Field evaluation of two new continuous-wave dye laser systems optimized for sodium beacon excitation

Adaptive correction of infrared images formed by large telescopes will be possible with the aid of a single laser guide star projected along the telescope axis. The fundamental limit set by focus anisoplanatism of a sodium beacon allows correction to the diffraction limit in the 1.65 and 2.2 micron bands under typical conditions at a good site. A 1 arcsec `star' of V magnitude ≈ 9 is then required to reduce photon noise to negligible level. In this paper we report on recent tests of continuous wave dye lasers at wavelength 589 nm, used to create sodium beacons above the Multiple Mirror Telescope. A coherent ring dye laser operating at 2.5 W average power and projected as circularly polarized light yielded a beacon of m_v = 9.85, and a Livermore-designed standing wave laser operating at 1.7 W and projected as linearly polarized light yielded m_v = 10.4. Beacon sizes close to 1 arcsec were achieved. The 9th magnitude goal should be achieved by using more advanced cw dye lasers with 20% conversion efficiency of a 25 W argon ion pump laser, and from improvements in beam projection efficiency. We describe a refractive beam expander under construction for the 6.5 m conversion of the MMT.