Adah takes a lick of Richie Townsend’s telescope mirror during its final rinse. Credit: Richie Townsend

This sweet photo makes me cringe and laugh at the same time. Amateur astronomer Richie Townsend of Duluth, Minn. was cleaning his 8-inch telescope mirror when his cat Adah walked up, saw her reflection and started licking away. Look closely and you’ll see a water film on the mirror. How could she resist a few licks off that silvery surface?

“I was nearly finished with the final rinse,” said Townsend. “Her “help” only required a small extra swipe with a cloth.”

Telescope mirror surfaces are ground to an accuracy of about one-millionth of an inch and coated on their front surface by a thin film of highly-reflective aluminum. They’re different from everyday mirrors, which are coated on the backside. Good thing. If a bathroom mirror were aluminized on the front side, you’d soon damage the coating after a few cleanings with Windex. Astronomical mirrors are coated on the front side to prevent precious light rays from being absorbed by a layer of glass.

Technicians inspect the 94-inch Hubble Space Telescope main mirror. Reflecting telescopes use mirrors like this one to gather light from distant stars and galaxies. The mirror’s surface is ground into a gentle curve called a parabola which allows light reflected off the mirror to come to focus in an image. An eyepiece magnifies the image for viewing. Credit: NASA

Aluminum coatings are still soft and vulnerable to scratching during the occasional cleanings they require. That’s why telescope mirror manufactures apply an additional overcoat of transparent silicon dioxide (silica). Silica is commonly found in nature as sand and quartz. The overcoat is tough enough to allow for gentle, infrequent cleanings.

A friend pointed out it was a good thing Adah didn’t cough up a hairball. Good thing indeed. Acids and salts can slowly eat away at mirror coatings. You might be surprised (or not) to know that acid, as in acid rain, is also found in dew. Repeating dewings of telescope mirrors on damp nights will slowly etch and flake coatings. Salty air near the oceans is no friend of mirrors either. It costs about $70 to re-coat a typical 8-inch telescope mirror.

That’s why amateur and professional astronomers alike baby their mirrors, trying to find a balance between overcleaning and keeping them dew-free. I’ve got a low-wattage light bulb rigged up under my main telescope mirror to keep it warm overnight so no dew can form. I unplug the light and let the mirror cool down before using the scope at night.

How a reflecting telescope works. Light enters the tube and strikes the main mirror, which directs the beam back up the tube to an angled flat mirror called the diagonal. The diagonal reflects the beam into an eyepiece which magnifies and focuses the image. Credit: LearnAstronomyOnline.com

Back in the early days of reflecting telescopes – those that use mirrors to gather and focus light from the stars – mirrors were made of a polished metal called speculum, a brittle alloy of copper and tin that tarnished quickly. Modern mirrors are made of Pyrex or plate glass and coated with aluminum in a special vacuum chamber. Over time, all coatings will tarnish and need to be stripped and replaced. Perhaps if we could gather enough cats, they would prove useful in the stripping process.

Henri 2, Paw de Deux

If you’re interested in how mirrors are coated, check out this well-illustrated article on how the 200-inch Palomar telescope gets its periodic re-aluminizing. And if you like cats  and humor, you must click on the existentialist cat video.