Preliminary closed-loop results from an adaptive optics system using a sodium resonance guide star

M. Lloyd-Hart, R. Angel, B. Jacobsen, D. Wittman, D. McCarthySteward Observatory, University of Arizona, Tucson, AZ 85721E. Kibblewhite, B. Carter, and W. Wild,Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL 60637ABSTRACTA sodium guide star has been used to sense and correct atmospheric aberration during two runs at the Multiple Mirror Tele-scope (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 tunedto the D2 resonance line, projected from a telescope centered and coaxial with the main array of six 1 .8 m mirrors. Scattering by themesospheric sodium layer produced an artificial beacon equivalent in brightness to a natural star of visual magnitude 12.5, and ofangular extent 1".2 full width at half maximum (FWHM). During the second run in 1994 February, a 1.7 W dye laser was used togenerate an artificial guide star of visual magnitude 10.4, and 1". 1 FWHM.1 In each case, the beacon was used by the MMT adap-tive optics system to compensate in real time for atmospherically-induced differential image motion between the six mirror ele-ments, at correction rates of up to 76 Hz. In the latter experiment, global wavefront tilt correction using a natural reference star wasadded, giving complete adaptive control. Simultaneously recorded images of a natural star coincident with the laser beacon showsignificantly reduced width and an increase in Strehi ratio of almost a factor of two.1. INTRODUCTIONAdaptive optics promises to revolutionize ground-based optical and infrared astronomy by removing the wavefront aberrationcaused by atmospheric turbulence, thus providing images and spectroscopy of unprecedented resolution. To correct in real time forthe aberration, an adaptive optics system requires a reference light source above the turbulence to be sensed, from which the instan-taneous shape of the distorted wavefront is deduced. By using a deformable optical element to correct the shape of the wavefrontfrom the reference source, one also corrects aberrations in light from nearby sources of interest, within the isoplanatic patch.2 Unre-solved natural stars are ideal as reference sources. However, the atmosphere is far from static, and the figure of the deformable ele-ment must be continuously updated at rates of at least 50 Hz for correction in the near infrared, or 200 Hz at visible wavelengths.Thus, the time available for integration of photons from the reference source is extremely limited. To be observable with adaptiveoptics using a natural field star, an object must be within the isoplanatic patch of a star sufficiently bright to provide adequate signal-to-noise ratio at the wavefront sensor. This requirement is so stringent that for large telescopes, at best only a few percent of the skyis available for imaging at the diffraction limit of resolution by this method, even at the more favorable infrared wavelengths.As a partial solution, an artificial beacon may be created by backscattered light from a ground-based laser beam. A number ofgroups have successfully demonstrated the use of a low-altitude beacon in closed-loop adaptive optics systems, relying on Rayleighscattering of a pulsed laser beam from molecules of atmospheric oxygen and nitrogen.3'4Two problems are associated with the use of laser guide stars. Firstly, because of the finite height of the beacon, backscatteredlaser light does not follow the same path through the turbulence as the light from astronomical objects at infinity. This effect, calledfocus anisoplanatism, introduces an error into the measured wavefront distortion which may be relatively large at the best astronom-ical sites where aberration from high atmospheric layers is often dominant. There is thus a need to place the beacon as high in theatmosphere as possible to minimize this limitation. This motivating force has lead to the development of sodium resonance lasers,which produce a beacon by exciting the D2 line of mesospheric sodium, at 590 nm wavelength.5'6'7'8The second problem arises because light from the artificial beacon undergoes refraction by the atmosphere in both the upwardand downward paths, so the component of wavefront tilt common to the paths is not sensed. Only the differential tilt between thetransmitting and receiving telescopes can be measured. For full adaptive correction therefore, one still requires a natural star tosense global wavefront tilt over the full aperture. Estimates of off-axis anisoplanatism show that guide stars bright enough to sensewavefront tilt errors should be found with around 50% probability for correction in the near infrared.92 EXPERIMENTAL GOALSVirtually all laser beacon experience to date has been with Rayleigh beacons from military sites with strong low level turbu-lence. Our goal in the present experiments, performed at the Multiple Mirror Telescope (MMT) in 1993 May and 1994 February,was to test experimentally the performance of a sodium laser system, with a large telescope and under the conditions prevailing at afirst class astronomical site. To date, the MMT adaptive optics system has been used to demonstrate a number of novel techniques364