Subaru Telescope
Hilo, Hawaii 96720, U.S.A.

AUGUST 29, 1998

SUBARU Telescope 8.3m Primary Mirror Finished

The largest monolithic mirror with the highest surface quality


The National Astronomical Observatory of Japan (NAOJ) has announced the completion of one of the world's largest monolithic optical-infrared mirrors, with a physical diameter of 8.3 meters (27 feet). The mirror is destined for the NAOJ Subaru Telescope under construction atop Mauna Kea, Hawaii.

A team of opticians at Contraves Brashear Systems (CBS) led by Dr. S. Smith completed the painstaking polishing of the huge primary mirror at the end of July 1998 at the Wampum underground facility of CBS in the suburbs of Pittsburgh, Pennsylvania. The engineering team of NAOJ, MELCO (Mitsubishi Electric Corporation), and CBS carried out intensive measurements to verify the surface figure of the primary mirror while it was mounted on its computer controlled support system manufactured by MELCO. They confirmed that the mean residual rms surface error can be reduced to as small as 12 nanometers* over an entire surface of 8.2 meters in diameter, making this the most accurate mirror of such size ever produced.
(* 1 nanometer or "nm" = ~0.00000004 inches)

To help understand this achievement, imagine enlarging the 8.2m surface to 82 km (51 miles), which is about the size of the Big Island of Hawaii. The mean residual error of 12 nm would then correspond to the task of smoothing out the landscape to follow a precisely specified curve with no hills or valleys (on average) taller or deeper than 0.12 mm (0.005"), the thickness of an ordinary sheet of paper! This tremendous achievement is the outcome of combining large mirror fabrication expertise found in the United States with Japanese expertise in making sophisticated control systems.

The Subaru Telescope project has now successfully completed its most difficult and important phase. The primary mirror will leave Wampum in September and arrive at the summit of Mauna Kea by the end of October. At the summit, the mirror will receive a thin coating of pure aluminum to make it highly reflective and finally be attached to the telescope's mechanical structure. Tests to evaluate the telescope's performance will follow. We are planning to obtain our first scientifically important images (i.e. "First Light") at the end of January next year.

Facts and Data:

1. Subaru Primary Mirror Size and Specifications

Constructed from Corning Ultra Low Expansion Glass (ULE) ...
Thermal Expansion Coefficient ... less than 10 parts per billion per deg Celsius.

Physical Size ...
Diameter: 8.3m (27'), with a central hole 1.2 m (4') in diameter
Thickness: 20cm (8")
Weight: 22.8 metric tons
Effective Mirror Surface: 8.2m diameter
Focal Length: 15 m (49')

Surface error (root mean square) ...
12 nm after 32 mode active correction of residual deformation

Strehl Ratio* ...
0.916 at 500 nm wavelength
* Strehl Ratio is a measure describing the deterioration due to the surface error. Ideal value is 1.0. The achieved value of 0.916 is outstanding for visual wavelengths.

The 80 % encircled energy** ...
0.04 arcsec at 500 nm wavelength
** This means that 80 % of visual photons collected by this primary mirror would fall onto a tiny spot that corresponds to 0.04 arcsec on the sky (about 0.00002 times the apparent diameter of the full Moon).

2. Comparison With Other Large Mirrors

European Southern Observatory (ESO) successfully started the test operation of their first 8.1m telescope, VLT1 in Chile at the end of May this year and has shown it can produce very sharp images. There will be three more 8m telescopes, VLT2, 3, and 4 to be completed by ESO. In addition, the GEMINI consortium is constructing twin 8m telescopes, one at Mauna Kea and the other in Chile. Some of the primary mirrors of these 8m telescopes are already completed. The Subaru primary mirror, however, is the largest and appears to have the smallest surface error among these 8m mirrors.

Subaru Telescope, with this marvelous primary mirror will have an imaging capability that rivals that of the Hubble Space Telescope (HST). The light gathering power of the Subaru Telescope is more than 10 times larger than that of the HST. Subaru Telescope, together with other 8m class telescopes and the two 10 m Keck telescopes with segmented mirrors, will contribute much to enhance our understanding on the structure and evolution of the Universe.

3. Outline of the Subaru Project

The 9 year project to construct the 8.2m Subaru Telescope began in the fiscal year 1991. The total budget of the project amounts to about 38 billion yen. It has the world's largest monolithic thin primary mirror, supported by 261 computer controlled actuators to achieve the ideal surface shape to collect light from distant faint objects. The enclosure of Subaru Telescope has a capability to suppress thermal turbulence around the telescope. Seven scientific instruments are under construction to allow various modes of observation.

Subaru Telescope, together with VLT, GEMINI, Keck, and HST, is expected to open up a new era for the exploration of the universe.

NAOJ established the Subaru Telescope Hawaii Facility in Hilo, Hawaii in April 1997 to support the construction and operation of this telescope. This is Japan's first governmental research institute built and operated in a foreign country. A total of 24 staff members moved from Japan and about two dozen local members are expected to be hired and work around the clock at Subaru Telescope. The director of Subaru Telescope is professor Norio Kaifu.

4. The First Light Event

The "First Light" of Subaru Telescope is expected to take place at the end of January 1999. Details of the schedule will be announced in October 1998. NAOJ is planning to arrange another press release and have live broadcasting of the First Light scene and initial images taken with Subaru Telescope via Japanese TV, NHK, and some other media.

Images supporting this release are available

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