Final review of adaptive optics results from the pre-conversion MMT

Transcription

1 Final review of adaptive optics results from the pre-conversion MMT M. Lloyd-Hart, R. Angel, T. Groesbeck, P. McGuire, D. Sandler a, D. McCarthy, T. Martinez, B. Jacobsen, T. Roberts, P. Hinz, J. Ge, B. McLeod b, G. Brusa c, K. Hege, and E. Hooper Center for Astronomical Adaptive Optics, University of Arizona, Tucson, AZ a ThermoTrex Corporation, Pacific Center Court, San Diego, CA b Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA c Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125, Firenze, Italy Presented at SPIE conference number 3126 on Adaptive Optics and Applications July 30, 1997

2 Final review of adaptive optics results from the pre-conversion MMT M. Lloyd-Hart, R. Angel, T. Groesbeck, P. McGuire, D. Sandler a, D. McCarthy, T. Martinez, B. Jacobsen, T. Roberts, P. Hinz, J. Ge, B. McLeod b, G. Brusa c, K. Hege, and E. Hooper Center for Astronomical Adaptive Optics, University of Arizona, Tucson, AZ a ThermoTrex Corporation, Pacific Center Court, San Diego, CA b Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA c Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125, Firenze, Italy ABSTRACT The FASTTRAC II adaptive optics instrument has been used at the Multiple Mirror Telescope (MMT) for the past 2 years to provide improved image resolution in the near infrared. Results have been obtained using both natural guide stars and an artificial sodium laser beacon. With the imminent closure of the MMT prior to its conversion to a single-mirror 6.5 m telescope, FASTTRAC II has come to the end of its life. The instrument has been to the telescope for a total of 8 runs, and during that time it has been of enormous value both as a learning aid, demonstrating the requirements of its successor on the 6.5 m, and as a scientific tool. At this meeting, we present a selection of astrophysical data derived from FASTTRAC II, including the first closed-loop demonstration of an adaptive optics system using a sodium laser beacon. The sodium laser has been used to obtain near diffraction-limited near-infrared images of the core of M13, allowing the construction of a color-magnitude diagram to below the main sequence turnoff. Results have also been obtained from several gravitationally lensed quasars, and the cores of nearby galaxies in the local group. We also summarize work characterizing atmospheric conditions at the site. These studies have proceeded in two areas - understanding the behavior of the phase perturbation with field angle and time, and characterizing the return from the sodium resonance beacon. Keywords: adaptive optics, atmospheric effects, control systems, image processing, laser guide stars, telescopes, turbulence 1. INTRODUCTION The existing six mirrors of the Multiple Mirror Telescope (MMT) will shortly be replaced by a single 6.5 m mirror, now being polished at the Steward Observatory Mirror Lab. 1 Earlier work 2, 3 has shown that an adaptive optics system using a single sodium laser projected co-axially with the telescope can provide imaging at the diffraction limit in the H and K photometric bands over most of the sky. Such a system is under construction by the Center for Astronomical Adaptive Optics (CAAO) in Tucson and San Diego. The system is designed to operate in the wavelength region from 1.5 to 10 µm, where scientific return can be maximized. A major design goal has therefore been the minimization of thermal background radiation. To that end, the system will incorporate an adaptive secondary mirror as the wavefront compensator, since such a design requires no reimaging optics at all between the telescope and the science camera. Constraints imposed by the system s limiting magnitude and the size of the isoplanatic patch require the use of an artificial beacon in place of a natural guide star to achieve significant sky coverage. The system design therefore includes a sodium resonance beacon, generated by a laser tuned to nm illuminating the mesospheric sodium layer. Imaging and spectroscopy of the corrected beam will be done in a dewar mounted at the Cassegrain focus. In order to test a number of components and concepts for the 6.5 m design, including the use of a sodium resonance beacon as an artificial guide star, a prototype system for operation at the MMT was built in early This instrument, called FASTTRAC II, 4 corrects twelve parameters of the wavefront, namely tip and tilt over each of the telescope s six primary mirrors. At 2 µm, the uncorrected, combined image at the MMT has the appearance of six sharp images jittering independently about a common, slower moving centroid. Under typical seeing conditions, a long exposure K-band image of a star has a full width at half maximum (FWHM) of 0.75 arc sec, whereas instantaneous images from the individual telescopes often show a nearly diffraction-limited core of 0.35 arc sec. This is consistent with the known telescope aberrations and measurements of Fried s coherence length r 0 of typically 0.9 m, half the current individual mirror diameter of 1.8 m. Thus, the simple correction provided by FASTTRAC II can in principle recover the sharp core in a long exposure image with the full telescope

