Herb's 8-inch mirror- Apr 18 2005

this Web page last updated Apr 28 2005. Copyright 2005 Herb Johnson


Here's some of the work done on April 18th 2005, at Gordon Waite's ATM session. My work, Gavin's parabolization, and Gordon's "hands-on" for my mirror are featured. Michael's polishing on his big mirror is pictured. Earlier work is on my mirror page. This is a rough draft, primarily to get the sequence of events straight. The narrative needs work.

It begins again...

On April 18th I started work on the mirror. The last "prescription" was MOT, a 1/3 C-O-C stroke with brief 1" W-stroke. Tonight, Gordon asked my previous "number" - the range from center to edge. I said it was .120; Gordon said "work it for half an hour". At 7:30 PM I microfaceted with about a 6-minute warm press, and 18 minutes of cold pressing, and then set to polish. As I worked, Gordon monitored my stoke, and called out when it was long, or when the forward stroke went furher than the back stroke. I'd do 6 or 7 strokes, then release the mirror to rotate with the turntable. But my procedures were still sloppy.

By 8:30 I was done polishing and we tested at 9PM. With a radius of 113-3/8" (f/7 focal length), Gordon said the edge was good, but it was still oblate by .050 inches. "Keep doin' whatcha doin'" he said, "for another 20 minutes."

I microfaceted again, but left the screen in for 20 minutes. "Twenty minutes!?!" So I cold pressed without the screen for 15 minutes. By 10PM I'd polished for 20 minutes, as before. But the results were disappointing, and puzzling to Gordon. My mirror had a good-sized belly button (peak in the center) and a raised zone at around 50% diameter. Overall, it was .140 oblate, less the bellybutton; a step backward once again for this mirror and my polishing.

The change did not make sense to Gordon. After all, the mirror was on TOP; how do you make it deeper when it's on top? I wasn't sure what more I could do. Over the last sessons of polishing, I was taking it more or less oblate. I knew my stroke was not right, and my microfaceting was not deep. Worse, my pitch lap was getting thinner, close to that 1/8" depth that Gordon required.

Gordon takes a hand

At 11PM, Gordon came to some kind of decision. He said "Let me polish your mirror - if you have the time. Should only take about 20 minutes." Easy enough for me to agree. A chance to see Gordon at work, AND to get my mirror back to a sphere? No problem! "What can I do to help?" I asked. "I'll do everything; just watch" was what he told me.

Gordon prepared the turntable for the lap, setting up the mirror and the microfacet screen, gooseing it with CerOx and covering it with Saran Wrap. He took my pitch lap and started to work on it with a razor blade, cutting open all the channels. By now they were starting to close: he cut them open about 1/8 inch. I asked if that was how wide they should be; he said "something like that". He rinsed the lap in warm tap water, scrubbing with a brass brush to score the squares and to get the pitch chips out of the channels. Then he immersed the lap in a warm water batch, a plastic container, for several minutes.

The lap bath was next to the turntable with the mirror. As soon as he pulled the lap from the water, he IMMEDIATELY threw it on the CerOx'ed mirror and pressed it down with his hand like doing CPR on an elephant - really putting his weight onto the tool. Some time ago he mentioned to me that the pitch cools down quickly, so it was important to work FAST to get the microfaceting deep into the pitch. After a minute or so he released the tool, covered tool and mirror in plastic wrap, and pressed 25 pounds of weight on it, for about two minutes. I asked if that was enough time. "It's instinct, not timing," he said. Then he removed the weight and screen, and pressed again without the screen.

