eso0236 — Organisation Release

New Vistas Open with MIDI at the VLT Interferometer

"First Fringes" in Mid-Infrared Spectral Region with Two Giant Telescopes

18 December 2002

Following several weeks of around-the-clock work, a team of astronomers and engineers from Germany, the Netherlands, France and ESO [2] has successfully performed the first observations with the MID-Infrared interferometric instrument (MIDI), a new, extremely powerful instrument just installed in the underground laboratory of the VLT Interferometer (VLTI) at the Paranal Observatory (Chile). In the early morning of December 15, 2002, two of the 8.2 m VLT unit telescopes (ANTU and MELIPAL) were pointed towards the southern star Epsilon Carinae and the two light beams were directed via the complex intervening optics system towards MIDI. After a few hours of tuning and optimization, strong and stable interferometric fringes were obtained, indicating that all VLTI components - from telescopes to the new instrument - were working together perfectly. Two more stars were observed before sunrise, further proving the stability of the entire system. The first observations with MIDI mark one more important step towards full and regular operation of the VLT Interferometer [3]. They are a result of five years of determined efforts within a concerted technology project, based on a close collaboration between ESO and several European research institutes (see below). Now opening great research vistas, they also represent several "firsts" in observational astrophysics, together amounting to a real breakthrough in the field of astronomical interferometry.

New views at mid-infrared wavelengths

MIDI is sensitive to light of a wavelength near 10 µm, i.e., in the mid-infrared spectral region ("thermal infrared"). This provides rich opportunities to study a wide range of otherwise inaccessible, crucial astrophysical phenomena, e.g., the formation of planets in dusty disks around newborn stars and the innermost regions around black holes. However, it is a great technical challenge to perform mid-IR observations. This is first of all because the terrestrial atmosphere, the telescopes, their mounts and, not least, the complicated optics system needed to guide the beams the long way from the telescopes to the MIDI instrument all glow bright at mid-IR wavelengths. Thus, even the most luminous mid-IR stellar sources "drown" in this bright background, calling for highly refined observational methods and data reduction procedures.

Fainter objects with large telescopes

This is the first time telescopes with mirrors as large as these have been used for mid-IR interferometry. The use of the VLT giants at Paranal now allows observing much fainter objects than before.

Sharper images with Interferometry

The distance between ANTU and MELIPAL during these observations, 102 metres, is a new world record for interferometry at this wavelength. The achieved angular resolution is indeed the one theoretically possible with this instrumental configuration, about 0.01 arcsec, better than what has ever been achieved before from ground or space at this wavelength.

MIDI is the first of two instruments that will be placed at the focus of the VLT Interferometer. It is a collaborative project between several European research institutes:

  • European Southern Observatory (ESO)
  • Max Planck Institut für Astronomie (MPIA) (Heidelberg, Germany)
  • Netherlands Graduate School for Astronomy (NOVA) (Leiden, The Netherlands)
  • Department of Astronomy - Leiden Observatory (The Netherlands)
  • Kapteyn Astronomical Institute (Groningen, The Netherlands)
  • Astronomical Institute, Utrecht University (The Netherlands)
  • Netherlands Foundation for Research in Astronomy (NFRA) (Dwingeloo, The Netherlands)
  • Space Research Organization Netherlands (SRON) (Utrecht, Groningen; The Netherlands)
  • Thüringer Landessternwarte Tautenburg (TLS) (Germany)
  • Kiepenheuer-Institut für Sonnenphysik (KIS) (Freiburg, Germany)
  • Observatoire de Paris (OBSPM) (Paris, Meudon, Nancay; France)
  • Observatoire de la Côte d'Azur (OCA) (Nice, France)

The first observations with MIDI will now be followed up by thorough tests of the new instrument before it enters into regular service. It is planned that the first community observations will be performed at the VLTI in mid-2003. Great efforts have gone into making observations with this complex science machine as user-friendly as possible and, contrary to what is normally the case in this technically demanding branch of astronomy, scientists will find interferometric work at the VLTI quite similar to that of using the many other, more conventional VLT instruments.

