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12\18 ESO - New Vistas Open with MIDI at the VLT Interferometer
Part 2 of 4

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 micron 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,
cf. PR Photo 30a-b/02 and PR Video Clip 03/02, 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
micron-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 micron
(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. PR
Video Clip 03/02.  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 micron 
(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
---------------------------------------------------------------
[PR Photo 30c/02]

Caption: PR Photo 30c/02 shows the MIDI instrument installed in the
VLTI Laboratory at Paranal. Easily recognizable are the massive
liquid-Helium cryostat in the background (which keeps the MIDI
detector at a temperature of less than 7 K), the optical table with
injection optics (that brings the two VLTI beams from the left of the
picture into the cryostat), and two small alignment telescopes
(yellow).

The MIDI instrument has been developed by a European consortium of
astronomical institutes, under the leadership of the
Max-Planck-Institut fuer 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 micron.

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. 
PR Photo 30c/02. 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 degC (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 deg (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.

(continued)

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