No. 7] Proc. Jpn. Acad., Ser. B 97 (2021) 337

Review Subaru Telescope —History, active/adaptive optics, instruments, and scientific achievements—

† By Masanori IYE*1,

(Contributed by Masanori IYE, M.J.A.; Edited by Katsuhiko SATO, M.J.A.)

Abstract: The Subaru Telescopea) is an 8.2 m optical/infrared telescope constructed during 1991–1999 and has been operational since 2000 on the summit area of Maunakea, Hawaii, by the National Astronomical Observatory of Japan (NAOJ). This paper reviews the history, key engineering issues, and selected scientific achievements of the Subaru Telescope. The active optics for a thin primary mirror was the design backbone of the telescope to deliver a high-imaging performance. Adaptive optics with a laser-facility to generate an artificial guide-star improved the telescope vision to its diffraction limit by cancelling any atmospheric turbulence effect in real time. Various observational instruments, especially the wide-field camera, have enabled unique observational studies. Selected scientific topics include studies on cosmic reionization, weak/strong gravitational lensing, cosmological parameters, primordial black holes, the dynamical/chemical evolution/interactions of galaxies, neutron star mergers, supernovae, exoplanets, proto-planetary disks, and outliers of the solar system. The last described are operational statistics, plans and a note concerning the culture-and-science issues in Hawaii.

Keywords: active optics, adaptive optics, telescope, instruments, cosmology, exoplanets

largest telescope in Asia and the sixth largest in the 1. Prehistory world. Jun Jugaku first identified a star with excess 1.1. Okayama 188 cm telescope. In 1953, UV as an optical counterpart of the X-ray source Yusuke Hagiwara,1 director of the Tokyo Astronom- Sco X-1.1) Sco X-1 was an unknown X-ray source ical Observatory, the University of Tokyo, empha- found at that time by observations using an X-ray sized in a lecture the importance of building a modern collimator instrument invented by Minoru Oda.2, 2) large telescope. He did so in the presence of the Yoshio Fujita3 studied the atmosphere of low- Emperor Showa and obtained a budget for the next temperature stars. He and his school established a year to build a telescope for astrophysical observa- spectroscopic classification system of carbon stars3),4) tions. This budget was the first case approved by the using the Okayama 188 cm telescope. Japanese congress for a newly established budget Among the three domestic sites evaluated, the scheme involving multiyear spending. The 188 cm Okayama Astrophysical Observatory site was con- telescope was manufactured by Grubb-Parsons in the sidered to be the best for installing the 188 cm U.K. and was installed in 1960 at the newly built telescope. A fair percentage of clear nights, ca. 30%, Okayama Astrophysical Observatory. It was the and stable astronomical “seeing” due to the moderate Seto Inland Sea climate were sufficiently good for *1 National Astronomical Observatory of Japan, Mitaka, stellar spectroscopy. However, the rapidly expanding Tokyo, Japan. Mizushima industrial complex increased night sky † Correspondence should be addressed: M. Iye, National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo brightness. Observing faint objects, like galaxies, 181-8588, Japan (e-mail: [email protected]). soon became increasingly difficult. The site was also a) https://subarutelescope.org/en/ not suitable for emerging infrared observations that 1 – Member of the Japan Academy in 1944 79. require a high-altitude site for low precipitable water. 2 Member of the Japan Academy in 1988–2001. 3 Member of the Japan Academy in 1965–2013 and the 1.2. Japan National Large Telescope Work- President of the Japan Academy during 1994–2000. ing Group. After completion of the 5 m Hale doi: 10.2183/pjab.97.019 ©2021 The Japan Academy 338 M. IYE [Vol. 97,

Telescope in 1948 at Palomar Mountain Observatory astronomical accuracy, roughly half of the total cost in California, a new generation of 4 m class telescopes would be for the product components, and the were constructed during the 1970s and the 1980s. remaining half would be for management. Therefore, Apart from the 4 m Mayall Telescope built on Kitt to keep the cost at an affordable level, the first Peak (1973)5) and the 3.9 m Anglo-Australian Tele- essential requirement was to reduce the mass of scope built on Siding Springs (1974),6) many others the primary mirror, around which the telescope were built on remote sites abroad, not limited to sites structure is designed. Toward this goal, three within the host country. These included the Blanco separate approaches were under development in the 4 m Telescope at Cerro Tololo (1974),7) European mid-1980s to design and build a large light-weight Southern Observatory (ESO) 3.6 m Telescope at primary mirror. La Silla (1977),8) 3.0 m NASA IRTF on Maunakea 1.3. Choice of the JNLT primary mirror. (1979),9) 3.6 m Canada-France-Hawaii Telescope on 1.3.1. Segmented mirror. Jerry Nelson of the Maunakea (1979),10) and 4.2 m William Herschel University of California announced a challenging Telescope on La Palma (1987).11) plan to build a low-cost 10 m telescope with 54 During the early 1980s, the Japanese optical segmented mirrors phased together to form a primary astronomy community discussed how to establish a mirror.13) The JNLT Working Group (WG) studied plan to build a new telescope. The initial plan was to this innovative concept14) during its early phase of build a 3.5 m telescope on a domestic site, since this the study. The JNLT WG found this approach to appeared to be a natural step forward after establish- be too challenging to adopt without first having ing the Okayama 188 cm telescope. However, there feasibility demonstrations, and dropped this option were also voices to locate a new telescope on one of from further study. the best sites abroad. Enlarging the imagined tele- The University of California and the California scope size to 7–8 m, while aiming at the world top Institute of Technology, however, made a brave class, was another point of discussion. To make such decision to adopt this plan.15) In 1985, they started a challenging jump, or to follow a secure path, was construction of a 10 m telescope with the financial aid a topic of much debate. Having no experience for of Keck Foundation. The segmented mirrors were the funding ministry to build a national research assembled in April 1992 and after years of effort to facility abroad was another concern. However, the fine-tune the system, an image quality of 0BB.42 (80% astronomical community decided to move ahead for a encircled energy) was attained to our surprise.16),17) larger telescope by the end of 1984. Thus, eventually, two 10 m Keck Telescopes— In August 1984, Keiichi Kodaira, the project Keck I in 1993 and Keck II in 1996—were completed leader, summoned a working group to start a design on Maunakea. study of a targeted 7.5 m telescope. The telescope Many engineering efforts and practical mathe- was tentatively named Japan National Large Tele- matics were behind the success of the segmented scope (JNLT), and was to be built on a great site mirror technology. The development of edge sen- abroad. This author chaired the working group and sors,18) careful mathematical analyses,14) and polish- held 50 meetings with scientists and engineers from ing the aspheric surface by bending mirrors during the academic community and industries. These polishing19) were all indispensable. This technology meetings were held in 1984–89 to define the outline became a legacy for next-generation telescopes: of the JNLT project. Spanish 10.4 m telescope (GTC, 2009),20) Thirty Yoshihide Kozai,4 the director of the Tokyo Meter Telescope (TMT),21) and European Extremely Astronomical Observatory of the University of Large Telescope (E-ELT).22) In fact, the original Tokyo, submitted a budget request; and the first Spanish 8 m telescope proposal was made based on preparatory study was authorized from 1987. The the thin meniscus mirror. However, Jerry Nelson, as budget request for construction12) was finally ap- an international reviewer, was successful to convince proved in 1990. the team to adopt the segmented mirror technology. A known empirical relation indicated that the The few lead years of the Keck Telescopes in cost of building a telescope increases at the rate starting scientific observations over the competing between the quadratic and cubic power of the Very Large Telescope (VLT), Subaru Telescope, diameter of the primary mirror. To the best of and Gemini Telescope, were a great advantage for California astronomers. The three national or inter- 4 Member of the Japan Academy in 1980–2018. national projects adopting thin meniscus mirrors No. 7] Subaru Telescope 339 referred to this situation with some sentiment of concept: to use a thin monolithic mirror supported by regret and respect that 8 m class telescopes were numerous actuators so that the mirror shape can be “Kecked”. actively controlled to maintain the optical surface 1.3.2. Honeycomb mirror. Built in 1948, the 5 m with satisfying an optical specification.30) This author Palomar telescope used another technology for its spent one year at ESO during 1983–84 and had primary mirror. To reduce the glass mass, numerous early discussions on this concept with his group. At clay cores were deployed in the melting furnace to that time, ESO was developing a 1 m test facility produce a honeycomb sandwich glass structure. to show that the mirror surface shape could be Roger Angel further developed this method by spin- controlled with supporting actuators.31),32) For a casting the glass to form a parabolic surface to save monolithic mirror, one can adopt zero-expansion on the polishing time.23) The melted glass was poured glass instead of borosilicate glass to ensure better between the cores and a months-long careful cooling thermal performance with a smaller internal stress. process followed so as to avoid the accumulation of The force control of the reflective surface shape of a internal stress in the glass. The clay cores were monolithic mirror is much more straightforward than removed from the back and a monolithic honeycomb controlling the phase of segmented separate mirrors. glass structure was completed with hollow spaces This approach was eventually adopted, not just for inside to make it lighter in weight. Roger Angel the 8.2 m Subaru Telescope, but also for the 8.1 m further developed this technology in his Mirror VLTs,33),34) and the 8.0 m Gemini Telescopes.35) Laboratory at the University of Arizona to produce light-weight honeycomb structure glass pieces and 2. Active optics to polish these pieces into astronomical primary The basic idea of active optics is simple and mirrors.24) mathematical. Any departure of the mirror surface Borosilicate glass, e.g., Ohara E6, which has a shape, z(r, 3), from the ideal hyperbolic shape low melting temperature and a low viscosity, was required for Ritchey-Chrétien optics can be expanded found to be suitable for casting in this furnace. in terms of Fourier-Zernike functions, which con- However, the thermal expansion coefficient of bor- stitute one of the orthonormal sets of eigenfunctions osilicate glass is on the order of 10!6/°C, about 100- of a circle area, by the following expansion: times more than those of so-called zero-expansion X1 0 0 glasses. The JNLT WG spotted the need to develop zðr; Þ¼ AnRnðrÞ the thermal control system for the primary mirror n¼2 so as to avoid significant thermal deformation under X1 X1 Am cos m Bm sin m Rm r ; varying ambient temperature during the night and to þ fð n þ n Þ n ð Þg maintain a good imaging quality, which appeared to n¼1 m¼1 ½1 be another rather challenging issue.25) m A finite element analysis of honeycomb mirror where Rn ðrÞ is the radial function of the Zernike m m deformation showed that although the thickness of circle polynomials, An and Bn are the expansion the honeycomb mirror is a few-times larger than the coefficients with n ! m being even. thin meniscus mirror that was studied for a The mirror shape error is therefore given by the comparison, the deformation would not be negligible, vector a F haii (i F 1, +, N), where the components m m and that the active optics discussed in the following ai are the coefficients An and Bn truncated to the section would be required anyway. The JNLT WG first N terms. Similarly, the correction force distri- concluded that force control is more manageable than bution is represented by another vector, f F hfji (j F thermal control. 1, +, M), where the components fj are the correction The honeycomb mirror technology was adopted forces of M actuators. in many U.S. university telescopes, for example, the The active optics uses Hooke’s law of deforma- 6.5 m Magellan 1 (2000) and 2 (2002) Telescopes,26) tion through the following equation: the Large Binocular Telescope with twin 8.3 m j a ¼ C f ½2 mirrors,27) the Large Synoptic Survey Telescope,28) i j 36) and the Giant Magellan Telescope with seven 8.4 m where Ci is the structure matrix of the mirror. This 29) j mirrors. structure matrix, Ci , can be obtained by measuring 1.3.3. Thin monolithic mirror. Raymond Wilson the mirror deformation response to the unit force at the ESO was the first promoter of the active optics exerted for each of the actuators. After the structure 340 M. IYE [Vol. 97,