3 2. THE FASTTRAC II INSTRUMENT Figure 1 shows a schematic view of FASTTRAC II. Light from the six separate Cassegrain telescopes is reflected to the central beam-combining pyramid by tertiary mirrors. The pyramid directs light off rapidly steerable facets to a pseudo-cassegrain f/8.4 focus where the six fields are made coincident. 5 Wavelengths longer than 1.1 µm are transmitted by a dichroic beamsplitter near the focal plane, which forms the entrance window of the science dewar containing a NICMOS3 array. Reimaging optics in the dewar provide a pixel scale of arc sec, giving a field of view of 23 arc sec. Visible light is reflected upward through a relay lens to an articulated mirror that can direct any selected portion of a 4 arc min field to the Shack-Hartman wavefront sensor (WFS). This mirror is precisely located at the pupil formed by the relay, so that the pupil reimaged onto the lenslet array in the WFS remains well aligned. A lens before the WFS places the entrance pupil at infinity, so that continuous translation of the WFS to maintain focus on the sodium layer (whose distance is a function of elevation), leaves the pupil still in focus on the lenslets. Inside the WFS, the star images formed by the lenslets are relayed by a microscope objective to a small format CCD array. The scale is chosen so that when the wavefront is unaberrated, each image is centered at the intersection of 4 pixels. The adaptive control servo moves the beam-combiner facets to make the 4 signals equal in each quad cell, thus keeping the six images at the focal plane in coincidence. For operation with the laser, the sodium guide star is reflected to the WFS by a dielectric notch reflector centered at 589 nm as the articulated pupil mirror. Other wavelengths are transmitted to a second articulated mirror, which directs light from a chosen field star to the global tilt sensor at a combined focal plane. The tilt sensor is another fast-scanned CCD array, operated as a single quad cell. Its measure of overall tilt is added to the signals from the WFS in the computer which applies corrections to the beam combiner facets. Figure 1. Schematic view of the Cassegrain instrument. After correction at the adaptive beam combiner, infrared light is brought straight into the science dewar to a NICMOS3 array, while visible light is reflected upward to the wavefront sensing portion of the instrument. Laser light is reflected to the wavefront sensor; light from natural field stars is sent via a steerable mirror to the global tilt sensor. 3. SODIUM LASER For work using the sodium resonance guide star, the beacon was generated by a continuous-wave laser beam. The standingwave laser uses rhodamine 6G dye pumped by a 25 W argon ion laser. About 3.2 W of power is developed at the sodium D 2 line, of which 1.8 W is transmitted through the beam projector to the sky. The beam expander is a refractor with 48 cm diameter aperture, mounted close to the optical axis of the MMT array. 6 The beam typically has an elliptical gaussian profile of cm FWHM and is focused to produce the smallest obtainable illuminated spot at the sodium layer. Guide star images recorded at the WFS have a typical FWHM of 1.1 arc sec, and appear as bright as a star of R ~ 10 seen through an R filter.

