Computer Science – Sound
Scientific paper
May 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004eso..pres...13.&link_type=abstract
ESO Press Release, 05/2004
Computer Science
Sound
Scientific paper
Successful "First Light" for the Mid-Infrared VISIR Instrument on the VLT
Summary
Close to midnight on April 30, 2004, intriguing thermal infrared images of dust and gas heated by invisible stars in a distant region of our Milky Way appeared on a computer screen in the control room of the ESO Very Large Telescope (VLT).
These images mark the successful "First Light" of the VLT Imager and Spectrometer in the InfraRed (VISIR), the latest instrument to be installed on this powerful telescope facility at the ESO Paranal Observatory in Chile. The event was greeted with a mixture of delight, satisfaction and some relief by the team of astronomers and engineers from the consortium of French and Dutch Institutes and ESO who have worked on the development of VISIR for around 10 years [1].
Pierre-Olivier Lagage (CEA, France), the Principal Investigator, is content : "This is a wonderful day! A result of many years of dedication by a team of engineers and technicians, who can today be proud of their work. With VISIR, astronomers will have at their disposal a great instrument on a marvellous telescope. And the gain is enormous; 20 minutes of observing with VISIR is equivalent to a whole night of observing on a 3-4m class telescope."
Dutch astronomer and co-PI Jan-Willem Pel (Groningen, The Netherlands) adds: "What's more, VISIR features a unique observing mode in the mid-infrared: spectroscopy at a very high spectral resolution. This will open up new possibilities such as the study of warm molecular hydrogen most likely to be an important component of our galaxy."
PR Photo 16a/04: VISIR under the Cassegrain focus of the Melipal telescope PR Photo 16b/04: VISIR mounted behind the mirror of the Melipal telescope PR Photo 16c/04: Colour composite of the star forming region G333.6-0.2 PR Photo 16d/04: Colour composite of the Galactic Centre PR Photo 16e/04: The Ant Planetary Nebula at 12.8 μm PR Photo 16f/04: The starburst galaxy He2-10 at 11.3μm PR Photo 16g/04: High-resolution spectrum of G333.6-0.2 around 12.8μm PR Photo 16h/04: High-resolution spectrum of the Ant Planetary Nebula around 12.8μm
From cometary tails to centres of galaxies
The mid-infrared spectral region extends from a few to a few tens of microns in wavelength and provides a unique view of our Universe. Optical astronomy, that is astronomy at wavelengths to which our eyes are sensitive, is mostly directed towards light emitted by gas, be it in stars, nebulae or galaxies. Mid-Infrared astronomy, however, allows us to also detect solid dust particles at temperatures of -200 to +300 °C.
Dust is very abundant in the universe in many different environments, ranging from cometary tails to the centres of galaxies. This dust also often totally absorbs and hence blocks the visible light reaching us from such objects. Red light, and especially infrared light, can propagate much better in dust clouds.
Many important astrophysical processes occur in regions of high obscuration by dust, most notably star formation and the late stages of their evolution, when stars that have burnt nearly all their fuel shed much of their outer layers and dust grains form in their "stellar wind". Stars are born in so-called molecular clouds. The proto-stars feed from these clouds and are shielded from the outside by them. Infrared is a tool - very much as ultrasound is for medical inspections - for looking into those otherwise hidden regions to study the stellar "embryos".
It is thus crucial to also observe the Universe in the infrared and mid-infrared. Unfortunately, there are also infrared-emitting molecules in the Earth's atmosphere, e.g. water vapour, Nitric Oxides, Ozone, Methane. Because of these gases, the atmosphere is completely opaque at certain wavelengths, except in a few "windows" where the Earth's atmosphere is transparent.
Even in these windows, however, the sky and telescope emit radiation in the infrared to an extent that observing in the mid-infrared at night is comparable to trying to do optical astronomy in daytime. Ground-based infrared astronomers have thus become extremely adept at developing special techniques called "chopping' and "nodding" for detecting the extremely faint astronomical signals against this unwanted bright background [3].
