Mauna Kea, the mountain upon which the Subaru Telescope stands, is a dormant volcano and its surface is covered with reddish brown cinder and volcanic ash. The winds are often strong at the summit, lifting some of the volcanic ash up into the air. Because the telescope enclosure is open all night long, some of this ash slowly collects on the surface of the primary mirror.

A view of the summit of Mauna Kea

Since the Subaru Telescope aims for high precision observations, the dust on the primary mirror is a great enemy. It would be very time-consuming to remove the 8.3 m Subaru primary mirror from the telescope for frequent cleaning. So, other methods were considered such as cleaning the primary mirror while it is still installing in the telescope:

• washing the dust away with water
• blowing the dust off with compressed air
• burning off the dust with a UV laser beam
• blowing the dust away with carbon dioxide (CO2)

We adopted the method of using CO2 because of its efficiency and proven performance. When liquid CO2 (-56.6C) is released out through a fine nozzle, it suddenly expands without exchanging heat (adiabatic expansion), becoming a mix of gaseous CO2 and dry ice. If we blow this mixture at the primary mirror, the dry ice wraps around the dust and both dust and ice are blown away from the mirror by the gaseous CO2. This method is called "CO2 Cleaning."

The process of CO2 Cleaning

At present, the CO2 Cleaning takes place once a month. The 8th CO2 Cleaning took place on February 4th and went very well. The next CO2 Cleaning is scheduled for early March 2000.

The primary mirror during the CO2 Cleaning

• The above picture with explanation