4 4.1 Astronomical results using a natural guide star 4. RESULTS The brown dwarf Gliese 229B High resolution imaging finds application in almost every area of astronomy. Of great interest at present is the direct imaging of faint companions to nearby stars in the search for brown dwarfs which may eventually lead to images of Jupiter-sized planets. 7 The ability of adaptive optics to aid in the search by reducing the level of the bright primary s seeing halo at the location of the faint companion is illustrated in Figure 2. This shows an image of the Gliese 229 system in an exposure recorded by FASTTRAC II. The guide star was Gliese 229A, an M1 dwarf with R magnitude 7.9. The corrected image of the star and the field around it was recorded in a 60 s K-band exposure. The brown dwarf companion at 7 arc sec separation 8 is seen with a resolution of 0.45 arc sec FWHM. Despite the proximity of the much brighter primary star (the contrast ratio between the two is 12,500 at 2.2 µm), the brown dwarf stands out at a signal-to-noise ratio of 75. The uncompensated image width at the time of this observation was about 0.7 arc sec. The improvement provided by FASTTRAC II was thus significant, but did not reach the 0.35 arc sec limit set by the size of the individual image cores. An investigation has shown that the reduction in performance is due to errors in the automatic registration of the six wavefront tilts. Static errors from coma-like aberrations in the telescope optics account for this. The centroid of the visible guide star image found by the WFS is displaced from the instantaneous infrared diffraction core when the wavefront is asymmetrically distorted. This effect accounts for offsets of typically 0.07 arc sec from the axis. Figure 2. K-band image of the Gl 229 system. For this 60 s exposure, wavefront tilts over the MMT segments were corrected using visible light from the bright primary star, badly saturated in this image. The faint companion, 7 arc sec away, appears with FWHM of 0.45 arc sec The core of M31 FASTTRAC II has been used in an investigation of the cores of several nearby galaxies. In these cases, the galactic nuclei themselves have been used as the reference objects for wavefront sensing. A striking example is provided by M31, the Andromeda galaxy. When the Hubble Space Telescope was used to examine M31, the core appeared to be split into two components with a separation of 0.5 arc sec. The photocenter defined by isophotes several arc seconds from the core is nearly coincident with the fainter of the two components, suggesting that this is the galaxy s dynamical center. Figure 3 shows results obtained with FASTTRAC II in the H and K bands, and compares them to HST images recorded in the V and I bands. Each image of Figure 3 shows the central 3 arc sec of the galaxy and in all cases the double nature of the nucleus is clearly seen. On the basis of the HST images, a number of scenarios have been postulated for the physical process occurring there, including the possibility of a merger with either a globular cluster or a captured dwarf galaxy. On the other hand, the apparent separation of the two components may be due to a dust cloud obscuring part of a single, though perhaps non-spherical nucleus. Though the new infrared data cannot provide a conclusive answer, it now seems unlikely that an obscuring dust cloud is responsible for the core s appearance. Such clouds are much more transparent in the near infrared than at visible wavelengths, and yet the magnitude difference between the two components remains almost unchanged at about 0.3 per square arc sec from 0.5 µm to 2 µm. The invariance of the brightness ratio also indicates that the stellar population of the two components is roughly the same. It is unlikely therefore that a globular cluster is merging with the galaxy s nucleus, since the population of globular clusters tends to consist of stars older and redder than those found at the centers of actively star-forming spirals such as M31.

5 a b c d Figure 3. Four images of the central 3 arc sec of the Andromeda galaxy M31. a) 600 s K-band image recorded by FASTTRAC II after deconvolution by the Lucy-Richardson algorithm clearly shows the double peak structure. b) A 600 s H-band FASTTRAC II image also shows the split. c) The same structure appears in this 1200 s exposure in I band from the HST s WF/PC-2 camera d) A second 1200 s exposure from WF/PC-2, this time in the V band. In all images, north is up and east is left The gravitational lens PKS Adaptive optics is well suited for studying gravitationally lensed images of quasars. In this class of objects, light from distant quasars or radio sources is deflected by the gravitational field of an intervening galaxy, causing multiple images of the quasar to be seen. Knowledge of the morphology of such lensing galaxies and their redshifts is of particular importance in the calibration of the cosmological distance scale. The mass of a typical lensing galaxy however causes a separation of the quasar images of only about 1 arc sec. It is therefore very difficult to discern the image of the galaxy itself in the midst of such a tight cluster of the typically much brighter images of the quasar. An example of such a lens system is PKS Discovered at radio wavelengths, this lens system is largely obscured at visible wavelengths by dust in our Galaxy. Furthermore, a foreground M star lies almost exactly on the same line-of-sight. Past attempts to detect the lens galaxy at visible or infrared wavelengths have been unsuccessful. 9 Figure 4 shows a K s -band image of a 6 arc sec field around PKS taken with FASTTRAC II in May Excellent seeing conditions allowed us to use a guide star 1 arc min away. Because of the low declination of the target, the elevation angle was only 36 at the time of observation. Despite the low elevation and long exposure time of 18 minutes, the image quality was 0.45 arc sec. A detailed analysis of the lens system remains to be done. However, we have used the iterative blind deconvolution algorithm 10 to enhance the resolution further, to 0.26 arc sec. This is very close to the 2 µm diffraction limit of the 1.8 m primary mirrors. Three objects appear below the foreground star, which are believed to be components of the lens. It is possible with further analysis the lensing galaxy itself will be detected.