Polishing begins

By 11:20 Gordon removed the weights, flipped the tool and mirror to put the mirror on top, and began polishing. I took notes and paid attention. I remember thinking: how would his strokes be different from mine? I took 6 or 7 strokes at a time, then released the mirror briefly to follow the turntable around. I've already said my strokes were sloppy, his would no doubt be straight and consistent, I'd seen that much of Gordon's work before. He started the timer and began: center-over-center, each stroke straight and about 1" over the edge, equally front and back. After four strokes, he'd turn the mirror in the direction of rotation and do another round. Four and rotate, four and rotate. Each set lasted a minute or two, by the timer. Sometimes his sets were longer strokes, about 1-1/2 inches, and slower. After several minutes, he'd add CerOx. At nine minutes in, he did a W-stroke; about one inch left and right of center, a little wider than he told me to do, to deal with the bellybutton. Four minutes of W-strokes, then back to center-over-center, by fours and fives.

As he polished I observed his movements. This was something he called out to me a few sessions ago. His stance was as he recommeneded: one foot well in front of the other, rocking on ankles and hips, arms pressing into the mirror. But his shoulders were hunched, not straight, and he was clearly putting his muscle into the mirror and its movement across the tool. His hands were mostly flat on the the mirror, holding it by his fingertips. That's something I could not master, even though my hands are not smaller than his. But I don't have the upper body weight, or arm muscles, that Gordon possesses.

At ten minutes in, Michael had to turn out the lights to read the Focault tester, and he called that out. "that's alright" said Gordon, "I can see by the bathroom light." We kept the bathroom door ajar for that purpose, but the room was dim anyway. Afterwards, I told Gordon that I though he was "operating by pure perception" instead of sight; I had to call out the timer for him as he continued in the dark.

Searching for something..?

At 15 minutes of polishing, by the timer, Gordon changed his strategy. After squirting the tool with CerOx, he pushed the mirror only by his fingertips and moved it slowing about the lap, like he was physically scanning it. He was clearly "looking" by feel, maybe by sight, for the form of the mirror, or maybe of the tool, or something - I did not ask, his attention was too focused for me to interrupt. Then he started a new stroke, one I'd never seen. It was a butterfly kind of stroke, a figure-eight from left to right, with the mirror never more than one inch off the edge. Four rounds of figure-8, then a rotate. No weight or pressure on the mirror, other than his fingertips could produce.

After about 90 seconds of that, he changed back to a one-inch stroke front to back, but with an offset from center of about two inches. I guessed he was working on the 50% zone. But half a minute into that, he stopped and said "Done!", to my surprise. In total, he polished for 17 minutes, true to his earlier word. He removed the mirror, covered the tool on the turntable with Saran Wrap, and washed the mirror in the sink.

By contrast, the Black Hole

Meanwhile, Gavin had finished his first run at parabolizing his spherical 8-inch mirror, and had set it up on the Focault stand. Gordon's attention turned to measuring Gavin's results. Unfortunately, the results were all too clear from the initial Ronchi test. There was a huge hole at roughly the central 25% zone on the Ronchigram. "Very pretty, like a spider", said Michael. I agreed, thinking it was also scary. Gavin got a lot of sympathy from Gordon and Michael, especially when Gordon said "Two hour's polishing" to bring down the rest of the mirror to the level of the hole. What would I see, I thought, after Gordon's work on my mirror?

The Moment of Truth

Just before midnite, we set up my mirror on the test stand. To set the right distance we used the Ronchi grating rather than the knife edge. The grating shows multiple vertical lines, fewer as the closer the source moves to the center of radius. As Gordon dialed in the the distance, the zebra strips became several bars, then a few - all straight and vertical. The Holy Grail of polishing was achieved: the spherical surface!

GOrdon raised his forearms in trimuph, like a prize fighter, shouting "Yes! Yes!" and strutted around the shop, stopping to slap Michael's raised hand. Gordon could not be happier. He went over to the monitor to look closely at the Ronchigram. I asked him to pose for a photo. Even my blurred image shows him smiling, next to the straight Ronchigram.

