Visible low-order wavefront sensor for the subaru LGSAO system

The Subaru laser guide star adaptive optics system (AO188) was installed at the Nasmyth focus of the Subaru Telescope on October 2006 and it is in operation with the natural guide star (NGS) mode. The operation of the laser guide star (LGS) mode started on January 2010. A visible low-order wavefront sensor (LOWFS) was built to measure tip-tilt and defocus terms of wavefront by using a single NGS within a 2.7 arcmin diameter field when an LGS is used for high-order wavefront sensing with the 188-element curvature based wavefront sensor. This LOWFS is a 2 × 2 sub-aperture Shack-Hartmann sensor with 16 photon-counting avalanche photodiode (APD) modules. A 4×4-element lenslet array is located after the 2 × 2 sub-aperture Shack-Hartmann lenslet array and it is coupled with the APD modules through optical fibers. The field of view of the LOWFS is 4 arcsec in diameter. It has own guide star acquisition unit, acquisition and pupil cameras, and atmospheric dispersion corrector. We describe the design, construction, and integration of this low-order wavefront sensor.