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 t he 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 K-s band with image resolution of 0.'' 51, when the uncorrected resol ution was 0.'' 72. Global tilt, not sensed by the laser, was measured from the image motion of a star 35 '' from the center of the field. De spite this separation, the star profiles do not vary significantly acr oss the image. Many more stars fainter than K-s = 17.5 can be identifi ed in the corrected image. The 0.'' 5 imaging capability demonstrated here, though not reaching the performance of existing faster, higher o rder systems with natural star wave-front sensors, is significant beca use it can be generally realized for very faint objects under normal o bserving conditions. This characteristic will carry over to higher ord er laser-based systems, making them very powerful. Our current 0.'' 5 resolution is much larger than the diffraction limit for the present M MT array (and its upcoming 6.5 m monolithic replacement) because disco ntinuities prevent the measurement of phase differences between the ar ray elements. Furthermore, small-scale wave-front aberrations caused b y atmospheric and static errors across individual mirrors were not cor rected. 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.