Performance of Subaru Cassegrain Adaptive Optics System

Abstract

Abstract The design and performance of the Cassegrain Adaptive Optics (AO) system for the 8.2 m Subaru Telescope are reported. The system is based on a curvature wavefront sensor with 36 photon-counting avalanche photodiode modules and a bimorph wavefront correcting deformable mirror with 36 driving electrodes. The engineering first light of the AO system took place in 2000 December. The AO system has been in service since 2002 April for two open-use instruments, an infrared camera and spectrograph and a coronagraph imager with adaptive optics. The Strehl ratio in the $K$-band is around 0.3 for bright guide stars under ${0\rlap {.}{}^{\mathrm {\prime \prime }}4} K$-band seeing condition. The control loop performs with 2060 corrections per second. High sensitivity of the wavefront sensor allows significant improvement in the image quality, even for faint guide stars down to $R = 18 \,\mathrm{mag}$. The FWHM of stellar images in a globular cluster was measured to derive an estimation of the isoplanatic angle and was found nearly constant out to $30''$ from the guide star, indicating that the height of the effective turbulent layer of that particular night was less than 1.8 km. The on-going upgrade plans for a fivefold increase in the number of control elements and for the installation of a laser guide AO system are described.