A ground-layer adaptive optics system with multiple laser guide stars


To determine the influence of the environment on star formation, we need to study the process in the extreme conditions of massive young star clusters (104 solar masses) near the centre of our own Galaxy1,2. Observations must be carried out in the near infrared because of very high extinction in visible light within the Galactic plane. We need high resolution to identify cluster members from their peculiar motions3, and because most such clusters span more than 1′, efficient observation demands a wide field of view. There is at present no space-based facility that meets all these criteria. Ground-based telescopes can in principle make such observations when fitted with ground-layer adaptive optics (GLAO)4,5,6, which removes the optical aberration caused by atmospheric turbulence up to an altitude of 500 m (refs 7–10). A GLAO system that uses multiple laser guide stars11,12,13 has been developed at the 6.5-m MMT telescope, in Arizona. In previous tests13, the system improved the resolution of the telescope by 30–50%, limited by wavefront error in the optics, but that was insufficient to allow rapid determination of cluster membership. Here we report observations of the core of the globular cluster M3 made after commissioning a sensor to monitor and remove slowly varying aberration in the optics. In natural seeing of 0.7′′, the point spread function at 2.2-μm wavelength was sharpened uniformly to 0.3′′ over a field of at least 2′. The wide-field resolution was enhanced by a factor of two to three over previous work13, with better uniformity, and extends to a wavelength of 1.2 μm. Entire stellar clusters may be examined in a single pointing, and cluster membership can be determined from two such observations separated by just one year14,15,16,17.

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Figure 1: The core of M3 imaged in the K band in two 60-s exposures in May 2009.
Figure 2: Comparison of open-loop and closed-loop near-infrared image widths.


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We thank the staff of the Steward Observatory Engineering and Technical Services division and the staff of the MMT Observatory for their support in the development and deployment of the MMT adaptive optics system. We are grateful to P. Strittmatter and R. Angel for reading the manuscript. The observations reported here were made at the MMT Observatory, a joint facility of The University of Arizona and the Smithsonian Institution. The work has been supported by the National Science Foundation.

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M.H. wrote the paper. N.M.M., C.B., M.H. and E.B. carried out the data reduction. N.M.M. designed the adaptive optics reconstructor matrices. K.P. analysed the real-time system performance. T.S. designed and built the laser launch optics and wrote the system’s operating software. D.M. and C.K. operated the infrared camera that recorded the cluster images at the telescope. All authors took part in the telescope runs during which the data presented here were acquired.

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Correspondence to M. Hart.

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Hart, M., Milton, N., Baranec, C. et al. A ground-layer adaptive optics system with multiple laser guide stars. Nature 466, 727–729 (2010). https://doi.org/10.1038/nature09311

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