Clean Mirrors 
If you don't keep your car's windshield clean, your view of what's ahead quickly gets murky. And the same thing happens with astronomical telescopes. Without a good cleaning every once in a while, their mirrors would get too grimy to provide clear views of the heavens.
A telescope's primary mirror is the part that gathers and focuses the light from distant objects. Like a windshield, it's exposed to the elements for hours at a time, so it gets coated with dust, pollen, bird droppings, and insects that splat up against it.
But you can't just pull out a bottle of Windex and some paper towels. The wrong materials could damage a mirror's coating. And if the cleaner is too abrasive, it could even alter the mirror's carefully figured shape, blurring the view.
So keeping a mirror clean requires some innovative solutions.
At McDonald Observatory, for example, technicians shoot a spray of dry ice at the mirror of the giant Hobby-Eberly Telescope. The small pellets of frozen carbon dioxide are abrasive enough to clear off the dirt and bugs, but not abrasive enough to scratch the mirror. And the pellets quickly vaporize, so there's no dirty water or cleaning fluid to dispose of.
Every few years, though, a mirror gets so tarnished that it needs a fresh coating. In most cases, the mirror is vacuum-coated with a fresh layer of aluminum or other reflective material. Then it's back to work -- with a clear view of the road ahead.
Script by Damond Benningfield, Copyright 2011