (grism).
  • PRIMA: The Instrument for Phase-Referenced Imaging and Microarcsecond Astrometry.
  • VINCI: The VLT Interferometer Commissioning Instrument. K-band.



VLTI users are actively supported by the VLTI Expertise Centers.