First System of Deployable Multi-Integral Field Units Ready
Summary
The ESO Very Large Telescope (VLT) at the Paranal Observatory is being
equipped with many state-of-the-art astronomical instruments that will
allow observations in a large number of different modes and wavebands.
Soon to come is the Fibre Large Array Multi-Element Spectrograph (FLAMES),
a project co-ordinated by ESO. It incorporates several complex components,
now being constructed at various research institutions in Europe and
Australia.
One of these, a true technological feat, is a unique system of 15
deployable fibre bundles, the so-called Integral Field Units (IFUs). They
can be accurately positioned within a sky field-of-view measuring no less
that 25 arcmin in diameter, i.e., almost as large as the full Moon. Each
of the IFUs looks like an insect’s eye and images a small sky area (3 x 2
arcsec^2) with a multiple microlens. From each IFU, 20 narrow light beams
are sent via optical fibres to an advanced spectrograph. All 300 spectra
are recorded simultaneously by a sensitive digital camera.
A major advantage of this technique is that, contrary usual spectroscopic
observations in which spectral information is obtained along a
(one-dimensional) line on the sky, it now allows (two-dimensional) area
spectroscopy. This will permit extremely efficient spectral observations
of many celestial objects, including faint galaxies, providing detailed
information about their internal structure and motions. Such studies will
have an important impact on our understanding, e.g., of the early
evolution of galaxies, the main building blocks in the Universe.
The IFUs have been developed by a team of astronomers and engineers [2] at
the Observatoire de Paris-Meudon. All IFU components are now at the ESO
Headquarters in Garching (Germany) where they are being checked and
integrated into the instrument [3].
PR Photo 03a/02: The GIRAFFE spectrograph in the ESO Assembly Hall
(Garching, Germany).
PR Photo 03b/02: Example of a future IFU observation in a sky field with
galaxies.
PR Photo 03c/02: An illustration of how the IFUs function.
PR Photo 03d/02: The IFU design.
PR Photo 03e/02: Computer simulation of the motions in a galaxy, as
deduced from IFU observations.
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The FLAMES instrument and its many parts[ESO PR Photo 03a/02] ESO PR Photo Caption: PR Photo 03a/02: The 03a/02 GIRAFFE spectrograph, a major component of the VLT Fibre Large Array Multi-Element Spectrograph [Preview - JPEG: 560 x 400 pix - (FLAMES), during the present 62k] assembly at the ESO Headquarters in [Normal - JPEG: 1120 x 800 pix - Garching (Germany). 544k] [Hi-Res - JPEG: 2885 x 2061 pix - 5.3M]
Late last year, the ESO Very Large Telescope (VLT) at the Paranal
Observatory received its newest instrument, NAOS-CONICA. The first tests
were very successful, cf. PR 25/01.
But this is far from the last. Work is now underway at several European and
overseas research institutes to complete the many other large astronomical
instruments planned for the VLT. Over the next years, these new facilities
will enter into operation one by one, further enhancing the capabilities of
this true flagship of European science.
One of these instruments is the Fibre Large Array Multi-Element Spectrograph
(FLAMES), to be installed at the 8.2-m VLT KUEYEN Unit Telescope. It will be
able to observe the spectra of a large number of individual, faint objects
(or small sky areas) simultaneously and incorporates several highly complex
components, e.g.,
- a Nasmyth Corrector – an optical system to focus the light that is
received from the telescope over a sky field of no less than 25 arcmin
in diameter, i.e., almost as large as the full Moon. It was installed
on KUEYEN in September 2001 - a Fibre Positioner (known as “OzPoz”). It is now being built by the
AUSTRALIS Consortium, lead by the Anglo Australian Observatory (AAO),
cf. ESO PR 07/98 - a high- and intermediate-resolution optical spectrograph, GIRAFFE, with
its own fibre system, developed by the Observatoire de Paris-Meudon in
close collaboration with ESO. It is now in the process of being
assembled in the ESO laboratories in Garching, cf. PR Photo 03a/01.
Work at the FLAMES facility will be supported by specialized data reduction
software developed by Observatoire de Genève-Lausanne in collaboration with
Observatoire de Paris-Meudon, and specialized observing software developed
at ESO. There will also be a fibre link to the UVES high-dispersion
spectrograph and there are plans for incorporating an intermediate
resolution IR spectrograph in the future; the ITAL-FLAMES consortium is now
preparing the associated instrument control and data reduction software
packages.
The Integral Field Units (IFUs) for FLAMES
[ESO PR Photo 03b/02] ESO PR Photo [ESO PR Photo 03c/02] ESO PR Photo
03b/02 03c/02
[Preview - JPEG: 573 x 400 pix - [Preview - JPEG: 538 x 400 pix -
94k] 63k]
[Normal - JPEG: 1145 x 800 pix - [Normal - JPEG: 1076 x 800 pix -
592k] 256k]
Caption: PR Photo 03b/02: An example of observations with Integral Field
Units (IFUs) at FLAMES (only 4 of the 15 units are shown here). Each IFU
is placed so that it records the light from 20 small adjacent sky areas
(each measuring about 3 x 2 arcsec^2). In this way, it is possible to
register simultaneously the spectrum of as many different regions of a
(distant) galaxy. PR Photo 03c/02 : How the IFUs work: each IFU consists
of a microlens that guides the light from a small sky area, normally
centred on a celestial object (e.g., a distant galaxy) and sends it on to
the entry of the spectrograph (inside the dotted box).
