System Overview of Subaru AO188
AO188 module
Subaru AO188 is an 188 element curvature sensor adaptive optics system that are operated in both natural guide star (NGS) and laser guide star (LGS) modes. AO188 module is mounted on the Nasmyth platform as shown in Fig.1. The system is based on a curvature wavefront sensor with 188 sub-apertures and an 188 element bimorph deformable mirror. It is the highest correction order of the curvature sensor AO system. The optics of AO188 is mounted on the 1.72 x 2.1 m optical bench (see Fig.2). The detail layout of the optics is shown in Watanabe et al. 2004, SPIE, 5490, 1096. Field of view of the optics covers 2.7 arcmin of the sky. We use this wide field optics mainly to acquire tilt guide stars for laser guide star operations. Calibration light sources which generate both natural guide star light and laser guide star light with artificial atmospheric turbulence is equipped. An image rotator unit is also equipped on the AO optical bench. The detail specification of the AO188 is summarized in Table 1.
Fig.1 Overview of the Subaru laser guide star AO system
Fig.2 Optical bench layout of AO188
Wavefront Sensor
Wavefront sensing of AO188 is based on curvature measurements of atmospheric turbulence. We use the curvature wavefront sensor with 188 elements for the high order wavefront measurement (HO-WFS) both in NGS and LGS modes. A lenslet array with 188 elements is used. Optical fibers are placed at the focus of each subaperture and feed to the photon-counting APDs.
For the LGS mode operation, we also need a natural guide star wavefront sensor (low order WFS) to measure wavefront tilt and defocus, which is not measured by laser guide star spot. Those stars for tilt measurement can be fainter than the magnitude required for HO-WFS. We use 2 x 2 SH wavefront sensor to measure the wavefront tip/tilt and defocus using 16 photon counting APDs. It has slight decrease of the throughput compared to the SH sensor with CCD, however it is adequate for fainter guide stars because it uses zero readout noise APDs.
Laser guide star system
A laser system is also installed at the Nasmyth floor. A sum-frequency solid state 589nm laser with average 4.5 W output power. The laser beam will be transferred to the laser launching telescope through a single mode photonic crystal fiber with 14 micron core diameter. The laser launching telescope with 50 cm aperture size will be mounted on the back side of the telescope secondary mirror.
Table 1. Specification of Subaru laser guide star AO system
| Location of the system | Nasmyth focus (IR) |
| Number of elements | 188 |
| Deformable mirror | Bimorph mirror with 188 elements, 90mm beam diameter (132mm blank) |
| High-order WFS | Curvature sensor with 188 photon counting APDs |
| Low-order WFS | 2x2 Shack-Hartmann sensor with 16 APDs for tip/tilt and defocus |
| Main Optics | 2.7' FOV, F/13.9 |
| Laser | Sum frequency generation 4.5W at 589nm (generates R=11 guide star) |
| Laser beam transfer optics | Single mode photonic crystal fiber |
| Laser launching telescope | 50cm telescope diameter behind the secondary |
| Real time computer | 4 Xeon CPUs (2.0GHz) with real time Linux OS |
| Control speed | 1000 correction/sec |
05 February 2009