National Astronomical Observatory of Japan

Dusting the Subaru Telescope Primary Mirror by Sprinkling it with Dry Ice


Maunakea, where the Subaru Telescope is built, has clear dry weather, making it favorable for astronomical observations. But because it’s dry, the wind can kick up clouds of fine dust (Maunakea volcanic cinder), causing concern that the dust might settle on the primary mirror during observations. Dusting is an important task for maintaining the observing capability of the telescope. But repeated cleaning could cause problems by scuffing the surface of the mirror. As the result of many experiments, we finally decided on a method to remove the dust by sprinkling the mirror with “snow” generated when solid and gaseous carbonic acid are produced from liquid carbonic acid.

Combining a Variety of Technologies: Cleaning the Subaru Telescope

The Subaru Telescope’s primary mirror, with an outer diameter of 8.3 meters, has a surface area equivalent to approximately 33 tatami mats (a ubiquitous type of traditional Japanese flooring, each mat is about 91 cm by 182 cm). Observations use the central 8.2 meters of the mirror. At lower elevation, telescopes would have suffered from other materials contained in the emissions from the factories and cars in cities. Seasonal plant pollen or sap from trees can also dirty the mirror. The extremely fine volcanic cinder on Maunakea contains glass particles. If a mist comes while dust is stuck to the mirror, it will be a big problem: the dust will damage the aluminum coating of the mirror. The telescope operators carefully monitor the weather so that things like this don’t happen.

Thinking that in this dry climate the dry ice used to clean semiconductor substrates would make a good cleaning method, NAOJ conducted the entire development of this system from the fundamental experiments to implementation. This method makes use of a special property of carbon dioxide known as sublimation. Evaporated carbon dioxide gushes out from high-pressure liquefied carbonic acid cylinders. When the small portion of this which has been flash-frozen into solid phase lands on the mirror and turns back into a gas; the resulting rapid expansion gently blows away the dust. Cleaning the large mirror area is simple thanks to integrating this cleaning mechanism into the telescope.

Text by: Saeko Hayashi (NAOJ, Subaru Telescope)
Translation by: Ramsey Lundock (NAOJ)