When it enters into operation later this year [3], GIRAFFE will become the
most efficient instrument of its kind available at the world’s large
optical/infrared telescopes. It will be especially suited for the study of
the dynamical properties of distant galaxies – their motion in space, as
well as the internal motions of their stars and gas clouds. Indeed,
observations of the velocity fields in a large variety of galaxies in the
early Universe (when its age was only one third to one half of its current
age) will be essential for a better understanding of those major building
blocks of the Universe.
This is first of all due to the unique system of 15 deployable fibre
bundles, the Integral Field Units (IFUs), that can be accurately positioned
within a field-of-view measuring no less than 25 arcmin across, cf. PR Photo
03b/02. Each IFU is a microscopic, state-of-the-art two-dimensional lens
array with an aperture of 3 x 2 arcsec^2 on the sky. It contains twenty
micro-lenses coupled with optical fibres leading the light recorded at each
point in the field to the entry slit of the spectrograph, cf. PR Photo
03c/02.
A great advantage of this technique is that, contrary to usual spectroscopic
observations in which spectral information is obtained along a
(one-dimensional) line on the sky, it now allows (two-dimensional) area
spectroscopy. It is therefore possible to obtain spectra of larger areas of
a celestial object simultaneously, and not just along one particular
diameter.
With 15 IFUs at their disposal, the astronomers will be able to observe many
galaxies at the same time – this will represent a tremendous gain of
efficiency with many more astrophysical data collected within the available
observation time!
The IFU design
[ESO PR Photo 03d/02] ESO PR Photo Caption: PR Photo 03d/02: Mechanical
03d/02 design of an IFU "button". Upper
right: photo of an "IFU entrance"
with the 20 square microlenses, each
[Preview - JPEG: 400 x 469 pix - measuring 1.8 x 1.8 mm^2.
86k]
[Normal - JPEG: 800 x 937 pix -
232k]
PR Photo 03d/02 shows the mechanical design of the entrance of one IFU. An
array of 20 square microlenses, each measuring 1.8 x 1.8 mm^2 is used to
concentrate the light in the corresponding, small sky field onto a prism
that passes the light on to 20 fibres. These are inserted and cemented into
a mechanical holder and the entire assembly is then mounted in an IFU
“button” that will be positioned in the focal plane by the OzPoz Positioner.
A magnet is incorporated at the base of the button to ensure a stable
position (a firm hold) on the focal plate during the observation. The
optical cementing is ensured with an UV curing and the fibre bundle is
cemented into the button with an epoxy glue in order to ensure excellent
stiffness of the complete assembly. The external diameter of the button is
about 6 mm, corresponding to about 11 arcsec on the sky, allowing quite
close positioning of the buttons on the focal plate.
An example of astronomical observations with IFUs
[ESO PR Photo 03e/02] ESO PR Photo Caption: PR Photo 03e/02 is a
03e/02 computer simulation of the velocity
field in a galaxy, as deduced on the
basis of IFU spectra. The blue area
[Preview - JPEG: 467 x 400 pix - has negative velocities and is thus
51k] the approaching side of the galaxy,
[Normal - JPEG: 933 x 800 pix - while the red area is receding. In
264k] this way, the direction of rotation
can be determined. The velocity unit
is km/s.
During the astronomical observation with the IFUs, the spectrograph slit
receives light from 15 sky areas simultaneously, each with 21 fibres (20
from the IFU and 1 that collects the light from the night sky in an adjacent
sky field) or 22 fibres (with the addition of 1 fibre with light from a
calibration lamp). Altogether, about 300 spectra are recorded
simultaneously.
By means of such observations, the astronomers can perform many different
studies, e.g., of the dynamics of star clusters and motions of stars and
interstellar clouds in galaxies.
PR Photo 03e/02 provides an example of a computer simulation of a resulting
diagramme in which the internal rotation of a distant spiral galaxy is
clearly visible. Red and yellow areas have positive velocities that are
approaching while the blue areas are receding). Of special interest will be
the study of the often violent motions when two or more galaxies interact
gravitationally.
Notes
[1]: This is a joint Press Release of ESO and the Observatoire de Paris (cf.
http://www.obspm.fr/actual/).
[2]:The GIRAFFE team at the Observatoire de Paris that has developed the
Integral Field Units (IFUs) discussed in this Press Release includes
Jean-Pierre Aoustin, Sebastien Baratchart, Patrice Barroso, Veronique
Cayatte, Laurent Chemin, Florence Cornu, Jean Cretenet, Jean-Paul Danton,
Hector Flores, Francoise Gex, Fabien Guillon, Isabelle Guinouard, Francois
Hammer, Jacques Hammes, David Horville, Jean-Michel Huet, Laurent Jocou,
Pierre Kerlirzin, Serge Lebourg, Hugo Lenoir, Claude Lesqueren, Regis
Marichal, Michel Marteaud, Thierry Melse, Fabrice Peltier, Francois Rigaud,
Frederic Sayede and Pascal Vola.
[3]: It is expected to ship the various components of the FLAMES instrument
to the VLT Observatory at Paranal (Chile) during the next month. “First
Light” is scheduled to take place some weeks thereafter, following
installation at the telescope and extensive system tests. ESO will issue
another Press Release with more details on that occasion.
Contacts
Francois Hammer
Observatoire de Paris-Meudon
Meudon, France
Tel.: +33 1 45 07 74 08
email: francois.hammer@obspm.fr
Gerardo Avila
European Southern Observatory
Garching, Germany
Tel.: +4989-3200-6394
email: gavila@eso.org
