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12\18 ESO - New Vistas Open with MIDI at the VLT Interferometer
Part 3 of 4

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
fuer Sonnenphysik in Freiburg (Germany), Observatoire de Paris-Meudon
and Observatoire de la Cote d'Azur in Nice (France), and Thueringer
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 fuer 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, cf. ESO PR 06/01. 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 micron 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 degC (4K - 10K) during operation. Also the optics in front
of the detector must be cooled to about -240 degC - 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 
micron) 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.

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.

Notes

[1]: This press release is issued in coordination between ESO and the
research institutes participating in the MIDI project in Germany (Max
Planck Institut fuer Astronomie (MPIA), Thueringer Landessternwarte
Tautenburg (TLS) and Kiepenheuer-Institut fuer 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 Cote 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).

(continued)

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