Japan's National Astronomical Observatory (NAOJ) has just completed one of the world's largest telescope mirrors, with a diameter of 8.3 metres (27 feet).

It is one of the smoothest surfaces ever fashioned by man. To understand how smooth imagine enlarging the 8.3 m surface to 82 km (51 miles), which is about the size of the big island of Hawaii.

There would be no hills or valleys (on average) taller or deeper than 0.12 mm (0.005"), the thickness of an ordinary sheet of paper!

Inspecting the finish

The mirror will leave the laboratory of Contraves Brashear Systems of Pittsburgh later this month to journey to the summit of the extinct Mauna Kea volcano in Hawaii.

At the summit, the mirror will receive a thin coating of pure aluminium to make it highly reflective and finally be attached to the telescope's mechanical structure.

Tests to evaluate the telescope's performance will follow. Scientists are planning to obtain the first scientifically important images (First Light) at the end of January next year.

The mirror is supported by 261 computer controlled actuators that can exert pressure on the mirror slightly distorting it to order. This way it can compensate for stresses and strains in the mirror that can affect its performance.

Pistons can flext the mirror

It will also be housed in a dome designed to limit air turbulence that can also affect what astronomers call "seeing".

With a light gathering power of more than 10 times that of the HST the so-called Subaru telescope will join other eight-metre class telescopes destined to rival and surpass the Hubble Space Telescope in some respects.

Earlier this year the European Southern Observatory (ESO) successfully started the test operation of their first eight point one metre telescope, the VLT1 in Chile.

There will be three more eight-metre telescopes, VLT2, 3, and 4 to be completed by ESO. In addition, the GEMINI consortium is constructing twin eight-metre telescopes, one in Hawaii and the other in Chile.