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Next-Gen adaptive optics.

The Subaru Telescope has donned a new pair of glasses called Raven, a multi-object adaptive optics (MOAO) system that enables astronomers to correct for atmospheric turbulence over an unprecedentedly large field of view.

MOAO systems use multiple guide stars to monitor atmospheric changes in the field of view so that the system can correct in response. The first MOAO-enabled image was obtained in 2010 on the 4.2-meter William Herschel Telescope in the Canary Islands. The system was a successful technical demonstration, but it wasn't meant to do astronomical science.

Recently, a team led by David Andersen (National Research Council, Herzberg Institute of Astrophysics, Canada), Colin Bradley, and Olivier Lardiere (both of University of Victoria, Canada) installed the Raven MOAO system on the 8.2-meter Subaru Telescope on Mauna Kea.

Raven has three sensors for tracking natural guide stars over a 3.5-arcminute field, each on a motorized "pick-off" arm; it also has a fourth sensor that's designed to work with the telescope's laser guide star. These state-of-the-art correction abilities enable the system to compensate for atmospheric turbulence using any stars in the field brighter than magnitude 14.

Raven also has two motorized pick-off arms for observing targets, each with a field of view of 4 arcseconds. These can also be pointed anywhere in the 3.5-arc-minute field. Raven's deformable mirrors then sharpen the light from these two targets simultaneously and send the corrected images on through--in this case, to the telescope's spectrograph, enabling astronomers to take high-resolution spectra of two targets at the same time.

Raven's MOAO system delivers images with roughly 0.15-arcsecond resolution --better than the typical 0.5-arcsecond achievable without AO, but not diffraction-limited. The resolution is worse than the 0.05 arcsecond of Keck, which observes in near-infrared as Subaru does and is often the gold standard for AO results. But the new system's designers chose to create something that would work with Subaru's existing spectrograph, not produce the highest-resolution images possible.

Full-fledged MOAO instruments dedicated for long-term science investigation might have on the order of eight laser-guide-star sensors and 20 science pick-off arms, Andersen says. These systems will be crucial for the next generation of large telescopes, which will have huge multiple mirrors and require the ability to produce AO corrections over a wide field of view.

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Title Annotation:TECHNOLOGY; Subaru Telescope's multi-object adaptive optics system called Raven
Author:Bochanski, John
Publication:Sky & Telescope
Geographic Code:1USA
Date:Dec 1, 2014
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