Development Plan
ULTIMATE-Subaru aims to deliver a new observational capablity of "wide-field", "high-sensitivity", and "high-resolution" in the near-infrared wavelength at the Subaru Telescope. The key technology to realize this capability is the development of the ground-layer adaptive optics (GLAO) system. GLAO is an adaptive optics system that greatly improves the image quality over a wide field of view, by correcting for the earth's atmospheric turbulence in real time (See also here for adaptive optics).
It is already possible to obtain high (diffraction limit) spatial resolution using a single reference star with the classical adaptive optics. However, since the levels of atmospheric turbulences are different depending on the direction toward the sky, a problem here is that the image quality can be improved only in a small area about 1-arcmin around the reference star.
GLAO uses multiple reference stars, to measure and correct for the atmospheric turbulence in the ground layer (an altitude of ~100 m or less). Because the atmospheric turbulence in the ground layer does not change significantly with the direction toward the sky from the telescope, it is possible to improve the image quality over a wide field of view. It should be noted that GLAO will not fully correct for the high-altitude atmospheric turbulence (and thus we cannot achieve diffraction-limit image quality), but we expect to achieve a spatial resolution of ~0.2-arcsec on average over a wide area (~20-arcmin) - the image quality comparable to the Hubble Space Telescope. The concept of GLAO is shown in the figure. Using the multiple wavefront sensors, we will measure the atmospheric turbulence of the ground layer level, and the adaptive secondary mirror (ASM) mounted on the telescope can change its mirror shape at very high speed, to compensate the turbulence over a wide field of view. A high-power laser system will also be used to generate multiple laser guide stars around the field of view of the telescope.
ULTIMATE-Subaru is a project to build the GLAO system which covers a field of view of up to 14x14 arcmin (20 arcmin in diameter) on the Subaru Telescope. The main components of the GLAO system are the wavefront sensor unit that measures the wavefront of the light and the laser guide star system that artificially generates a reference light source for measuring the wavefront. We will also install the adaptive secondary mirror (as the pupil of the telescope) to correct for the atmospheric turbulence over the wide field of view. The adaptive secondary mirror is equipped with 924 actuators behind a thin shell mirror with a diameter of 1.26 m and a thickness of 2 mm, The mirror can change its shape at a high speed of 1 kHz or higher. For the laser guide star system, a high-power laser with a wavelength of 589 nm is launched from the telescope to excite sodium atoms in the sodium layer of the atmosphere at an altitude of about 90 km, creating an artificial wavefront reference star (laser guide star). We will launch 4 lasers from the side of the telescope to create 4 laser guide stars.
Light from 4 laser guide stars will be monitored by four Shack-Hartmann wavefront sensors mounted near the focal plane of the telescope. In addition to the light from 4 laser guide stars, the wavefront sensor unit uses 4 natural stars to measure low-order wavefront errors that cannot be measured with the laser guide stars. The measured wavefront data is sent to a real-time computer, extracting only the atmospheric turbulence of the ground layer, and it deforms the shape of the adaptive secondary mirror accordingly. Such measurement and correction "loops" are performed in the frequency of 0.5 to 1.0 kHz. The wavefront sensor units are installed at the Cassegrain focus and the Nasmyth focus of the Subaru Telescope, and they deliver the GLAO-corrected light to science instruments mounted at each focus.
We are also developing science instruments to be operated with GLAO on the Subaru Telescope. First, the existing wide-field near-infrared imaging spectrometer (MOIRCS) will be used for the commissioning of GLAO after relocating it from the Cassegrain focus to the Nasmyth focus. MOIRCS will be used not only for the commissioning of GLAO, but also for the early science of ULTIMATE in imaging mode and near-infrared multi-object spectroscopic survey. Also, at the Cassegrain focus, a wide-field near-infrared imager (WFI) dedicated for GLAO observations with a maximum field of view of ~200 square arcmin (14arcmin x 14arcmin) will be installed as the flagship instrument of ULTIMATE. See also here for the details of the instruments.
Finally, the ULTIMATE-Subaru is not only for wide-field capabilities. By launching the laser guide stars toward a narrow region of the sky, we can completely corrects for the atmospheric turbulence in the small area and delivers nearly diffraction-limited image quality over a wide wavelength range from visible to near-infrared light. We plan to implement this narrow-field mode (LTAO mode) that provides extremely high spatial resolution. An idea of the new LTAO-assisted, high-sensitivity spectroscopic instrument that simultaneously observes from optical to near-infrared wavelength (NINJA) has been proposed. We emphasize that Subaru Telescope is not a "wide-field-only" telescope, but will continue to be the world's best telescope with a variety of observational capabilities.