Fig. 1. Structure of the electromechanical actuator developed for Subaru Telescope.38)

Fig. 3. Control loop of the Subaru Telescope active optics system.38)

ration (MELCO). The actuators work actively in the axial support and passively for the lateral mirror support. This system, hence, avoids any S-distor- tion,37) which arises for the mirrors supported laterally at its periphery during tilted orientations. Figure 3 shows the force control loop of JNLT, which is calibrated by optical Shack-Hartmann camera measurements every time the primary mirror under- goes some operation including re-aluminization.38) Owing to its stable configuration, it was proven that there is no need to make nightly calibrations of the Fig. 2. Deployment of 261 actuators to support the primary structure matrix. 38) mirror of the Subaru Telescope. The actuators, inserted from Although the success of controlling the 1 m the back pocket holes, support the mirror at the local center of 31) its gravity plane, actively in the axial direction and passively in test mirror shape in a vertical position was the lateral direction. encouraging news, the JNLT WG realized the need to conduct an independent verifying experiment of active optics with an original facility at tilted matrix is known, the correction force distribution for orientations. In 1988, the National Astronomical a measured deformation of the mirror surface can be Observatory of Japan (NAOJ) reorganized from the derived by Tokyo Astronomical Observatory (see section 3.4) j 1 and MELCO developed an experimental telescope to f ¼½C a: ½3 i verify that the active optics can control the mirror 2.1. Active optics R&Ds. Figure 1 shows a figure, even at tilted orientations of the telescope schematic drawing of the electromechanical actuator (Fig. 4). A thin spherical mirror of 62 cm in diameter developed for the JNLT active optics system. A force was fabricated and supported by nine prototype sensor developed by Shinko Denshi, which measures active optics actuators39) developed by MELCO and the frequency modulation of an elinvar-alloy tuning by three fixed supporting points. NAOJ developed fork under tension, enabled force measurements with a Shack-Hartmann camera40) to measure the mirror a resolution of 0.01 N over a 0–1,500 N range. This shape from the top of the test telescope. force sensor was integrated in the actuator to control Figure 5 shows the result of successful bending the supporting force with a 10!5 resolution at 1 Hz. of the 62 cm spherical mirror to generate the specified 2 Figure 2 illustrates the deployment of 261 amplitude of astigmatism B2 ¼780 nm, while keep- actuators to support the primary mirror at its local ing other terms at zero. The measured results with 2 center of gravity, designed by Noboru Itoh, the this simple system were, B2 ¼779 nm, and the lead engineer of MITSUBISHI ELECTRIC Corpo- amplitude of other terms was less than 49 nm. This No. 7] Subaru Telescope 341

Fig. 6. Time profiles of measurements of the astigmatic defor- 2 2 mation amplitudes A2 and B2, the Strehl intensity Sint, the temperature of the ambient air Tair and the mirror surface Tmir are shown for every 3 min, starting from 17:00 local time for 24 hours. The measured values of the astigmatic deformation scatters during the period when the mirror surface is warmer than the air, Tmir > Tair, indicating that the microturbulence generated from the mirror surface disturbs the measurement, Fig. 4. (Color online) Active optics experiment setup with a which is known as “mirror seeing”.36) 62 cm spherical mirror supported by nine actuators and three fixed points and the Shack-Hartmann camera mounted on the top of the telescope structure. hours. This experiment was planned with an ex- pectation that the nighttime measurements would provide cleaner data due to reduced car traffic near to the campus and the absence of human activity- related noises. As is shown in Fig. 6, contrary to WG’s expectation, the nighttime measurement from around 19:00 until around 10:00 the next morning was more chaotic than that during daytime measure- ments. This phenomenon was interpreted as being due to microturbulence generated from the mirror sur- face. The mirror temperature drops belatedly than the cooling environmental air due to its thermal inertia. Therefore, the cooling mirror remains warmer than the ambient air during the nighttime. The Fig. 5. Measured result of the mirror shape control to generate 2 on/off transition of the “mirror seeing” is very well an astigmatism deformation of B2 F!779 nm. correlated with the temperature inversion between the air and the mirror, as shown in this figure. result ensured that one can correct arbitrary lower- This finding suggests that all observations using order Zernike terms of deformation of the primary existing ground-based telescopes were conducted mirror by implementing a larger number of actuators through the “mirror seeing” turbulence generated for JNLT. from their own mirrors. Based on this finding, a Because the measuring sensitivity and stability design provision was made for JNLT to keep the of the mirror surface shape by the Shack-Hartmann primary mirror at 2 °C below the foreseen nighttime sensor were critical for the active optics system, in temperature during the daytime by air conditioning 1989, automatic nighttime measurements of the the mirror inside the closed mirror cell. mirror surface shape were conducted every three The 62 cm experimental telescope setup was minutes at the NAOJ Mitaka campus to calibrate used to verify the active optics performance at tilted the measuring stability. Figure 6 shows the measured orientations41) and the effect of wind loading.42) 2 2 amplitude profiles of astigmatism, A2 and B2, for 24 Thus, adopting active optics for JNLT was decided. 342 M. IYE [Vol. 97,

2.2. Seven foci. The additional merit of their dedicated secondary mirrors. However, the adopting active optics is its flexibility in configuring existing secondary mirrors are used to deliver a the telescope foci. The primary mirror and the good image quality by adjusting the aspheric hyperbolic Cassegrain secondary mirror form constant of the primary mirror and shifting the Ritchey-Chrétien optics optimized to provide a position of the secondary mirror to remove any wide-field of view of 6 arcmin in diameter for the spherical aberration. Cassegrain focus. Due to the structural design, the The primary focus requires a dedicated field distance from the intersection of the azimuthal corrector lens system that also enables atmospheric axis and the elevation axis of the telescope to the dispersion corrections by shifting a lens unit laterally Nasmyth foci is 1.6 m larger than the corresponding according to the elevation of the telescope.43),44) distance to the Cassegrain focus. Therefore, the Ritchey-Chrétien optics for the Nasmyth focus 3. Project promotion should have a slightly different aspheric constant. 3.1. Site survey. The JNLT WG made a Because infrared observations require a secondary worldwide site survey to identify the best site for mirror as an entrance pupil to shut out the thermal building Subaru Telescope. Maunakea, Hawaii, background, the size of the infrared secondary is Chilean sites on the Andes, and Canary Islands were smaller than the optical secondary mirror. The image studied with the support of Japan Society for the at the Nasmyth foci rotates during exposure, and Promotion of Science (JSPS) for international studies image rotators are equipped to de-rotate the image during 1986–88. The group eventually chose Maun- for the observing instruments. akea, Hawaii, as its most preferable site for con- Table 1 summarizes the optical parameters for struction owing to its superb observing conditions in the seven foci: primary focus (F/1.87), optical and the northern hemisphere. A high percentage of clear infrared Cassegrain foci (F/12.2 and F/12.4), optical nights, dark sky, low water vapor, and superb seeing and infrared Nasmyth foci with dedicated image statistics were appreciated. A relatively shorter rotators (F/12.7 and F/13.9), and optical and distance from Japan and cultural closeness with the infrared Nasmyth foci without an image rotator (F/ presence of Japanese-American community in Hawaii 12.5 and F/13.6). JNLT, therefore, has three were additional merits for the decision. dedicated secondary mirrors: optical Cassegrain 3.2. Japan–U.K. collaboration. Professor secondary (CasOptM2), optical Nasmyth secondary Richard Ellis, the chair of Large Telescope Panel of (NasOptM2), and infrared Cassegrain secondary U.K. Science and Engineering Research Council, and (CasIRM2); for which the Ritchey-Chrétien optics other scientists visited Tokyo Astronomical Observ- is configured, respectively, by changing the aspheric atory in 1987 to discuss a potential Japan–U.K. equal shape of the primary mirror. partnership collaboration framework for building an The infrared Nasmyth foci with and without the 8 m class telescope. Although this offer was appre- image rotator and the optical Nasmyth focus without ciated, Tokyo Astronomical Observatory decided to the image rotator are configured without producing decline the offer for two reasons. First, JNLT will be the first national science project to build a facility abroad and building a national facility abroad is a Table 1. Optical parameters of the seven foci. Nasmyth Opt1 and Nasmyth IR1 are equipped with respective dedicated image high enough legal challenge, since there was no rotator units. Nasmyth Opt2 and Nasmyth IR2 are not equipped previous example. Second, Tokyo Astronomical with image rotators and do not constitute Ritchey-Chrétien Observatory had no past experience to conduct this optics level of huge international collaboration. Instead, Japan and the U.K. agreed to start (1) (2) (3) (4) collaboration on science programs and the develop- Focus b1 b2 Top unit ment of instruments. This author and Richard Ellis ! Prime 1.0082456 NA Corrector Lens submitted proposals to JSPS and Particle Physics ! ! Cassegrain Opt 1.0082456 1.91608 CasOptM2 and Astronomy Research Council (PPARC), respec- ! ! Cassegrain IR 1.0084109 1.91787 CasIRM2 tively, to enable this bi-lateral collaboration. A series ! ! Nasmyth Opt1 1.0083945 1.86543 NasOptM2 of Japan–U.K. NDN meetings were established in ! ! Nasmyth IR1 1.019724 1.91787 CasIRM2 1993, and among others the mosaic charge-coupled ! ! Nasmyth Opt2 1.0061814 1.86543 NasOptM2 device (CCD) camera developed by Maki ! ! Nasmyth IR2 1.017608 1.91787 CasIRM2 Sekiguchi 45),46) was brought to La Palma to conduct No. 7] Subaru Telescope 343 an imaging survey of clusters of galaxies in 1995–97 the focal length of 15 m so as to minimize the design on the 4.2 m William Herschel Telescope.47) change to the enclosure and to the cost. This size 3.3. Agreement with the University of change was decided by considering the announced Hawaii. In 1980, Yasumasa Yamashita, the division size of 8.1 m VLT and the 8.0 m Gemini telescope. director of optical astronomy of Tokyo Astronomical The aspheric constant for the Cassegrain Ritchey- Observatory, made the first contact with John Chrétien system would be b F!1.008350515. Jefferies, the former director of the Institute for The Subaru Telescope base facility was com- Astronomy (IfA) of the University of Hawaii, for a pleted in an astronomical complex of the University potential construction of the Japanese telescope of Hawaii, Hilo campus, in 1996. After five years of facility on Maunakea. After exchanging letters and constructing and installing facilities at the summit having meetings for several years, the Operation and of Maunakea, the engineering first-light observation Site Development Agreement (OSDA) was finally was successfully conducted on December 24, 1998. signed in 1988. This agreement declared that NAOJ 4.1. 8 m-club. In 1993, the Association of will try to secure funds to construct Subaru Tele- Universities for Research in Astronomy (AURA) and scope on Maunakea, while IfA would provide all of National Optical Astronomy Observatory (NOAO) the necessary help to get it constructed and operated. of the United States decided to construct twin 8.0 m In return for access to the summit ridge and a site telescopes, Gemini, with one on Maunakea and the for a Subaru base facility in Hilo with an annual other on Cerro Tololo to cover both in the northern nominal lease payment of U.S. $1 each, it was agreed and southern skies. Their decision to employ the that IfA would have guaranteed access for 52 thin mirror active optics concept was challenged by observing nights a year. some from the user community of honeycomb 3.4. Budget request. Tokyo Astronomical mirror telescopes. The Houck Committee in the Observatory submitted the budget request through U.S. Congress summoned a hearing to confirm the the University of Tokyo for preparatory studies of appropriateness of adopting the thin mirror option. JNLT from 1987. In July 1988, Tokyo Astronomical Upon a call from the Gemini group, a meeting with Observatory became an interuniversity facility, the the VLT and Subaru representatives to discuss the National Astronomical Observatory of Japan merits and demerits of thin mirror projects was (NAOJ). A full budget request was submitted in arranged in the U.K. The meeting was named the 1989. By that time, the JNLT concept was fixed to a “8 m-club”, and three more meetings were held within 7.5 m active optics telescope to be built on Maunakea a year to discuss engineering issues, which are with four foci equipped to allow the installation of common or different among the three projects. various imaging and spectroscopic instruments for The Subaru Telescope was unique to implement optical and infrared observations. the prime focus for wide-field imaging observations. The astronomical community was in full support VLT and Gemini chose a design strategy to build of this project and the Science Council of Japan multiple telescopes at low cost, four and two, (SCJ) issued a strong recommendation to get it respectively, so as to increase the total observation funded. The Ministry of Education, Science and time. The higher construction cost of the Subaru Culture (MESC; reestablished in 2001 as the Telescope was criticized during the budget request Ministry of Education, Culture, Sports, Science and phase. The funding ministry, MEXT, however, Technology (MEXT)) started funding the construc- approved the project. The choice of VLT and Gemini tion of Subaru Telescope during 1991–99 as one of for lightweight telescopes made retrofitting a wide- the eight major Frontier Science Projects approved. field camera at the top of the telescope unfeasible because of their less sturdy structure and no space 4. Construction available to accommodate the protruding camera At the start of construction in 1991, the project inside the just-sized enclosure. Thus, Subaru Tele- team solicited a new name for JNLT and chose scope became a unique 8–10 m class telescope, “Subaru Telescope” after the ancient Japanese word enabling wide-field imaging survey observations, for the Pleiades constellation. which eventually turned out to be successful. The primary mirror was originally targeted for Another concern expressed concerning the a size of 7.5 m in diameter (F/2.0) with a focal length Subaru Telescope architecture was its challenging of 15 m. It was enlarged in the early phase of active support concept of drilling 261 pockets in the construction to 8.2 m in diameter (F/1.83), keeping back of the primary mirror to support the mirror at 344 M. IYE [Vol. 97,

