It's hard to keep a telescope mirror clean!

Via the New York Times, this very amusing column: 

Few things in science appear to be as delicate or precarious as the giant mirrors at the hearts of modern telescopes. These mirrors — doughnuts of glass meters in diameter, weighing tons and costing millions of dollars — are polished within a fraction of a wavelength of visible light into the precise concavity required to gather and focus starlight from the other end of the universe.

When not at work, they are sheltered in lofty domes that protect them from the distortions of humidity, wind and changes in temperature. But this cannot shield them from all the vicissitudes of nature and humanity, as I was reminded on a recent visit to the Las Campanas Observatory in Chile.

As my hosts showed off one of their prized telescope mirrors — 20 feet of shiny, immaculately curved aluminum-coated glass — I couldn’t help noticing a small, suspicious smudge. It looked like the kind of smear you might find on your windshield in the morning, especially if you had parked under a tree.

“Birds,” one astronomer grumbled when asked what it was.

Do read the article, which has all kinds of details of objects damaging telescope mirrors.