VISIR: an extremely complex instrument
VISIR - the VLT Imager and Spectrometer in the InfraRed - is a complex multi-mode instrument designed to operate in the 10 and 20 μm atmospheric windows, i.e. at wavelengths up to about 40 times longer than visible light and to provide images as well as spectra at a wide range of resolving power up to ~ 30.000. It can sample images down to the diffraction limit of the 8.2-m Melipal telescope (0.27 arcsec at 10 μm wavelength, i.e. corresponding to a resolution of 500 m on the Moon), which is expected to be reached routinely due to the excellent seeing conditions experienced for a large fraction of the time at the VLT [2].
Because at room temperature the metal and glass of VISIR would emit strongly at exactly the same wavelengths and would swamp any faint mid-infrared astronomical signals, the whole VISIR instrument is cooled to a temperature close to -250° C and its two panoramic 256x256 pixel array detectors to even lower temperatures, only a few degrees above absolute zero. It is also kept in a vacuum tank to avoid the unavoidable condensation of water and icing which would otherwise occur.
The complete instrument is mounted on the telescope and must remain rigid to within a few thousandths of a millimetre as the telescope moves to acquire and then track objects anywhere in the sky. Needless to say, this makes for an extremely complex instrument and explains the many years needed to develop and bring it to the telescope on the top of Paranal. VISIR also includes a number of important technological innovations, most notably its unique cryogenic motor drive systems comprising integrated stepper motors, gears and clutches whose shape is similar to that of the box of the famous French Camembert cheese.
VISIR is mounted on Melipal
ESO PR Photo 16a/04
ESO PR Photo 16a/04
VISIR under the Cassegrain focus of the Melipal telescope
[Preview - JPEG: 400 x 476 pix - 271k]
[Normal - JPEG: 800 x 951 pix - 600k]
ESO PR Photo 16b/04
ESO PR Photo 16b/04
VISIR mounted behind the mirror of the Melipal telescope
[Preview - JPEG: 400 x 603 pix - 366k]
[Normal - JPEG: 800 x 1206 pix - 945k]
Caption: ESO PR Photo 16a/04 shows VISIR about to be attached at the Cassegrain focus of the Melipal telescope. On ESO PR Photo 16b/04, VISIR appears much smaller once mounted behind the enormous 8.2-m diameter mirror of the Melipal telescope.
The fully integrated VISIR plus all the associated equipment (amounting to a total of around 8 tons) was air freighted from Paris to Santiago de Chile and arrived at the Paranal Observatory on 25th March after a subsequent 1500 km journey by road. Following tests to confirm that nothing had been damaged, VISIR was mounted on the third VLT telescope "Melipal" on April 27th. PR Photos 16a/04 and 16b/04 show the approximately 1.6 tons of VISIR being mounted at the Cassegrain focus, below the 8.2-m main mirror.
First technical light on a star was achieved on April 29th, shortly after VISIR had been cooled down to its operating temperature. This allowed to proceed with the necessary first basic operations, including focusing the telescope, and tests. While telescope focusing was one of the difficult and frequent tasks faced by astronomers in the past, this is no longer so with the active optics feature of the VLT telescopes which, in principle, has to be focused only once after which it will forever be automatically kept in perfect focus.
First images and spectra from VISIR
ESO PR Photo 16c/04
ESO PR Photo 16c/04
Colour composite of the star forming region G333.6-0.2
[Preview - JPEG: 400 x 477 pix - 78k]
[Normal - JPEG: 800 x 954 pix - 191k]
ESO PR Photo 16d/04
ESO PR Photo 16d/04
Colour composite of the Galactic Centre
[Preview - JPEG: 400 x 478 pix - 159k]
[Normal - JPEG: 800 x 955 pix - 348k]
Caption: ESO PR Photo 16c/04 is
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