I have been preparing to grind my own primary mirror for a long time. I've collected many fine books on telescope making and mirror grinding, including Richard Berry's Build Your Own Telescope, Sam Brown's All About Telescopes, and Jean Texereau's How to Make a Telescope. I read through all of these books, ordered a 6" mirror kit from Willmann-Bell, and built a grinding stand and Foucault tester. But something always kept me from starting: graduate studies, work, other hobbies. Another cause for my procrastination was a nagging little fear of failure. All of the books make very clear that there are a huge number of variables and environmental factors to control in order to craft a fine optic. How can a beginner keep in mind all of Texereau's myriad warnings and still make progress? What I needed was some sort of crash course to get me going.
Then, last November, the Delmarva Stargazers announced the First Annual Mid-Atlantic Mirror Grinding Seminar. They would provide the materials and guidance over three days to help novice mirror grinders turn out respectable mirrors. In addition to the Stargazers’ experienced glass pushers, Steve Swayze and Peter Ceravolo were going to be present to impart their wisdom. This was the kind of impetus I needed to get moving: lock me in a lodge a long way from home and don't let me out until the mirror is finished. I immediately sent a check to reserve an 8" grinding kit and a room for two nights at the lodge.
Once the check was mailed, I was committed to making a mirror. But I wanted to get a little bit of experience before I went to the seminar, so I took my four-year-old 6" mirror kit to the National Capital Astronomer's mirror making club at American University. There, Jerry Schnall and Guy Brandenburg got me started. By the end of the first session, I felt like I was really getting the hang of it. I had the f/8 curve fully hogged out, and I hadn't ruined my blank yet. Over the next two weeks, working at home and at AU, I gradually made my way through the various grit sizes. I got more and more excited as I finished each abrasive and saw the mirror's surface get smoother and more reflective. By the end of the third week I was ready to polish. By then it was also time for the seminar, and I felt pretty confident that I would return with a nice 8" mirror, ready to put the ultimate figure on the 6" that was already underway.
I arrived at Mallard Lodge in Smyrna, Delaware just as the Delmarva Stargazers were getting the mirror grinding stands set up. I picked out a stand at a comfortable height and listened as Don Surles, the president of the DMSG, gave us some quick instructions. Then we beveled the edges of our preground f/8 blanks and tools and started grinding. Since the blanks already had rough curves ground in, we were able to start working at a relatively fine grit, saving several hours of caveman work. Throughout the grinding process, the Stargazers floated among the students, checking our progress and offering helpful suggestions. Fine grinding proceeded very quickly, and by about 4 o'clock (taking time out for a late lunch), I was ready to start polishing.
To speed the polishing process, the kits we used included eyeglass polishing pads. These are self-adhesive paper pads shaped like flower petals. We stuck them all over the faces of our plate glass tools and proceeded to polish with cerium oxide. By the time dinner was served, the center of my mirror had a beautiful polish, but the edge still showed a gray haze, indicating there were many fine pits left from fine grinding.
|Grazing reflection of light bulbs in a fine-ground mirror. (Photo by the author)|
After a delicious dinner, I gave my back and shoulders a much needed rest as the first of the weekend's presentations was given. David Lane of the Royal Astronomical Society of Canada, and author of the planetarium program Earth Centered Universe, showed a number of beautiful slides he had taken on a trip to the Canada-France-Hawaii telescope on Mauna Kea.
After David's talk, I continued polishing. The edge of my mirror got shinier at an agonizingly slow pace, so I took it to Steve Swayze for suggestions. He set it up in the mirror test stand and crouched down behind his Ronchi tester. "Oh my. What is going on here?" he said in a puzzled tone. "You have a severely hyperbolic figure." Uh-oh, that didn't sound too good, so I looked for myself. While I had hoped to see straight, vertical "jailbars" of light, what I faced were pairs of concentric oval rings. I had mistakenly made my polishing lap too small, causing the center of my mirror to be polished far more than the edge. To fix the problem, I would have to abandon the polishing pads and continue polishing on a standard pitch lap. At that point I gave up for the night, sneaking into my bunk around 12:30 am.
I got up early Saturday morning and prepared to make a pitch lap, which
is a notoriously messy job. Don Surles helped me heat up the pitch,
pour it onto the tool, and press channels into the pitch layer with a rubber
mat. When it was finished, I had a checkerboard of squares about
1/4 inch thich and 11/2 inches wide distributed over the glass tool.
After about an hour of polishing on the pitch, my edge finally reached full polish. Checking the figure on the Ronchi tester once again, the inner 3/4 of the ovals had turned into nice straight bars, while the outer zones still showed severe curvature. I continued polishing for the rest of the afternoon, stopping to test every once in a while. It appeared that the techniques I was using were going to give me a nice spherical mirror, so I continued working in the same manner throughout the afternoon.
|Don Surles getting ready to press the channels into a pitch lap. (Photo by the author)|
Once again, after dinner we had presentations. Dave Groski spoke about a design for a solar prominence viewer that could be built by an amateur for about $320. This project may be the focus of next year's Delmarva telescope making workshop. After Dave's brief presentation, Peter Ceravolo described his efforts to film the delicate structure in Comet Hyakutake's tail. Those efforts resulted in a very striking short film, which shows fine filaments of dust undulating behind the comet’s head.
After the presentations, I tested my mirror once again. The edge
was almost spherical, but a broad hill had appeared in the center.
Once again, Steve gave me instructions about how to proceed and I went
back to work for a while. At about 10:30, I tested the mirror one
last time before going to bed. The hill had not changed much, but
at least the edge was maintaining its good figure.
|Steve Swayze demonstrates polishing technique to a student (Photo by Doug Miller for the Delmarva Stargazers)|
The next morning, I continued my efforts to reduce the hill in the center of my mirror. I was particularly paranoid about overcorrecting my hill and digging a hole, so I proceeded very slowly. Every 30-45 minutes I took my mirror back to Steve for testing. Eventually, the hill got reasonably small and the edges of the mirror began to parabolize. Meanwhile, we were hearing reports about a great snowstorm that was on its way to the Mid-Atlantic and Northeast. I was feeling rather rushed to finish my mirror and make the 21/2 hour drive back Alexandria before the snowstorm hit. I decided it was time to stop when my novice eye could no longer discern the errors that Steve described to me as he examined the mirror under the Ross Null test. I thanked the Stargazers for their well organized seminar and wonderful meals, then headed home.
Since returning from the mirror grinding seminar, I have continued to attend the NCA's mirror making class on Friday evenings at American University. Guy Brandenburg helped me pour another pitch lap, I got the 6" mirror polished, and I am now working on figuring it. I have also tested the 8" mirror, and have found that it does not quite meet the 1/4-wave criterion for diffraction limited performance. Once the 6" is finished, I plan to revisit the 8" to improve its figure.
I can envision mirror making becoming an addiction. The way a
beautifully polished surface emerges from a plain disk of glass is magical.
The realization that, using only hand tools, one can make a glass surface
accurate to a few nanometers is intoxicating. Furthermore, the whole process
has turned out to be a lot less complicated than I had feared. While
you can't hope to simultaneously control all of the factors that affect
the mirror, all you have to do is follow reasonable working habits and
tackle any problems as they arise. There are two pearls of old mirror
making wisdom that one should keep in mind: "Anything you grind into
the mirror, you can grind out again", and "Grind more, worry less".
I definitely plan to grind more.
|For More Information…
NCA Mirror Making classes, led by Guy Brandenburg and Jerry Schnall,
are held on Friday evenings from 7:00 until about 10:30 in American University’s
McKinley Hall, room 9 (in the Basement).
|Some good websites: