After a spa treatment and a fresh coat of aluminum, the Hale telescope’s 200-inch mirror boasts a new shine. The butt end of the telescope, from whence the mirror came, looms in the background of the “before” picture (top).

The Shining

There’s a fresh gleam in the eye of the Hale telescope. In November, the 200-inch mirror at the Palomar Observatory was treated with a good scrub and a shiny new coat of aluminum.

After staring into space for nearly 300 days out of the year, the mirror accumulates dirt and dust that even weekly cleanings can’t get rid of. So, every year or two, the mirror is unmounted, washed, and given a new reflective surface.
First, the staff uses a cart to lower the 14.5-ton piece of glass from the scope. They then clean the mirror

with soap and water, keeping the surface wet at all times to prevent spots. Next, an acid wash called “Green River” strips away the old aluminum. After some more cleaning, drying (with paper towels), and inspection, the mirror’s ready for its new shine.

A 17.5-ton bell jar is lowered over the mirror. It takes a day to suck out enough air to make the vacuum the mirror needs to be recoated. Inside the chamber, hundreds of tungsten coils covered in aluminum are heated to more than 500° C. The heat vaporizes the metal, and the aluminum deposits onto the mirror, forming a layer less than eight millionths of a centimeter thick.—MW