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Ground-Layer Adaptive Optics (GLAO): the Key to ULTIMATE-Subaru
Ground-Layer Adaptive Optics (GLAO): the Key to ULTIMATE-Subaru
ULTIMATE-Subaru aims to realize an observational capability in the near-infrared wavelength range with wide field of view, high sensitivity, and high spatial resolution. The key to realizing this is Ground-Layer Adaptive Optics (GLAO). GLAO is a type of adaptive optics (AO) that corrects wavefront distortions caused by Earth's atmosphere in real-time, significantly improving the image quality (for more details on adaptive optics, please refer to this page).
Conventional adaptive optics use a single wavefront reference star to achieve high spatial resolution close to the diffraction limit of the telescope. However, because the strength of atmospheric turbulence varies depending on the sky direction, conventional methods can only improve image quality within a narrow region of about 1-arcmin around the reference star.
GLAO, on the other hand, uses information from multiple reference stars to measure and correct only the turbulence in the atmospheric layer near the ground (the ground layer) at altitudes of approximately 100 meters or lower. Since ground-layer turbulence does not vary significantly with the telescope's viewing direction, it enables improvement of the image quality across a wide field of view. While GLAO cannot correct for turbulence in higher atmospheric layers and thus cannot achieve diffraction-limited resolution, it is known that at the summit of Maunakea, where the Subaru Telescope is located, ground-layer turbulence is dominant. As a result, by correcting only ground-layer turbulence, it is expected to achieve a spatial resolution of ~0.2-arcsec (comparable to Hubble Space Telescope) across a 20-arcmin field of view (in diameter).
ULTIMATE-Subaru aims to implement a GLAO system on the Subaru Telescope that will cover the world's largest field of view, up to 14 arcmin × 14 arcmin (diagonal ~20 arcmin). As shown in the figure above, the key components of the GLAO system of ULTIMATE-Subaru include:
(1) Adaptive secondary mirror (ASM),
(2) Laser guide star facility that creates reference light sources for wavefront measurement (LGSF),
(3) Wavefront sensor unit for measuring wavefront distortions (WFS), and
(4) Wide-field science instruments.
(1) Adaptive Secondary Mirror (ASM):
(1) Adaptive Secondary Mirror (ASM):
To correct atmospheric turbulence across a wide field of view, the Subaru GLAO system incorporates the adaptive secondary mirror (ASM), which corresponds to the telescope's entrance pupil. This adaptive mirror has a diameter of 1.26 meters and a thickness of only 2 mm. It is equipped with 924 actuators on its back, allowing it to adjust its shape at a high speed of over 1 kHz.
(2) Laser Guide Star Facility (LGSF):
(2) Laser Guide Star Facility (LGSF):
The laser guide star generation system projects a high-power laser with a wavelength of 589 nm into the sky. This excites sodium atoms in the upper atmospheric layer at an altitude of about 90 km, creating artificial wavefront reference stars (laser guide stars). In the Subaru GLAO system, four laser beams are projected from the sides of the telescope to create four laser guide stars.
(3) Wavefront Sensor Unit (WFS):
(3) Wavefront Sensor Unit (WFS):
Light from the four laser guide stars is directed to four Shack-Hartmann wavefront sensors installed near the telescope’s focal plane. In addition to measuring wavefront distortions using the laser guide stars, the wavefront sensor unit also utilizes light from four natural guide stars to detect low-order wavefront errors that cannot be measured with laser guide stars alone. This results in a total of eight wavefront sensors. The wavefront data collected by these sensors is sent to a real-time computing system, which extracts only the turbulence from the ground layer and adjusts the shape of the adaptive secondary mirror accordingly. This measurement and correction loop operates at a frequency of 0.5 to 1.0 kHz. The wavefront sensor unit is mounted at the Cassegrain focus of the Subaru Telescope and provides turbulence-corrected light to the science instruments.
(4) Wide-Field Science Instruments:
(4) Wide-Field Science Instruments:
ULTIMATE is also developing a wide-field science instrument to be operated with GLAO system on the Subaru Telescope. At the Cassegrain focus, a wide-field near-infrared imaging instrument (Wide-Field Imager, WFI) with a maximum field of view of 14 arcmin × 14 arcmin will be installed. As the flagship instrument of ULTIMATE, WFI will enable unprecedentedly wide-field infrared survey observations. The WFI consists of four identical optics to cover the wide field of view, with each optics equipped with a HAWAII-4RG detector. To achieve the scientific goals of ULTIMATE-Subaru, WFI will support a variety of broad-band, medium-band, and narrow-band filters (λ ~1.0–2.5 μm). The improved image quality delivered by GLAO will also significantly enhance its sensitivity, making it possible to achieve a 5σ point-source detection limit of ~25–26 magnitude (AB) in just a few hours of observation.
ULTIMATE-Subaru: beyond the wide-field capability
ULTIMATE-Subaru: beyond the wide-field capability
ULTIMATE-Subaru offers more than just wide-field observational capabilities. The adaptive optics system developed for GLAO will also implement a narrow-field mode (LTAO mode), where laser guide stars are concentrated in a small region of the sky to fully correct atmospheric turbulence. This enables the telescope to achieve high spatial resolution close to the diffraction limit over a wide wavelength range from visible to near-infrared light. Using this mode, sensitivity to point sources will be maximized, and a new high-sensitivity spectrograph (NINJA) with a simultaneous wavelength coverage from visible to near-infrared wavelengths.
Subaru Telescope is not merely a "wide-field survey telescope"; Subaru can respond to a variety of demands of researchers, and remain the world's leading telescope. ULTIMATE-Subaru is thus essential to achieving this goal.
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