Press Releases 2001

"First Light" for the VLT Interferometer

Excellent Fringes From Bright Stars Prove VLTI Concept

"First Fringes at the VLTI": A great moment!

The moment of "First Fringes" at the VLTI occurred in the evening of March 17, 2001 . The bright star Sirius was observed with two small telescopes ("siderostats"), specially constructed for this purpose during the early VLTI test phases. ESO PR Video Clip 03/01 includes related scenes and is based on a more comprehensive documentation, now available as ESO Video News Reel No. 12.

The star was tracked by the two telescopes and the light beams were guided via the Delay Lines in the Interferometric Tunnel to the VINCI instrument [1] at the Interferometric Laboratory. The path lengths were continuously adjusted and it was possible to keep them stable to within 1 wavelength (2.2 µm, or 0.0022 mm) over a period of at least 2 min.

Next night, several other stars were observed, enabling the ESO astronomers and engineers in the Control Room to obtain stable fringe patterns more routinely. With the special software developed, they also obtained 'on-line' an accurate measurement of the angular diameter of a star. This means that the VLTI delivered its first valid scientific result, already during this first test .

First observation with the VLTI

Following careful adjustment of all of the various components of the VLTI, the first attempt to perform a real observation was initiated during the night of March 16-17, 2001. "Fringes" were actually acquired during several seconds, leading to further optimization of the Delay Line optics.

The next night, March 17-18, stable fringes were obtained on the bright stars Sirius and Lambda Velorum .

The following night, the first scientifically valid results were obtained during a series of observations of six stars. One of these, Alpha Hydrae , was measured twice, with an interval of 15 minutes between the 2-min integrations. The measured diameters were highly consistent, with a mean of 0.00929±0.00017 arcsec. This new VLTI measurement is in full agreement with indirect (photometric) estimates of about 0.009 arcsec.

The overall performance of the VLTI was excellent already in this early stage. For example, the interferometric efficiency ('contrast' on a stellar point source) was measured to be 87% and stable to within 1.3% over several days. This performance will be further improved following additional tuning.

The entire operation of the VLTI was performed remotely from the Control Room, as this will also be the case in the future.

Another great advantage of the VLTI concept is the possibility to analyse the data at the control console. This is one of the key features of the VLTI that contributes to make it a very user-friendly facility.

Overview of the VLT Interferometer

The interferometric principle is based on the phase-stable combination of light beams from two or more telescopes at a common interferometric focus , cf. PR Photo 10c/01 .

The light from a celestial object is captured simultaneously by two or more telescopes. For the first tests, two "siderostats" with 40-cm aperture are used; later on, two or more 8.2-m Unit Telescopes will be used, as well as several moving 1.8-m Auxiliary Telescopes (ATs), now under construction at the AMOS factory in Belgium.

Via several mirrors and through the Delay Line, that continuously compensates for changes in the path length introduced by the Earth's rotation as well as by other effects (e.g., atmospheric turbulence), the light beams are guided towards the interferometric instrument VINCI at the common interferometric focus. It is located in the subterranean Interferometric Laboratory , at the centre of the observing platform on the top of the Paranal mountain. Photos of some of the VLTI elements are shown in Appendix B.

The interferometric technique allows achieving images, as sharp as those of a telescope with a diameter equivalent to the largest distance between the telescopes in the interferometer. For the VLTI, this distance is about 200 metres, resulting in a resolution of 0.001 arcsec in the near-infrared spectral region (at 1 µm wavelength), or 0.0005 arcsec in visual light (500 nm). The latter measure corresponds to about 2 metres on the surface of the Moon.

The VLTI instruments

The installation and putting into operation of the VLTI at Paranal is a gradual process that will take several years. While the present "First Fringe" event is of crucial importance, the full potential of the VLTI will only be reached some years from now.

This will happen with the successive installation of a number of highly specialised instruments, like the near-infrared/red VLTI focal instrument (AMBER) , the Mid-Infrared interferometric instrument for the VLTI (MIDI) and the instrument for Phase-Referenced Imaging and Microarcsecond Astrometry (PRIMA).

Already next year, the three 1.8-m Auxiliary Telescopes that will be fully devoted to interferometric observations, will arrive at Paranal. Ultimately, it will be possible to combine the light beams from all the large and small telescopes.

