The University of Arizona

Glass Sphere in Optical Sciences a Monument to Persistence

By Jeff Harrison, University Communications | April 20, 2011

The beauty of the newest sculpture in the Museum of Optics belies the hundreds of hours needed to coax it from a chunk of glass.

Geoff Bret Harte, with the finished glass sphere in the Museum of Optics. (Photos by Beatriz Verdugo, UA News)
The sphere rests on a stand that was designed and built by Scott Benjamin in the College of Optical Sciences.

Among the displays in the new Museum of Optics in the College of Optical Sciences is a large optical glass sphere resting atop a steel tripod inside the west wing lobby of the Aden Meinel Building.

The polished sphere looks out onto the University of Arizona Mall and inverts what people see looking through it. Stephen Jacobs, the emeritus professor of optical sciences who envisioned this object d'art, said, "It turns the world upside down."

The sphere's smooth clear finish is testament to the hundreds of hours of painstaking physical labor required to make it presentable. Most of the work was done by Geoff Bret Harte, an optician assistant at the college.

Guided by José Sasián, an optical sciences professor and a former optician, Bret Harte spent literally hundreds of mind-numbing hours going over the sphere's surface.

The project began when Jacobs convinced the Japanese company Ohara Glass to donate a sizable chunk of optics-grade glass languishing in an Ohara warehouse. Ohara makes the specialized glass used by the Steward Observatory mirror lab for the giant telescope mirrors  manufactured under the stands of Arizona Stadium.

The glass was shipped to an Oregon company that was able to turn it into a rough sphere but was unable to polish it. Designing and building a new machine to polish a glass sphere that size was prohibitive. Sasián volunteered to guide the project.

"The obstacles were that it was hard work because it had to be done by hand, the problem of handling the sphere without damaging it, and, of course, who would be doing the actual work," he said.

Sasián made a support structure to mount and handle the sphere and the rest fell to Bret Harte.

"Getting someone was challenging as we would do training with the aim of gaining an optician for the long run, and most importantly, a person who was responsible and careful. We were lucky to find Jefe, who turned out to be superb," he said, using Bret Harte's nickname.

Bret Harte, who previously ran his own house-painting business, said he started the job with a crash course in fabricating, grinding and polishing optical glass, using a 1950s-era textbook as a guide. He began by making a simple telescope mirror from two 4.5-inch disks, a project familiar to many optical science students.

"Painting fits my wiring as a human," he said. "I'm very patient and detail oriented and very methodical in everything I do, traits that are critical to an optician. I had no background in optics, but those traits really crossed over into this work."

The mirror project taught him how to use abrasives to slowly grind away excess glass to form concave and convex shapes, and how to keep a surface clean to avoid making scratches.

The sphere, however, isn't something that can be held and worked in the palm of a hand. With a diameter of nearly 2 feet, it weighs an estimated 350 pounds. And when it arrived in Tucson, it had a glazed, yellowed, scratched and pitted surface that had to be stripped down and polished before it could be displayed.

"The scratches weren't very deep, but nobody knew how the glass would grind," Bret Harte said. "It was up to José to figure out the starting point. We had to go just beyond the deepest scratch, just as if we were sanding a piece of wood."

Bret Harte's colleagues in the optical shop fashioned a concave tool that conformed to the convex surface of the sphere. Starting with a 40-grit abrasive – particles about 425 micrometers in size embedded in a paste – Bret Harte set about on the task that would take him several months to complete.

In order to maintain a perfect spherical shape, Bret Harte mapped the sphere's surface into quarters and kept track of the 15-minute segments spent on each quarter in a log book. There were also frequent checks with Sasián and the opticians.

Bret Harte worked the tool back and forth, one section at a time, carefully rotating the sphere with a wooden device called a Johnson bar, watching his hand motions and using smaller and smaller abrasives, down to 25 microns.  Eventually, the finished sphere began to emerge from its rough exterior.

"That was critical. Because it is such a massive piece of glass, if I didn't continually rotate it and monitor my time, it would have gotten a flat side," he said.

About three months in, after 100 hours of hands-on grinding, Bret Harte finally switched to the polishing phase.

"There are a lot of changes then. When it got down to a 5-micron surface, the sphere looked like frosted glass. If I got it wet, I could almost see through it. It was beautiful, and I was very excited to get to that point."

Using the same tool, Bret Harte switched from abrasives to a special pitch used to polish optical glass. As the surface became clearer, he periodically checked his work by shining a flashlight on the sphere's surface. The scattered light test, as it's called, detects tiny pits and other imperfections in what otherwise look like perfectly polished glass.

Contaminants that occasionally made their way into the grinding and polishing compounds also caused delays. Since the sphere is only for display and didn't require the precision of telescope optics, the aesthetics were the primary consideration and the few microscopic flaws that remain are virtually undetectable.

"There were about six scratches that I lost sleep over," Bret Harte said. "I had no idea where they came from. It certainly added to the physical and mental stress of it."

Also adding to the stress was the question of how the sphere would look in natural light. The lab where the fabrication process took place is deep underground, lit only by fluorescent lights and the occasional flashlight check.

Bret Harte estimated that he lost about 15 pounds during the project, manhandling an object about twice his weight and using a 10-pound glass polishing tool to grind and polish the sphere by hand for several hundred hours.

"The isolation was really hard for me. The process itself is amazingly simple, and it's amazing and interesting that such precision can be obtained from a process that is hundreds of years old. But it was physically trying and I think a lot of people would have cracked. I spent over 600 hours in intimate contact with this piece of glass, touching it, inspecting it, looking at it at close range," Bret Harte said.

"Jose got me started and then left. It was scary, a new environment and a new medium for me. But ‘out of the frying pan, into the fire' can bring out the best in us."

Sasián concisely summed it up, saying "I think we all are pleased with the job."