Making Tactile Models with a 3D Printer
The Subaru Telescope: Structure of the Telescope

Please note the following features of the Subaru Telescope while touching the 3D models. See the Appendix for more information about terms used in this page.

  1. No Telescope Tube
  2. Short Distance Between the Primary Mirror and the Top Ring
  3. One of the Largest Monolithic Mirrors in the World
  4. Hyper Suprime-Cam
  5. Exchanging Instruments
  6. The Motion of the Telescope
  7. Appendix

1. No Telescope Tube

The 3D model of the Subaru Telescope. A commercial telescope has a tube to eliminate stray light, mainly caused by unwanted reflections inside of the telescope. The Subaru Telescope, like other large telescopes, eliminates the tube to reduce weight. Instead of a tube, the optical systems of the Subaru Telescope and its observational instruments are designed to prevent stray light. For example, the small tube just above the primary mirror (indicated with a pink circle in Fig. 1) eliminates stray light.
Instead of a tube, the Subaru Telescope has a truss structure composed of triangles connecting the top ring and the "mirror cell" around the primary mirror.
Fig. 1 The 3D model of the Subaru Telescope (detailed version). The small tube indicated by a pink circle blocks stray light.

The Subaru Telescope has a strong structure to allow a heavy camera (see "4. Hyper Suprime-Cam") to be mounted at the top of the telescope. Compared to other 8-10 meter class telescopes, the Subaru Telescope has a thicker and more rigid truss structure.

2. Short Distance Between the Primary Mirror and the Top Ring (Short Focal Length)

The distance between the primary mirror, which initially reflects the light from a celestial body, and the top ring is about 15 meters (49 feet). At the center of the top ring exists the prime focus where the light reflected from the primary mirror forms an image. In other words, the "prime focus," the focal length of the primary mirror, is 15 meters (49 feet). This is proportionally shorter than previously constructed reflectors. For example, the focal length of the primary mirror for the 188-cm telescope at Okayama Astrophysical Observatory is 9.15 meters (30 feet). The ratio of the focal length to the aperture (diameter) of the primary mirror of the Subaru Telescope (1.8 = 15 meters / 8.2 meters) is less than half of that of the 188-cm telescope (4.87 = 9.15 meters / 1.88 meters). This small ratio is thanks to advances in technology that enabled the fabrication of a primary mirror with a shorter focal length (a shorter radius of curvature).

3. One of the Largest Monolithic Mirrors in the World

The diameter of the Subaru Telescope's primary mirror is 8.2 meters (27 feet), making it one of the largest monolithic mirrors in the world. The larger the primary mirror is, the more light gathering power and the better spatial resolution the telescope has. In other words, the Subaru Telescope excels at observing small and faint objects.

This primary mirror is made of ultra-low thermal expansion glass (ULE® glass) with nearly zero expansion in response to temperature changes. It took seven (7) years to complete and it is one of the smoothest mirrors in the world. If we made a model of the primary mirror the size of the Big Island of Hawai‘i or the Kanto Plain in Japan (about 100 kilometers/62 miles), the average bump on the model would only have the thickness of an ordinary sheet of paper. The surface of the mirror is aluminized and is cleaned with dry ice snow (CO2) every two weeks to maintain a high reflectivity. The mirror is also re-aluminized every two (2) or three (3) years. The CO2 cleaning and re-aluminization are done inside of the telescope enclosure on Maunakea.

The primary mirror of each of the 3D models is made from a vinyl-chloride transparency half-sphere. It has a different, smooth texture from other parts, making it easy to identify its location.

4. Hyper Suprime-Cam (HSC)

The Subaru Telescope has an epoch-making giant digital camera with an extremely wide field of view compared to other 8-10 meter telescopes. In general, the larger the telescope, the smaller the portion of the sky it can observe at once is. In spite of its large primary mirror, the Subaru Telescope's prime focus camera mounted at the top of the telescope structure can capture a wide area of the sky. The first-generation prime focus camera (Suprime-Cam) started working in 1999, just after the Subaru Telescope’s first-light and produced many good results in discovering candidates for "the most distant galaxy" and in other research fields. The new prime focus camera called Hyper Suprime-Cam (HSC) started observations in 2013 and Suprime-Cam retired in 2017.

HSC mounted at the prime focusHSC is a gigantic camera. It is as tall as an adult; the length of the lens barrel is 154 cm (5.4 feet), and it weighs 3 tons. It is a world-leading digital camera with an extremely wide field of view which is equivalent to the area of nine (9) full moons. The specially developed 116 CCDs have 870 million pixels. This powerful camera enables more efficient observations.
Fig. 2 HSC mounted at the prime focus of the Subaru Telescope. The blue top ring of the telescope can be seen around HSC.

