Pushing Glass at Stellafane 2000

by Ken Slater

Opening early and closing late due to crowd demand, the Mirror Making Demonstrations under the big tent at Stellafane 2000 were again extremely popular. Featuring hands on mirror grinding, brief lectures and demonstrations of tool making, pitch lap pouring and optical testing, there was something interesting for conventioneers of all experience levels.

Where are the Machines?

Probably the most common question heard under the tent is "You can do this by hand?" or " Where are the Machines?". Many people incorrectly assume it takes expensive, precision machinery to create the precesion optical surface needed for a quality telescope mirror. They are often surprised to learn that powered machinery is unnecessary and little more than two glass blanks, grit, water and human energy get the job done, and done very well. With many barrels and benches set up as grinding stations, we explain this simple, traditional process while grinding real glass. Most visitors will take up the offer to try it themselves, and soon we have both young and old taking "walks around the barrel" -- many find it hard to stop as the rhythmic grinding motions cast their age-old magic spell.

How do two Flats make a Sphere?

The next most common question regards the creation of a spherical section in a mirror blank and tool that start out flat. This seems best answered through demonstration, rather than theory, and a few words about applied forces while demonstrating a chordal stroke seem to satisfy the curious. Looking through the center of the mirror blank while it is being pushed around the edge of the tool, it seems clear to all that the mirror center will start to hollow out. I have often felt that until this inevitable second question is asked, my audience has not quite accepted my assertion that a mirror can be made by hand.

How Long will it Take?

As the realization that mirror making does not seem to be too hard sinks in, and that perhaps it might be attempted, the third and final question is inevitably voiced: "How long will it take?". In actuality, this is the hardest of the three questions to answer, because so much depends on the work habits and skill of the individual. I've been telling people about 100 hours of labor over the course of a year for that first 6 or 8 inch mirror, which I don't think is too far from what I have observed from the mirror making classes we give. You can see disappointment in faces as this answer is absorbed -- so many seem to feel this is far more time than they have available, yet it is only 2 hours a week! A few understand, and two friends I demonstrated mirror making to last year both brought beautiful homemade scopes back to Stellafane this year!

Optical Testing

We had a great set of equipment for demonstrating optical testing this year! The Optical Interferometer built by Scott Milligan and Dave Kelly was always surrounded by interested amateurs. Scott and Dave should be commended for demonstrating how an advanced piece of test equipment like this could be built on a reasonable budget by an advanced amateur. At the other end of the test equipment scale, Rick Hunter assembled a flashlight, prism and Ronchi grating with some duct tape into a "Ronchi Flashlight", which did a quick, qualitative test of a mirror with little fuss or bother. We also had three Foucault testers set up and running, and demonstrated their use and shadow interpretation. Optical testing seems complicated and mysterious to many novices, and having the equipment set up and demonstrating it's use helps bring down another barrier to starting your own mirror.

Tools & Laps

Junie Esslinger demonstrated how to make a plaster tool. Plaster & tile tools have still not made it into any of the popular telescope making books (only much less widely available magazine articles), so this technique is not known to many mirror makers. We get a lot of questions about them while working under the tent, and we make a special effort to spread the word about how well they work, and how easy they are to make. Our master lap maker, Phil Rounsville, again made pouring pitch look easy, and he was surrounded by eager apprentices during his lap making demonstration.

Big Glass

A big piece of glass seems to have a magic attraction -- people just seem to move toward it as soon as they spot it. Since 1997, we have had a 27" blank being rough ground under the tent. This year, as it was ready for 220 grit, we didn't want to risk a scratch in the dusty tent, so it was left safely in the bunkhouse. Instead, we took two 16" full thickness blanks out of storage, and they seemed to have just as much magic to attract conventioneers. We had one on a barrel which got most of the adults pushing glass, and one on a bench so that our younger attendees could easily push glass also (with a cast iron barbell weight as tool). And grind they did! Thanks to all the many club members who patiently helped instruct and educate our guests, we seemed to have fulfilled our goal -- to take some of the mystery out of pushing glass, and plant the seed that mirror making is a satisfying and attainable goal. Lets hope we see a few more telescope on the hill next year that got their start in the Mirror Making Demonstration area this year!

Ken Slater, Springfield Telescope Makers

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