The telescope as seen from below. (National Astronomical Observatory of Japan)
The giant mirror. (National Astronomical Observatory of Japan)
Subaru has been able to make these discoveries by making use of cutting-edge technology. What does this technology involve?
The larger the aperture (width) of a telescope, the better it can see dim objects. But when a mirror becomes bigger, it also becomes heavier. It is easy for a heavy mirror to become warped by just a slight change in temperature. It also becomes more difficult to make the surface of the mirror precise by polishing it, and the base that supports the mirror must be very large.
Subaru uses a special kind of glass that can withstand changes in temperature. This glass is 20 centimeters (about 8 inches) thick, much thinner than anything that has been made for previous telescopes. The primary mirror was made by a U.S. company and took more than three and a half years to complete. Even a very small irregularity would prevent the light captured from a star from properly focusing, giving a distorted image. In order to prevent this from happening, a laser was used to check the surface of the mirror many times in the final stage of production. Finally, engineers used the very finest grade of a special kind of powder to polish the surface of the mirror by hand. The stress involved in this work was so great that some staff members of Japan's National Astronomical Observatory, which watched over the work of polishing the mirror, were hospitalized after developing stomach ulcers.
But Subaru depends on more than just its mirror. Japanese companies like Sony, Fujitsu, Hamamatsu Photonics, and famous makers of optical equipment Canon and Nikon played a role in the development of the instruments that record and analyze the data that is gathered with the mirror.
The actuators that maintain the mirror's integrity. (National Astronomical Observatory of Japan)
One of the revolutionary technologies that was used in making Subaru is the use of devices known as actuators to support the primary mirror. Mitsubishi Electric developed a structure in which the giant mirror is supported by 261 actuators from beneath. One of the most difficult problems that arose in the construction of Subaru was how to control a mirror with a diameter of 8.2 meters. Due to concerns about the weight and precision of mirrors, it had been thought until recently that the mirror at the Palomar Observatory, which has a diameter of 5 meters (about 16 feet), was the largest that could be built. Mirrors built in the past required a thickness in proportion to their diameter; the wider the mirror, the thicker it had to be. An 8.2-meter mirror would weigh well over 100 tons if it were built this way. In fact, Subaru's primary mirror is 20 centimeters thick and weighs only 23 tons. But because of its remarkable thinness, this mirror would change shape slightly under its own weight when it was moved, so engineers came up with the idea of a system of 261 movable actuators that would constantly analyze the weight they support and make adjustments to correct for any warp that develops. By doing this, the mirror's surface error was held below 0.1 micron (0.00001 millimeters). Making the world's largest mirror usable required the application of new technology and the hard work of a team of engineers.
Telescopes from all over the world on the snow-covered peak of Mauna Kea.
(National Astronomical Observatory of Japan)
The location that was chosen for Subaru was the top of Mt. Mauna Kea in Hawaii, an ideal place for observing the cosmos. Mauna Kea is 4,205 meters (about 2.6 miles) above sea-level, and the air is clean and dry. At night the area is very dark, making it easy to observe the stars. The weather is clear more than 60% of the time, so there are about 260 days a year that can be used for observations.
At the top of the mountain, however, the wind speed averages 7.5 meters per second, which can affect the temperature in the dome that houses the telescope and distort the face of the mirror. Images of the stars are sometimes even blurred by the shaking caused by the wind.
Many different ideas were thought up to counter this problem. One was to protect the dome itself from the wind by making it in the shape of an ellipse. Another was to install a wind-breaking screen at the opening through which light enters the telescope. Subaru truly brings together all kinds of cutting-edge technology.