6 Figure 4. The gravitationally lensed quasar PKS is seen here in an 18 minute exposure in the K s band. The brightest object in the field is one of the components of the lensed image. Immediately above this, a foreground M star appears which partially obscures our view of the lensed system. A second star can be seen just to the lower right of the primary quasar image, and just beyond that is the secondary quasar image. The field of view in this picture is 6 arc sec. Resolution in the raw image was 0.45 arc sec. The deconvolved image seen here has a resolution of 0.26 arc sec, and it is shown on a logarithmic gray scale. 4.2 Observations with the sodium guide star Observations such as those of Section 4.1 are only possible with a relatively bright natural guide object. Because of the small size of the isoplanatic patch (around 1 arc min at 2 µm), this restriction limits natural guide star observations to a few percent of the sky. This constraint can be largely overcome for infrared imaging with large telescopes when laser-generated artificial beacons are used to measure wavefront errors. 2 As a test of the FASTTRAC II sodium laser system, we have obtained images of a rich field in the core of the nearby globular cluster M13. Figure 5 shows two exposures taken in the K s band with and without adaptive compensation by the laser beacon and natural tilt star. Corrections to both differential and global tilt were applied at 30 Hz. The uncompensated image in Figure 5a was derived by combining three consecutive 20 s exposures. The point-spread function has a FWHM of 0.72 arc sec, which indicates that seeing conditions at the time were close to median for the site. The compensated image is shown in Figure 5b, again the result of combining three 20 s exposures. For this image, the laser guide star was placed at the center of the field of view (the beam is of course invisible to the infrared camera), and global tilt information was derived from a natural star (V 12.7) separated from the laser by 35 arc sec, outside the field of view. The mean FWHM of the stellar profiles in the image with AO correction is 0.51 arc sec. The reduction in width is accompanied by an improvement in the Strehl ratio or peak brightness of 1.7. We have processed both images with the iterative blind deconvolution algorithm, 10 to bring out fainter objects. The results for the AO corrected image are shown in Figure 5c, which has a mean FWHM for the stellar images of 0.36 arc sec. The reality of the additional fainter stars has been checked by comparison with V-band images of M13 obtained with the WF/PC-1 camera of the HST. 11 We show the positions of stars identified by HST, to a limiting magnitude of V = 19, as dots overlaid on the deconvolved image. The DAOPHOT package 12 under IRAF has been used to derive photometry from the deconvolved image. In conjunction with the HST data, the results have been used to plot a color-magnitude diagram of V vs. V-K s for our observed field. The preliminary result is shown in Figure 6. A very well defined red giant branch leads down to the main-sequence turnoff slightly below our limit for complete sampling of K s 17. Eight horizontal branch stars blueward of the RR Lyrae instability are seen,

7 a b c Figure 5. K s -band images before and after correction with the adaptive optics system, using a sodium laser beacon as the wavefront reference source. a) With no correction, the stellar images are seeing limited at 0.72 arc sec in a 60 s exposure. b) With correction on the basis of the sodium laser beacon, and global image motion corrected by reference to a natural star, the images in this 60 s exposure have been improved to 0.51 arc sec. c) After PSF subtraction using the iterative blind deconvolution algorithm on the corrected image, the image size is further improved to 0.36 arc sec. The positions of 570 stars from a V-band image from the HST s WF/PC-1 camera are superposed as dots. All images are shown on a logarithmic gray scale to give a dynamic range of 1000, which exaggerates the halos of the bright stars. North is approximately 15 clockwise from vertical; east is left.

