**How Do You Clean This Thing?
The E-ELT will collect nearly as much dust as it does starlight, but simply washing the mirror with water isn't an option because the cracks between each panel could allow moisture into the electronics. Instead, telescope cleaners will spray ultrapure, cooled carbon dioxide "snow" over the primary mirror. The CO2 binds to the dust particles as it falls onto the mirror's surface and, as it evaporates, takes most of the dust with it. With normal wear and tear, the mirrors will need a fresh coating of reflective material, most likely aluminum, every few years.