A wonderful moment

Another vital step has been accomplished as planned towards full operation of the ESO Very Large Telescope (VLT) and the associated VLT Interferometer (VLTI) at the Paranal Observatory in Chile, one of the world's foremost astronomical facilities. Indeed, plans had been made more than one year ago for this milestone event to take place at the end of 2002.

In the early morning of December 15, 2002, at 02:45 local time (05:45 UT), a team of astronomers and engineers from Germany, Netherlands, France and ESO celebrated the first successful combination of mid-infrared "light" beams from ANTU and MELIPAL, two of the four 8.2-m VLT Unit Telescopes .

This special moment, referred to as the "First Fringes", occurred when infrared radiation at a wavelength of 8.7 µm from the bright star Epsilon Carinae was captured simultaneously by the two telescopes (situated 102 metres apart) and then directed via a complex optics system towards the MID-Infrared interferometric instrument (MIDI), a new, extremely sensitive and versatile instrument just installed in the underground VLT Interferometric Laboratory. Strong interferometric fringes, well visible on the computer screen to the delighted team, were obtained repeatedly by the MIDI instrument and the recorded data were of excellent quality.

A great achievement

This is the first time ever interferometry in the near-infrared 8.7 µm-band (technically: the "N"-band") with large telescopes has been accomplished and the first time at 100-m baselines.

For this to happen, it was necessary to keep the difference in the length of the light paths from the two telescopes to the focus of the MIDI instrument stable and equal to within a small fraction of this wavelength during the observations, in practice to about 1 µm (0.001 mm). The team spent the first few hours of the night tuning the system, positioning the many optical components and optimizing the various feed-back mechanisms that involve precision-guided mirrors below the two telescopes and the so-called "delay lines" in the underground Interferometric Tunnel [3].

After a few attempts and successive on-line optimization, modulated "fringes" - the typical signature of interferometric measurements - became visible on the screens of the instrument computers, demonstrating conclusively the validity of the overall concept, cf. ESO Press Video eso0236 . The rest of the night was used to further trim the VLTI and MIDI. The team also observed two other objects before sunrise, the young binary star Z Canis Majoris and the enigmatic Eta Carinae - for both, interferometric fringes were convincingly obtained.

The perfection of all of the 32 optical elements needed to guide the starlight towards MIDI for these observations contributed to this, as did the availability of advanced user-friendly control software, specially developed for the VLTI and its instruments in order to facilitate the future observations, also by non-specialists.

Advantages of MIDI

With its high sensitivity to thermal radiation, MIDI is ideally suited to study cosmic material (dust and gas) near a central hot object and heated by its radiation .

In the case of astronomical observations in the visible spectral region, such material is usually hidden from view because of a strong obscuring effect that is caused by the dust it contains. Most optical observations of star-forming clouds only show the dark contours of the cloud and nothing about the complex processes that happen inside. Contrarily, this obscuring effect of the dust is often entirely insignificant at the longer mid-infrared wavelengths around 10 µm (0.01 mm) at which MIDI observes, allowing direct studies of what is going on inside.

MIDI science targets

Thanks to interferometry and the large collecting surface of the VLT telescopes, MIDI achieves unsurpassed image sharpness (about 0.01 arcsec) and sensitivity at these "revealing" wavelengths, promising extremely detailed views, also of faint and distant objects. Clearly, the associated opportunities for exciting research are almost unlimited.

Some of the first targets for the fully operational MIDI instrument will thus include the enigmatic dust rings now believed to be located around giant black holes at the centers of quasars and strong radio galaxies.

Equally interesting will be in-depth studies of those disks of matter that are known to accompany the creation of new stars and from which exoplanets are forming . And with MIDI, it will now be possible to investigate the outer zones of the extended atmospheres of giant stars where the dust grains form in the first place - those complex particles that, loaded with water ice, minerals and simple organic molecules, eventually move into interstellar space and later play a crucial role in the formation of stars and planets.

MIDI - a new and powerful instrument for the VLT Interferometer

The MIDI instrument has been developed by a European consortium of astronomical institutes, under the leadership of the Max-Planck-Institut für Astronomie (MPIA) in Heidelberg (Germany). Following the installation in 2001 by ESO of the VLTI test instrument, VINCI, to verify and tune the exceedingly complex optical system [3], MIDI is the first of two scientific instruments that will be devoted to interferometric observations with the VLT Interferometer during the coming decade. The other is AMBER which will combine three beams from different telescopes and will be sensitive in the wavelength region of 1-2.5 µm.

