They are designed to operate fully exposed in the harsh conditions of the Array Operations Site, to survive strong winds, and to keep their smooth reflecting surfaces accurate to at least 25 micrometers (less than the typical thickness of a human hair) under a wide range of air temperature of -20 to +20 degrees. (alma-telescope.jp)
It is accurate to less than half the thickness of a human hair. (sciencedaily.com)
Recently, holograms that are mere hundredths of the thickness of a human hair have been made from metal deposited onto materials such as silicon. (sciencedaily.com)