Subaru Telescope adds laser guide star

Nov. 21, 2006
November 21, 2006, Mauna Kea, HI--Researchers at the 8.2 m Subaru Telescope have achieved "first light" with the instrument's new adaptive-optics (AO) system that includes a laser guide star. The AO-fortified telescope captured images of the Trapezium region of the Orion Nebula at a resolution of 0.06 arcsec, ten times better than the telescope's usual resolution.

November 21, 2006, Mauna Kea, HI--Researchers at the 8.2 m Subaru Telescope have achieved "first light" with the instrument's new adaptive-optics (AO) system that includes a laser guide star. The AO-fortified telescope captured images of the Trapezium region of the Orion Nebula at a resolution of 0.06 arcsec, ten times better than the telescope's usual resolution.

Ordinarily, the resolution of the Subaru Telescope is hindered by atmospheric turbulence, in which warm and cool pockets of air mingle to create a nonuniform and constantly changing refractive-index distribution between the telescope and the heavens. While the extinct volcano Mauna Kea was selected as one of the best earthly astronomical observation points (due to the mountain's height and the relatively smooth, uniform flow of air above it), no ground-based site escapes the degrading effects of the atmosphere. An AO mirror dynamically changes shape, compensating for the roiling changes in refractive index.

Subaru's first-light image, taken October 9 of this year, is compared to a non-AO-compensated image taken when the telescope first began observing in 1999 (see figure). Both contrast and detail are greatly boosted by the AO system (left), which allows the telescope to reach the resolution it could achieve if the whole giant instrument could somehow be transported intact to a position somewhere in space.

Subaru's AO-development team has been working for the past five years on replacing its older 36-element AO system with an improved 188-element system (the elements are mirror actuators). At the same time, the team also developed and installed a new laser-guide-star system that allows astronomers to create an artificial star 90 km up, in the most-tenuous reaches of the atmosphere; the wavefront received from the artificial star is sensed and the resulting information used to drive the mirror actuators.

Laser guide stars have been used for years; a very robust and powerful yellow-emitting laser , which produces a 589 nm line especially well-suited for artificial guide stars (in this case created by exciting sodium atoms in the mesosphere), was developed at the Air Force Research Laboratory (Kirtland AFB, NM).

Subaru's laser-guide-star system is the fourth in the world to be completed for eight- to ten-meter telescopes; its use of a type of solid-state-laser and optical-fiber technology developed in Japan represents an original contribution to the field. The Keck Telescope, a neighbor to the Subaru on Mauna Kea, also has an AO system.

The AO-enhanced Subaru telescope will enable astronomers to study objects that were previously unobservable, such as the detailed structure of faint distant galaxies and stellar populations of nearby galaxies. They will also be able to do more-detailed imaging and spectroscopy of quasars and gamma-ray bursters.

Subaru is operated by the National Astronomical Observatory of Japan, a member institute of Japan's National Institutes of Natural Science. The research and development of the new systems was supported by a grant from MEXT, the Japanese Ministry of Education, Culture, Sports, Science and Technology.

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