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Sodium Laser Guide Star Brightness, Spotsize, and Sodium Layer

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Sodium Laser Guide Star Brightness, Spotsize, and Sodium Layer So dium Laser Guide Star Brightness, Sp otsize, and So dium Layer Abundance a a a a a b Jian Ge , B.D. Jacobsen ,J.R.P. Angel ,P.C. McGuire ,, T. Rob erts , B. McLeo d a a,c M. Lloyd-Hart , D. Sandler a Steward Observatory, The UniversityofArizona,Tucson, AZ 85721, USA b Center for Astrophysics, MS 20, 60 Garden Street, Cambridge, MA 02138, USA c ThermoTrex Corp oration, 10455 Paci c Center Court, San Diego, CA 92121, USA ABSTRACT We rep ort on new results of simultaneous measurements of so dium layer abundance and the absolute return ux from laser guide stars created by a mono chromatic 1 W cw laser, tuned to the p eak of the so dium D2 hyp er ne structure. The return was conducted at the MMT and the so dium abundance was measured at the CFA 60 inch telescop e, ab out 1 km away from the MMT, with the Advanced Fib er Optic Echelle (AFOE) sp ectrograph. The laser frequency stability,which can greatly a ect the return ux, was monitored at the same time in order to improve the measurement accuracy. After the correction of laser frequency jitter and atmospheric transmission, the absolute 6 1 2 ux return for circularly p olarized lightis1.7110 photons s m per watt launched, p er unit column density, 9 2 whichwe take as our measured mean over the year of N(Na) = 3.710 cm at Tucson. The solidi cation of a nal well-determined relationship b etween the so dium laser guide star brightness and so dium layer column density is pivotal in the design of the next generation laser guide star adaptive optics systems. We also rep ort the measurements and analysis of the relationship b etween the pro jected b eam waist of the so dium laser and the resultant sp ot size on the so dium layer under typical atmospheric conditions. Knowledge of the waist size is crucial to the design of the laser b eam pro jector for optimal p erformance. By pro jecting the laser through di raction limited optics of roughly 3 r diameter, wehaveachieved the smallest arti cial b eacon yet recorded, ab out 0 0.8 arcsec. Keywords: Adaptive Optics, Laser Guide Star, So dium Layer Abundance, Point Spread Function 1. INTRODUCTION Adaptive optics (AO) systems can signi cantly reduce atmospheric distortion e ects in astronomical imaging if a bright light source is available within arcmin eld-of-view of a scienti c ob ject for measuring wavefront phase errors. However, the sky coverage for the AO systems relied on natural bright eld stars is only a few p ercentage of the sky (e.g. Ro ddier 1995). In order to signi cantly increase the sky coverage with AO correction, laser-generated arti cial guide stars or \b eacons" are required for sensing the wavefront errors (e.g. Sandler et al. 1994). The most promising arti cial laser guide star is generated by fo cusing a so dium laser b eam tuned to the so dium D line at 589 nm on 2 the mesosphere so dium layer at ab out 90 km altitude. Several groups in the world have b egun to explore the use of so dium laser guide star technique for astronomical adaptive optics (e.g. Jacobsen et al. 1993; Max et al. 1994; Jelonek et al. 1994; Avicola et al. 1994). In the Fall 1996, our group successfully closed the laser guide star adaptive optics tip-tilt system lo op resulting in the rst rep orted improvements in image quality(Lloyd-Hart et al. 1997). In early 1997, Livermore laser guide star group also successfully closed their high order AO lo op and made signi cant image corrections (Olivier et al. 1997). The recent closed lo op exp eriments with so dium laser guide stars at Max-Plank-Institut fur Astronomie have also demonstrated a factor of two image improvements in the near-IR (Hippler et al. 1998). These exp eriments demonstrate the p ossibility of so dium laser guide star technique in adaptive optics applications. However, there is as yet no consensus on the optimum laser and p ower. The choice dep ends not only on the laser p ower, but also on Other author information: (Send corresp ondence to Jian Ge) J. G.: Email: [email protected]; WWW: http://qso.as.arizona.edu/ jge; Telephone: 520-621-6535;Fax: 520-621-1532 the inherently di erent pulse formats and frequency purity of di erent laser typ es. Theoretical calculations predict di erences in return for the same p ower and column density up to a factor 5 according to pulse format (Milonni &Telle, 1996, private