Six Telescopes Acting as One

Image credit: USNO

Astronomers from several US observatories announced that they have successfully merged the light from six independent telescopes to form a single, high-resolution image of a distant multi-star system. To create an image with this level of detail, a single telescope would need to be 50-metres across – bigger than anything that currently exists. This technique, called interferometry, has been done with pairs of telescopes before, but never with as many as six.

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 mea-surements 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 telescopes.

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 projects.

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 stars.

“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.”

Original Source: US Naval Observatory News Release