This blog is mainly about Telescope making, and some things about my politics. At last we finally have a President that can say "Fool me once; shame on you. Fool me twice; shame on me." instead of mixing up with an old Who song.

Sunday, July 20, 2008

Mirror Mantra

Sailors on tall ships had them while trimming sails. Old Irish women had them while weaving cloth. Field hands had them while picking crops. Chain gangs had them while tarring roads. Bakers had them while kneading bread. Hikers had them while on the trail. When I mow the lawn, I have several. There is even a modern one for computer programmers. "Working 40 hours on the Xerox line" by Stan Rogers.


And thus, today, when I dosed my new tile tool with a charge of #80 grit and some water and started the old back and forth, these rhythmic songs started creeping into my brain like a slow distant freight train.

It was hot out there. About as hot as I would attempt it. Over 90 degrees in the shed by the time that I called it quits at 3pm. My hands were wet with water, and that helped, but boy, I really need to wait until after August for this thing. Or at least wait for a cloudy day.

But to keep going and to keep even pressure and even strokes, it was the meaningless mantra going through my head that eased all the work.

I don't like music when I work. I find that I get into listening to music way to much. I ponder it when I do. Maybe it's that I don't like things to interrupt the music. But when I do work in my shed, on the telescope or whatever, or when I am programming, I like things very quiet so that I can concentrate. And then the voices start in my head. I talk to myself in meaningless ways - giving myself encouragement, and admonishing myself on mistakes made. Or I count how many strokes it takes to make one round (between 60 and 73). There is the rhyme and rhythm of counting that all blurs into simple meaninglessness after you miss-count but continue making the noises in your head anyway, just for the hell of it, and because you aren't done yet.

Porter was wrong. The mirror isn't the thing. Its the cadence... now that, the cadence is the thing.

More pictures next time. They are out in my shed in the camera on the workbench, and I am too hot and too lazy to go out and get it right now. Not too exciting. Same old tool. I have it wrapped in clear packing tape now, and I'm not so sure I like that idea too much. The slightest wrinkle causes little tunnels where grit can reside. So when I finally get to the #120 and later ones, I am going to have to strip all that off, clean the tool and start over from the wax stage each time. I keep telling myself that I did manage to save a lot by not using dental $tone. I'm cheap - its worth it.

Where I left off with the mirror was hogging it with a 2" pipe cap. I thought it would take forever to get this new tool to the point where every tile is fully touching, but after today, it looks like most of them are there, and even some of the ones I thought would simply never make contact are half way there. I'm pretty impressed with tile tools, let me tell you.

All of the worry about having the tool stick - like when you have two peices of glass - is completely gone. You spend your time grinding instead of worrying. You start thinking of how you can press down on it more, rather than thinking how much can this this sucker drag. I am truly amazed.

After a while, I noticed that the glazing was being removed in a ring around the center of the tool. It was a stark reminder of how grinding of the mirror really works.

If the tool is grinding away at an annulus ring, then the mirror on top, must be grinding a hole in the center to match it. Sure enough, when I took my first spherometer measurements, it was .038 in the center and .036 at the edge. .038 is a little too deep for me, and I wanted to remove that hump of glazing on the tool, so I tried it about 10 rounds with the tool on top. I was worried that the packing tape would be slippery when wet, but it was actually just fine to hold on to, dry hands or no. After 10 rounds, I checked it again. .0365 in the center, and .0345 at the edge. And I got rid of that glazing.

One thing I noticed was that using a big tool like this makes the surface a lot more even than you can control with a 2" pipe cap or 3" flange. As my data proves, the numbers were all over the board. But with just a gut feel even after such a short time "lapping in " my new tool, its obvious that these kinds of anomolies will subside. I checked all over the mirror, and its really very even from the center all the way out to the 10.5" ring. None of the measurements I took inside that ring varied from .0365 at all. At the 11" ring, none of them varied from .0350, and at the edge, none of them varied from .0345".

So, the next plan...
1. Wait for a cooler day.
2. Grind with tool on top for a while to get the the measurement between .0320 and .0325 all over if I can.
3. Switch up to #120 :)


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