A Big Bottle of Windex?

Like other glass surfaces, telescope mirrors need an occasional cleaning. It's a little more complicated than just using some Windex and a paper towel, though. To clean the surface of the Hobby-Eberly Telescope at McDonald Observatory, for example, technicians use a spray of carbon dioxide. The tiny pellets of dry ice wipe away dirt and grime, then vaporize into the atmosphere. [Marty Harris/McDonald Observatory]