Four eyes are better

September 26, 2002

During the nights of September 15/16 and 16/17, 2002, preliminary tests were successfully carried out during which the light beams from all four VLT 8.2-m Unit Telescopes (UTs) at the ESO Paranal Observatory were successively combined, two by two, to produce interferometric fringes. This marks a next important step towards the full implementation of the VLT Interferometer (VLTI) that will ultimately provide European astronomers with unequalled opportunities for exciting front-line research projects.

It is no simple matter to ensure that the quartet of ANTU, KUEYEN, MELIPAL and YEPUN, each a massive giant with a suite of computer-controlled active mirrors, can work together by sending beams of light towards a common focal point via a complex system of compensating optics. Yet, in the span of only two nights, the four VLT telescopes were successfully "paired" to do exactly this, yielding a first tantalizing glimpse of the future possibilities with this new science machine.

While there is still a long way ahead to the routine production of extremely sharp, interferometric images, the present test observations have allowed to demonstrate directly the 2D-resolution capacity of the VLTI by means of multiple measurements of a distant star.

Much valuable experience was gained during those two nights and the ESO engineers and scientists are optimistic that these test observations with the numerous components of the VLTI will continue to progress rapidly. Five intense, technical test periods are scheduled during the next six months; some of these with the Mid-Infrared interferometric instrument for the VLTI (MIDI) which will soon be installed at Paranal.

Later in 2003, the first of the four moveable VLTI 1.8-m Auxiliary Telescopes (ATs) will be put in place on the top of the mountain; together they will permit regular interferometric observations, also without having to use the large UTs.

The full text of this ESO Press Release, with two photos (in different sizes and resolutions) and all weblinks, is available at: http://www.eso.org/outreach/press-rel/pr-2002/pr-16-02.html

Combining the VLT telescopes

Less than one year after the first combination of two 8.2-m VLT telescopes - described in detail in ESO Press Release 23/01 - successful tests have now been carried out, during which all of the four telescopes were combined pairwise in rapid succession.

Of the six combinations possible (ANTU-KUEYEN, ANTU-MELIPAL, ANTU-YEPUN, KUEYEN-MELIPAL, KUEYEN-YEPUN and MELIPAL-YEPUN), only the last one could not be used, because of the current geometrical configuration of the three delay lines installed so far.

The combination of the light beams from two (or more) VLT Unit Telescopes is a daunting task. It involves pointing them simultaneously towards the same celestial object, ensuring optimal optical adjustment of the computer-controlled telescope mirrors (including the shape of the 8.2-m primary mirror by "active optics"), performing extremely smooth and stable tracking of the object as the Earth turns, guiding the light beams via additional ("coude") mirrors into the "delay lines" installed in the Interferometric Tunnel below the telescope platform, keeping the total path lengths equal to within a fraction of a micron during hours at a time and finally, to register the interferometric fringes at the focal point of the VINCI instrument [1], where the light beams encounter each other.

Next year, the first adaptive optics systems for the VLTI will be inserted below the telescopes. By drastically reducing the smearing effects of the turbulent atmosphere through which the light has to pass before it enters the telescopes, this will further "stabilize" the imaging and increase the sensitivity of the VLTI by a factor of almost 100.

First results with four Unit Telescopes

At this moment, three delay lines have been installed, but for the present first test, the VLTI engineers and astronomers used the telescopes in pairs, in order to set-up the various equipment configurations properly. In this way, they could also start "teaching" the computer control software to handle this very demanding process as efficiently and user-friendly as possible in the future. With the arrival of the science instrument AMBER in mid-2003, up to three beams can be combined simultaneously.

It turned out that the various predictions of mirror positions and angles were quite accurate and only a moderate amount of time was needed to "obtain fringes" in all different configurations. Measurements were then made on a number of stars, among them the brightest star in the southern constellation Eridanus (The River), known as Alpha Eridani or Achernar, that was observed several times with the different telescope pairings. This star is a hot dwarf (spectral type "B5 IV") that is located at a distance of about 145 light-years. It has also been extensively observed during earlier VLTI tests. It is a very suitable object for the present resolution tests as its angular diameter is only about 0.002 arcsec and it therefore remains unresolved at the near-infrared wavelength of the K-band used (2.2 micron).

