Adaptive optics system for Cassegrain focus of Subaru 8.2 m telescope

The adaptive optics system for Subaru 8.2 m telescope of the National Astronomical Observatory Japan has been developed for the Cassegrain near-infrared instruments,CIAO (coronagraph imager with adaptive optics) and IRCS (infrared camera and spectrograph). 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 = 16 mag 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, and will eventually be upgraded to a laser guide star system in cooperating an infrared wavefront tilt sensor to provide nearly full sky. The construction of this common use system to Subaru telescope is now underway in our laboratory in Tokyo, Prior to starting the fabrication of this common use system, a full size prototype system was constructed and tested with the 1.6 m infrared telescope at our observatory in Tokyo. This system has the identical optical design, deformable mirror,loop control computer to those for the Subaru system, while the wavefront sensing detectors were less-sensitive analog APDs. We succeeded in getting closed loop images of stars in K band with diffraction limited core. The Strehl ratio was around 0.5 and the factor of improvement was about 20 at K-band under the average seeing of 2 arcsec during the observation The loop speed of the system was 2 K corrections per second.