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). 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

Image rotator

A K-type image rotator (IMR) that consists of three flat mirrors mounted on a rotation stage is used to track parallactic angle. Currently, following tracking modes are available.
  • Field rotation tracking for sidereal and non-sidereal objects
  • Pupil rotation tracking for sidereal objects (so-called ADI mode)
  • No tracking (stop IMR rotation)
The pupil rotation tracking mode for non-Sidereal objects will be implemented soon.

Atmospheric dispersion corrector

Atmospheric dispersion corrector (ADC) is installed in the science path of AO188. It has a conventional design, which consists of two identical doublet prisms. This ADC corrects the atmospheric dispersion in both visible and infrared wavelength over 0.45-2.2 μm for both science instrument and wavefront sensors. When the zenith angle is small and then the correction is not needed for the science instrument, the science-path ADC can be removed and other ADCs in the WFSs are used for the correction in the visible wavelength. Field and pupil tracking modes for sidereal objects are available. If you want to use the ADC for non-sidereal objects, please make contact with the support astronomer.

Deformable mirror

AO188 uses a 188-element bimorph type deformable mirror (DM) that was manufactured by CILAS. The bimorph mirror is made of two PZT plates with a diameter of 130 mm (beam size 90 mm) and a total thickness of about 1.9 mm. Control electrodes are located between the two plates. The DM is mounted on a tip/tilt mount to correct a tip/tilt error accumulated on the DM.

Beam splitter

Two kinds of beam splitters called BS1 and BS2 are installed in AO188. BS1 is used to split the input beam into two beams for the wavefront sensor and science camera. Currently, we have two BS1s with different cut-off wavelengths of 900 nm and 640 nm for using NIR (IRCS and HiCIAO) and optical (Kyoto-3DII) science instruments. For the BS2, a flat mirror is used for the NGS mode to reflect the input beam toward high-order wavefront sensor (HOWFS) and a notch filter that reflects only 589 nm light toward HOWFS and transparents light of other wavelengths is used to feed a laser guide star to HOWFS and feed a tip/tilt guide star to low-order wavefront sensor (LOWFS). All beam splitters are installed in BS automatic exchanger units that allow to switch observation mode easily during a night .

Guide star acquisition unit

To feed a guide star to the wavefront sensor, we are using a guide star acquisition unit that consists of two gimbal mirrors and a linear stage. The gimbal mirrors adjust the position and ray tilt of the guide star. The linear stage is mounted under the one of the gimbal mirrors and adjusts the focus position of the guide star by changing the separation of two mirrors. There are two acquisition units called AU1 and AU2 for acquiring a guide star for high-order and low-order wavevfront sensors, respectively. AU1 acquires a LGS or NGS within a FOV of 2 arcmin, while the AU2 acquires a Tip/Tilt guide star within a FOV of 2.7 arcmin.

High order wavefront sensor

High-order WFS is a visible 188-element curvature based wavefront sensor with photon counting APD modules, and measures high-order terms of wavefront using either of a single LGS or NGS. It basically consists of a fast tip/tilt mirror with an Offner relay to stop the LGS image at the focus, relay lenses to convert the focal ratio, an ADC for the NGS beam, a vibrating membrane mirror (VM), collimating mirrors, a lenslet array coupling to APDs through optical fibers, an acquisition CCD camera with a 20 arcsec FOV, a high-resolution camera with a pixel scale of a few milli-arcsec, and a pupil camera.

Low order wavefront sensor

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. The LOWFS basically consists of relay lenses to convert the focal ratio, and ADC, a collimating lens, 2x2 sub-aperture Shack-Hartmann lenslet array, and 4x4 lenslet array which connects to 16 APDs through optical fibers. It also has an acquisition CCD camera with a FOV of 20 arcsec and pupil camera for field acquisition and alignment.

Laser guide star system

A laser system is installed at the Nasmyth floor next to the AO188 optical bench. A sum-frequency solid state 589nm laser with average 6.8 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. Overall throughput for the laser transfer fiber and optics is about 60% and then the on-sky output power is 4.0W which corresponds to 10.5 mag (at zenith) in R band.

4 August 2010

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