Fig. 7. Side view of the Subaru Telescope structure (NAOJ). its center-of-gravity plane. This design was also of an anxious concern for the Subaru project team, which considering an inherent risk of breakage of the precious primary mirror. The Subaru Telescope group, however, chose this design for obtaining Fig. 8. (Color online) Subaru Telescope installed in the enclo- better performance. Precautions were taken by sure. Black-painted “Great Walls” on both sides of the telescope removing microcracks at the ground pockets through structure introduces laminar wind flow through the telescope so as to reduce the “dome seeing” turbulence (NAOJ). acid etching and designing the support mechanism such that the exerted strain would not exceed 1,000 psi. The team considered that this design with additional care concerning thermal control to reduce atmospheric turbulence in and around the telescope would help to attain a superb image quality. 4.2. Telescope structure. The telescope structure was designed to deliver a good image quality to four foci. Keeping the optical alignment at any orientation in a varying thermal environment was a fundamental requirement. The telescope drive should ensure a high pointing and tracking accuracy for stable long-time exposure. To meet these scientific requirements, the error budget to attain the targeted performance was developed. Hydrostatic bearings and a direct drive were employed to drive the telescope. The finite-element method was widely applied to the full model of the telescope so as to evaluate its static and dynamic performance and, Fig. 9. Hex disk deployment configuration for fusing into an 49) subsequently, improve the actual design of the 8.3 m monolithic mirror blank. telescope structure.48) The final structure of the telescope has a 22.2 m 4.3. Mirror blank. Corning Glass Work height and 27.2 m width with a total moving mass produced 44 disks of 2 m in diameter, made of Ultra of 555 tons. Figure 7 illustrates the outline of the Low Expansion (ULE) glass, for the Subaru primary telescope structure. The telescope structure was once mirror blank. These disks were sliced in a hexagonal assembled and tested concerning its drive system in shape or smaller pieces, and their coefficients of Japan in 1996, and then disassembled for shipment thermal expansion (CTE), a few ppbs/°C, were to Hawaii for the final installation. Figure 8 shows carefully measured. These pieces were placed in a the actual telescope structure installed in the fusing furnace to form a monolithic disk of 8.3 m enclosure. in diameter and 30 cm thickness (Fig. 9).49) As the 31 No. 7] Subaru Telescope 345 hexagonal segments, each having a small CTE, but NAD83 system are at longitude W155°28B34BB and not exactly zero, are of the same shape and latitude N19°49B32BB with an altitude of 4,163 m at interchangeable, one should be able to find the best the elevation axis. To prevent an uneven settlement deployment configuration so that the resulting of volcanic soil of the construction site, the soil monolithic disk has the best thermal deformation cement method was used to improve the soil stiffness. behavior. A simulated annealing method was used to The telescope pier of 14 m height from the ground to find the best deployment configuration among ca. elevate the telescope above the ground layer was 100,000 patterns evaluated.50),51) This optimization built on a round concrete mat slab of 43 m in reduced the thermal deformation amplitude of the diameter constructed 7 m underneath the ground. final mirror to be 91/30 of those for a randomly The enclosure has its own foundation separated from arranged distribution. This study helped to attain a the telescope pier to avoid any vibrations induced better image quality. from the enclosure. 4.4. Mirror polishing. In 1994, the mirror A water tunnel test52) and wind tunnel tests blank was delivered to Contraves, near Pittsburgh, were used to compare the wind flow properties for polishing work. The mirror blank of 8.3 m in around various dome shapes. A cylindrical shape diameter and 30 cm thickness was then sagged down design was chosen so as to avoid the turbulent to a meniscus shape on a mold inside a heating ground-layer from flowing upward along the conven- furnace. It was then ground to a thickness of 20 cm. tional semispherical dome. The MELCO designed A total of 261 pockets of 15 cm depth were drilled and Coast Steel Fabricators Limited (cf. https://en. from behind to house actuators. A careful acid- wikipedia.org/wiki/Dynamic_Structures) fabricated etching process was used to remove any microcracks. the enclosure to withstand an earthquake of 1,000- The final mirror had an 8.2 m diameter useful surface, year turnaround probability and a 70 m/s high wind. which was polished to a hyperboloidal shape with a Taisei Corporation supervised the summit site work central curvature of radius of 30 m (a focal length of and the construction of the enclosure. The telescope 15 m). Years of the careful polishing process ended structure was also designed to meet safety require- with the final surface residual error, measured on ments. Specially designed mechanical fuses were the active support system in August 1998 at the installed at the three fixed points of the primary acceptance test, as small as 14 nm rms after the active mirror to release excessive force at these points if a removal of low-order deformation components strong earthquake was to occur. During construction, (Fig. 10). there was a fire accident and we regret that three 4.5. Facilities. The enclosure of Subaru local workers lost their lives. Telescope is located on the summit ridge of the The height of the telescope elevation axis at 24 m Maunakea. The coordinates of the telescope in the above the ground was chosen by taking the vertical distribution of turbulence due to the ground layer. The height of the enclosure is 43 m and the diameter of the enclosure rotation rail is 40 m. The total moving mass of the enclosure is 2,000 tons. The enclosure rotates in synchronization with the move- ment of the telescope. Various design considerations were made on thermal analysis and wind loading to ensure better image quality.53) Two black-painted “Great Walls” built on both sides of the telescope regulate the laminar wind flow through the telescope, and insulates the telescope thermally from other parts inside the enclosure. The wind screen reduces high wind to the telescope; and several wind hatches help to regulate the wind flow, depending on the direction and speed of the night wind while observing. Figure 11 shows a side view of Fig. 10. (Color online) Surface figure error map of the finished primary mirror. The residual figure error is as small as the enclosure. 14 nm rms. Three fixed points at the peripheral positions show On one side, behind the great wall at the top larger residuals (NAOJ). unit floor, a rotary facility houses three secondary 346 M. IYE [Vol. 97,

Fig. 12. Point spread function of a star image taken by IRCS in Fig. 11. The Subaru Telescope enclosure has a biparting dome the K-band under an excellent night condition shows a FWHM shutter and a wind screen. Several side windows are used to of 0.198 arcsec.56) Note that the observation was made before the fl ensure the regulated wind passing inside the enclosure to ash AO-system became available. internal turbulence and keep the inside temperature following the ambient temperature (NAOJ). system by a query server in Hawaii.55) Subaru observational data are following the general rules of mirror units and two prime focus instruments. A the Flexible Image Transport System (FITS) header specially designed top unit exchanger enables the safe standard. exchange of these top units. Another facility device, 4.7. The first light. The primary mirror a Cassegrain Instrument Automatic eXchanger was finished near Pittsburgh and shipped, via the (CIAX), helps to exchange Cassegrain instruments Mississippi River and the Panama Canal to the from the instrument-housing stations on the observ- Kawaihae port of Hawaii Island. In November 1998, ing floor behind the great wall, where the cryogenic, it was transported to its final site using a special electric, and network support keep the instruments trailer vehicle equipped with computer-controlled for the standby condition ready for observations and jacks to avoid any excessive tilt of the load while maintenance.54) ascending the mountain. The primary mirror was The surface of the primary mirror is cleansed by cleansed and aluminized to prepare for the first-light aCO2 dry-snow cleaning facility installed near the observation in the following month. mirror cover every two weeks to keep the reflectivity The engineering first light was conducted on at above 80%. The primary mirror undergoes a re- December 24, 1998. The first-light scientific observa- aluminizing operation every three years. An 80-ton tions made in January 1999 provided a superb image top crane of the enclosure lowers the mirror on its quality of the Subaru active optics telescope.56) An cell to the basement. A mirror-cleaning facility and inauguration ceremony was held on September 17, an aluminizing chamber firing aluminum vapors on 1999, in the presence of Princess Sayako, many the mirror are used for the maintenance. international delegations, and the Representatives of 4.6. Control and software. The design of the the Native Hawaiian community on the summit of Subaru Telescope control system consists of several Maunakea. segmented groups of computers connected to each The FWHM of the K-band image was confirmed other using UNIX-based Remote Procedure Call to be as sharp as 0.198 arcsec (Fig. 12). The limiting (RPC). The observational data are archived at the magnitude in the R band for a one-hour exposure by summit and transferred to the base facility at Hilo by the Subaru Telescope reached as deep as 28-mag, an optical fiber link. The Subaru Telescope Archival comparable with that achieved by the Hubble Space System (STARS5) in Hilo is mirrored by a system at Telescope (HST) from space for the equivalent band Astronomical Data Center of NAOJ in Mitaka. Any and exposure time.57) The spatial resolution was registered user can download desired observational twice better for HST due to the absence of at- data after the 18-month proprietary period using the mospheric seeing, but the sensitivity was comparable SMOKA (Subaru-Mitaka-Okayama-Kiso Archive) because of the 13-times larger light-collecting power that contributed to enhancing the signal-to-noise 5 https://stars2.naoj.hawaii.edu/stars1min.html ratio. No. 7] Subaru Telescope 347