5. Exchanging Instruments

The Subaru Telescope has a variety of instruments, and they can be exchanged depending on the purpose of the observations. At the prime focus in the center of the top ring, we can mount either a secondary mirror or a camera. Likewise, multiple instruments can be mounted at the easy-to-access Cassegrain focus just below the primary mirror cell. At the summit area of Maunkea where the atmospheric oxygen is only 60% of that available at sea level and humans tire easily, a robot called the Cassegrain Instrument Auto eXchanger (CIAX) brings an instrument to the Cassegrain focus safely and efficiently. See the related links below about the instruments mounted at each focus and exchanging instruments with CIAX.

Like the real Subaru Telescope both the detailed and the simplified models have a secondary mirror ("teleMove_E") which can be switched out to mount HSC ("teleMove_E_HSC"). (In Fig. 1 HSC is mounted.) Although there are multiple instruments for the Cassegrain focus, only one instrument ("teleMove_D") is provided in the model.

6. The Motion of the Telescope

すばる望遠鏡の模型。望遠鏡は上下左右に動く。 The Subaru Telescope tracks a celestial body by combining azimuthal (horizontal) and elevational (vertical) motions. In spite of its rigid structure and heavy weight of more than 500 tons, the telescope moves very smoothly, because it floats on the toric "rail" with pressurized oil pads to minimize friction, and is driven by a magnetically impelled linear motor. While observing, it tracks a bright star close to the observational target to adjust the tracking accuracy every 0.1 seconds. Thanks to the smooth motion and highly accurate tracking, the Subaru Telescope can obtain a sharp (high-resolution) image.
Fig. 3 The motion of the Subaru Telescope. The telescope moves from side to side and up and down. The six (6) oil pads are marked with pink circles.

Like the real Subaru Telescope, the 3D models can move from side to side and up and down. (They move only about 90 degrees vertically, the same as the real telescope.) The telescope floats on the toric "rail" (the grey part in Fig. 3) with the six (6) oil pads (marked with pink circles). The grey part of the 3D model looks and feels like a plate; however, it is actually a cylindrical concrete pillar to support the telescope, which extends to the ground. It is constructed on a foundation independent from that of the enclosure to avoid transmitting vibrations generated when the dome rotates.

7. Appendix

Refractor and Reflector

An astronomical telescope observing visible (optical) light is called an optical telescope. (The Subaru Telescope is also used for infrared observations, and is called an optical-infrared telescope.) There are two kinds of optical telescopes: one is a refractor (refracting telescope) with a convex lens, and the other is a reflector (reflecting telescope) with a concave mirror. For example, Galileo Galilei's telescope about 400 years ago was a refractor, and the Subaru Telescope is a reflector.
For more details, see the "400 Years of the Astronomical Telescope" poster, which was developed in 2009 (International Year of Astronomy). This poster was distributed during Scicence and Technology Week in Japan 2009 promoted by Ministry of Education, Culture, Sports, Science and Technology (MEXT).

The Subaru Telescope's Four (4) Foci and the Primary, Secondary, and Tertiary Mirrors

The Subaru Telescope has four (4) foci: the prime focus, Cassegrain focus, and two Nasmyth foci.
The schematic image of the Subaru Telescope.
Prime focus: This focus is at the top of the telescope. The light reflected from the primary mirror forms an image at the prime focus. An instrument used at the prime focus is designed to take advantage of the wide field of view.

Cassegrain focus: When the secondary mirror is mounted at the prime focus, the light reflects from it, passes through the hole at the center of the primary mirror, and then forms an image. This focus is located at the bottom of the vertically movable section of the telescope. It is easy to access this focus, and a variety of instruments can be interchanged here.

Nasmyth foci: When a tertiary mirror* is placed in front of the Cassegrain focus, the course of the light changes by 90 degrees and forms an image at one of the Nasmyth foci. Because at these foci the position of an instrument does not change as the telescope tilts, heavy instruments can be installed here.
The two Nasmyth floors of a 3D model to either side of the movable section are easy to touch and identify. The "subaru_teleBase_C" parts are the Nasmyth floors.

* The 3D printer models do not include the tertiary mirrors. The tertiary mirrors are stored on the inner wall of the tube indicated with a pink circle in Fig. 1. One of the mirrors is placed in front of the Cassegrain focus only when using one of the Nasmyth foci.

Fig. 4 The schematic image of the Subaru Telescope. (From the Subaru Telescope official site.)


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