communication), but these haveyet to b e con rmed exp erimentally.Thus, exp eriments that measure column density and details of the excitation and scattering prop erties of so dium atoms in the so dium layer are very imp ortant to re ne the design parameters of the laser and assess the p ower requirement. Simultaneous lo cal observations of the so dium column density and the laser return are necessary to obtain the fundamental relationship b etween the magnitude of the laser guide star and so dium abundance. Previous studies have shown that the column density of the so dium layer varies with time, including long term seasonal variations and short term variations (e.g. Pap en 1996; Ge et al. 1997), and also varies with latitude (Hunten 1966; Magie et al. 1978; Pap en et al. 1996; Ge et al. 1997). A simultaneous measurementofcolumndensity and guide star brightness has not b een done b efore. This together with the measurements of so dium layer abundance variation (Ge et al. 1997) will provide a reference for the design of the so dium laser for the MMT 6.5 m AO system as well as other so dium AO systems in the world. In order to obtain the strongest signal from a so dium laser guide star for wavefront sensing and correcting, the images at a Shack-Hartmann wavefront sensor should b e as sharp as p ossible. A doubling of the image would require a quadrupling of laser p ower to obtain the same signal/noise ratio (e.g. Sandler et al. 1994). The instantaneous sp ot size on the so dium layer is directly related to the so dium laser b eam waist size. The optimal so dium laser b eam waist width varies with the instantaneous atmospheric conditions. Therefore, a series of exp eriments need to b e conducted to nd out the relationship b etween the optimal so dium b eam waist and sp ot size and howtight a sp ot the laser b eam pro jector generates on the sky. In this pap er we will rep ort on simultaneous so dium laser return and mesosphere so dium abundance measure- ments, instantaneous so dium b eam waist width and sp ot size measurements. 2. SIMULTANEOUS SODIUM LASER RETURN AND COLUMN DENSITY MEASUREMENTS The most prop er way to determine howmuch laser p ower is required for certain laser guide star AO p erformance is to directly calibrate the laser p ower with the simultaneous measurements of so dium laser guide star ux and so dium layer abundance. The continuous-wavedye laser we used for the MMT FASTRACIIAO system can provide a single longitudinal mo de with tunable frequency at high p owers after it passes through a traveling-wave ring cavityand frequency tuning elements (Rob erts et al. 1997). It is p erhaps the simplest laser for the simultaneous measurements. The observational results can b e easily understo o d and provide a go o d reference for predicting other laser guide star p erformance. Furthermore, the results provide a direct testing of current theoretical mo dels for so dium laser guide stars (Milloni & Telle 1997, private communications). The simultaneous measurements of so dium laser return ux and mesosphere so dium abundance were conducted in March, May, and Septemb er, 1997. The so dium laser return ux was measured at the Multiple Mirror Telescop e (MMT) on Mt. Hopkins. The laser output p ower was measured b efore and after each collecting data set with typical 10 image frames, whichwere typically taken in totally ab out 3 minutes. The exp osure time of each frame is 3-5 seconds. The average laser p ower pro jected on the sky is ab out 1 watts for the all three runs. Only the so dium return ux during photometric conditions were used for the nal data analysis. In fact, during each return exp eriment the laser b eacon images were used for cho osing those photometric measurements b ecause any thin clouds passing through the laser b eams would showvery strong Rayleigh scattering and signi cantly reduce the return ux from the so dium sp ot. Esp ecially during the Septemb er run, b ecause thin clouds covered part of the sky,we to ok advantage of the laser b eacon high sensitivity to thin clouds to use the laser return images to help nd clear sky patch for the return measurement. The so dium layer abundance was measured at the CFA60inch telecop e with the Advanced Fib er Optic Echelle (AFOE) sp ectrograph. The telescop e is only ab out 1 km away from the MMT on the same mountain. Therefore, similar so dium layer patches were observed by b oth telescop es. The sp ecrograph resolving p ower is R = 50,000 (Brown et al. 1994).
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