How to Clean a Huge 8.3 m Mirror Surface

July 10, 2014

The primary mirror of the Subaru Telescope, which spans 8.3 m in physical diameter (and 8.2 m in effective aperture for observations) is one of the world's largest and smoothest single-piece mirrors. It's location atop 13,796 foot (4205 m) Mauna Kea, a dormant volcano, means that the mirror is vulnerable to dust, which is composed primarily of reddish-brown cinder (i.e., volcanic ash) that covers the mountain's surface. Strong summit winds can transport the dust to the mirror's surface, high above the ground, when the shutters of the enclosure are open for observations at night. Such extraneous materials can interfere with unobstructed observations from the telescope; a dust-covered mirror means less reflectivity and less light-collecting power. These environmental conditions necessitate the periodic cleaning of the mirror to ensure high-performance observations. How does Subaru Telescope keep such a large optical surface clean?

One alternative is to remove the mirror from the telescope and clean it separately from the telescope structure. Although Subaru Telescope staff thoroughly wash, inspect, and recoat the mirror every 3 or 4 years, it would be very time-consuming to remove the mirror from the telescope for frequent cleaning. Early on, telescope staff considered other methods for cleaning it while it was still installed on the telescope. In contrast to other methods--washing the dust off with water, blowing it away with compressed air, and kicking it off with a UV laser beam--blowing the dust away with carbon dioxide (CO2) snow was adopted because of its efficiency and proven performance.

Video: CO2 cleaning of the Subaru Telescope's primary mirror shows the operation of the curved wands that disperse CO2 onto the mirror. Each of the four wands sweeps across a quarter of the surface. A portion of the video is accelerated to 8x speed. (Credit: NAOJ)

Standard maintenance of the telescope now includes on-site cleaning of the mirror every few weeks. The structure of the telescope contains four curved wands that are hooked up to cylinders of liquid CO2. When the CO2 gas comes out from numerous small nozzles along the curved wand, it suddenly expands without exchanging heat and some of the gas converts ("sublimates") into a more solid form ("snow" or dry ice). The volume of the solid component expands about 700 times when it touches the mirror and then converts back to its gaseous form, leaving no residue as it changes state. This expansion is powerful enough to remove dust from the mirror yet soft enough for repetitive applications that will not scratch the mirror coating. Each of the four wands above the mirror sweeps across a quarter of its surface in succession until the entire area has been cleaned. The wands then retire to their storage position outside of and above the edge of the mirror until their next round of cleaning.

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