First Astronomical Images Sharpened with Adaptive Optics using a Sodium Laser Guide Star

Adaptive optics with a sodium resonance laser guide star was used at the Multiple Mirror Telescope (MMT) in 1996 April to image the core of the globular cluster M13 (NGC 6205). A 23'' field was recorded in the Ks band with image resolution of 051, when the uncorrected resolution was 072. Global tilt, not sensed by the laser, was measured from the image motion of a star 35'' from the center of the field. Despite this separation, the star profiles do not vary significantly across the image. Many more stars fainter than Ks = 17.5 can be identified in the corrected image. The 05 imaging capability demonstrated here, though not reaching the performance of existing faster, higher order systems with natural star wave-front sensors, is significant because it can be generally realized for very faint objects under normal observing conditions. This characteristic will carry over to higher order laser-based systems, making them very powerful. Our current 05 resolution is much larger than the diffraction limit for the present MMT array (and its upcoming 6.5 m monolithic replacement) because discontinuities prevent the measurement of phase differences between the array elements. Furthermore, small-scale wave-front aberrations caused by atmospheric and static errors across individual mirrors were not corrected. But our system, by its simultaneous correction of differential slopes derived from the laser beacon and global tilt from the natural guide star, illuminates the principles and sources of error common to all future laser systems.