The MIDI instrument weighs about 1.5 tons and is mounted on a 1.5 x 2.1 m precision optical table, placed at the centre of the underground VLT Interferometric Laboratory at the top of the Paranal mountain, cf. ESO Press Photo eso0236 . The large cube at the back of the table is a vacuum vessel that allows cooling of the infrared detector and the surrounding optics to temperatures of -270 to -240 °C (4K to 35K on the absolute temperature scale), which is necessary for observations at these infrared wavelengths.

Despite its large dimensions, MIDI has to be very carefully adjusted to the light beams arriving from the telescopes, with initial precision exceeding 0.01° (angles) and 0.1 mm (position). The electronic equipment necessary to run the instrument is installed in a separate room in order to reduce any disturbances from heat, noise and vibrations to the lowest possible level. During the observations, the astronomers operate the entire instrument, as well as the VLT Interferometer, from a building below the mountain top, more than one hundred metres away.

This state-of-the-art instrument is the outcome of a close collaboration between several European research institutes [1], greatly profiting from their combined expertise in many different technological areas. This involves the construction of large astronomical instruments for infrared observations, involving operation in vacuum and at low temperatures (MPIA in Heidelberg, Germany), designing and manufacturing optics for the extreme cryogenic environment (ASTRON in Dwingeloo, The Netherlands), designing and creating the complex software needed to run the instrument in a user-friendly way (NEVEC in Leiden, The Netherlands, and MPIA), as well as other specialised contributions from the Kiepenheuer-Institut für Sonnenphysik in Freiburg (Germany), Observatoire de Paris-Meudon and Observatoire de la Côte d'Azur in Nice (France), and Thüringer Landessternwarte in Tautenburg (Germany). This wide collaboration was carried out in close cooperation with and profiting from the professional experience of ESO that has built and now operates the Paranal Observatory, ensuring the proper interfacing between MIDI and the VLTI needed for high-performance interferometric measurements.

Brief history of the MIDI project

Work on the mid-infraredinterferometric instrument MIDI started in 1997 when MPIA proposed to ESO to build such a facility that would conform with ESO's plans for interferometric observations with the VLT telescopes and which would most probably become the first of its kind worldwide.

Soon thereafter, the Netherlands Science Organization NOVA with ASTRON and NEVEC and the other partner institutes in France, the Netherlands and Germany joined the project. With Christoph Leinert and Uwe Graser from MPIA teaming up to lead the project, more than two dozen engineers, astronomers and students worked intensively for three and a half years on the planning, design and production, before the integration of this highly complex instrument could start at the Max-Planck-Institut für Astronomie in Heidelberg. This took place in September 2001 and was followed by a period of extensive instrumental tests.

Much preparatory work had to be done at Paranal in parallel, to be ready for a smooth installation of MIDI [3]. After a positive, concluding status review of MIDI by ESO in September 2002, the many parts of the complex instrument were packed into 32 big wooden boxes, with a total weight of 8 tons, and sent from Heidelberg to Paranal by air freight.

The installation of MIDI in the VLT Interferometric Laboratory began as scheduled in early November. The first test measurements were carried out during the first days of December with two 40-cm siderostats, the same that were used to obtain "first fringes" with the VINCI test instrument in March 2001. These initial measurements led to stable, good-quality fringes on the bright stars Alpha Orionis (Betelgeuse) and Omicron Ceti (Mira).

The total cost of MIDI is of the order of 6 million Euros. Of this, 1.8 million Euros are for equipment, materials and optical parts, with the remaining for salaries during the extensive planning, construction and testing of this front-line instrument.

Some related technical achievements

Astronomical observations of electromagnetic radiation at mid-infrared wavelengths near 10 µm are difficult, because this is the spectral region of thermal radiation from our environment .

