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.
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.
What was the very beginning of the universe?
I'd like to describe two topics that were clarified based on data obtained with high dispersion spectroscopy. The first one is relating to the production of the elements and the chemical evolution in the universe. Immediately after the Big Bang at the beginning of the universe, almost no element existed except hydrogen and helium. On the other hand, we know that today over 100 different elements exist on earth. However, the abundance of heavy elements with large atomic numbers is low for stars that were formed long, long ago. This shows that heavy elements were produced with the evolution of the universe. Our current understanding is that the majority of elements heavier than helium were produced in the core of stars over their lifetimes (stellar evolution) and released into space when stars ended their lives with Super-Nova (SN) explosions. New-generation stars are born by collecting elements ejected from SN. Thus, newly-born stars contain much more heavy elements than old stars. Stellar evolution contributes to the increase in the abundance of heavy elements.
First-generation stars contain only hydrogen and a tiny amount of helium. If stellar mass is great enough, the temperature of the core becomes sufficiently high through gravitational energy, initiating a nuclear fusion reaction. In other words, hydrogen burns to convert it into helium. In massive stars, this helium burns to produce other heavy elements. The stellar light is the release of this nuclear fusion energy. The reaction converts elements into those with an atomic number of up to 26 (iron). Massive stars containing iron finish their lives as supernova explosions, resulting in the release of various elements less heavy than iron into space. These elements again aggregate due to the gravitational force to form next-generation stars, which then distribute the produced elements into space repeatedly in a cycle of birth and death of individual stars that also results in an increase in the abundance of heavy elements.
The origin of the universe clarified by observing old stars with HDS
High dispersion spectral observations of stars reveal the abundance of elements when individual stars are born. By observing a great number of stars which were born at various times, we can understand how elements increased in amount (chemical evolution). HDS, which was developed in cooperation with Nikon, is a powerful instrument for this study.
As an example, here is one recent result which was reported in April 2005. We discovered a star with the least amount of iron ever observed. Iron abundance represents the amount of elements heavier than helium. The star's iron content is only one 250,000th that of the sun. This data will be very valuable in helping us understand elemental abundance at the earliest stages of the universe.
Another exciting result obtained by high-dispersion spectroscopy
The second topic relating to high-dispersion spectroscopy is the extra-solar planet search. HDS is an optical spectrograph with high wavelength resolution that enables us to measure the intensity of light with each 1/100,000 wavelength interval. By using this high spectral resolution, we have the chance to find extra-solar planets like earth.
How can we find extra-solar planets?
For an observer far away from the sun, spectral lines of the sun shift to blue when the sun is approaching the observer. On the contrary, spectral lines shift to red when the sun is receding.
For observers far away from the solar system, it is difficult to detect the faint light from the earth because of strong radiation from the sun. However, observers could discover the existence of planets by measuring the sun's periodic motion. Now, assuming our solar system consists of only the sun and the earth, we would say that the earth orbits the sun. However, this statement is not correct in the strictest sense. Actually, both the sun and the earth orbit around the center of gravity of the two bodies. However, the center of gravity is close to the center of the sun because the sun is so huge compared to the earth. Since the sun is also orbiting around this center of gravity, its motion can be detected as periodic oscillation in the observed solar spectrum. When the sun approaches the observer, spectral lines of the sun shift to blue, and when the sun recedes, the lines shift to red. This is known as the Doppler shift. Therefore, by observing the periodic variation in the Doppler velocity of the spectral lines we can find the existence of planets orbiting bright stars. Very small Doppler velocity variation can be observed with HDS enhanced with optional tools.
We have observed many candidate stars that might have orbiting planets and searched for the periodic spectral variations caused by orbiting planets. On the basis of data on periodic variation, we can estimate the mass of a planet and the separation between a star and a planet.
The extra-solar planet search is currently one of the most exciting projects. Over 100 extra-solar planets have been found so far, almost all of which are gigantic planets like Jupiter. We expect to detect earth-like planets in the near future by improving the accuracy of the Doppler velocity measurements
Principle of the grating spectroscopy
“High dispersion spectrograph,” or HDS, is an instrument used to obtain high wavelength resolution spectra using a reflective Echelle grating, on which evenly spaced grooves disperse light depending on the wavelength.