8 Figure 6. Color-magnitude diagram for 154 stars in the field-of-view of Figure 5. The K s magnitudes were obtained from photometry of Figure 5c; the V magnitudes derive from HST images. An approximate evolutionary track for the upper main sequence and the red giant branch is shown as a solid line. Two blue straggler candidates are circled in the lower left. and we identify two possible blue straggler stars (circled in Figure 6). This is consistent with measurements from HST data of the specific incidence of blue stragglers in the core of M13 by Cohen et al. 11 which lead us to expect between one and two in our field. 4.3 Atmospheric results In addition to the astronomical program carried out by FASTTRAC II, extensive effort has been devoted to characterizing the atmospheric conditions at the MMT site, with a view to determining system requirements for adaptive optics on the 6.5 m conversion. Results have been obtained in two areas of interest, namely the behavior of the atmospheric phase fluctuation, and explicit measurements of the column density of atomic sodium in the mesosphere Characterization of the atmospheric aberration The effects of atmospheric turbulence are generally calculated within the theoretical framework developed by Tatarski 13 and Fried. 14 This theory derives wavefront aberrations caused by Kolmogorov turbulence in the free atmosphere, and parametrizes the turbulence in terms of the quantities r 0 and τ 0, the spatial and temporal coherence scales, and θ 0, the angular correlation scale. A further quantity d 0 determines the magnitude of focus anisoplanatism, the measurement uncertainty which is the fundamental limitation in a laser-based adaptive optics system. Measurement of r 0 and τ 0 is particularly straightforward with an adaptive optics system, since they may be derived directly from data recorded by the WFS. Figure 7 shows the structure function of the phase fluctuation at a point in the pupil plane of the telescope, computed from motion of the six individual telescope images measured by the WFS. The structure function of the phase φ at a point r in the plane has the form S( t) = [ φ( r, t) φ( r, t + t) ] 2 where the angle brackets indicate ensemble averaging over time t. This quantity saturates for large t at approximately 2(D/r 0 ) 5/3 where D is the telescope diameter, which allows r 0 to be determined. The correlation time is simply the value of the time delay t at which the expected phase change is 1 radian. For the particular instance of Figure 7, the seeing was rather good, and we find τ 0 = 43 ms, and r 0 = 1.3 m at a wavelength of 2.2 µm. The typical values are given in Table 1.

9 Figure 7. Structure function of the atmospheric phase fluctuation at a point as measured at the MMT. The derivation of r 0 and τ 0 are illustrated. The phase error is shown for a wavelength of 2.2 µm. The wavefronts arriving at the telescope from a pair of stars separated by angle θ are different. This anisoplanatism sets a limit on the value of θ for wavefront sensing with natural field stars. Even when laser guide stars are used, the effect is of importance because of the need to measure overall wavefront slope. To explore the effect of anisoplanatism, we have made measurements of the relative motion of binary stars with various separations. 3 Figure 8 plots the rms tilt difference as a function of separation for the tilt components Z 2 and Z 3 (perpendicular and parallel to the separation vector respectively). The difference is shown in terms of the diffraction-limited image width at 2.2 µm for a single aperture telescope of the same diameter as the MMT array. Of particular interest is the Strehl ratio of the corrected image. On the top axis we show the resulting long-exposure Strehl ratio which would be attained if the higher order modes of the wavefront were corrected perfectly (the ideal case in a laser system) and only these errors in both Z 2 and Z 3 remained. Finally, we have measured the parameter d 0 for our sodium laser beacon. The mean square phase error caused by focus anisoplanatism, arising because the laser beacon Figure 8. The wavefront error due to off-axis anisoplanatism introduced by the two tilt modes is shown here as a fraction of the diffraction-limited image width λ/d, where λ = 2.2 µm and D = 6.86 m. is not at infinity, is given by σ 2 fa = (D/d 0 ) 5/3. The wavefront over the full MMT array was measured simultaneously by a natural star and the sodium beacon by aiming the laser at the star and separating the images from the six telescopes on a wide-field fast-framing CCD. 3 The relative motions of the six star and six beacon images were analyzed in terms of Zernike polynomials to determine the accuracy with which measurements made with the laser reflected the wavefront from the star. This is illustrated by Figure 9, which plots the results for the focus term over a typical 1.4 s period, in terms of the rms wavefront deviation across the full 6.86 m aperture. Measurements made with the laser track those made with the natural star very closely. For the first six Zernike modes after tilt, we find rad 2 of phase error due to focus anisoplanatism under median seeing conditions. Extrapolating to all modes, we expect σ 2 fa = 0.13 rad 2, yielding a value for d 0 of 23.5 m. Table 1 summarizes the mean measured values of the important atmospheric quantities.

