Physics – Optics
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002eso..pres...25.&link_type=abstract
ESO Press Release, 12/2002
Physics
Optics
Scientific paper
"First Fringes" in Mid-Infrared Spectral Region with Two Giant Telescopes
Summary
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 eta 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.
PR Photo 30a/02: MIDI " First Fringes " of eta Carinae. PR Photo 30b/02: The happy team at the moment of "First Fringes". PR Photo 30c/02: MIDI in the Interferometric Laboratory at Paranal. PR Video Clip 03/02: Optical path scan with "First Fringes" appearing on the computer screen. A wonderful moment
ESO PR Video Clip 03/02 [384x288 pix MPEG-version]
ESO PR Video Clip 03/02 (480 frames/0:19 min) [MPEG; 384x288 pix; 6.6M] [RealMedia; streaming; 56kps] [RealMedia; streaming; 200kps]
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 eta 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 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. 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 sensitiv
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