Polishing

2014.03.18. Having completed two hours of 3 micron, I am moving to polishing. Stage one: convert tile tool to polishing tool by covering tool with optical polishing pads.

2014.04.04. There is this almost magical moment in the course of polishing when as the rinse dries between wets that the surface goes from frosted to clear. That happened today after a half hour of polishing (and so 2.5 hours with the polishing pads and CeO²). It’s a fun moment (as I remembered from my experience with the JTK2). The mirror doesn’t quite pass the laser test, i.e., casting a laser at a low angle across the mirror reflects off the surface partially and causes the few pits to sparkle. And with a bright light at a low angle and viewing from a low angle across the surface of the mirror, we saw a lightly cloudy band around the edge. Still, at this point we put it up on the tester. First, the radius of curvature turned out to be 151″, putting the mirror at f/6. I’ll want to test again: not because it falls short of the f/6.25 that I originally was thinking about, but because it falls short of the f/6.1 that I was consistently measuring through the TOT fine grinding over the previous month. The Ronchi looked crazy, as one would expect after pad-polishing. Maybe, maybe I should anticipate a TDE as polishing continues. But there was no evidence of zones. One last note: I had planned to polish for two hours. But nearly dry pad polishing may, in terms of brute force, be more work than hogging out. I called it quits at 1′ 45″. Take a peak in the gallery for its bodily effects … and no, it’s not the killer triceps I’m still waiting on.

2014.05.06. Since I’ve been working with the polishing pads for about eight hours, the decision has been made to move from polishing pads to a pitch lap. In the outermost rim there are possibly a pit or two every once in a while; the pitch lap can rub them out. Making the lap went smoothly enough. You’ll notice bubbles in the surface, which seem in the meantime to have been flattened out. For warm pressing the heat lamp is too little; so I’m using now the bucket I did my rinses in during grinding as a tub for the tool’s warm bath.

 

My notes on polishing. This covers from the day I began with Cerium Oxide (25 March) till I began parabolizing (27 May): 14.25 hours polishing.