more difficult than in the K-band, since it requires 28-times more sensing and correcting elements and 5.3-times faster control for the same wind velocity. It is important to note the similarity and difference of the active and adaptive optics.64) The “active optics” controls the wavefront distortions in a telescope introduced by mechanical, thermal, and optical effects in the telescope at frequency band- widths slower than 1 Hz. Alternatively, the “adaptive optics” controls rapidly varying atmospheric wave- Fig. 13. (Color online) A possible manual alignment method to front distortions, called “astronomical seeing”,at reflect the sunlight onto a war ship. A soldier can be trained to much higher band widths of 10–1,000 Hz. “Adaptive watch the sun spot produced through the hole on his cheek or optics” should respond much more rapidly than fl cloth and guide the spot into the hole through the back re ection “active optics”. Hence, the correction device should of the mirror. This figure was reproduced based on the figure in Ref. 58. be much smaller and responsive than the primary mirror of the telescope, although the basic principle is common. This confusing terminology, however, is 5. Adaptive optics widely adopted due to historical reasons. 5.1. Early history. Based on a Greek historian Some developments were made for defense Lucian’s description, Albert Claus suggested that applications under strict information control during Archimedes was the first to use adaptive optics by the 1970s and 1980s. The satellite tracking system deploying Syracuse soldiers to line up and reflect of the U.S. Air Force telescope on Haleakala was sunlight with “burning mirrors” to set Roman battle- equipped with such an adaptive optics system. In ships on fire.58) He conjectured that a handy size 1990, soon after the launch of HST, its primary double-sided mirror with a central hole can be used to mirror was unfortunately found to be mis-figured. An visually adjust its angle so that the specular reflection additional problem that NASA scientists spotted is pointing to the target (Fig. 13). Though it is an was unforeseen jitter vibration of the space telescope. interesting hypothesis, the availability of such The first generation of its solar panel boom was found mirrors at that time is highly dubious. The attack, to shrink/expand thermally every time the HST if any, might have been made in a different way by went in and out of the shadow of the Earth. The Air firing flaming balls.59) Force Maui Optical Site (AMOS) telescope with its The California astronomer Horace Babcock was adaptive optics was used to image the solar panel of actually the first to propose and develop a modern the HST to measure the “glint walking” along the adaptive optics system concept to compensate for bent boom from the specular reflection of the sunlight any atmospheric disturbance of the optical wavefront while the HST is staring at some observing target in real time to restore the diffraction-limited imaging (Fig. 14).65) capability.60) The Russian meteorologist Valerian Independent of the defense R&D, the astronom- Tatarski mathematically expressed the character- ical community also developed adaptive optics istics of the wavefront perturbation generated while systems during the 1980s. ESO announced the first the light beam passes through a turbulent medium.61) demonstration result where they used a 19-element D. L. Fried defined a scale length, called the Fried piezo-driven deformable mirror coupled through a parameter, r0, of a circular area within which the rms fast digital signal processor with wavefront sensing at wavefront aberration due to a turbulent atmosphere 25 subapertures.66) is less than 1 radian.62) The typical nighttime median, The wavefront error due to atmospheric turbu- r0, in meters at a wavelength of 6 (µm) for the zenith lence can also be expressed by Eq. [1]. By applying distance, O, can be approximated63) by the modal correction from the lower-order modes by r 0:114 =0:55 6=5 sec 3=5 m : 4 using adaptive optics, the residual wavefront error ½ 0median ¼ ð Þ ð Þ ð Þ ½ decreases from the uncompensated variance of 2 5=3 On a fair night at Maunakea, the Fried parame- uncomp ¼ 1:03ðD=r0Þ to tip-tilt the corrected 2 5=3 ter for the 2.2 µm K-band is, therefore, about 0.4 m, variance of tip-tilt ¼ 0:134ðD=r0Þ , where the num- while that for the 0.55 µm V-band is about 0.1 m. ber of corrected modes is Nm F 2. For a larger Nm, the Implementing adaptive optics in the V-band is much corrected variance can be approximated by 348 M. IYE [Vol. 97,

Fig. 14. (Color online) The specular reflection of the sun light from HST, which is observing an observational target, was measured using adaptive optics imaging. The thermally warped “ ” HST solar array boom produces walking glints , in contrast to Fig. 16. Shack-Hartmann sensor and curvature sensor. The fi the line glint of the unbent boom. The gure was redrawn based Shack-Hartmann camera measures the displacement vectors of fi on the original gure in Ref. 65. the individual subaperture images of the guide star, which refer to the local tilts of the wavefront. The curvature sensor measures the nonuniform illumination at the pupil plane that is produced by the local curvature of the wavefront. The actual measurement with a curvature sensor can be done by modulating the image plane of the sensor before (①) and after (②) the pupil plane. An additional vibrating mirror accomplishes this image plane shifting without moving the sensor. A provision to evaluate the difference between the two images is necessary.

the curvature distribution, from the illumination variations observed at the pupil plane. Figure 16 Fig. 15. (Color online) The residual wavefront error, <, due to shows the principles of the two methods. turbulent air decreases as the number of corrected modes, Nm, A deformable mirror takes care of any wavefront F increases. Nm 2 corresponds to corrections of the tip and tilt correction. A thin membrane mirror, actuated by only. Nm F 3 corresponds to the tip-tilt plus defocus corrections. Next, corrections of astigmatism, coma, spherical aberration etc. numerous rod-type piezoelectric actuators or by follow as Nm increases. With about 100 modes corrected, the bimorph type piezoelectric actuators, is widely image size sharpens to one-tenth of the size of the original used. Excellent textbooks on adaptive optics are blurred image. available.63),68) 5.2. Subaru adaptive optics system. Some pffiffi 2 0:2944N 3=2 D=r 5=3: 5 early R&D toward adaptive optics, including the ¼ m ð 0Þ ½ development of an image stabilizer for the Okayama Figure 15 shows the image-sharpening achieved 188 cm telescope and an original stacked piezo- by increasing the number Nm of the controlled modes. electric deformable mirror, were conducted at NAOJ François Roddier of the University of Hawaii and Communications Research Laboratory in the introduced a new idea concerning the curvature 1980s. Based on these experiences, the involved sensor to measure the wavefront.67) The Shack- group developed the first-generation adaptive-optics Hartmann sensor measures the distribution of the system at the Cassegrain focus of the Subaru first-order derivative of the wavefront, namely, the Telescope.69),70) The strategy to implement second- local gradient vector, from the image displacements generation adaptive optics at the Nasmyth focus of of a reference star for each subaperture. In contrast, Subaru Telescope was established.71) the curvature sensor measures the distribution of the MEXT’s programs of Grant-in-Aids for Spe- second-order derivative of the wavefront, namely, cially Promoted Research (2002–06) and for Scien- No. 7] Subaru Telescope 349

Fig. 18. Optical layout of the AO188 system. The incident beam from the telescope Nasmyth focus goes through the image rotator and the collimated beam is fed, through the ADC, to the deformable mirror for adaptive correcting of the wavefront error. The aberration-corrected beam is then delivered to the infrared scientific instrument. The optical beam selected by a beam splitter is fed to a high-order wavefront sensor and to a low-order wavefront sensor to derive the signal for driving the deformable mirror.82)

Fig. 17. The configuration showing the Laser Guide Star Adaptive Optics system. The sodium laser beam produces a natural guide star shining at the sodium layer at 90 km height. The real-time digital controller drives the The wavefront sensor measures the optical disturbance at the bimorph mirror, using signals from the wavefront kHz level, using either a natural star or a laser guide star. This sensor, to compensate for any wavefront distortion information is used to drive the deformable mirror so as to at 1 kHz. The aberration-corrected beam is finally compensate for any disturbance in real time (NAOJ). fed to the infrared Nasmyth instrument station. The infrared instruments (HiCIAO, IRCS and others) tific Research (S) (2007–11) enabled the implemen- can then make good use of the diffraction-limited tation of the Laser Guide Star Adaptive Optics image.75) Olivier Guyon made an original addition system with 188 control elements (LGSAO188) for of the extreme adaptive optics technology to enable Subaru Telescope. Figure 17 shows a conceptual higher level exoplanet detection.76) system outline of the LGSAO188. The actual layout The first light of the Subaru AO188 system of the AO188 optics is shown in Fig. 18.72) The image observation was made in 2006. The AO first-light rotator is a three-mirror optical device that de- image of the Trapezium in the Orion nebula revealed rotates the object image rotation induced for the a sharp image size of 0BB.06 in the K-band, while a alt-azimuth telescope, like Subaru Telescope. The fair natural seeing image taken in 1999 was blurred atmospheric dispersion corrector (ADC) compensates 10 times to 0BB.6 (Fig. 19). A 7-million-dollar invest- the object image dispersed due to the chromatic ment on the LGSAO188 improved the vision of the dispersion of the atmosphere. 400-million-dollar Subaru Telescope by an order of a A method to design the optimal configuration magnitude. of the electrodes for a bimorph deformable mirror 5.3. Laser guide star. French astronomers with 36 electrodes for the Cassegrain adaptive optics Renaud Foy and Anton Labeyrie were the first in the system was developed.73) A new deformable mirror astronomical community to mention the possibility with 188 electrodes for the AO188 system of Subaru of using the Rayleigh back-scattering light from a Telescope was fabricated by a French company, laser beam as a light source to probe turbulence in CILAS, based on the design by NAOJ.74) the atmosphere up to 100 km in altitude.77) Laird 350 M. IYE [Vol. 97,

Fig. 19. Comparison of the Orion trapezium images with (left; taken in 2006) and without (right; taken in 1999) the adaptive optics system shows a significant image quality improvement by a factor of 10 (NAOJ).

Fig. 20. Formation of a sodium laser guide star at 90 km in Thompson and Chester Gardner of the University of altitude was confirmed. Note the yellowish spot ahead of the Hawaii then reported their experiments at the diminishing point of Rayleigh scattering from the laser beam. Maunakea site to test the feasibility of using a laser to generate an artificial guide star in the mesospheric sodium layer.78) The sodium layer is a layer of neutral The availability of a laser guide star increased atoms of sodium at 90 km above the sea level with a the chance of making adaptive optics observations. thickness of 5 km, produced by meteor ablation. LGSAO188 increased the sky coverage to about a Sodium atoms excited by a 589 nm laser beam reemit 50% level, but not to reach the 100% level. The laser the sodium D line emission. guide star generated from the telescope moves its Although not known to the astronomical com- position in the sky in a synchronous way to the munity, similar research and experiments had been vibrations of the telescope. Therefore, the jittery conducted confidentially for defense applications. motion of the telescope cannot be measured using the The “detente” movement between the United States laser guide star, itself. To measure the jittery motion and the Soviet Union in the late 1970s and the of the telescope and make a relevant correction for it, fact that U.S. tax was spent doubly for developing monitoring a natural guide star, rather than a laser the same technology in astronomy and defense, guide star, is indispensable. The required brightness enabled disclosing the classified technology, and for a natural guide star for tracking the telescope subsequently papers on the laser-guide-star system vibration is R 5 19, a much less stringent require- were published.79),80) ment for a guide star for full compensation of the Yutaka Hayano developed a prototype sodium atmospheric turbulence, R 5 11. Nevertheless, the laser-guide-star system for Subaru Telescope,81) chance of having a guide star close enough to the which evolved into a useful version in 2009.75),82) target of observation sets this limit on the availability Figure 20 shows the test generation of an artificial of LGSAO188. These limits can be relaxed by sodium laser guide star in 2006. introducing higher power laser. To observe using the adaptive optics system, a A transponder-based airplane detector was guide star bright enough (R 5 11) and close enough developed to power off the laser for the safety of any to the target (separation of less than 0.5 arcmin) is airplane flying at the nighttime across Maunakea. needed. Adaptive-optics observations of exoplanets use a sufficiently bright mother star as a natural 6. Science instruments guide star. Adaptive-optics observations of extra- The scientific instrument plan for the Subaru galactic targets relied on a narrow chance of Telescope was discussed by the Subaru Advisory observing a natural guide star that is both bright Committee in the early 1990s. Limited financial and close enough to the target. Such a chance was on support for research and development studies was average less than 1%. facilitated using the NAOJ fund. The Subaru Tele- No. 7] Subaru Telescope 351

