European Southern Observatory Very Large Telescope

The ESO Very Large Telescope (or just Very Large Telescope or ESO VLT) consists of four 8.2-meter reflector telescopes located at the Paranal Observatory on the Atacama Desert in Chile, operated by the European Southern Observatory (ESO). The four can be used separately or together. They observe at optical and infrared wavelengths. The four, which were completed individually over the years 1998-2000, are:

• UT1, "Antu"
• UT2, "Kueyen"
• UT3, "Melipal"
• UT4, "Yepun"

Among their instruments are:

• 4LGSF - lasers to provide artificial stars (for AO).
• AOF - adaptive optics (AO) facility.
• CRIRES (now CRIRES+) - cryogenic infrared echelle spectrograph.
• ERIS - enhanced resolution imager and spectrograph.
• ESPRESSO - echelle spectrograph for rocky exoplanet and stable spectroscopic observations.
• FLAMES - fibre large array multi-element spectrograph: includes UVES and GIRAFFE.
• FORS2 - focal reducer and low dispersion spectrograph.
• HAWK-I - high acuity wide field K band imager.
• KMOS - cryogenic near-infrared multi-object spectrograph.
• MAVIS - MCAO assisted visible imager and spectrograph (MCAO for multi-conjugate adaptive optics).
• MOONS - multi-object optical and near-IR spectrograph.
• MUSE - integral field spectrograph.
• SPHERE - spectro-polarimetric high-contrast exoplanet research: includes SAXO, IRDIS, IFS, and ZIMPOL.
• UVES (part of FLAMES) - ultraviolet and visual echelle spectrograph.
• VISIR - VLT spectrometer and imager for the mid-infrared.
• X-Shooter - wide-band spectrograph.
• GRAVITY - near-infrared interferometer combining signal from all four telescopes for high-precision astrometry (VLTI).
• MATISSE - multi aperture mid infrared spectrograph experiment (VLTI).
• PIONIER - precision integrated-optics near-infrared imaging experiment (VLTI).
• GPAO - GRAVITY+ AO (VLTI).

Some subsystems and AO systems that I'm not sure are current:

• CIAO - coudé infrared adaptive optics.
• GIRAFFE (part of FLAMES) - medium-high resolution visible light spectrograph.
• ARGUS - large integral field unit; works with GIRAFFE.
• MEDUSA (part of FLAMES, feeding GIRAFFE) - fiber feed for up to 132 separate objects.
• GALACSI - ground atmospheric layer adaptive optics for spectroscopic imaging; works with MUSE.
• GRAAL - ground-layer AOM (adaptive optics module) assisted by lasers; works with HAWK-I.
• IRDIS (part of SPHERE) - infrared dual-band imager and spectrograph.
• IFS (part of SPHERE) - integral field spectrograph.
• SAXO (part of SPHERE) - AO system.
• ZIMPOL (part of SPHERE) - Zurich imaging polarimeter.

On the auxiliary telescopes (see below):

• NAOMI - new adaptive optics module for interferometry.

Among their retired instruments:

• AMBER - astronomical multi-beam recombiner.
• ISAAC - infrared spectrometer and array Camera.
• NACO - Nasmyth AO system.
• FORS1 - former twin of FORS2.
• MAD - AO instrument.
• MIDI - mid-infrared spectrograph.
• SINFONI - spectrograph for integral field observations in the near-infrared.
• ULTRACAM (moved to other telescope).
• VIMOS - visible multi-object spectrograph (originally called VIRMOS).
• VINCI - initial test instrument.
• PRIMA - phase-referenced imaging and microarcsecond astrometry.

The term VLTI for Very Large Telescope Interferometer refers to it as an optical interferometer, e.g., when using GRAVITY. It features a 120-meter baseline. The observatory-complex includes four additional ("auxiliary") 1.8-m telescopes on tracks, allowing use of VLTI even while the four main telescopes are carrying out other observations. The term CCL for Combined-Coudé Laboratory refers to a room to which light from all four telescopes can be routed, which currently houses ESPRESSO; I believe rather than use for interferometry, CCL's purpose is to increase the light-collection by using more than one telescope, or possibly cover a larger FOV. Even with light from just one telescope, CCL provides an isolated, protected environment for delicate instrument operation. The term GRAVITY+ is used for an upgrade project for both VLTI and GRAVITY, but I'm not certain whether it replaces or merely adds to the pre-upgraded instrument.