In fact, the combination of these data (including also some that were obtained in October 2001) now makes it possible to reconstruct the first interferometric "point-spread function (PSF)" of a star obtained with the VLTI, cf. PR Photo 22b/02. This is like an "interferometric image", except that the disk of this particular star remains unresolved.

The angular resolution is inversely proportional to the aperture of a telescope for single telescope observation, and to the length of the "baseline" between two telescopes for the interferometric observation. However, observing interferometrically with two telescopes will improve the resolution only in the direction parallel to this baseline, while the resolution in the perpendicular direction will remain that of a single telescope. But then the use of other telescope pairs with different baseline orientations "adds" resolution in other directions.

The reconstructed PSF of Achernar shown in PR Photo 22b/02 is obviously still very incomplete, due to the technical nature of the present tests and the limited time that was spent observing the star in each configuration. However, it already presents a powerful illustration of the extreme imaging sharpness that will be achieved with the VLTI.
-end-
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 Max-Planck-Institute for Extraterrestrial Physics (Garching, Germany). The IR detector and the IRACE detector electronics were supplied by ESO.

Additional Contact Information

For technical issues:

Andreas Glindemann
European Southern Observatory
Garching, Germany


For scientific issues:

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

ESO

Related Telescope Articles from Brightsurf:

ATLAS telescope discovers first-of-its-kind asteroid
University of Hawai'i telescope discovers extraordinary asteroid with comet-like features that has researchers puzzled.

Precision calibration empowers largest solar telescope
An article published in the SPIE publication Journal of Astronomical Telescopes, Instruments, and Systems (JATIS), 'Polarization Modeling and Predictions for DKIST Part 5: Impacts of enhanced mirror and dichroic coatings on system polarization calibration,' marks a substantial advance in ensuring the accurate solar information measured and collected by the Daniel K.

Subaru Telescope captures 1800 exploding stars
The Subaru Telescope has captured images of more than 1800 exploding stars in the Universe, some located 8 billion light years from Earth.

Quasar jets confuse orbital telescope
Astrophysicists from the Moscow Institute of Physics and Technology, the Lebedev Physical Institute of the Russian Academy of Sciences (LPI RAS), and NASA have found an error in the coordinates of active galactic nuclei measured by the Gaia space telescope, and helped correct it.

Cosmic telescope zooms in on the beginning of time
Observations from Gemini Observatory identify a key fingerprint of an extremely distant quasar, allowing astronomers to sample light emitted from the dawn of time.

Both halves of NASA's Webb Telescope successfully communicate
For the first time, the two halves of NASA's James Webb Space Telescope -- the spacecraft and the telescope -- were connected together using temporary ground wiring that enabled them to 'speak' to each other like they will in flight.

Balloon-borne telescope looks for cosmic gamma rays
Cosmic gamma rays can provide us with important insights into the high-energy phenomena in our universe.

Natural telescope sets new magnification record
An international team of astronomers, led by Harald Ebeling of the Institute for Astronomy at the University of Hawaii at Manoa, has discovered one of the most extreme instances of magnification by gravitational lensing.

NASA's Webb Telescope to witness galactic infancy
Scientists will use NASA's James Webb Space Telescope to study sections of the sky previously observed by NASA's Great Observatories, including the Hubble Space Telescope and the Spitzer Space Telescope, to understand the creation of the universe's first galaxies and stars.

New way to weigh a white dwarf: Use Hubble Space Telescope
For the first time, astronomers have used a novel method to determine the mass of a type of star known as a 'white dwarf' -- the shrunken corpse of a dead star that used to be like our sun.

Read More: Telescope News and Telescope Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.