If our eyes were sensitive to that radiation, everything around us would be brilliantly bright, including the sky at night, and no stars would then be visible to the naked eye. Sensitive imaging detectors for these wavelengths have become available during the past years, but to work satisfactorily, they must be cooled to a very low temperature around -265 °C (4K - 10K) during operation. Also the optics in front of the detector must be cooled to about -240 °C - otherwise all images would be immediately overexposed, due to the added thermal radiation from those lenses and mirrors.

In practice, the technical solution to this fundamental problem is a so-called closed-cycle cooler that works with high-pressure helium gas and achieves the required low temperatures on several "cold fingers" inside the instrument. However, the associated moving pistons cause vibrations which must be reduced to a minimum by means of special damping materials and connections for the cooler and the instrument. Otherwise this motion would be detrimental to the sensitive measurements, which require near-perfect mechanical stability, to within a fraction of the infrared wavelength, i.e., to 0.001 mm (1 µm) or better.

Similarly, slight bending effects of the instrument parts during cool-down from room temperature would also compromise the measurements. This has been avoided by manufacturing the support of all optical parts near the detector from one single, carefully selected block of special aluminium.

Still, as the light from the star being observed falls on the detector inside MIDI, it will be surrounded by strong thermal radiation from the terrestrial atmosphere in this direction and all uncooled ("warm") mirrors in the light path. The transfer of the digitally recorded images from the detector to the computer data storage must therefore occur at very high speed, one image per 0.001 sec, and always be strictly synchronized with a modulation inherent in the measurement process.

This requires powerful, highly specialized and yet flexible electronics - this crucial part of the new instrument was developed over the past years at MPIA. With this and many other technical innovations successfully completed, and with the first on-the-sky observations just accomplished to the full satisfaction of the MIDI team, this new, powerful instrument will soon be ready to enter into new and unknown research territory. Hundreds of astronomers in the ESO members countries and their colleagues all over the world are now eagerly waiting to get their hands on this new facility.


[1] This press release is issued in coordination between ESO and the research institutes participating in the MIDI project in Germany (Max Planck Institut für Astronomie (MPIA), Thüringer Landessternwarte Tautenburg (TLS) and Kiepenheuer-Institut für Sonnenphysik (KIS)), in the Netherlands ( Netherlands Graduate School for Astronomy (NOVA), Department of Astronomy - Leiden Observatory, Kapteyn Astronomical Institute, Astronomical Institute, Utrecht University, Netherlands Foundation for Research in Astronomy (NFRA) and Space Research Organization Netherlands (SRON)) and in France ( Observatoire de Paris (OBSPM) and Observatoire de la Côte d'Azur (OCA)). German-language versions are available from MPIA. A Dutch-language version is available from NOVA.

[2] The members of the MIDI team are listed at the corresponding MIDI webpage. Key personnel: Christoph Leinert (MPIA - PI Project Scientist) and Uwe Graser (MPIA - PI, Project Manager), Andrea Richichi (ESO Instrument Scientist for MIDI) and Francesco Paresce (ESO VLTI Project Scientist).

[3] The progress of the VLT Interferometer (VLTI) and its many parts has been described at the VLTI website, as well as in various ESO press releases:

More information

Information about MIDI and its many components is available at several dedicated websites, including those at MPIA, NOVA, NFRA and FLUOR. Photos are available in the related ESO press releases [3] and in the VLT Photo Gallery.


Christoph Leinert (Principal Investigator)
Max-Planck Institut für Astronomie
Heidelberg, Germany
Tel: +49-6221 528 264

Andrea Richichi (ESO VLTI)
Garching, Germany
Tel: +49-89-3200-6803

Jakob Staude (MPIA PR Dept.)
Max-Planck Institut für Astronomie
Heidelberg, Germany
Tel: +49 6221 528 229

Richard West (ESO EPR Dept.)
Garching, Germany
Tel: +49-89-3200-6276

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About the Release

Release No.:eso0236
Legacy ID:PR 25/02
Name:Instrumentation, MIDI
Type:Unspecified : Technology : Observatory : Instrument
Facility:Very Large Telescope, Very Large Telescope Interferometer



"First Fringes" in mid-infrared spectral region with two giant telescopes
"First Fringes" in mid-infrared spectral region with two giant telescopes

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