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Fibers help Keck telescopes see further

13 Jan 2006

Astronomers hail the coupling of the Keck telescopes with optical fibers a success in this week's Science.

An international team of astronomers has linked the two 10m-diameter Keck telescopes in Hawaii using single-mode fibers. The resulting interferometer is said to be the first milestone on the way to producing an array of telescopes that offers sub-milliarc second angular resolution. This level of performance will allow researchers to study black holes in other galaxies and other astronomical phenomena in unprecedented detail. (Science 311 194)

A telescope's diameter limits its ability to resolve astronomical objects. A way around this however is to coherently recombine the light from a number of telescopes. This essentially creates a giant interferometer with an angular resolution superior to any one single telescope.

The 'OHANA project (Optical Hawaiian Array for Nanoradian Astronomy) hopes to connect the seven large telescopes on the Mauna Kea summit in Hawaii. The resulting array will have a diameter of 800m and provide angular resolutions below 1 milli-arc second at near infrared wavelengths.

"The potential of fibers is to transport beams over large distances at minimum loss," Guy Perrin from Paris Observatory told Optics.org. "Getting down to a milli-arc second in the near infrared requires propagating beams over large distances to make them interfere. Classical bulk optics interferometers have low transmission and would have poor sensitivity."

Perrin and colleagues used two 300m long single-mode fluoride glass fibers to combine light from the Keck telescopes at wavelengths between 2 and 2.3 microns. The fiber inputs were placed at the adaptive-optics corrected focus of each telescope and the light was carried to the interferometric laboratory in the basement of the telescope building.

After an initial test observing the 107 Herculis star, the team is optimistic. "This first result essentially demonstrates that linking telescopes with fibers is possible," said Perrin. "This is important in the perspective of future kilometric arrays. Despite cloudy conditions, we could demonstrate a good transmission and that makes me optimistic with respect to the potential sensitivity of fiber-based interferometers."

The team is now looking forward to an observing night in May where it hopes to study young stellar objects. "The next technical step is to link the Canada-France-Hawaii (CFH) and Gemini North telescopes," concluded Perrin. "The longer term goal is to realise the full 'OHANA interferometer."

Jacqueline Hewett is technology editor on Optics.org and Opto & Laser Europe magazine.

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