MID-infrared Interferometric instrument

MIDI, overseen by Instrument Scientist Thomas Rivinius and Instrument Fellow André Müller at Paranal Observatory, is one of three interferometers installed for use with the Very Large Telescope Interferometer (VLTI), alongside AMBER and PIONIER.

"MIDI covers the mid-infrared wavelength range (8–13 micrometres) for the VLTI, making it ideal for observing relatively cool (around 30°C), dusty environments around young stars, evolved stars, and active galactic nuclei (AGN)," explains André. "This differs from AMBER, the other instrument on VLTI, which operates in the near-infrared to observe hot dust at a temperature of around 1200°C and molecules that have been directly exposed to the central star."

MIDI, like other interferometers, doesn’t produce an image as seen with other VLT instruments such as FORS2 or HAWK-I. Instead, it produces a series of interference fringes, the details of which can be found on the main interferometry page. By combining the incoming light from a combination of two Unit Telescopes (UTs) and Auxiliary Telescopes (ATs), the baseline of the observations— the distance between the two telescopes in use — can be changed to capture different pieces of data about an astronomical object.

As well as interferometry, MIDI also possesses some basic spectroscopic capabilities which can split the incoming light into its different colours, or wavelengths, just as a prism does. This capability is sufficient to pick out features that show the presence of silicates, which are the main ingredient of cosmic dust. MIDI can show us how and where they are produced in the disc surrounding a young star, which is an important factor for models of planet formation.

"The strength of the interferometric signal observed at various telescope separations and  orientations, can be interpreted and modelled to give information about the physical dimensions and shape of an object, for instance whether it is symmetrical or asymmetrical, and its chemical composition in relation to its distance from the target star," André concludes.

In other words, MIDI does not produce super-sharp images — in fact, it does not produce images at all! But each measurement obtained with one pair of telescopes gives one piece of information on how large the astronomical object appears at a given wavelength and in a given direction. By combining many such observations like a jigsaw, it is possible to rebuild a detailed model of the appearance of the object at high resolution. Therefore, MIDI would not be able to take an image of astronauts on the Moon, but it would potentially be able to distinguish the two headlights of their moonbuggy!

Science highlights with MIDI

  • Dust around black holes is discovered to be pushed away from above and below, rather than sit in a donut shape around it (eso1327)
  • New technique discovered for determining the shape and size of asteroids, and is used to observe a peculiar one named Barbara (eso0904)
  • VLTI watches a dust shell form and expand around a nova (eso0822)
  • MIDI clearly resolves star WOH G64 to discover it is not as big as previously thought! (eso0815)
  • VLTI observes how a young star gains material that enables it to mature into an adult (eso0803)
  • A celestial chrysalis is discovered that may help explain the shaping of planetary nebulae (eso0743)
  • List of the scientific papers produced by MIDI, via the ESO library’s TelBib database.

A typical MIDI detector image (life-size), given by the "field"camera mode of MIDI. The two disks correspond to the field-of-view of the two entrance beams (without beam-combiner before the detector), illuminated by the thermal background of the environment(walls of the laboratory). The vertical stripes correspond to the readout channels of the detector, which have slightly different gains,especially at the edge. Substraction by reference image allows to discard these artifacts.


The authoritative technical specifications as offered for astronomical observations are available from the Science Operation page.

Site: Paranal
Telescope: Very Large Telescope Interferometer
Focus: Coudé
Type: Interferometer and spectrograph
Wavelength coverage: Mid-infrared, 8–13 μm
Spatial resolution: Interferometric
Spectral resolution: R = 30 (prism), R = 230 (grism)
First light date: December 2002 (now decommissioned)
Images taken with the instrument: Link
Images of the instrument: Link
Press Releases with the instrument: Link
Science goals: Young stellar objects, exoplanets, active galactic nuclei, AGB stars, post-AGB stars


Germany: Max-Planck-Institut für Astronomie, Heidelberg; Kiepenheuer-Institut, Freiburg; Thüringer Landessternwart, Tautenburg

The Netherlands: University of Amsterdam (API); Leiden Observatory; Kapteyn Astronomical Institute, Groningen; Netherlands Research School for Astronomy (NOVA), Leiden; Netherlands Institute for Radio Astronomy (ASTRON), Dwingeloo; Netherlands Institute for Space Research (SRON), Utrecht/Groningen; Astronomical Institute, Utrecht University

France: Observatoire de Paris-Meudon, Meudon; Observatoire de la Côte d'Azur, Nice