Precision Integrated-Optics Near-infrared Imaging ExpeRiment

PIONIER, or the Precision Integrated-Optics Near-infrared Imaging ExpeRiment is an instrument on the Interferometer of the Very Large Telescope (VLTI), together with AMBER and MIDI. PIONIER represents a huge milestone for the VLT. In October 2010, for the first time, the beams from four telescopes were recombined in this instrument, thus achieving the full potential of the VLTI.

These means that PIONIER can increase the efficiency of observing the Universe as more baselines can be used at once. Each pair of telescopes from which light is combined constitutes a baseline, and the length of the baseline is determined by the separation of the telescope pair.  “The greater the number of baselines, the more information you get on the morphology of your image, e.g., the level of detail for the objects,” says Julien Milli, ESO astronomer at Paranal. PIONIER is the only instrument to have six potential baselines available. To make a musical analogy: the object represents the complete song, and each baseline represents one of the notes that make up the piece. The more baselines we have, the more notes we have, and the more complete our version of the song is.

Another highlight of PIONIER is its spectral coverage. “You can spread the spectrum of your signal over three channels and therefore get information on the spectral dependence of your image, which is the intensity as a function of the wavelength. This helps us to characterise the warm dust around a star, providing relevant insights on the formation process, says Julien Milli.

PIONIER’s huge capacity for capturing detail allows astronomers to study distant objects, as well as regions where planets or stars may be forming, with an unprecedented level of sensitivity and precision.

How does PIONIER work? It is an interferometer, so once the light reaches the instrument, it is sent across an optical circuit, smaller than a credit card, that brings together the light waves from up to four different telescopes in a very precise way so that they create an interference pattern. An interference pattern consists of fringes, i.e. alternative dark and bright stripes with a given contrast between them, so the final result is not a conventional image.

PIONIER was developed at IPAG in Grenoble, France, and it complements the work of AMBER and MIDI instruments.

Science highlights with PIONIER

  • Granulation patterns directly observed for first time on the surface of a star outside the Solar System (eso1741)
  • Sharpest view ever of dusty disc around aging star (eso1608)
  • VLTI Detects Exozodiacal Light (eso1435)
  • Best images ever of a star that has lost most of its material to a vampire companion (eso1148)
  • Hundred-metre virtual telescope captures unique detailed colour image (eso0906)


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

Site: Paranal
Telescope: VLT Interferometer
Focus: VLTI Laboratory
Type: Near-infrared instrument
Wavelength coverage: 1.5–2.4 μm
Spatial resolution: Set by the longest baseline, 2.5 milliarcseconds
Spectral resolution: Low
First light date: October 2010 (ann1081)
Images taken with the instrument: Link
Images of the instrument: Link

Videos of the instrument:


Press Releases with the instrument: Link
Data papers: Link
Science goals: Diagnostics of close environments of planet-forming stars on the scale on one astronomical unit.


PIONIER is funded by Université Joseph Fourier, LAOG, INSU-CNRS (ASHRA-PNPS-PNP) ANR 2G-VLTI ANR Exozodi.