The Multiple Mirror Telescope (MMT) has been used in experiments with sodium laser guide stars to sense and correct atmospheric image distortion. The major goal was to develop and test concepts for a full adaptative 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 guide star, produced by a continuous-wave dye laser beam projected out along 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 demonstration 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 apertures. Two adaptive servo loops were closed simultaneously. The laser beacon provided a measure of the differential wave-front tilt between 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 Strehl ratio was measured, and the resolution improved from 0.''58 to 0.''41. 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 aberration of starlight across the full 6.9 m aperture of the MMT was examined. 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 simultaneously. The small difference between the wave fronts, caused by focus anisoplanatism, was analyzed in terms of Zernike coefficients and was found to correspond to a Strehl ratio of 77% in the K band over the full aperture sampled, despite poor seeing during this measurement. From more extensive measurements of binary star wave fronts, we deduce that focus anisoplanatism for the 6.5 m telescope will correspond to a Strehl ratio of typically 88% at K under normal seeing conditions.In a laser-based adaptive system, a natural guide star is still required to sense overall wave-front slope. Our measurements of binary stars also yielded the image degradation to be expected from differences in the overall slopes between the wave fronts from the object of scientific interest and the natural guide star. A Strehl ratio of 80% at K was deduced for an offset of 40'', implying that good sky coverage will be possible. 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.

A 0.5 W beam from a continuous-wave dye laser, tuned to the D 2 resonance of atomic sodium, has been used to generate an artificial guide star by illuminating the mesospheric sodium layer, 90 km above the site of the Multiple Mirror Telescope (MMT). The resonant backscattered light appeared as a source of 1".3 FWHM, equivalent in brightness to a natural star of magnitude 12.5 seen through a standard V filter. This artificial source was used by the MMT adaptive optics system to compensate in real time for atmospherically-induced image motion between the six 1.8 m primary mirrors of the telescope, at correction rates of between 5 and 30 Hz. In related experiments, we have recorded images of the laser return and a coincident natural star using a high-speed CCD speckle camera, in which the six individual beams from the telescope are deliberately separated in the focal plane. From these images we have made the first direct measure- ments of the ability of the laser guide star to sense correctly five low order Zernike aberrations over large aperture.

Artificial beacons generated at the sodium layer have many advantages for adaptive optics (AO) systems in astronomy. The source can be sufficiently bright to achieve high Strehl ratios at infrared wavelengths and because the target object can often be used as the tracking source, photometric field errors can be much reduced compared to off-axis natural beacons. The problem has been to develop a laser that could be tuned to the sodium D2 line and focused into a compact spot without saturating the layer. Both CW and sum frequency lasers are well suited for this work will be used for the ChAOS AO system described elsewhere in this conference. In this paper we discuss some of the parameters that determine how well CW work, the limitations of pulsed lasers and the development of a 10 watt diode laser pumped sum frequency laser for use in astronomy.

We have obtained the first measurements with a sodium laser beacon of focus anisoplanatism over large aperture, at the Multiple Mirror Telescope. In complementary studies, the atmospheric turbulence at the high altitude and on the large scale responsible for the measured focus anisoplanatism was explored by observations of binary stars of different separations. We confirm the predictions of Kolmogorov theory, and derive an effective height for the turbulence of 5050 m above the telescope. These results confirm that the sodium laser guide star planned for use with the 6.5 m telescope conversion of the MMT in 1996 will allow diffraction limited infrared observations in the H and K bands.

A sodium guide star has been used to sense and correct atmospheric aberration during two runs at the Multiple Mirror Telescope (MMT). For the first run in 1993 May, the artificial star was created by a 0.5 W beam from a continuous- wave dye laser tuned to the D2 resonance line, projected from a telescope centered and coaxial with the main array of six 1.8 m mirrors. Scattering by the mesospheric sodium layer produced an artificial beacon equivalent in brightness to a natural star of visual magnitude 12.5, and of angular extent 1'.2 full width at half maximum (FWHM). During the second run in 1994 February, a 1.7 W dye laser was used to generate an artificial guide star of visual magnitude 10.4, and 1'.1 FWHM. In each case, the beacon was used by the MMT adaptive optics system to compensate in real time for atmospherically- induced differential image motion between the six mirror elements, at correction rates of up to 76 Hz. In the latter experiment, global wavefront tilt correction using a natural reference star was added, giving complete adaptive control. Simultaneously recorded images of a natural star coincident with the laser beacon show significantly reduced width and an increase in Strehl ratio of almost a factor of two.

ABSTRACTAn overview of the Wavefront Control Experiment (WCE) adaptive optics system ispresented. We discuss our initial tests of the system and illustrate adaptive correctioncapability of the system using its internal star source simulator. Future laboratory tests are planned prior to adapting the system to a coude feed of a 4 1 reflecting telescope at Yerkes Observatory where atmospheric compensation with astronomical sources will be investigated.1. BACKGROUNDThe Wavefront Control Experiment is a space-qualified, high temporal bandwidth, compact adaptive optics package that was developed as a stand alone but integral component within the Strategic Defense Initiative Organization (SDIO) directed energy Starlab experiment. This experiment was to be conducted in near earth orbit using the Space Shuttle; the WCE was designed to operate in the zero gravity but shirtsleeve environment of the Spacelab module. The goal of the Starlab program was to exploreacquisition, tracking, and pointing capabilities in orbit within the context of the SDIOdirected energy mission. The exhaust plume of a Starbird booster rocket was to be acquired