Table 2. Imaging capability of Subaru cameras. Column (4) Table 3. Spectroscopic capability of Subaru spectrographs. refers to the maximum number of filters mountable on the holder Columns (3) and (4) refer to the slit width and the slit length, units respectively

(1) (2) (3) (4) (1) (2) (3) (4) (5) Instrument FoV Pixel scale Filters Instrument Resolution Width Length Pixel HSC 1°.5? 0BB.17 6 FOCAS 250 0BB.4 6B 0BB.1 FOCAS 6B? 0BB.1 14 7500 0BB.4 multi slit IRCS 21BB,54BB 0BB.020, 0BB.052 18 HDS 32000 1BB 10BB,60BB 0BB.138 MOIRCS 4B # 7B 0BB.116 3 160000 0BB.2 IRCS 50 0BB.9 6BB.5–20BB 0BB.020 (grism) 14000 0BB.1 0BB.052 IRCS 5000 0BB.54 3BB.5–9BB.4 0BB.055 fi scope and its instrumentation plan were rst (echelle) 20000 0BB.14 introduced at an international conference organized MOIRCS 463 0BB.5 multi slit 0BB.116 83) in Tokyo in October 1994. A decision was made 3020 0BB.5 by the Subaru Advisory Committee in June 1995 to select seven first-generation instruments (Subaru Prime Focus Camera (Suprime-Cam),84) Faint Ob- ject Camera And Spectrograph (FOCAS),85) High Dispersion Spectrograph (HDS),86) OH airglow Sup- pression spectrograph (OHS),87) InfraRed Camera and Spectrograph (IRCS),88) Coronagraph Imager with Adaptive Optics (CIAO),89) and COoled Mid- Infrared Camera and Spectrograph (COMICS)90)), and two observatory instruments (AO36,70) MIRTOS91)) to be funded for construction among the proposed 14 instruments. The performances and actual usage of the initial set of seven open-use instruments were reported.92),93) Although the lifetime of a good telescope can be over five decades, the lifetime of scientific instru- ments is generally about a decade. This is because Fig. 21. (Color online) Observational parameter space coverage scientific needs are ever evolving, and the availability of currently available open use instruments. The abscissa is the of new technology urges astronomers to build new wavelength, and the ordinate is the spectral resolution (NAOJ). instruments more suited for new scientific motiva- tions. At the time of writing this review in 2021, the CCD cameras45) on the 105 cm Kiso Schmidt Tele- Subaru Telescope was serving five facility instru- scope, the 4.2 m William Herschel Telescope and the ments (Hyper Suprime-Cam (HSC), FOCAS, HDS, 2.5 m Sloan Digital Sky Survey telescope.94) Satoshi IRCS, and Multi-Object Infra Red Camera and Miyazaki established an evaluation system of CCD Spectrograph (MOIRCS)). A few other instruments detectors and developed customized CCDs for are also offered by developer groups for open-use astronomical applications under collaboration with observers. Table 2 summarizes the imaging capabil- Hamamatsu Photonics.95) ity of these facility imaging instruments. Table 3 Miyazaki and his group built the Suprime-Cam summarizes the spectroscopic capability of a facility for integrating 10 devices of 4k # 2k CCDs to cover instrument suite for 2021. a35B # 28B field of view.84) Its unique capability to Figure 21 shows the wavelength-spectral reso- cover a wide-field of view was a legacy not competed lution parameter space covered by the currently by any of the 8–10 m class telescopes. Systematic available open-use instruments offered by the ob- projects that take advantage of the wide-field deep servatory. survey capability of the distant universe were 6.1. Hyper Suprime-Cam (HSC). Maki successful. Some of the scientific achievements are Sekiguchi made a pivotal development of mosaic discussed in the next section. 352 M. IYE [Vol. 97,

Fig. 24. Faint Object Camera And Spectrograph (FOCAS) mounted beneath the Cassegrain focus.85) Fig. 22. (Color online) Cross-section of the HSC.96) The wide- field corrector of HSC is shown in the lower part of the figure.

wavelength region 420–1,070 nm at F/2.23, with the function of atmospheric dispersion compensation. The filter exchanging unit of HSC accommodates six filters to enable imaging observation from a set of 5 broad-band filters and 18 narrow-band filters. A total of 330 dark nights is dedicated for a systematic deep survey program (HSC-SSP; see section 7.2) using HSC to image millions of galaxies. The redshift of these galaxies is well inferred using a photometric redshift technique97) to produce a 3D map of galaxies. HSC is now the most heavily used instrument on the Fig. 23. (Color online) 116 CCDs at the focal plane of HSC.96) Subaru Telescope. 6.2. Faint Object Camera And Spectrograph. FOCAS85) enables the imaging of a 6B field of view They built an even more ambitious camera, with various filters, slit spectroscopy, and spectro- HSC,44) as a key instrument for the World Premier polarimetry for a wavelength region 370–1,000 nm International Research Center Initiative. This pro- with a choice of grisms. These grisms are direct vision ject was jointly led by Kavli Institute for the Physics dispersive elements designed by combining a trans- and Mathematics of the Universe (IPMU) and the missive dispersion grating and a prism98) so that the NAOJ, including indispensable contributions from central wavelength of the dispersive band goes the High Energy Accelerator Research Organization straight in the optical axis. The adoption of grisms (KEK), the Academia Sinica Institute for Astronomy as dispersive elements enables easy switching be- and Astrophysics in Taiwan (ASIAA), and Princeton tween imaging and spectroscopic observations by University. Figure 22 shows the cross-section of HSC replacing filters and grisms (Fig. 24). In addition to with its field corrector lens unit.96) The main optics the long-slit spectroscopy, multi-object spectroscopy was designed and fabricated by Canon. The wide- (MOS) for up to 40 objects in the field is possible. field camera has 116 devices of 4k # 2k CCDs The MOS mask plate can be designed and prefabri- deployed at the prime focus corrected for a 1.5° field cated using the laser facility available in the summit of view (Fig. 23).96) The Wide-Field Corrector enclosure. The main optics and the body were comprises seven lenses with five aspheric surfaces to manufactured by NIKON, and the CCD camera deliver a high-quality image over the field for the and the MOS unit were fabricated by NAOJ. No. 7] Subaru Telescope 353

Fig. 25. (Color online) Several orders of the echelle spectrum of a comet are shown. The wavelength increases from left to right in each order and progressively upward in this image. The continuum spectrum shows the reflected sunlight from a comet with the absorption line. The emission lines from molecules, e.g., 99) NH2 etc., of the comet are superposed on the spectrum.

6.3. High Dispersion Spectrograph. The HDS unit manufactured by NIKON86) is deployed at the optical Nasmyth platform for conducting high- resolution echelle spectroscopy at a wavelength range of 310–1,000 nm. Figure 25 shows several orders of an echellogram image obtained by HDS. The highest spectral resolution of 160,000 is available for a slit Fig. 26. InfraRed Camera and Spectrograph (IRCS).88) width of 0BB.2, using an image slicer.99) HDS has been used for the abundance analysis of various types of stars, especially for metal-poor stars and for precise radial velocity measurements using an iodine cell for wavelength calibration.100) 6.4. InfraRed Camera and Spectrograph. IRCS was designed and fabricated at the Institute for Astronomy, the University of Hawaii, as an infrared Cassegrain instrument (Fig. 26).88),101) IRCS offers infrared imaging and spectroscopic observation for using an ALADDIN II 1,024 # 1,024 InSb array. After completing the AO188 system, IRCS was transferred to the Infrared Nasmyth platform behind the AO188. Therefore, IRCS provides diffraction- Fig. 27. (Color online) Multi-Object InfraRed Camera and 102) limited imaging and spectroscopy at 20 mas/pixel Spectrograph (MOIRCS). and 52 mas/pixel sampling in the near-infrared wavelength 0.9–5.6 µm band with a choice of grism or an echelle disperser. these were replaced with new generation instru- 6.5. Multi-Object InfraRed Camera and ments. Spectrograph. MOIRCS is a near-infrared instru- Eight visiting instruments are under operation ment with the wide-field imaging mode and the for Subaru Telescope. Six of these (CHARIS,103) MOS mode.102) Dual optical trains cover a 4B # 7B FastPDI, IRD,104) MEC, REACH,104) and Subaru field of view for 0.85–2.5 µm using two 2,048 # 2,048 Coronagraphic Extreme Adaptive Optics HgCdTe HAWAII-2 arrays (Fig. 27). (SCExAO)105)) are dedicated instruments using 6.6. Other instruments. So far, ten early Subaru AO188. SWIMS106) is a near-infrared multi- generation instruments of Subaru Telescope have object spectrograph developed for the 6 m telescope been decommissioned. The list includes AO36, of the University of Tokyo Atacama Observatory. CIAO, COMICS, HiCIAO, CISCO/OHS, FMOS, The design and development of these new instru- Kyoto3DII, RAVEN, and Suprime-Cam. Some of ments are carried out by various groups in and outside Japan with close arrangements using the 6 https://subarutelescope.org/Observing/Instruments/index. Subaru Telescope. Information on these instruments html is available on the Subaru Web Page.6 354 M. IYE [Vol. 97,

five broad-band filters (B, V, R, iB, zB-bands) and two fi 7. Major scienti c achievements narrow-band filters (NB816 and NB921108)). The During 2000–20, 2,282 refereed papers were objective was to construct a deep galaxy catalog published around Subaru Telescope, of which 51% containing more than 105 galaxies with photometry were headed with Japanese first authors and 78% and follow-up spectroscopy for the selected subsam- include international coauthors. The distribution of ples. Two narrow-band filters were designed to pick the number of papers by journal is ApJ(41%), up photometrically the Lyman , emitters at redshifts MNRAS(17%), PASJ(16%), AJ(8.0%), A&A(7.8%), of z F 5.7 and z F 6.6 for which their Lyman , ApJS(3.4%), PASP(1.2%), Icarus(0.8%), Na- emission would fall in the narrow-band paths of ture(0.8%), Science(0.6%), and others(3.8%). NB816 and NB921. The results of Lyman , emitter The distribution of the number of papers by the survey observations were reported for z F 5.7109) and instrument is Suprime-Cam(37.6%), HSC(11.6%), for z F 6.6,110) respectively. HDS(11.4%), FOCAS(10.5%), IRCS(9.6%), CIAO/ Kashikawa highlighted the significant decrease HiCIAO(6.1%), MOIRCS(5.4%), COMICS(3.7%), in the luminosity function of the Lyman , emitter CISCO(2.7%), FMOS(2.4%) and OHS(1.0%). Su- population from z F 5.7 to z F 6.6 (Fig. 28).110) They prime-Cam and HSC produced just half of the ascribed it to the end of reionization of the refereed papers. intergalactic medium. The educational impact was also outstanding, The author of this paper and his group made an producing 156 PhDs. The total citation counts on the additional survey at z F 7.0 using NB973 and refereed papers amount to 110,841. The subjects reported finding a single source, IOK-1, at z F involving science cover from the solar system to the 6.964, that remained the most distant galaxy deep universe. Described below are some examples spectroscopically confirmed until 2011.111) Figure 29 of selected topics. compares the distribution of Lyman , emitters at 7.1. Epoch of cosmic reionization. According three epochs: z F 5.7, 6.6, and 7.0. An abrupt drop in to the major current model, the Universe started the number density between redshifts 6.6 and 7.0 as with inflation and a Big Bang 13.8 billion years ago, is shown in Fig. 29 indicates that the transparency of which is followed by a rapid expansion and cooling. the intergalactic medium against Lyman , photons At 0.38 million years after the Big Bang, at a redshift changed during this period. of z 9 1,100, the temperature of the Universe had 7.1.2. Tension on the reionization epoch. Masami dropped to about 3,000 K, cooled enough for protons Ouchi derived a luminosity function, clustering and electrons to combine to form neutral hydrogen measurements, and Lyman , line profiles based on atoms. From this epoch, the photons were decoupled 207 Lyman , emitters at z F 6.6 to conclude that 112) from further cooling matter. Since there were not yet major reionization occurred at zre 6 7. any celestial shining bodies, this cold early Universe An early analysis of the cosmic microwave is referred to as the “dark age”. The current scenario background data obtained by Wilkinson Microwave of structure formation expects gravitational growth Anisotropy Probe (WMAP) reported the derivation of dark-matter fluctuation to exist, leading to the of cosmological parameters, that showed the epoch of 113) formation of primordial galaxies at around z 9 20. cosmic reionization at zre F 10.5 ’ 1.2, somewhat UV radiation from the newly formed massive stars discrepant with the results obtained from Lyman , and quasars would have warmed up the intergalactic emitters census.114) However, a recent update based 115) hydrogen, and “cosmic reionization” would have on Planck 2018 results show zre F 7.64 ’ 0.74, occurred. consistent with the Lyman , census result. The early SDSS spectroscopic observations have shown discrepancy between the two methods is thus from “Gunn-Peterson” tests of rest-frame UV spectra resolved. of high-redshift quasars that the reionization process 7.1.3. The most distant galaxies. The Lyman , was completed by z F 6.107) Observational studies for emitter IOK-1 at z F 6.964111) remained the highest z 6 6 became practical with the advent of Subaru redshift galaxy during 2006.9–2010.12. In 2010, all of Telescope. the top 10 entries of the most distant galaxies 7.1.1. Subaru Deep Field. Nobunari Kashikawa spectroscopically confirmed were monopolized by and his group launched a large program to make a Subaru Telescope discoveries. This was the high time deep galaxy survey over a blank field, named Subaru that Subaru Telescope proved its capability in the Deep Field (SDF), using Suprime-Cam imaging in wide-field survey. After IOK-1 lost the top position, No. 7] Subaru Telescope 355

