Innovative instrument distinguishes individual stars

In December’s issue of the journal Astronomy & Astrophysics, astronomers from the University of California in the USA reported that a new instrument combining adaptive optics and interferometry, facilitated by the use of fibre optic cables, has distinguished individual stars in a nearby star system for the first time. This demonstrates promise for eventually identifying planets around other stars in the universe.

The team, led by UC Berkeley assistant research astronomer Gaspard Duchêne, used a prototype instrument called the Fibered Imager foR Single Telescope (FIRST) that was mounted three years ago on the Shane three-metre telescope at the University of California Lick Observatories in San Jose.

The FIRST instrument uses fibre optic communication cables to channel visible light from 18 different spots on the main mirror of the telescope to a detector, where the light beams interfere to reveal high-resolution detail. The instrument on the Subaru telescope also uses 18 fibre optic cables to sample spots on a larger eight-metre main mirror. The three-metre Lick and eight-metre Subaru telescopes are already equipped with adaptive optics, which creates sharper images by removing the jiggle in stars caused by turbulence in the atmosphere. FIRST takes advantage of the stability provided by the adaptive optics system to inject the light from the star into the precise center of the fibres, which have a core only four micrometres in diameter.

One of the key components of the system is a tiny movable mirror, a micro electro mechanical systems (MEMS) device that directs starlight into the optical fibres, which then channel the light with little loss to the interferometer.

The advantage of FIRST is that it can resolve very close objects, such as close binary stars, or the disks of dust and gas that circle stars in the process of forming planets. Alternative techniques used in other telescopes are limited by the turbulent glare from the stars, but this is effectively removed by the use of optic fibres in FIRST.

‘With the FIRST instrument at Subaru telescope, we expect to be able to resolve giant and supergiant stars and observe the close environment of debris disks around young stars,’ said co-author Franck Marchis, a research astronomer at the SETI Institute.

At the moment, however, FIRST cannot resolve objects that differ in brightness by more than 50 to 100 times. Planets the size of Jupiter are typically 10,000 to 100,000 times fainter than their stars, while Earth-size planets are a million times fainter. ‘If we could add enough fibres, we could get very high contrast; that is the goal,’ Duchêne said. ‘If we can scale this up to look for planets, it would be very, very exciting.’

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