10 Figure 9. Deviation of the wavefront from a plane caused by focus aberration across the 6.86 m aperture of the MMT as measured by the sodium beacon (solid line) and a coincident natural star (dashed line) simultaneously. The results, measured in the visible, have been scaled in this plot to a wavelength of 2.2 µm. r 0 (m) τ 0 (ms) θ 0 (arc sec) d 0 (m) Table 1. The four quantities characterizing the statistical behavior of the wavefront at the MMT have been measured. Their mean values are quoted here for a wavelength of 2.2 µm Measurement of the sodium column density In common with FASTTRAC II, the 6.5 m will take advantage of the naturally-occurring mesospheric sodium layer to generate an artificial guide star. Laser power is expensive to purchase and maintain, so it is of great interest to know how much is required to create a beacon of given brightness. The physics of the resonant excitation are well understood, 15 but until recently, explicit measurements of the column density of atomic sodium had not been made at an astronomical site. Such measurements have now been made at the MMT, in conjunction with photometric observations of the return from the FASTTRAC II laser. The results allow us to calculate directly the constant of proportionality between the column density and the expected return per watt of projected power. 16 To measure the sodium column density, we have relied on atmospheric absorption of starlight. Using the Advanced Fiber Optic Echelle spectrograph (AFOE) at the 1.5 m telescope on Mt. Hopkins, close to the MMT, we have obtained spectra at resolution of 50,000 around the sodium D 1 line. Figure 10 shows an example from a 2 hour integration on the star Altair. The column density is calculated from the line s equivalent width, under the assumption that the line falls on the linear portion of the curve of growth. While the spectra were being recorded, the FASTTRAC II laser on the MMT was aimed close to the same star, so that the beam would pass through roughly the same volume of mesospheric sodium. The tilt star acquisition camera (Figure 1) was used to record the return from the laser beacon through a standard R filter. Images of the beacon were compared to images of R-band photometric standard stars to calibrate the photometry. The results are shown in Figure 11 for circularly polarized light. (In the case of a linearly polarized beam, optical pumping effects reduce the strength of the return by ~30%.) The measurements were made on two nights in March and May 1997, and all lie close to a straight line fit predicting ph/s/m 2 /W per unit column density. The wide spread in the observed column density in the May data seems to be real, since the strength of the laser return is equally affected. Thus, variation in the brightness of the sodium beacon by a factor of 2 to 3 must be expected during the course of a night s observing.

11 Figure 10. Spectrum of Altair recorded over a 2 hour integration with the AFOE spectrograph at the 1.5 m telescope on Mt. Hopkins, which lies at about 1 km line-of-sight distance from the MMT. The sodium column density has been calculated using the equivalent width of the D 1 atmospheric absorption line. Figure 11. Observed absolute flux from the FASTTRAC II sodium laser beacon as a function of the column density of atomic sodium. 5. SUMMARY From the outset, FASTTRAC II was intended to be a temporary instrument, lasting for the remaining lifetime of the MMT. It was expected to perform two functions, namely to take advantage of the excellent seeing on Mt. Hopkins in a variety of scientific programs while testing a number of system concepts and hardware components for the 6.5 m system, and to measure critical atmospheric parameters that bear directly on the final design of the new system. During its brief life, FASTTRAC II has fulfilled both of those goals. In this paper, we have presented a small sample of the results obtained in both areas. The key features of the adaptive optics system for the 6.5 m MMT allow diffraction-limited imaging across a broad range of wavelengths, and most of the observable sky, with very low light loss and contamination by thermal background radiation. This will be achieved through the use of a deformable secondary mirror, and a single sodium resonance laser projected along the telescope s axis. By using the laser, the system s limiting magnitude will be improved by six magnitudes, which represents an enormous increase in the fraction of the sky open to high-resolution imaging.