Fig. 28. Cumulative Lyman , luminosity functions, as observed in the SDF. The squares represent the photometric luminosity function for z F 5.7, and the long-dashed curve shows its fitted Schechter function. Open and filled circles show the observed and the completeness-corrected photometric luminosity func- tions for z F 6.6, and the solid curve shows its fitted Schechter function. The triangles are the spectroscopic luminosity function for z F 6.6.110)

Fig. 29. The distribution of Lyman , emitters, as confirmed in 116) GN-108036 at z F 7.213 and SXDF NB1006-2 the SDF at three epochs, z F 5.7,109) 6.6,110) and 7.0,111) with at z F 7.215117) retrieved the world record position limiting narrow-band magnitudes of 26.0, 26.0 and 24.9, during 2012.1–2012.6 and 2012.6–2013.10, respec- respectively. Although the sensitivity for detection is 1.1- magnitude more shallow for z F 7.0, the clear drop in the tively. , , number of Lyman emitters indicates cosmic reionization Since the Lyman emission from sources beyond proceeding significantly at 6.6 5 z 5 7.0. z F 7.3 would enter the infrared wavelength region where the Si CCD detector loses sensitivity, objects at higher redshifts could only be studied by infrared ground observation113) established the presence of instruments for which the field of view is much dark energy and dark matter. Subaru Telescope narrower than for Suprime-Cam or HSC. The infra- contributed to the supernova cosmology’s Nobel red camera mounted on HST has been used to Prize with FOCAS spectroscopy, including the one identify high-redshift candidates from infrared photo- at z F 1.35, the most distant supernova spectroscopi- metry. Some of those objects were verified spectro- cally confirmed with ground-based telescopes at that scopically to be at high redshifts. time.123) Inoue et al. made the first successful detection Toward elucidating the actual distribution of of the [OIII] emission line of SXDF NB1006-2 using dark matter, the wide-field, high spatial-resolution, the submillimeter interferometer ALMA.118) This multi-band imaging of HSC is most relevant for the approach produced further discoveries of MACS gravitational weak-lensing analysis of the dark 1149-JD1 at z F 9.1096119) and MACS J0416.1-2403 matter distribution at a high redshift universe. at z F 8.3118,120) respectively. At the time of writing The Hyper Suprime-Cam Subaru Strategic this review, GN-z11 is the furthest galaxy spectro- Program (HSC-SSP) was designed to make 300 scopically confirmed at z F 10.957 based on its nights of observation at over 1,400 deg2 field in five emission lines, [CIII]61907 and CIII]61909.121) broad bands (grizy) down to a 5< limiting magnitude 7.2. Dark matter and dark energy. Super- of r 9 26.124) An additional 30 nights were assigned to nova cosmology122) and cosmic microwave back- this program in 2019. The main goals are to study 356 M. IYE [Vol. 97,

Fig. 30. (Color online) (upper) Three-dimensional dark matter mass map derived from weak-lensing analysis. (lower) The corresponding three-dimensional galaxy mass map derived from the photometric luminous red galaxies sample. The enhanced regions with contours from 2< to 6< are shown.126) Note that condensation of dark matter in the universe grows from high redshift to low redshift. Fig. 31. (Color online) Constraints of cosmological parameters in the +m-<8 plane (upper) and in the +m-S8 (, F 0.45) plane (lower) for BCDM models. See Ref. 127 for details. the distribution of galaxies, the evolution of the large- scale structure, studies on dark matter and dark analysis of the power spectrum of the weak-lensing energy, and the local galactic archeology. The first effect, in four tomographic redshift bins (z F 0.3–1.5) data release for the first 1.7 years of observations of 9 million galaxies with shape parameters and for 61.5 nights yielded a catalog containing a total of photometric redshifts derived from the 90 nights of 10 million objects for UltraDeep, Deep, and Wide HSC observation, was carried out.127) A careful fields.125) For galaxies, the five-band photometry of comparison with various mock models of $CDM led individual objects enabled a photometric redshift to an independent derivation of the cosmological estimation by comparing the observed spectral parameters +m and S8 from the local Universe energy distribution with those derived from stellar (z 9 1), which are delicately discrepant from those evolution models of various ages and redshifts. values derived from cosmological microwave back- 7.2.1. Weak-lens mapping of dark matter. The ground (CMB) analysis (z 9 1,000) (Fig. 31). weak-lensing effects of distant galaxy shapes from the Similarly, a measurement of the Hubble con- first-year HSC imaging data (167 deg2) were meas- stant, using a time-delay analysis of gravitationally ured to reconstruct the mass distribution over a wide lensed quasars by HST and ground-based telescopes solid angle.126) Figure 30, obtained by combining the with adaptive optics was performed.128) They found H 73:3þ1:7 !1 !1 photometric redshift information, suggests the red- 0 ¼ 1:8 km s Mpc , in agreement with local shift evolution of dark matter aggregation on a measurements of H0 from type Ia supernovae, but cosmological scale. in 3.1< tension with the value H0 ¼ 67:4 0:5 7.2.2. Tension on cosmological parameters. km s!1 Mpc!1 derived from Planck CMB observa- Despite the great success of the standard $ cold tions. dark matter ($CDM) model to describe the expan- Although various approaches to derive cosmo- sion history and the growth of the large-scale logical parameters have been converging to give structure of the Universe, the exact nature of the much more consistent results during these two dark matter and dark energy remains unknown. The decades, some tensions still appear between the weak gravitational lensing effect provides a measure results obtained from a z 9 1,000 universe and from to compare the observed mass distribution of the a z 9 1 universe. Universe with those predicted from various models to 7.2.3. Primordial black holes. The existence of a identify most plausible cosmological parameters. The significant amount of dark matter is a common No. 7] Subaru Telescope 357

GW170817 was an epoch-making discovery, which was also contributed by Subaru observations.131),132) Figure 33 shows two images of GW170817 taken two days and eight days after the event, showing the decay in luminosity and change in color. A theoretical study showed that optical and near-infrared light curves of the optical counterpart of GW170817, lasting for more than 10 days, are explained by 0.03M of ejecta containing lanthanide elements.133) Their radiative transfer simulations of kilonova, where the radioactive decays of r-process nuclei synthesized in the merger power optical and infrared emissions, are compatible with the observa- tions. This result indicates that neutron-star mergers are the origin of the r-process elements in the Universe. Fig. 32. (Color online) New stringent limits on the mass 7.3.2. Lensed quasar. LGSAO analysis of 28 spectrum of low-mass primordial black holes derived from lensed quasars elucidated their configurations of the observations of microlensing events of stars in M31.129) The lensing galaxies and the lensed images.134) Among fi gure was redrawn by M. Takada. them, SDSS J1310D1810 is a quasar at a source redshift of zs F 1.393, which is strongly lensed by the gravitational field of a lensing galaxy at zl F 0.373 in understanding. However, apart from its non-baryonic front of the quasar, forming four clear images within and non-relativistic nature, the identification of dark a 1.8 arcsec separation. With precise astrometric and matter remains one of the most important quests in photometric data on four images, one can constrain modern physics. Experimental searches for weakly the gravitational-lensing models, as well as the mass interacting massive particles (WIMPs) have not been and shape of the lensing galaxy. Figure 34 shows the successful. One of the alternative candidates, theo- observed image and the constructed lensing model retically proposed, is primordial black holes (PBH) of SDSS J1330D1810. A close analysis revealed a that formed in the early universe. If a large number need to add satellite galaxies in addition to the main of such PBHs exist in the universe, they should be lensing galaxy in order to precisely reproduce the detected through the microlensing effect they would observed results. Monitoring these objects to measure produce when they pass in front of stars. the time delays among the lensed images can be an A unique observation of a 7-hour monitoring independent method to derive the Hubble constant. experiment of a hundred million stars in the 7.3.3. Asymmetric explosion of supernovae. Stars neighboring galaxy M31 with HSC was made to more massive than 10 solar mass end their lives when search for microlensing events due to low-mass the nuclear fuel in the innermost region is burnt out. PBHs.129) They found only one candidate event, Without sufficient pressure to balance the gravity, compared to the expected 1,000 events if PBHs make their core collapses to a neutron star or a black hole. up all dark matter in the Milky Way and M31 halo FOCAS spectroscopy of some core-collapse super- regions. As a result, for the first time, they obtained novae (CC-SNe) show a highly aspherical explosion, stringent upper limits on the abundance of PBHs in and suggest their link to long-duration gamma-ray !11 !6 135) the mass range of [10 ,10 ]M (Fig. 32). bursts (GRBs). Many of the observed oxygen 7.3. Gravitational wave events, quasars, and emission lines show double-peaked profiles, a distinct supernovae. signature of an aspherical explosion (Fig. 35). They 7.3.1. Binary neutron star merger. The detection conclude that all CC-SNe from stripped-envelope of gravitational waves from merging binary black stars are aspherical explosions, and that SNe holes was one of the great scientific discoveries during accompanied by GRB exhibit the highest degree of the 2010s. A real breakthrough was another detection aspheric nature. of a gravitational wave event, GW170817 from 7.4. Galaxy interactions. merging binary neutron stars.130) Identifying an 7.4.1. Gas streak from galaxies. Multi-band optical counterpart of the gravitational wave event imaging observations revealed an unusual complex of 358 M. IYE [Vol. 97,

Fig. 33. Optical counterpart of the gravitational wave event GW170817 observed two days and eight days after the explosion. These pseudo-color images were produced from z- band image taken by the Subaru/HSC and H and Ks band images taken by SIRIUS/IRSF.131)

