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Advanced instruments for the Subaru Telescope

New focal plane tools and an adaptive optics system will aid in visible- and IR-wavelength imaging of faint and distant astronomical objects.
27 August 2008, SPIE Newsroom. DOI: 10.1117/2.1200808.1203

The Japanese Subaru Telescope is one of the world's largest optical/IR telescopes, with an 8m-diameter primary mirror, and is located atop the dormant Mauna Kea volcano in Hawaii. It has been in operation since 19991 and has seven first-generation instruments with imaging and spectroscopic capabilities attached at its four foci (see Figure 1). Subaru has produced many remarkable astronomical discoveries with those instruments, studying objects from the solar system to the distant universe. One example is the discovery of the most distant galaxy, or the earliest galaxy in the universe known to humankind. The universe was only 750 million years old when light from this galaxy began its journey to us, compared to its current age of 13.7 billion years.2

Not surprisingly, after 10 years of observation that have given us many answers, we have more new questions about the universe. We need to look into space more deeply, examining more samples with higher precision. At Subaru, we are planning and constructing new instruments with improved performance using state-of-the-art technologies. One approach is to build instruments that cover a wider field of view with more CCDs or IR array detectors with a larger format. These tools enable us to observe more stars and galaxies at a time, which means we can increase the exposure time for fainter or distant objects within a limited observation time. Another approach is to make images sharper by removing the effects of atmospheric turbulence using adaptive optics (AO).3

Figure 1. The Subaru Telescope's new instruments are installed at the four foci. The direction of light is switched during the daytime to allow the instruments to be used at night without losing observation time.

Subaru has been developing and commissioning new instruments along these lines. The first, the Multi-Object Infrared Camera and Spectrograph (MOIRCS),4 has been in operation since 2005. The instrument covers a 4×7arcmin field in near-IR bands and can take 50 spectra at a time.5 Figure 2 illustrates the instrument's capabilities. The field of view is seven times larger than our previous instrument, and now has the widest field of view among the near-IR instruments for 8–10m-class telescopes. MOIRCS is now being extensively used as a powerful tool for deep imaging of the universe.

A second project is a new AO system, AO188,6 which aims for higher image correction using a 188-actuator deformable mirror. (Our current AO system, AO36, uses 36 actuators.) AO188 enables us to obtain diffraction-limited images (60milliarcsec at 2.2μm) in the near-IR bands (see Figure 3). It will also have a laser guide-star system and will be able to observe distant galaxies without the use of a nearby natural guide star, which the current system requires. In the near future, AO188 will be used with another new instrument, the High-Contrast Coronographic Imager for Adaptive Optics (HiCIAO),7 to search for extrasolar planets.

Figure 2. Multi-object spectra taken with MOIRCS. Up to 50 spectra can be obtained simultaneously.

Figure 3. Improved image quality with AO. Seeing-limited image without AO (right) and diffraction-limited image with AO (left).

We are also developing the Fiber Multi-Object Spectrograph (FMOS).8 This instrument's 400 optical fiber entrances are located at the Subaru's prime focus. The fibers are precisely manipulated by unique piezo-based actuators and positioned on the objects in the 30arcmin field of view, as illustrated in Figure 4. Light from stars and galaxies will be introduced through the fibers to the near-IR spectrographs. The power of FMOS lies in its ability to take spectra of 400 galaxies at a time. No other instrument can perform near-IR spectroscopy of more than 100 objects at a time. This instrument will begin operation in 2009.

Figure 4. Four hundred fibers of the FMOS at the prime focus. The positions of the fibers are precisely manipulated to be positioned on images of astronomical objects.

The largest ongoing project at Subaru is HyperSuprime-Cam (HSC),9 the successor of Suprime-Cam at the prime focus. Suprime-Cam is a CCD camera with a 32×24arcmin field of view, the widest of any prime focus instrument for an 8m-class telescope. It has been very productive for research that requires a large number of samples, such as tens of thousands of distant galaxies for the study of the early universe. HSC will have a 1.5-degree field of view, which is 10 times larger than Suprime-Cam. It will use about 100 CCDs on the focal plane. We are expecting the first light in 2012. Another future instrument, a wide-field multi-object spectrograph (WFMOS), is now being studied. This multi-fiber visible instrument with around 4000 fibers would use the corrector lenses of HSC. It would make a critical contribution to our understanding of the source of dark energy, now the most enigmatic mystery of the universe.

Focal plane instruments are essential components of modern astronomical telescopes. Their designs are directly connected to science goals. Improving their performance is the only way to stay competitive with other large telescopes striving to open new frontiers of science, a challenge Subaru continues to address.

Hideki Takami
Subaru Telescope
National Astronomical Observatory of Japan
Hilo, HI

Hideki Takami earned a PhD in physics from Kyoto University in 1988 for research in far-IR astronomy, and worked for the Communications Research Laboratory of the Ministry of Posts and Telecommunications from 1989 to 1994. He has worked for the National Astronomical Observatory of Japan from 1994 to the present. In 2002, he began working on the Subaru Telescope in Hawaii, and in 2007 became a professor and associate director of the telescope. His research areas are AO and IR astronomy.