Take a look at the diagram. Incident parallel light falling onto two adjacent grooves A and B on the surface of the grating results in an optical path difference of Δ after diffraction. If the path difference (Δ) is an integral multiple of the wavelength, the diffracted light waves are mutually strengthened in such directions that the equation Δ=mλ (m: integer, λ: wavelength) is satisfied, which is called m-th order diffraction.
When the diffraction angle is specified multiple orders (m=0, 1st, 2nd, ...), diffracted light can be detected simultaneously. If the diffraction angle deviates a little, the wavelength of the diffracted light changes a little, so the incident light can be dispersed depending on the wavelength. You have probably learned that light has a dual nature, particle and/or wave. Diffraction is the typical nature of the wave.
The above figure shows that there is an optical path difference of Δ between light beams diffracted at two adjacent grooves A and B. If the optical path difference (Δ) is an integral multiple of the wavelength (m times), the light is strengthened. When m=1 (i.e. λ0=Δ), the diffraction is called 1st order diffraction. Higher order (2nd , 3rd, ..., mth order diffraction) corresponds to the wavelengths of λ0/2, λ0/3, .....λ0/m. Light beams with wavelengths of λ0+Δλ, (λ0+Δλ)/2, (λ0+Δλ)/3, etc. are strengthened in the slightly deviated direction.
The merits of higher order diffraction
HDS has a high wavelength resolution capable of measuring light intensity at every 1/100,000 wavelength interval. Since the wavelength difference (Δλ/m) for a given diffraction angle shift (Δ) is smaller for higher order diffraction, high-dispersion is easily realized for high order diffraction.
However, for the high order diffraction spectrum, many orders (...(m-2)th , (m-1)th, mth ,(m+1)th, (m+2)th...) are superposed, so we have to separate individual orders. The cross-disperser grating separates this superposed spectrum into individual orders in a direction perpendicular to the direction dispersed with the Echelle grating. Since the directions of the dispersion caused by Echelle and cross-disperser are perpendicular from each other, final spectrum (Echelle spectrogram) extends two dimensionally, which can be obtained by the 4,000x4,000 pixel CCD put on the focal plane of camera optics.
HDS optical layout
Left: Sample of Echelle spectrogram extended two dimensionally
Right : A portion of raw HDS spectrum of comet LINEAR C/1999 S4). The longest wavelength is at the upper-right corner, and that of the shortest wavelength is at the lower-left corner. Bright spots in this image correspond to NH2 emission lines.
How did you become involved in the development of HDS?
I started studying infrared astronomy at graduate school. This field was at a primitive stage at that time, and we were obliged to make our own instruments. There weren't many astronomers in Japan in those days that had experience in developing spectrometers, so my experience was valuable for the HDS project. I participated in the HDS project in 1995 because I had experience in making infrared spectrometers, though only small ones.
The name of Nikon was, of course, familiar to me. Nikon's technology was fairly well known to Japanese astronomers because instruments developed by Nikon had contributed to Japanese astronomy even before Subaru's construction. However, I only really came to understand Nikon's abilities after we began collaborating. I think our demands exceeded our budget from Nikon's view-point and I was afraid we would get Nikon employees into trouble (laughs). Looking back on our collaboration, we owe the success of the HDS project to the excellence of the Nikon staff assigned to the project. I would like to express my gratitude to Nikon's staff. Actually, I'm quite sure that this interview is due to the success of the HDS project achieved because of the remarkable efforts made by Nikon's staff. There are still many unknowns in astronomy but I hope our ongoing collaboration with Nikon will contribute to future progress in astronomy.
Some images are courtesy of the National Astronomical Observatory of Japan
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after diffraction. If the path difference (Δ
) is an integral multiple of the wavelength (m times), the light is strengthened. When m=1 (i.e. λ0=Δ
) is smaller for higher order diffraction, high-dispersion is easily realized for high order diffraction.