Fig. 35. Observed [OI]66300, 6363 emission-line profiles (black curves), classified into characteristic profiles: single-peaked (denoted by S, top nine SNe), transition (T, middle four SNe), and double-peaked (D, bottom five SNe). Line profiles of the bipolar model with different viewing directions are shown for a highly aspheric model (red curves with the direction denoted by the red text) and for a relatively less aspheric model (denoted Fig. 34. (left) LGSAO188 image of SDSS J1330D1810 with four in blue curves and text).135) lensed images, A–D, and a lensing galaxy, G. (right) Gravita- tional lensing model of SDSS J1330D1810. The cross shows the position of the lensing galaxy, and the squares show the positions of lensed images of a quasar.134) narrow blue filaments, bright blue knots, and H, emitting filaments extending up to 80 kpc from an EDA galaxy RB199 in the Coma Cluster.136) Also reported is a finding of H, filaments from 14 galaxies in the Coma Cluster.137) Figure 36 shows an impres- , ,– sive collimated H, filament extending 60 kpc from Fig. 36. (left) B, R and H color composite. (right) H R image showing H, emission extending out to 60 kpc from the parent the core of the galaxy GMP2910. The origin of such a galaxy GMP2910 in the Coma Cluster.137) structure is hitherto enigmatic, but is likely to be related to the ram-pressure stripping of gas clouds from a high-speed collision of a galaxy with a hot ages and star-formation history.138) The high spatial intracluster medium. resolution needed to resolve individual stars and 7.4.2. Stellar population of dwarf galaxies. Dwarf the high sensitivity of the Subaru Telescope enable galaxies, found during the last decade or two in the studying the color-magnitude diagram well below the Local Group, are relevant fossil objects to study the main-sequence turn-off point (MSTO). formation and evolutionary history of galaxies. Okamoto et al. found, for instance, Boötes I Detailed studies concerning the stellar popula- ultra-faint dwarf spheroidal galaxy has no intrinsic tion of ultra-faint dwarf galaxies became feasible by color spread in the width of its MSTO, suggesting comparing the observed color-magnitude diagrams that it is composed of a single old population with various stellar population models with different (Fig. 37).138) This finding indicates that the gas in No. 7] Subaru Telescope 359 their progenitor was depleted more efficiently than The Kepler mission,144) finding exoplanets by mon- those of brighter galaxies during the initial star- itoring their transits in front of mother stars, formation phase. The spatial distribution of the increased the number of detected exoplanets by stellar population indicates a tidal interaction with well over 1,000 within 16 months of operation.145) our Milky Way Galaxy. The pulsar timing method146) and the microlensing 7.4.3. M31 outer halo. An extensive imaging method147) are additional methods used to detect survey of the stellar halo population of M31 was exoplanets. With a rapid increase in entries in the made along its minor axis out to 100 kpc (Fig. 38), list, it is now a common understanding that many and two new substructures were found139) in the stars have planets. The NASA Exoplanets Archive northwest halo in addition to the two substructures now registers 4,352 exoplanets. Figure 41 shows the reported in the southeast halo.140) period-mass distribution of detected exoplanets. These substructures are probably disrupted Chushiro Hayashi7 established a legacy scenario remains of dwarf galaxies trapped into M31 in the (Kyoto Model) to explain the physical processes to past. The color magnitude diagrams of the southern form the Solar system.148) The discovery of various Giant Stream show high metallicity peaked at planet systems, including hot Jupiters that are not [Fe/H] 6!0.5 and an age of 98 Gyr. The metallicity expected in the Kyoto Model, highlights the need for of those substructures is not uniform, indicating that more versatile planet formation schemes. the halo population has not had sufficient time to 7.5.1. Imaging exoplanets. Motohide Tamura dynamically homogenize the accreted populations. reviewed the SEEDS project, direct imaging program The faint and smooth metal-poor stellar halo extends to study exoplanets and proto-planetary disks, using up to the survey limit of 100 kpc. To probe the outer CIAO and AO188 of the Subaru Telescope.149) To halo distribution of matter and dark matter, further date, 12 undisputed exoplanets have been imaged observation with higher sensitivity is needed. with the help of adaptive optics systems, including 7.4.4. Milky Way halo and dwarf galaxies. HDS some from the Subaru SEEDS program. spectroscopy of six extremely metal-poor red giants The first discovery of a brown dwarf or a planet in the Sextans dwarf spheroidal galaxy was con- with Subaru/HiCIAO imaging observation was ducted.141) They found that their [Mg/Fe] abundance brought for an object at a projected separation of ratios at the low metallicity end ([Fe/H] 5!2.6) are 29 au from a Sun-like star GJ758.150) The H-band significantly different from those of the main part of luminosity of GJ758B suggests a temperature of the Galactic halo (Fig. 39). These results imply that 549–637 K and a mass of 12–46 MJ, depending on its classical dwarf spheroidal galaxies, like Sextans, do assumed age of 0.7–8.8 Gyr. not resemble the building blocks of the main part of The discovery of a Jovian planet around a Sun- the Galactic halo. Further studies are needed on the like star, GJ504 (Fig. 42), followed. They derived 160þ350 4þ4:5M roles of dwarf galaxies in the early Galaxy formation. an age of 60 Myr, a mass of 1:0 J , and a 510þ30 K 151) Another HDS spectroscopy of a metal-poor star temperature 20 for this planet. There J1124D4535, with 1/20 solar metallicity, discovered remains, however, some dispute about its system age. by LAMOST observation, was performed.142) They The discovery of a spiral structure in circum- found that its abundance ratio of the r-process stellar disks of AB Aur and SAO element, Eu, with respect to Fe is more than one 206462(HD135344B) (Fig. 43), respectively152)–154) order of magnitude larger than that of the Sun, while opened a new challenge concerning the formation the , elements show a striking deficiency. Atomic mechanism of such a structure. A similar spiral abundance ratios (Fig. 40) like this star are found in structure and ring structure with gaps are found in stars of the Ursa Minor dwarf galaxy, which suggests other young stars with HiCIAO, and now with that this star was originally a member star of a ALMA. The formation mechanisms of these spiral dwarf galaxy that was merged in our Milky Way and ring structures are being discussed in terms of the Galaxy. density wave theory. 7.5. Exoplanets. The discovery of a Jupiter- To enable even a higher resolution and contrast, mass planet by measuring the radial-velocity modu- a team led by Olivier Guyon is developing a new lation of 51Peg, caused by the orbital motion of the camera, SCExAO.105) The SCExAO is a high- planet, opened a new look into exoplanet studies.143) contrast high-efficiency coronagraph exploiting Phase Induced Amplitude Apodization (PIAA) 7 Member of the Japan Academy in 1987–2010. Coronagraph. It is placed behind the AO188 and in 360 M. IYE [Vol. 97,

Fig. 39. Abundance ratios [Mg/Fe] as a function of the iron abundance [Fe/H]. Red filled circles with error bars are new measurements for six extremely metal-poor Sextans stars. Measurements for Galactic halo stars are shown by open circles, asterisks, crosses, and small circles, while plus symbols are for disk stars.141) Fig. 37. Color-magnitude diagrams of a Boötes I ultra-faint dwarf spheroidal galaxy. (a) Entire region, (b) central region, (c) model tracks with different ages fitted, and (d) model track for M92 overplotted.138)

fi fi Fig. 38. Locations of surveyed elds (rectangular elds) around Fig. 40. Abundance ratios [Mg/Fe], [Eu/Fe], and [Eu/Mg] as M31 overlaid on a color-coded stellar density map (taken from shown in panels a, b, and c, respectively, of J1124D4535 (red Fig. 50 of Ibata et al. (2007) ApJ 671, 1619). Red, green, and circle) appear consistent with those measured for stars in Ursa ! / blue colors show, respectively, stars with 0.7 < [Fe H] < 0.0, Minor dwarf galaxy (red open star symbols).142) Figure was ! / ! ! / ! 1.7 < [Fe H] < 0.7, and 3.0 < [Fe H] < 1.7. The loca- redrawn by W. Aoki. tions of the Giant Stream and a few other streams are shown.139) front of various new scientific instruments to enable Pic155) brought a finding of substructure. They fitted the imaging of exoplanets close to their mother star. the observed spectra with a model composed of three COMICS 10 µm long-slit spectroscopic observa- emission components from: 0.1 µm glassy amorphous tion of the edge-on proto-planetary disk around O silicate grains, 2.0 µm glassy olivine amorphous No. 7] Subaru Telescope 361

grains and Mg-pure crystalline ollivine (forsterite, Mg2SiO4) grains. The distribution of crystalline grains and micron-size grains were continuous. They, however, found that the 9.7 µm emission from 0.1 µm amorphous silicate grain shows three peaks indicative of the presence of planetesimal ring belts. 7.5.2. Dynamics of exoplanet systems. Hirano et al. reported a joint analysis of Subaru spectrosco- py to measure the Rossiter-McLaughlin (RM) effect and the Kepler photometry during the planet transits of the Kepler Object of interest (KOI)-94 system.156) The RM effect is a spectroscopic shift of the stellar radial velocity observed due to a partial eclipse of the stellar surface by the transit of the planet, which conveys information concerning stellar rotation. The Fig. 41. Distribution of exoplanets in the orbital period vs. mass KOI system shows that the orbital axis is aligned plane, showing their variety and detection biases, depending on with the stellar spin axis. This system has four observational methods. This figure was reproduced from NASA planets and dual transits by two of the four planets Exoplanet Archive (https://exoplanetarchive.ipac.caltech.edu/ that were observed during which the two planets exoplanetplots/). overlapped to produce a planet-planet eclipse. Further monitoring of this system will elucidate the details of this rare planetary system. Narita et al. reported the first detection of the retrograde orbit of a transiting exoplanet, HAT-P- 7b, from their RM effect analysis.157) The further detection of a backward-spinning star with two coplanar planets is also reported.158) These findings add constraints on the formation scenario of plane- tary systems. 7.6. The solar system. Sheppard et al. reported on the finding the most distant planetoid, Fig. 42. Exoplanet GJ504b (upper right). The HiCIAO corona- 2018 AG37, tentatively named as “Farfarout”,ata graph optics masks the bright central star, GJ504, so as to distance 132 au (Fig. 44). Its elongated orbit reaches 151) enhance the dynamic range of vision for fainter objects. as far as 175 au with a pericenter at 27 au. A close gravitational interaction in the past with Neptune is inferred. The size of “Farfarout” is estimated to be at 400 km across from its reflective brightness. “Farfarout” is even more remote than the previous solar system distance record-holder, 2018 VG18,

