ADAPTIVE OPTICS EXPERIMENTS USING SODIUM LASER GUIDE STARS

The Multiple Mirror Telescope (MMT) has been used in experiments with sodium laser guide stars to sense and correct atmospheric image distor tion. The major goal was to develop and test concepts for a full adapt ative optics system, to be used in 2 yr when the present array of six 1.8 m telescopes will be replaced with a single 6.5 m mirror. The guid e star, produced by a continuous-wave dye laser beam projected out alo ng the optical axis of the telescope, was as bright in the V band as a natural star of m(n)u = 10.4. Our tests culminated in the first demon stration of a sodium laser guide star used to improve the image of an astronomical telescope, in this case formed by two of the six 1.8 m ap ertures. Two adaptive servo loops were closed simultaneously. The lase r beacon provided a measure of the differential wave-front tilt betwee n the two apertures, and a natural guide star was used to measure the overall wave-front tilt. A factor of 2 improvement in the K-band Streh l ratio was measured, and the resolution improved from 0.''58 to 0.''4 1. The experiment demonstrated all the features needed for correction of the 6.5 m telescope to the diffraction limit using a sodium beacon. The accuracy with which the laser beacon measures the atmospheric abe rration of starlight across the full 6.9 m aperture of the MMT was exa mined. This was done with the artificial beacon and a coaxial natural star, and using the six elements of the MMT as a large Shack-Hartmann wave-front sensor to measure the shape of both wave fronts simultaneou sly. The small difference between the wave fronts, caused by focus ani soplanatism, was analyzed in terms of Zernike coefficients and was fou nd to correspond to a Strehl ratio of 77% in the K band over the full aperture sampled, despite poor seeing during this measurement. From mo re extensive measurements of binary star wave fronts, we deduce that f ocus anisoplanatism for the 6.5 m telescope will correspond to a Streh l ratio of typically 88% at K under normal seeing conditions. In a las er-based adaptive system, a natural guide star is still required to se nse overall wave-front slope. Our measurements of binary stars also yi elded the image degradation to be expected from differences in the ove rall slopes between the wave fronts from the object of scientific inte rest and the natural guide star. A Strehl ratio of 80% at K was deduce d for an offset of 40'', implying that good sky coverage will be possi ble. In general, our results are consistent with calculations based on measurements of atmospheric turbulence at the best sites. Our direct measurements over such a large aperture show clearly the effects of a finite and variable outer scale of turbulence.