12 ACKNOWLEDGMENTS This work has been supported by the Air Force Office of Scientific Research under grant number F , and by the National Science Foundation under grant number AST K. Hege. thanks E. M. Hege and the University of Arizona Foundation for support of the iterative deconvolution image calibration. We are very grateful to the staff of the MMT for their support in this most demanding work. Observations reported here were made at the Multiple Mirror Telescope Observatory, a joint facility of the University of Arizona and the Smithsonian Astrophysical Observatory. REFERENCES 1. H. M. Martin, J. H. Burge, D. A. Ketelson, and S. C. West, Fabrication of the 6.5-m primary mirror for the Multiple Mirror Telescope conversion, Proc. SPIE conf. on Optical Telescopes of Today and Tomorrow, ed. A. Ardeberg, 2871, 399, D. G. Sandler, S. Stahl, J. R. P. Angel, M. Lloyd-Hart, and D. W. McCarthy, Adaptive optics for diffraction-limited infrared imaging with 8-m telescopes, J. Opt. Soc. Amer. A, 11, 925, M. Lloyd-Hart et al., Adaptive optics experiments using sodium laser guide stars, Astrophys. J. 439, 455, P. M. Gray et al., FASTTRAC II near-ir adaptive optics system for the Multiple Mirror Telescope: description and preliminary results, Proc. SPIE conf. on Adaptive Optical Systems and Applications, ed. R. K. Tyson & R. Q. Fugate, 2534, 2, L. M. Close, G. Brusa, D. G. Bruns, M. Lloyd-Hart, and D. W. McCarthy, An adaptive beam-combining mirror for the MMT, Proc. SPIE conf. on Adaptive Optical Systems and Applications, ed. R. K. Tyson & R. Q. Fugate, 2534, 105, B. P. Jacobsen et al., Field evaluation of two new continuous-wave dye laser systems optimized for sodium beacon excitation, Proc. SPIE conf. on Adaptive Optics in Astronomy, ed. M. A. Ealey & F. Merkle, 2201, 342, J. R. P. Angel, Ground based imaging of extrasolar planets using adaptive optics, Nature, 368, 203, T. Nakajima, B. R. Oppenheimer, S. R. Kulkarni, D. A. Golimowski, K. Matthews, and S. T. Durrance, Discovery of a cool brown dwarf, Nature, 378, 463, S. Djorgovski, et al., A search for the optical/ir counterpart of the probable Einstein ring source , Mon. Not. Roy. Ast. Soc., 257, 240, S. M. Jefferies and J. C. Christou, Restoration of astronomical images by iterative blind deconvolution, Astrophys. J., 415, 862, R. L. Cohen, P. Guhathakurta, B. Yanny, D. P. Schneider, and J. N. Bahcall, Globular cluster photometry with the Hubble Space Telescope. VI. WF/PC-1 observations of the stellar populations in the core of M13 (NGC 6205), Astron. J., 113, 669, P. B. Stetson, DAOPHOT: a computer program for crowded-field stellar photometry, Pub. Ast. Soc. Pac., 99, 191, V. I. Tatarski, Wave Propagation in a Turbulent Medium, (New York: McGraw-Hill) D. L. Fried, Statistics of a geometric representation of wavefront distortion, J. Opt. Soc. Amer., 55, 1427, P. W. Milonni and J. M. Telle, Analysis of sodium layer scattering physics, Proc. ESO workshop on Laser Technology for Laser Guide Star Adaptive Optics, Garching, in press. 16. J. Ge et al., Mesosphere sodium column density and the sodium laser guide star brightness, Proc. ESO workshop on Laser Technology for Laser Guide Star Adaptive Optics, Garching, in press.