The measure of mirror

After the celebration, Gordon and all looked more closely at the Ronchi result. It showed no turned edge, but there was some small zoning from the previous 50% zone which was slightly visible on the Focault test. We set up the 5-zone mask to make specific measurements, and Gordon called out the numbers from center to edge. Michael would enter them into the computer program, to see how close the curve would be to spherical. Identical numbers - a relative measure of focal length - would be a sphere.

From the center, Gordon saw "..point 160". That was where I had the belly-button. "..152...153...160", the last being the remains of the raised 50% zone. "...161" was the last number, for the edge. "What would that be," said Gordon. "1/7th wave? 1/8th?" Michael entered the rest of the numbers, and called out "one over 10.8th wave", and Gordon raised his arms again and shouted "King of the World!" and took a few moments of satisfaction. "What about the paraboloid?" he asked Michael. "That would be...1/7th? 1/8th?". Michael ran the numbers again, selecting the parabolic calculations, and read off "one over 2.1 wave paraboloid."

An 11th-wave sphere, from an oblate speroid many waves off, in 20 minutes. "Why can't I do that with MY mirrors?" Gordon said afterwards.

The Paraboloid?

Gordon wanted to have some more fun. "You know, Herb", he purred, "you are only 15 minutes from parabolozing - if you do it right." Michael said, "you could proabably goad him to do it." Gordon offered me some amusing persuasions if I'd parabolize tonight. But I wasn't going there, and I said "My wife already said to me tonight, 'if you work after midnight, you'll regret it, you'll make mistakes'". It's a rule of thumb I've used for years, as it's often true. I certainly did not disprove it from working at Gordon's on previous nights. Also, I saw that evening what 20 minutes of Gavin's work had done to HIS mirror.

Besides, my thoughts were more on what Gordon had done, and how he had done it, and how I'd remember or understand his methods. I could see what he had done - I had minute-by-minute notes. But how did he decide what to do, and when? I asked as much, and Gordon replied: "It's more by feel and experience, than by sight or by numbers." I knew some of the obvious "secrets". The length of the stroke was related to the zones you tried to remove; W-strokes are good for removing high spots in the middle. All those errors are measured by the Focault test. But Gordon's strokes were so *precise*, of equal length either side of center. Yet Gordon said, "it's important to keep your work *random*." I said, "but your strokes weren't random AT ALL, they were PRECISE! consistent!" Gordon shrugged.

"what about that figure-8 stroke?", I continued. "you never mentioned *that*". Gordon considered this and said "At that point, I'm trying to find the highs and lows. I'm working the mirror with my fingertips. Bu that time, the pitch tool is fully in contact with the mirror, but the tool is uniform. It becomes the standard, to use to look for peaks and valleys. On peaks, the mirror drags; on valleys it skips. These are the areas you want to work as you find them."

I said: "That's when you were one, with the mirror." Gordon was amused but he said, "The whole point of my doing the mirror was for you to learn." I said I saw what he did, I took notes, but I wasn't sure how much of it I could apply and learn. "After a point, it's like watching someone fish. It's hard to see all the fine points at once." Also, I said, Gordon could work the mirror "harder, as you've got more upper arm strength, and upper body mass." "Well, eat more and you will too" Gordon said, patting his stomach. He's not really that bulky: I'm pretty thin, it's relative.

By then it was well past midnight, and I had to think about my hour's drive home. The three of us, Michael and Gordon and I, sat and chatted pleasantly about the evening's events. Then I called my wife to say I was leaving and then did so.


blow-by-blow for April 18th

Here's how Gordon went from oblate to sphere in 17 minutes' polishing, no stops.