Great research promises

Together, they will be able to achieve an unprecedented image sharpness (angular resolution) in the optical/infrared wavelength region, and thanks to the great light-collecting ability of the VLT Unit Telescopes, also for observations of quite faint objects. This will make it possible to carry out many different front-line scientific studies, beyond the reach of other instruments. There are many promising research fields that will profit from VLTI observations, of which the following serve as particularly interesting examples:

• The structure and composition of the outer solar system, by studies of individual moons, Trans-Neptunian Objects and comets.
• The direct detection and imaging of exoplanets in orbit around other stars.
• The formation of star clusters and their evolution, from images and spectra of very young objects.
• Direct views of the surface structures of stars other than the Sun.
• Measuring accurate distances to the most prominent "stepping stones" in the extragalactic distance scale, e.g., galactic Cepheid stars, the Large Magellanic Cloud and globular clusters.
• Direct investigations of the physical mechanisms responsible for stellar pulsation, mass loss and dust formation in stellar envelopes and evolution to the Planetary Nebula and White Dwarf stages.
• Close-up studies of interacting binary stars to better understand their mass transfer mechanisms and evolution.
• Studies of the structure of the circum-stellar environment of stellar black holes and neutron stars.
• The evolution of the expanding shells of unstable stars like novae and supernovae and their interaction with the interstellar medium.
• Studying the structure and evolution of stellar and galactic nuclear accretion disks and the associated features, e.g., jets and dust tori.
• With images and spectra of the innermost regions of the Milky Way galaxy, to investigate the nature of the nucleus surrounding the central black hole.

Clearly, there will be no lack of opportunities for trailblazing research with the VLTI. The "First Fringes" constitute a very important milestone in this direction.

Appendix A: How does it work?

The principle behind interferometry is the "coherent optical interference" of light beams from two or more telescopes, due to the wave nature of light. The above illustrations serve to explain what the astronomers observe in the simplest case, that of a single star with a certain angular size, and how this can be translated into a measurement of this size.

In PR Photo 10d/01 , the difference between two stars of different diameter is illustrated. While the image of the smaller star displays strong interference effects (i.e., a well visible fringe pattern), those of the larger star are much less prominent. The "visibility" of the fringes is therefore a direct measure of the size; the stronger they appear (the "larger the contrast"), the smaller is the star.

If the distance between the two telescopes is increased when a particular star is observed ( PR Photo 10e/01 ), then the fringes become less and less prominent. At a certain distance, the fringe pattern disppears completely. This distance is directly related to the angular size of the star.

Appendix B: Elements of the VLT Interferometer

Contrary to other large astronomical telescopes, the VLT was designed from the beginning with the use of interferometry as a major goal . For this reason, the four 8.2-m Unit Telescopes were positioned in a quasi-trapezoidal configuration and several moving 1.8-m telescopes were included into the overall VLT concept, cf. PR Photo 10f/01 .

The photos below show some of the key elements of the VLT Interferometer during the present observations. They include the siderostats , 40-cm telescopes that serve to capture the light from a comparatively bright star ( Photos 10g-i/01 ), the Delay Lines ( Photo 10j/01 ), and the VINCI instrument ( Photo 10k/01)

Earlier information about the development and construction of the individual elements of the VLTI is available as ESO PR 04/98 , ESO PR 14/00 and ESO PR Photos 26a-e/00.

Note

[1]: The VINCI instrument was built under ESO contract at the Observatoire de Paris (France) and the camera in this instrument was delivered by the MPI for Extraterrestrial Physics (Garching, Germany). The detector electronics was supplied by ESO.

Technical information about the photos

PR Photo 10a/01 shows the computer screen during the "first fringes" of the VLTI, as presented by the off-line data analysis software. The data displayed represent about two minutes of observation. Fringes are acquired by moving a mirror back and forth around the position of equal pathlength for the two telescopes. One such scan (lasting about 0.5 sec) can be seen in the upper right window, while a combined ('waterfall') display of a total of 200 scans is shown in the upper left window. From each scan is computed a value for the contrast of the fringes (yellow dots).

Contacts

For technical issues:

Andreas Glindemann
European Southern Observatory
Paranal, Chile
Tel.: +56-55-43-5000/5240
email: aglindem@eso.org

For scientific issues:

Francesco Paresce
European Southern Observatory
Garching, Germany
Tel.: +4989-3200-6297
email: fparesce@eso.org

The ESO PR Video Clips service to visitors to the ESO website provides "animated" illustrations of the ongoing work and events at the European Southern Observatory. The most recent clip was: ESO PR Video Clips 02a-b/01 with views from the Paranal Observatory and the Interferometric Tunnel (26 January 2001).

Information is also available on the web about other ESO videos.