Ground-Layer Adaptive Optics with Multiple Laser Guide Stars

We report the first closed-loop results obtained from an adaptive optics system with multiple laser guide beacons. The system is mounted on the 6.5 m MMT in Arizona, and is designed to explore advanced altitude-conjugated techniques for wide-field image compensation. Five beacons are made by Rayleigh scattering of laser beams at 532 nm integrated over a range from 20 to 29 km by dynamic refocus of the telescope optics. The return light is analyzed by a unique Shack-Hartmann sensor that places all five beacons on a single detector, with electronic shuttering to implement the beacon range gate. The wavefront sensor divides the 6.5 m telescope pupil into 60 subapertures, and wavefront correction is applied with the telescope's unique deformable secondary mirror. The first application of the system is to correct boundary-layer turbulence, resulting in image quality of 0.2 arcsec in the near infrared bands from 1.2 to 2.5 μm. In this mode we do not try to reach the diffraction limit of the 6.5 m aperture, but instead aim for improved seeing over a field of view much larger than the isoplanatic patch. In this paper we present images of the central 2 arcmin region of the globular cluster M13 in the halo of the Milky Way, and an open cluster, where correction is almost uniform across the full field. The system has particular scientific application to extragalactic survey work, typically done in dark fields where guide stars are very faint, and where large samples of objects are required.