Press Release

Optical Telescope Array Now Counts Six!

By SpaceRef Editor
March 7, 2002
Filed under , ,

Astronomers from the U.S. Naval Observatory (USNO), the Naval Research
Laboratory (NRL), and Lowell Observatory announced today that they have
successfully combined the light from six independent telescopes to form a
single, high-resolution image of a distant multiple-star system. This is
the first time that this has ever been accomplished in the optical region of
the electromagnetic spectrum. The Navy Prototype Optical Interferometer
(NPOI) at Lowell Observatory’s Anderson Mesa site near Flagstaff, Arizona
observed the triple star system Eta Virginis, located about 130 light-years
away from Earth.

“This development makes it possible to ‘synthesize’ telescopes with
apertures in excess of hundreds of meters,” says Dr. Kenneth Johnston,
Scientific Director of the Naval Observatory. “It will lead to the direct
imaging of the surfaces of stars and of star spots, analogous to the
sunspots on the Sun. This technology can also be applied to space systems
for remote sensing of the Earth and other objects in the solar system, as
well as stars and galaxies.”

Optical interferometers combine the light from several independent
telescopes to form a “synthetic” telescope whose ability to make a
high-resolution image is proportional to the maximum separation of the
telescopes. They are the answer to the prohibitive costs and immense
technical difficulties of building extremely large, monolithic single-mirror
telescopes. Since the rate at which a giant telescope aperture is
synthesized with an interferometer array is equal to the number of
combinations between any two telescopes of the array, the combination of the
six NPOI telescopes has more than quadrupled NPOI’s capability to collect
data over its competitors.

USNO and NRL, in collaboration with Lowell Observatory and with funding from
the Office of Naval Research and the Oceanographer of the Navy, joined
forces in 1991 to build the instrument. Stellar observations have been
conducted with a three-station array since its “first light” in 1996.

However, due to the technical difficulty associated with linking even a
small number of separate telescopes, the high-resolution capabilities of
optical interferometers have only been used to date on relatively simple
stellar sources. Basic questions, such as a star’s apparent diameter or the
existence and motions of nearby stellar companions, are easily answered for
such sources. However, to increase the spatial resolution and sensitivity
to stellar structure, interferometers must link more telescopes together to
provide an even sampling of the synthesized aperture. Three combined
telescopes provide three measurements in the synthesized aperture,
but six telescopes provide 15 combinations.

To merge the six beams, the NPOI team has designed a new type of hybrid beam
combiner. In addition, new hardware and control systems have been developed
to uniquely encode every possible telescope combination in the recorded data
so that the information necessary for the alignment and superposition of the
starlight wave-fronts and the image reconstruction may be properly decoded.

The field of interferometry is a rapidly developing one, with giants like
the twin Keck 10-meter telescopes having achieved “first fringes” last year,
and the European Southern Observatory’s VLTI planning to combine the light
from four 8-meter telescopes. More modest but versatile imaging
interferometers like CHARA, COAST, and IOTA have also been operating for a
few years, but NPOI is the first to combine light from a full array of six

In the near future, NPOI will be commissioning all of the remaining stations
onto which any of the six telescopes can be mounted for a maximum array size
of 430 meters, the largest baseline of all current imaging interferometer

Stellar astrophysics will be revolutionized by the capability to directly
image stars other than the Sun. Ultimately, when employed in space with the
experience collected from ground-based experiments, optical interferometry
may develop the capability to image Jupiter-sized planets orbiting distant

“Remember the early days of radio interferometry and look at the world- wide
arrays we routinely use today,” says Dr. Johnston. “We’ve gone from simple
two-element arrays to continent-sized ones with 10 or more antennas that
produce extremely fine-scale images of distant quasars. We are standing on
the brink of achieving similar results for visible-light sources.”

For more information, please contact:

Christian A. Hummel, USNO

Washington DC

(202) 762 0314

James A. Benson, USNOFS

Flagstaff AZ

(928) 773 4868

Notice to Editors:

An online version of this release, with images of the Eta Virginis
triple-star system and the NPOI facility at Anderson Mesa, may be found at

Hardcopy images may be obtained by contacting Geoff Chester at the address

SpaceRef staff editor.