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 Galaxy. 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 motions, 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), which removes the optical aberration caused by atmospheric turbulence up to an altitude of ∼500m (refs 7-10). A GLAO system that uses multiple laser guide stars has been developed at the 6.5-m MMT telescope, in Arizona. In previous tests, 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 work, with better uniformity, and extends to a wavelength of 1.2μm. Entire stellar clusters may be examined in a single pointing, and cluster membership can be determined from two such observations separated by just one year.
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