11:00 trim lap, scrub with brass brush under warm tap water., put in warm bath
11:15 screen on mirror with CerOx, tool from bath immediately to mirror and JUMP ON TOP OF IT.
11:17 press with weights, then remove screen and flip mirror and tool
11:19 press with weights (mirror now on top)
11:21 start polishing, 4-count c-c stroke about 1.5", rotate mirror against rotation of table.
+2 min longer stroke abt. 2",
+3 min shorter stroke under 1", putting shoulders into it
+6 min 3-strokes and rotate
+6.5   adjust mirror holders and add CerOx
+7 min 4 stroke, 1.5", rotate WITH rotation of table
+8 min longer stroke
+9 min more CerOx, W-stroke +/- under 1"
11 min still W stroke
13 min more CerOx, 1.5" c-c stroke, 4-stroke and rotate
15 min more CerOx, figure-8 stroke left-right up to 1" from edge, 4-count and rotate
16.5   offset stroke 2" from center, 1" long: light strokes holding edge of mirror
17 min DONE

Images for April 18th

[8-inch mirror]

On Herb's mirror at 9PM:
facet_apr05.jpg shows the microfaceting, I did it twice this evening.
9pm_ron.jpg is a Ronchi at 9PM after 20 minutes of c-c mirror on top
9pm_fou1.jpg is a Focault at 9PM: see the belly-button in the center? and the shallow zone at 50% diameter?
The numbers for this round: center @ .130, the hilltop is .080, that's a .050 difference. Better than last time's value of .120, so I'm making progress.

On Herb's mirror at 11PM:
11pm_ron.jpg shows that after another 20 min of c-c polishing, I've DEEPENED the oblate spheroid..huh?
Here's the numbers: 260 for the outermost zone, .330 for the 50% ring, .400 for the ring around the center peak, .470 at the very peak. So my general difference is now .140, plus a belly button at .070 more. It's gettting worse again, more oblate. This with the mirror on top. Gordon is perplexed.

Gordon says "you got some time? Let me polish your mirror - just 20 minutes."
I watched, took notes. Gordon cleaned up the lap, microfaceted it, and worked the mirror over it.
gordon_happy.jpg Why is Gordon happy, in this slightly blurry image? Why is he shouting "King of the World!"? Check the monitor...
mid_ron.jpg The Ronchi tells the story. 17 minutes of Gordon's magic mirror moves..
mid_fou.jpg ..and you can see he achieved a sphere! Numbers say it's 1/11 wave sphere, even 1/2 wave paraboloid!
mid_fou_con.jpg A higher contrast Focaultgram shows a little hollowing at 50%
The numbers are listed below in "zonal measurements" for Apr 18th. My notes on Gordon's work will follow soon.

On Gavin's mirror:
gavin_fou.jpg is Gavin's work to parabolize what was a sphere. Center is a little deep.
gavin_ron.jpg is Gavin's deep hole in a Ronchigram.
The numbers from hell: .450 for the outer sphere, .080 for the bottom of the hole.
..but you should not feel so all alone...EVERYbody must get stoned...

Gordon told me that, a week later, Gavin came back to show his additional two hours of work on his mirror. Says Gordon, "He succeeded pretty well. The mirror was basically back to spherical, except with a little bit of zoning. The "spider" was gone, so a slight bit more smoothing and he's back on the right road. See, there's not much that you can polish in that can't be polished out. The trick is "slow and steady" so no problem gets out of hand. Especially when parabolizing, until you have experience you want short sessions and lots of testing to stay in control."

Michael's mirror
michael_ron.jpg Michael's big mirror is making good progress, in this midnite Ronchigram.

zonal measurements on Herb's mirror:

		Jan 17th	Jan 31st	Mar 7th		Apr 18 Gordon	deltas for perfect parabola 
		11:30PM		midnite		11:40PM		midnite		113-3/16" f.l. 8-inch mirror
cntr:		.311		.402		n/a		.160		.000
zone:		.351		.416		n/a		.152		.017
zone:		.386		.438		n/a		.153		.014
zone:		.418		.452		n/a		.160		.014
outr:		.448		.480		n/a		.161		.014
		-----		-----		-----		-----		-----
diff:		.137		.078		.065		small		.060

Herb Johnson

Contact information:
Herb Johnson
New Jersey, USA
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Copyright © 2005 Herb Johnson