Advanced Technologies

The Smoothest Mirror

The primary mirror is the heart of a telescope. Casting the 8.2 meter mirror blank took three years, and polishing and final processing took another 4 years. The result is one of the world's smoothest single piece mirrors.



A map of the surface error of the mirror shows that the average bump is only 0.012 μm, or about one in five thousandths of the thickness of a human hair. If the primary mirror were the size of the Big Island of Hawaii, the average bump would only have a thickness of an ordinary sheet of paper.

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The mirror traveled through land and water from New York where it was cast, to Pennsylvania where it was polished, and finally to Hawaii.

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Active Optics

261 robotic fingers fit into the holes drilled into the back of the mirror to keep it in a perfect shape no matter where the telescope is pointing in the sky. This technology, called active optics, makes the building of large aperture telescopes possible. A mirror that doesn't have to maintain its shape can be thin, and therefore relatively light weight and maneuverable.

Actuators through the uncoated primary mirror.

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Accurate Tracking

Subaru Telescope has the strength and rigidity necessary to support an instrument at prime focus. The telescope, which weight over 500 metric tons, sits on a thin layer of oil and is moved by a magnetically impelled linear motor so that it can move smoothly and accurately with minimal friction. This design has translated into superior tracking and pointing accuracy.

High tracking accuracy cannot be achieved without a strong foundation. This photograph shows the pier that supports the telescope after its completion.

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The telescope during test assembly in a shipyard in Japan(1995). Now that the telescope is in the enclosure on the summit, it is no longer possible to get a full view of the telescope. Compare the size of the structure to the person in the foreground.

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Cylindrical Enclosure to Reduce the Turbulence

Subaru adopted a cylindrical enclosure design to reduce air turbulence inside the enclosure. Fluid experiments and computer simulations show that wind flows smoothly around cylindrical enclosures and that air in the enclosure can be flushed efficiently without bringing in turbulence from outside.

The shape of Subaru's enclosure is unique among telescopes on the summit of Mauna Kea.

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Fluid flow experiments and computer simulations were essential for determining the design of the telescope enclosure.

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Time Saving Innovations

Instrument exchange is a delicate process in which optical elements must be carefully aligned and electrical wiring and hoses for coolant securely attached. In the physically challenging high altitude environment of the summit, automation helps engineers accomplish the work quickly and accurately.

The Cassegrain Instrument Automatic Exchanger can exchange Cassegrain instruments in two hours.

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Prime focus is located at the center of the telescope's top ring. The Top Unit Exchanger helps exchange prime focus instruments, such as Suprime-Cam and the secondary mirrors.

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Maintenance is essential for keeping the telescope in optimal condition. The telescope incorporates nozzles that spray dry ice snow onto the surface of the mirror to remove dust. Every two years or so, re-aluminization restores the mirror's reflectivity.

The surface of the mirror is cleaned with dry ice snow every two weeks.

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Topics: The Subaru Primary Mirror "CO2 Cleaning" (Feb. 10, 2000)

The lower level of the telescope enclosure houses facilities for re-aluminizing the mirror.

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Topics: Infrared Secondary Has a New Silver Coat (May 16, 2003)
Movie: The aluminizing work in 2003 (24MB)

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