Mirror-making is a consuming activity; it takes a lot of time, patience, learning, and some more patience. I've completed only one mirror, and another is in progress, but I've participated in the making of dozens while helping to teach a class at the local haven of mirror-making, the North Carolina State University (NCSU) Crafts Center. We estimate that over 60 mirrors have been completed there--about 45 in classes and 15 or so in extracurricular activities. In fact, about 15 more are in progress now. The effort has grown over the past five years into a collaborative effort that has taken advantage of the talents of many. Many opinions have been formed as experiments with materials and techniques have been debated and examined.
A successful mirror-making program requires three essential ingredients; experienced teachers, an appropriate facility, and students! Besides myself the class is taught by one of the Raleigh Astronomy Club's (RAC) optical gurus, Dr. Gary Held. Gary learned the art at the Adler Planetarium in Chicago and has made many mirrors and some lenses. In five years of mirror-making, I can't remember his giving any bad optical advice. Gary knows his stuff, and he's kind enough to share it. I would never have undertaken my first mirror had he not provided expert advice and endless patience. In addition to Gary and myself, the late Paul Kosht, who was a member of the first class, taught one of the classes while I was pregnant and unable to teach.
Another essential ingredient is having an appropriate facility where a class can be conducted. NCSU Crafts Center Director Jim Pressley, who is also a member of the RAC, became interested in mirror-making classes about five years ago. He allowed the first class to proceed and perhaps became the most enthusiastic member of that class. He has since spent zillions of hours making and refiguring several mirrors. He has a lot more experience than I!
The mirror-making program is held in two partially underground rooms at the Crafts Center. They have concrete walls and floors which provide a thermally stable setting, and one in which a slip of the old pyrex blank can result in disaster. The grinding room hosts eight 55 gallon oil drums and some small work tables. The testing room contains a long table for measuring focal radius, and another where Foucault testing is done. A cardboard tunnel has been placed over the optical path on the testing table. This helps minimize some of the thermal disturbances caused by people in the room. When the 12 week class is not in session, the grinding and testing rooms are available for informal "lab" sessions (veritable breeding grounds for TM's). The year-round availability of the mirror-making program--the rooms and all the ancillary equipment--keeps enthusiasm alive and growing.
Class members make mostly the typical 6 inch f/8 paraboloidal mirror. Several 8, 10, and 12.5 inch mirrors have been made, and work has also been done on a 2 inch elliptical (for a Gregorian) and a 16 inch Cassegrain. A couple of the more active and generous graduates have even refigured mirrors sent to us by other amateurs in the area. Grinding proceeds on the familiar barrel top arrangement. Smoothly painted plywood tops are available for grinding and fine-grinding; tops with cleats are available for polishing and figuring. Cleats spaced for different diameter blanks fit on any barrel. A photo-negative table and eye loupe are very useful when looking for pits during grinding.
Abrasives are to some extent a matter of preference. Lab users supply their own while the class members use Willmann-Bell kits. The kits are used as a matter of convenience, although their price has grown since our first class. Experiments with different abrasives have ended in some debate, but some ideas are generally agreed upon. For example, many of us feel that 320 and 500 grit silicon carbide sometimes produce excess fracturing and pitting problems. Most prefer to use Aluminum Oxide if possible. We have also found that using 3 micron aluminum oxide lapping powder to end fine grinding shortens polishing time. We have used several different abrasives for polishing and figuring. Some have used cerium oxide and a pitch lap to polish out, followed by red rouge and a pitch lap to figure. Others have used cerium oxide and self-adhesive felt polishing pads to polish out, followed by rouge and a pitch lap to figure. The consensus seems to be that while the polishing pads seem to work faster, they produce a very rough surface that has to be smoothed with rouge and a lap. Since the pads require extra work on a lap to smooth, I prefer to skip the pads and use only a lap.
The testing equipment available to us is perhaps what distinguishes our operation from many others. Gary's homebuilt slip-type tester is hard to beat. The light and knife are mounted on an aluminum stock platform with rails underneath that ride on ball bearings. The motion is controlled in both planar axes by micrometer heads. Motion is perfectly smooth, but the micrometers are hard to read! Our current class tester has a lab quality x-y table which has larger micrometer heads and gives very precise motion. Its light source and knife edge were built and donated by RC member and class graduate Greg Gittings. I should mention that both Gary and Greg have good machining tools and skills.
The pads are easy to apply so that polishing begins quickly, but I've made so many laps now that it doesn't take long to get started on a pitch lap instead. Lap making is greatly expedited by the use of molds available from Telescopics. The molds work well with standard sized blanks as long as the molds are kept clean. We have found that chipping while removing the mold from the new lap is minimized by using red rouge. Since that's pretty messy in a class situation, we have found that Zirconium Oxide works almost as well. Cerium Oxide as provided in the kits does a pretty rough job. I don't think there are any clear cut preferences for the type of pitch. We've used tempered burgundy, hard burgundy, and black blocking pitch. Success seems to vary according to room temperature, and sometimes a batch of pitch has to be tempered with turpentine anyway.
Our second notable testing resource was also provided by a RAC member and class graduate, Kurt Eichenberger. Kurt attended our first class and saw an immediate need for speeding up the data reduction process. Gary's hand-held HP calculator was fast enough, at least in Gary's hands, but eight students clamoring for answers prompted Kurt to write a spreadsheet modeled after Texereau's method. The spreadsheet provides data in tabular format. With a little practice, one can glance at the graph and know exactly what the readings mean. We provide a printed copy of the final test results to students.
Kurt also provided us with zonal test masks. He can plot nearly any diameter mask on cardstock using his CAD software. Then it is a matter of cutting out the holes with an Exato knife and entering the zone sizes and locations into the spreadsheet. Enter a few zone readings, and presto, reduced data emerges. Resourceful, these architects.
Video is beginning to play a role in our testing. A small surveillance camera can be attached to a special tester beside the light source so that the image of the slit test can be shown on a monitor and recorded onto tape. This arrangement is nice for doing before/after comparisons and for instructing several people on the nuances of shadow interpretation.
Incidentally, each year the RAC participates in Astronomy Day at the North Carolina Museum of Natural Sciences. A mirror-making exhibit is included each year, and is perhaps the most popular of our exhibits. Four years ago, grinding began on a 6 inch f/8. Each year, RAC members who are also mirror-makers grind all day while the public watches and asks questions. That mirror is ready for polishing now, but I don't think we have the nerve to try that in a public exhibit. Maybe next year, we'll start a new one and let the public participate. The exhibit is too popular not to continue!
The five years of experience has taught us many things, but it is the enthusiasm and talent of those involved that continues to build the base of expertise. We learn something with every new class, and the TM's using the lab during off-season are masters of innovation. Some innovations have been tried, some failures have occurred, but mostly we just have fun. Class graduates often stop in to see what's new and contemplate what their next mirror should be. It's a great place to socialize, to talk optics, and astronomy, and to try new techniques. Using a telescope with optics made with your own hands is a special feeling that's hard to describe. This article could not have been possible without the contributions of Gary Held, Jim Pressley, Greg Gittings, and the many other participants in the NCSU mirror-making program.Thanks to all of them for their input. ---reprinted with the permission of Amateur Astronomy Magazine and editor Tom Clark, and the author.