Adaptive optics and laser guide star: Astronomical application for Subaru 8.2 telescope

Abstract

The adaptive optics system for Subaru 8.2 m telescope of the National Astronomical Observatory of Japan has been developed to obtain nearly diffraction limited images for the near-infrared instruments. The system consists of a wavefront curvature sensor with 36 subaperture photon-counting avalanche photodiode modules and a bimorph deformable mirror with 36 electrodes. The expected Strehl ratio at K band exceeds 0.4 for objects that are located close enough to a bright guide star as faint as R = 16th magnitude at the median seeing of 0.45 arcsec at Mauna Kea. The system will be in operation in 1999 as a natural guide star system. To overcome a problem of the natural guide star AO system that the observable area is limited to small part of the sky, less than 40 percent at K band, it has been started to develop a laser guide star system extending it to nearly 100 percent. The system generates a bright spot at sodium layer at 90 km height by exciting the sodium stoms using 589 nm laser beam. The proposed laser system is LD pumped continuos wave dye laser with 2 to 4 W power which will produce 9th magnitude stars.