Prior to 1996 we had polished for 72 hours using the 42" tool and then continued for 85 hours with the smaller 24" tool. Most of this work was done with various center strokes in an effort to bring the center closer to our final parabola.
We began the year having already worked the glass for 157 hours. Further, the 8 optical test we had done up to that time confirmed that we were well on our way.
The first 7 months of 1996 continued the center work. We tried to run 12 hour marathon sessions during that time. We also experimented with a variety of different strokes in an effort to get just the right wear pattern.
This effort got a big boost from Anthony Stillman when he wrote a computer program that simulated the wear we could expect when we used any given tool on any given stroke. This allowed us to pick strokes that more closely matched the wear removal pattern we wished.
(One unexpected result of this was the prediction that an exact center-over-center stroke does NOT have its highest wear in the center. This prediction was later confirmed by small rings that showed up in the optical tests. As a result, we have been able to choose strokes that don't suffer from this anomaly and have since removed the rings.)
During this time we also: replaced the worn overarm pivot with one made from a heavy duty swivel body; did 6 optical tests; dealt with the problems of band-induced astigmatism; and spent more time than you might think arguing about the meaning of our tests & the shape of the glass.
Another interesting thing happened when Ernie Piini asked me to do an interview for a local cable TV show that he was associated with. While I don't think of myself as a particularly photogenic guy, I agreed. What resulted was a somewhat silly interview but the tape they produced showed our operation better than any to date. I have already used it in presentations and I hope it will become the basis for future tapes on the Group70 project.
By the time we did our 14th optical test (the one of 7/20), we came to the conclusion that it was time to work the edge. With the benefit of Anthony's program, we already knew that the 24" tool could NOT create the desired wear profile on the edge. Therefore, we needed a new tool.
Another result from Anthony's program was the observation that tool rotation contributes much more to both the wear rate and the pattern of wear than does the movement of the tool over the glass. This suggested the need for a powered tool.
That, together with the desire to work smaller zones led us to decide we needed a 16" powered tool.
Well, making the decision is one thing. Making the tool is quite another.
At this point Donn Mukensnable came to the rescue. He designed the tool; found a motor for it; parceled out the work needed among the members of the polishing team; and led the effort to bring it all together.
It worked beautifully. One day, we all came together with each of our parts and, what do you know, everyone had done their assigned task on time and all the parts fit together perfectly.
Not only that, once we started the tool, it took only 10 minutes before it seated (due, I think, to Mark Thein's wonderful tool pitch casting); it worked wonderfully the entire time; and it STILL works better than any tool we've ever had.
(Actually, we did have one problem that day. When we first turned on the tool, we blew a fuse in the power supply. It turned out that we were using too small a fuse for the current we were drawing. But, one quick trip to OSH and we were up & running.)
Since then we have used the new tool to bring down the edge a bit. As of the time I write this (12/4) we have used this tool for 4 hours on the edge and the 3 optical tests we have run suggest we are just getting started.
This edge work is more difficult & nerve-wracking than working the center was. We are painfully aware that this is the time when many amateurs turn down their edge. As a result, we spend much more time testing & arguing than we do polishing.
While this is slowing the apparent progress of the figuring, I believe it is ultimately in our best interest. We will finish the mirror much faster if we approach the correct figure slowly than we will if we turn down the edge and re-figure the rest of the mirror to match.
Both testing and polishing are becoming more difficult. But soon, I hope, a well figured 70" mirror will be the result.
Can you imagine the party we will throw then?