The large optical-infrared telescope “Subaru” is located on the summit of Mauna Kea in Hawaii, in the middle of the Pacific Ocean. The reason why Subaru is located in such an isolated environment, high atop a mountain 4,200 m above sea level, is that the summit offers incomparable advantages for astronomical observation in terms of the high number of clear nights throughout the year and transparency of the atmosphere because of low humidity. The summit of Mauna Kea is famous as an observation site that satisfies astronomical requirements. It is no wonder that there are various telescopes there from all over the world.
Subaru, Japan's renowned telescope, is equipped with a High Dispersion Spectrograph (HDS). HDS plays an active role in measuring high dispersion spectra in order to evaluate the elemental abundance of very old stars formed at the beginning of the universe. This enables the study of the process of chemical evolution in the universe. We interviewed Dr. Kunio Noguchi, professor of the National Astronomical Observatory of Japan, to learn about HDS and some of the most important discoveries made with it.
Aiming for Top-Level Astronomical Observations
How the national project “Subaru” began.
I became involved in the HDS project in 1995. By then, the construction of Subaru had already started, so I am not a witness to the entire history of the Subaru project. At that time, most Japanese astronomers were asking themselves what is most important for the future of optical and infrared astronomy in Japan. In addition to traditional optical astronomy, newly developed infrared astronomy was growing in importance. Thus, the Subaru was designed to be suitable for both optical and infrared astronomy. The project was officially called the “Japanese National Large Telescope (JNLT) Project” and the telescope was named “Subaru” when completed.
Astronomy in Japan before “Subaru”
The Subaru telescope on the summit of Mauna Kea in Hawaii
In the 1970s, 3- to 4-meter class telescopes became popular for observing more distant astronomical objects at higher resolutions. Telescope aperture is one of the most important factors when observing deep space. Since the light-collecting power of the telescope depends primarily on the surface area of the primary lens or mirror, the largest 1.8 m aperture telescope in Japan at that time could not compete with the world's largest telescopes. The performance of observing instruments mounted on a telescope is also important. The accuracy of the results obtained with 1.8 m telescope could be more than 10 times less than the world's best telescope. Other than the telescope aperture, the climate and weather conditions are also important. Japan is not suited to astronomical observations because of the high humidity. At the beginning of the JNLT project, we had two options. One was to construct a 4-meter class telescope in Japan, which would allow for easy access to the telescope. The other was to construct a top-class telescope at a location best suited to astronomical observations. The latter could be a difficult and challenging project.
In a highly humid environment, the performance of a telescope is lessened due to the poor transmittance of the atmosphere, especially in the infrared. Therefore, the climate in Japan is not suited to infrared observations. Mauna Kea in Hawaii was deemed one of the best sites because the altitude of the Mauna Kea summit is 4,200 meters above sea level, higher than that of Mt. Fuji, which provides excellent conditions for astronomical observations. Eventually, it was agreed that the goal of the JNLT project should be to construct the world's most advanced telescope at a site best suited for obtaining the highest quality astronomical data. This was truly a challenging project at that time for Japanese astronomers, who had no experience constructing big telescopes.