By Tom Burns
February 2, 2014
As I type these lines, I am looking at a photo of the great John Dobson and me sitting on the front stoop of Perkins Observatory. Dobson died recently at 98.
The first thing you noticed about him was his worn and calloused hands.
You noticed the long fingers, twisted and arthritic from decades of doing what he did best.
You noticed the tips of the fingers. They were flat and scarred, literally torn off from too many accidents that happen when he plied his trade.
Those were hands that have known great pain and great joy.
Those were the hands of John Dobson, master telescope mirror maker.
Those were hands with magic in them.
Dobson is mostly remembered for his innovative Dobsonian telescope mounts. He built his “Dobs” out of scrap plywood, cardboard Sonotubes, and Formica sink cutouts liberated from construction sites. His mirrors were made not from standard Pyrex glass but from surplus portholes. By today’s standards, they are huge, crude, and cheap contraptions, but they worked – and made telescopes accessible to everyone.
He got kicked out of many National Parks for public, after-dark activities. According Once, a ranger said, “The sky is not part of the park.”
Dobson’s replied, “Ah, but the park is part of the sky.”
Sad to say, hand-made telescope mirror making is a dying art. Optics firms can make the mirrors relatively inexpensively.
You start with two thick disks of glass. One disk will be the mirror; the other is the tool you use to grind it.
The tool is mounted on a 55-gallon drum. Grinding compound is placed on the tool. As you slowly walk around the barrel, you push the mirror back and forth against the tool. After many hours, a concave, spherical surface begins to form on the mirror.
It is excruciatingly slow work. Mirror making has been called “a million trips around a barrel.”
The rough grinding process creates a surface that is deeply pitted and scarred like John Dobson’s hands – hardly the kind of surface that will reflect the light from stars.
When a curve of the proper depth has formed, you begin to use finer and finer grades of grinding compounds until the surface begins to look a little like glass again.
Then the surface must be polished to glassy smoothness. Hot, melted pitch, a kind of tree sap, is poured on the tool and then shaped to the spherical contour of the mirror as the pitch hardens. Making this “pitch lap” can take many hours.
Now the polishing of the mirror begins. Rouge, basically the same stuff that is used in makeup, is placed in a liquid solution between the lap and mirror.
When the surface is completely polished, it is still not acceptable as an astronomical mirror. A spherical surface will not bring the light to a focus.
In the longest stage of mirror making, called “figuring,” the center of the mirror must be deepened to a parabola. The surface must be exact to within a few millionths of an inch.
Then you take the precious fruit of many months of labor and ship it to a company that puts a coating of highly reflective aluminum only a few microns thick on the surface of the mirror.
Ohio has had its share of master mirror makers. I learned what I know about mirror making from Dick Suiter.
He guided me as best he could, but I still messed up my 10-inch mirror rather severely.
After 8 months of work, I was in the final figuring stage. I dug a large, deep hole a few hundred-thousandths of an inch in the center. It was a disaster of major proportions.
As Dick said to me at the time, “Tom, you can always take glass off, but you’re going to have a heck of a time putting it back on.”
But he tried to help anyway. He squinted for a long time through the tester. Then, with his two index fingers lightly pressed on a tiny, 2-inch pitch lap he had made and muttering some mystic, incantatory cusswords under his breath, he polished the mirror using a secret, magical stroke developed over many years of bailing out beginners like me.
He polished for only a few minutes, and we set the mirror up on the tester. He looked, smiled, paused for dramatic effect, and invited me to look.
I still don’t know how he did it. Dang it, somehow Dick had managed to put the glass back on the mirror.
It must have been the magic in his hands.
Tom Burns is the director of Perkins Observatory. He can be reached at firstname.lastname@example.org.