25.03.2014 CeO 1/3 C-C TOT 2:00:00 I filled in a few gaps on the surface of the tool with “petals” of optical pad before starting. Because the pushing would be so touch, I screwed three feet on the rotating board to hold the mirror in place as I would push. I lightly moistened the mirror with Cerium Oxide. I started polishing. Every quarter hour I remoistened lightly the mirror and shifted the mirror on the wooden disk.
04.04.2014 CeO 1/3 C-C TOT 1:45:00 Polishing with pads continues. There is this almost magical moment in the course of polishing when as the rinse dries between wets that the surface goes from frosted to clear. That happened today after a half hour of polishing (and so 2.5 hours with the polishing pads and CeO²). It’s a fun moment (as I remembered from my experience with the JTK2). The mirror doesn’t quite pass the laser test, i.e., casting a laser at a low angle across the mirror reflects off the surface partially and causes the few pits to sparkle. And with a bright light at a low angle and viewing from a low angle across the surface of the mirror, we saw a lightly cloudy band around the edge. Still, at this point we put it up on the tester. First, the radius of curvature turned out to be 151″, putting the mirror at f/6. I’ll want to test again: not because it falls short of the f/6.25 that I originally was thinking about, but because it falls short of the f/6.1 that I was consistently measuring through the TOT fine grinding over the previous month. The Ronchi looked crazy, as one would expect after pad-polishing. Maybe, maybe I should anticipate a TDE as polishing continues. But there was no evidence of zones. One last note: I had planned to polish for two hours. But nearly dry pad polishing may, in terms of brute force, be more work than hogging out. I called it quits at 1′ 45″.
08.04.2014 CeO 1/3 C-C TOT 2:00:00 I did four thirty minutes shifts of polishing with polishing pads. My left shoulder aches. I tried to keep the polishing fairly dry in the first hour, but it ended up being moister in the second hour for the sake of my muscles. Again we didn’t have a laser handy, so we inspected with a microscope. I have difficulty distinguishing some dust from pits. Guy is optimistic enough about progress that he proses I move to a pitch lap next time. But we’ll reinspect next time and make the call then.
11.04.2014 CeO 1/3 C-C TOT 1:00:00 I arrived prepared only to make a pitch lap. But as we had a laser finally, I made a laser test. The mirror didn’t pass. There was reflection on the top surface at many points as well as sparkles, in both cases more toward the edge than the center. So I decided to keep polishing with pads. Thus an hour’s work this evening.
15.04.2014 CeO 1/3 C-C TOT 1:00:00 I polished for another hour this evening. Both microscope examination and laser test indicated pits. Both examinations after an hour’s polishing suggested progress. A Ronchi exam indicated, on the one hand, clearer vertical lines in and out of focus. On the other hand, a central “certs” tablet and serifs at the circumferance were attention catchers. Discussion with the two Bills suggested nothing serious yet; however, it would seem prudent next time to work MOT. Also, note to self, I was trying hard to push down with the tool. Perhaps that effort was too much, and I should bring a lighter touch to the polishing next time.
22.04.2014 CeO 1/3 C-C MOT 1:00:00 A creaking in my grinding box has led me to stop using and to remove the pine disk, the four stops which held the disk in place, and the three furniture feet on which the disk rested. Now the tool sits directly on the top of the box and is held in place by three hickory “stops” screwed into the top and a shim that holds the tool in place as I grind (and which I remove to spin the tool). There are still pits, though few, and distributed more in the outer zones than the inner ones. There are sleeks, it would seem. Other than those pits and in the center, the surface looks quite  smooth. Scratches are minimal. At the end of the evening, we looked at the Ronchi. I will continue MOT. I think I will make slightly shorter strokes and keep things slightly wetter.
29.04.2014 CeO 1/3 C-C MOT 1:30:00 The rotation of the tool is certainly more inconvenient without the pine disk. Visual inspection of the surface reveals few if any pits. Dust and pits are hard to distinguish. Guy thinks I may be ready to move on to a pitch lap. I proposed waiting till next time and double checking first thing to see whether the pits had grown or reproduced ;>) in the intervening days before making the move to a pitch lap.
06.05.2014 CeO 1/3 C-C MOT 1:00:00 On inspection we decided that the few remaining pits along the outmost circumference could be rubbed out with the pitch lap. So we spent an hour making the pitch lap and another hour using the newly formed lap. Warm presses to be done by water in a bucket.
13.05.2014 CeO 1/3 C-C MOT 1:00:00 I work in three 20-minute segments. There’s considerable delay between segments because the queue at the testing contraption is long: three of us are figuring. The lines are pretty wacky, but I’m told that they’re typical of a mirror recently polished with polishing pads. Otherwise, no obvious zones, TDE, or astigmatism.
20.05.2014 CeO 1/3 C-C MOT 1:00:00 Like last time, three 20-minutes segments. I wander around the corner of the table to make the turning variations a little easier. At the end of the evening, Guy judged the limited progress a function of weakly mated lap and mirror. We concluded with a cold press and committed ourselves to moving forward. Next week I should perhaps try TOT.
27.05.2014 CeO 1/3 C-C TOT 1:00:00 today I worked through three twenty-minute polishing sequences. There were thus four Ronchi measurements. I started with a warm pressThe first polishing sequence involved a standard 1/3+ c-c stroke. The Ronchi effects were minimal: the inner six-inch diameter seemed pretty good, but the outer three inches of radius were deeply curved plus with serifs. With an interest in making the lines and their curve more consistent I made the strokes longer in the second sequence, following another warm press and after the warm hard press I left weights for the sake of a cold press. In the middle of this round I felt the tool snagging a bit atop the mirror and so I did another cold press. After this second sequence, in which the strokes were a little longer than perhaps I should have and in the end with some effect: the inner circle expanded in size (good) and the curves in the outer circumference smoothed, but the serifs were enhanced, suggesting a TDE. Guy recommended adhering strictly to a one-third c-c stroke. I did a full ten minutes of weighted cold press, and kept my strokes to two inches on either side with slight w-ing in the storke. The final Ronchi test was very satisfying. The lines were very straight, with very slight outward curvature inside of radius of curvature. We checked the results against the ideal parabola. Guy recommends for next time: 2.5 inch stroke overhangs on either side, sligtht w-ing, TOT. Today’s results were very satisfying indeed.

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