Fig. 44. Images taken on January 15, 2018 (left) and January 16, 2018 (right) show the moving object 2018 AG37. The figures are edited from those posted at Subaru Telescope website (https:// Fig. 43. Spiral structure in the circumstellar disk around a subarutelescope.org/en/results/2021/02/10/2932.html) (Credit: Herbig Ae star SAO206462 revealed by HiCIAO.154) Scott S. Sheppard). 362 M. IYE [Vol. 97, nicknamed as “Farout’’ by the same group. These Currently, on average, observing nights are objects offer a clue to search for the undetected allocated to Open Use (49%), SSP (17%), the Planet Nine. University of Hawaii (15%), Engineering (10%) and Batygin et al. pointed out that the observed Director’s Discretion Time (10%), respectively. Re- physical clustering of orbits with a semi-major axis mote and service observations are gradually increas- exceeding 9250 au, the detachment of perihelia of ed to ensure a more efficient operation. The select Kuiper belt objects from Neptune and the implementation of unmanned summit operation is dynamical origin of highly inclined/retrograde long- planned on a limited basis. period orbits are elusive of the existence of Planet The observed data are preserved to the propos- Nine.159) Searching for additional objects, like 2018 ers of the respective observing programs as propri- AG37, is feasible only with wide-field cameras, like etary data for 18 months. After this proprietary HSC. period, data are made available to any registered researcher in the world through the data archive 8. 20 years of operation system. For most of the data, a SMOKA system is 8.1. Policy and statistics. After the first light available.55) Additionally, for instance, the photo- in January 1999, it took 1.5 years to tune the metric redshifts for HSC-SSP imaging data are performance of the Subaru Telescope and to com- available from the dedicated site.97) mission some of the observational instruments. Subaru Telescope experienced two major earth- Eventually, the Subaru Telescope was made avail- quakes. The telescope tracking was found to have a able for open use since December 2000. To obtain significantly larger error right after the operation was open-use telescope time, astronomers must submit resumed after a magnitude 6.7 earthquake occurred an observing proposal with a scientific rationale and on October 15, 2006. This was caused by a 1.3 mm feasibility of the proposal. Several international displacement of the telescope position on its pier. A experts are asked to review and evaluate each correction of the coordinates of the telescope resolved proposal. The evaluation reports are assembled to the issue. Another magnitude 6.9 earthquake hit the choose winners by the Subaru Time Allocation telescope on May 4, 2018 causing partial damage to Committee (TAC) under the Science Advisory the enclosure. Committee (SAC). The oversubscription rate is Figure 45 shows the number growth curves of between 3 and 5. TAC took the initial policy for annual refereed papers of 8–10 m telescopes. This two years to allocate observing time up to five nights for each program to encourage potential users to get familiar with the facility. To facilitate observational programs that require a larger number of nights, the SAC prepared the Intensive Program framework (since 2003) and the SSP framework (since 2007) for more systematic observational programs. So far, four SSP programs have been approved: (1) SEED SSP (2009–15), “Strategic Explorations of Exoplanets and Disks with Subaru”,149) made 120 nights of observation of exoplanets and proto- planetary disks using the stellar coronagraph instru- ments, CIAO and HiCIAO, behind the AO188. (2) FastSound (2011–14), “A Near-Infrared Galaxy Red- shift Survey for Cosmology at z 9 1.3 using Subaru/ FMOS”,160) finished 40 nights of observations using FMOS. (3) HSC-SSP (2014–21), “Wide-field Imaging with Hyper Suprime-Cam: Cosmology and Galaxy Evolution”,124) is about to complete 330 nights Fig. 45. (Color online) Comparison of the growth in the number observation. (4) IRD-SSP (2019–25), “Search for of refereed papers per telescope as a function of the year from commissioning. Subaru Telescope (red double line) is competing Planets like Earth around Late-M Dwarfs: Precise with Keck (pink solid line) and VLT (green broken line) and 104) Radial Velocity Survey with IRD”, is under way to exceeds Gemini (light blue line with square symbols) and CFHT conduct 175 nights of observation. (blue broken line). No. 7] Subaru Telescope 363 graph shows the scientific competitiveness of Subaru Telescope. 8.2. Future plans. Observational astronomy has made giant leaps during the last several decades. Many space missions have enlarged the wavelength coverage and the survey volume coverage in obser- vational astronomy. Neutrino and gravitational wave detection became possible and their connection to observational astronomy is referred to as multi- messenger astronomy. Subaru Telescope started as an all-mighty telescope, but concentrating on its unique wide-field survey capability will be the main Fig. 46. (Color online) CG rendering of the TMT to be built on role in the coming decades. Maunakea (TMT International Observatory). Hattori et al. summarized the plan of Subaru instrumentation as of December 2020.161) The past two decades achievements have revealed the advant- 8.3. Maunakea and Hawaii culture. The age of the unique wide-field capability of the Subaru TMT International Observatory, of which the NAOJ Telescope. The plan, therefore, places an emphasis is a founding member, plans to build a next- on using the currently available HSC for wide-field generation telescope with a segmented primary imaging survey programs. mirror of 30 m in diameter. The TMT with its 4- The Prime Focus Spectrograph (PFS), which is times higher spatial resolution and 13-times larger currently under commissioning and will start scien- light-collecting power enables a point-source detec- tific observations in 2023, has been developed based tion sensitivity 180-times more than Subaru Tele- on an international collaboration led by Kavli-IPMU scope. As Subaru Telescope remains the only tele- and NAOJ. The consortium includes universities and scope in the northern hemisphere capable of making institutes of U.S.A., U.K., France, Germany, Taiwan, deep and wide survey observations, it will bring and Brazil. PFS enables the spectroscopy of 2,400 forward important candidates of observations for sources simultaneously for a spectral region 0.38– TMT (Fig. 46). Thus, the combination of Subaru 1.26 µm with its multifiber feed mechanism.162) The Telescope and TMT, fully equipped with adaptive combination of HSC and PFS will be a powerful tool optics, has been expected to show new visions of the for the coming decade of the Subaru Telescope. Universe. Takada et al. have summarized the science goals, The Maunakea site of Subaru Telescope has using the planned PFS survey: the history and fate of a land sublease agreement with the University of the Universe and the physical process and role of dark Hawaii valid until 2033. The University of Hawaii is matter in governing the assembly of galaxies over the preparing an official process to apply for an extension cosmic time including the Milky Way Galaxy.163) of the master lease from the State of Hawaii. The third wide-field instrument under R&D When TMT International Observatory planned study, ULTIMATE, is an imager and a multi-object to have a groundbreaking event at the summit of spectrograph covering a 20 arcmin field of view for Maunakea in October 2014, the access road near the 1.0–2.5 µm with the help of ground layer adaptive summit was blocked by hundreds of protestors. TMT optics.164) was chosen as a visible target of protest movement Strategic observing time exchanges of the against the historical injustice to the native Hawaiian Subaru Telescope with Keck Telescopes and Gemini community. Several lawsuits and illegal blockings of Telescope have been arranged to benefit community the access road suspended the start of the construc- needs on both sides. Future scientific collaborations tion work. Due to the sensitive nature of the issue, with the Large Synoptic Survey Telescope (LSST) of the local authorities have not been able to resolve Vera C. Rubin Observatory, under construction in this deadlock situation. the southern hemisphere, and astronomical space Subaru Telescope and the TMT teams expanded missions to be launched soon, including the 1.2 m much effort to establish a healthy relationship with Euclid mission, the 2.4 m Rowan Space Telescope the local community. The history of ancient Hawaii and the 6.5 m James Web Space Telescope, are under began with Polynesians and Micronesians navigating discussion. canoes to travel across the ocean and reaching the 364 M. IYE [Vol. 97,

Hawaii islands of around AD 400. Nainoa Thompson The Japanese astronomical community made a has shown that navigating canoes between islands giant leap through the Subaru Telescope project to was possible without modern instruments. Careful reach the top level in world optical/infrared obser- observations of stars, winds, and waves enabled them vational sciences. NAOJ gained experiences to to navigate among the islands. He called the method conduct this level of major scientific exploration on wayfinding or star navigation.165) In this sense, we an international platform. The final notes show some understand that the native Hawaiians culture and issues that the future astronomy community needs modern astronomy have common roots. to face. Although this cultural issue is beyond the control of the astronomical community, it is vital Acknowledgments for the astronomy community to find a way to The project design, construction, and operation continue astronomical observations on Maunakea of Subaru Telescope have been possible only with the toward the future. Science and culture are not dedications of thousands of individuals from NAOJ, mutually exclusive and can coexist. This author universities, involving industries, governmental enti- wishes everyone involved in the future of Maunakea ties, and communities, not limited to Japan, for over to find an agreeable best solution, and to sustain the the four decades. Their contributions in engineering, availability of this precious astronomy site for the sciences, administration, and general support, are benefit of all humankind. all indispensable. The author humbly acknowledges their valuable contributions. The financial support 9. Conclusion from the Ministry of Education, Culture, Sports, This paper records the history of Subaru Tele- Science and Technology (MEXT) for the construc- scope from its beginning in the early 1980s until 2020 tion and operation of Subaru Telescope has been the through the author’s perspective, while covering the key to success of Subaru Telescope and is greatly engineering aspects and some of the scientific results appreciated. that could be achieved through the evolving tele- The choices of the scientific achievements scope. included in this review are inevitably biased to the The key engineering of Subaru Telescope was the taste of the author. He sincerely apologizes to those implementation of technology to enable active con- whom he has failed to cite explicitly for their trol of the optical shape of the primary mirror so as to contributions for the project and for scientific provide the best image quality under varying gravita- information. tional and thermal conditions. Another important The author is grateful especially to Prof. development of the adaptive optics system with a Richard Ellis for his careful reading and constructive laser-guide-star facility made a 10 times improvement comments as a referee and to Dr. Masafumi Yagi for of the image quality by cancelling out the atmospher- his numerous helpful comments. Dr. Fred Myers gave ic disturbance to the image in real time. many suggestions to improve language. He thanks Various scientific discoveries and achievements another anonymous referee for useful comments and made in observational cosmology and in studies of thanks Drs. W. Aoki, I. Iwata, K. Nariai, T. Sasaki, exoplanets are highlighted in this review. There M. Takada, W. Tanaka, and M. Yoshida for their are, however, tremendous collections of scientific helpful comments in the early draft of this paper. The papers produced through the observations of Subaru author acknowledges Ms. C. Yoshida for her compi- Telescope, and some of the selected papers are also lation of records on published papers. introduced in this review. We are honored and grateful for the Hawaiian The capability of wide-field survey sciences with community providing us with the opportunity of Suprime-Cam, HSC, and PFS established the unique observing the Universe from Maunakea, having the roles of Subaru Telescope in the era of TMT in the superb cultural, historical, and natural significance. northern hemisphere. This is shown by the fact that We hope Maunakea remains the symbol for the HSC and PFS gathered strong interest from the coexistence of science and culture. international astronomical community. The needs for identifying relevant observational targets in the References fi northern sky from deep and wide- eld surveys are 1) Sandage, A., Osmer, P., Giacconi, R., Gorenstein, other rationales to support the accomplishment of P., Gursky, H., Waters, J. et al. (1966) On the Subaru Telescope for TMT. optical identification of Sco X-1. ApJ 146, 316– No. 7] Subaru Telescope 365

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Profile

Masanori Iye was born in Sapporo in 1949 and graduated from the University of Tokyo in 1972. He received his Ph.D. in 1977. It was a theoretical study concerning the origin of the spiral structure of galaxies. He became an assistant professor in the Department of Astronomy, the University of Tokyo in 1978, an associate professor of the Tokyo Astronomical Observatory in 1986, and a professor of the National Astronomical Observatory in 1993. Iye was also a visiting researcher in the Institute of Astronomy, Cambridge University in 1982–83 and in the European Southern Observatory in 1983– 84. Upon his return, he developed the first cryogenic CCD camera in 1986 and chaired a working group to define the Subaru Telescope project employing the active optics scheme. He devoted himself to constructing the telescope and its science instruments on Maunakea, Hawaii until 2002. He then led the development of a laser-guide-star adaptive optics system and enhanced the telescope vision by ten times in 2011. He found in 2006 the most distant galaxy at 13 billion light years away and elucidated the epoch of the cosmic reionization. From 2005, he promoted the Thirty Meter Telescope (TMT) project and served as the first Vice-Chair of the TMT International Observatory Governing Board. For his accomplishments, he received the Nishina Memorial Prize, the Toray Science and Technology Prize, the Medal with Purple Ribbon, the Japan Academy Prize, and other honors. Iye is a Member of the Japan Academy, a Member of the International Astronomical Union (IAU), and a Fellow of the International Society for Optics and Photonics (SPIE).