We at Shinko Denshi have adapted our tuning-fork sensor to serve as part of the active support mechanisms for the primary reflecting mirror of Subaru, the world's largest, most accurate optical telescope. The National Astronomical Observatory of Japan built Subaru on top of Hawaii's Mauna Kea (4,200m above sea level) at an enormous outlay of 40 billion yen in order to observe the edge of the universe, which is estimated to be fifteen billion light years away. The telescope was completed in 2000 and is now operational.
The primary reflecting mirror is made of ultra-low expansion glass and has a diameter of 8.2m and a thickness of 20cm. Because the mirror weighs a massive 23 tons, its surface is subject to gravitational distortion that affects the focus. In order to solve this problem, the telescope needed precision technology to reduce the distortion to a level less than ±2mm within a surface diameter of 100km. Specifically, the 261 active support mechanisms required force sensors with extreme accuracy and long-term stability to detect and respond to distorting forces. In the past, this was not possible to accomplish with load cells in a strain gauge system, but we were able to answer the challenge by adapting and integrating our tuning fork force sensor into the control system, providing a resolution accuracy of 1/150,000.
SUBARU's Major Specifications
|Primary mirror effective diameter||8.2m|
|Material||Ultra-low thermal expansion glass|
|Tracking accuracy||0.1 arcsec-|
|The primary mirror support system||Active support mechanism (261 actuators)|
261 actuators play an important role in supporting Subaru's primary mirror. Each actuator generates a force by its motor and transfers it to the primary mirror through the sensor. At the same time, it feeds data back to itself. The information on gravitational distortion and thermal deformation of the mirror is transferred to a computer, which instantly gives instructions to the actuators to keep the mirror in the ideal shape.