Ed Grinds A Mirror: A Serial Article

By Ed Ting Latest Installment: 10/30/00

Index To Mirror Grinding Section:
  • Page 1 (This Page): Grinding, Polishing, Testing (5/00-10/30/00)
  • Page 2: Ed Builds a Telescope (11/00)
    This summer (2000) I joined a mirror-grinding class hosted by our local club's ATM committee. Despite having looked through hundreds of telescopes through my observing career, I had to admit I had never ground my own mirror; in fact, my knowledge of the whole subject was rather sketchy at best. It was time to remedy this.
    The 6" mirror blanks
    (Note: This article will appear in installments over the next few months. Check back here from time to time to see how I'm progressing. Good or bad, I promise you will hear all the details! -Ed 5/29/00) Chapter One: Rough Grinding, May 28, 2000 I arrived on a cool Sunday afternoon in May, and started grinding with five other club members in a local barn. Right away, there was pressure on me; I had missed the first two-hour grinding session and everyone was well ahead of me on the coarse (80) grit. As a result, I used somewhat more pressure on the blank, and took fewer breaks than the others. On the spherometer we were using, the correct depth to be reached was 20.8 thousandths of an inch on the coarse grit. Most of the others were already in the 15-17 range, and one person finished within the first hour of the second session. Since I was starting at ground zero, this put even more pressure on me to catch up.
    Measuring the depth of the center. The mirror is currently at f/55. Anyone have a 27 foot-long tube?
    Mirror grinding is a noisy, messy, wet activity. It doesn't take up enough of your attention to fully occupy you, but it does take up just enough that you cannot do something else like watch TV, listen to the radio, etc. You're pretty much stuck there looking at your hands, and adding water or grit once in a while. This goes on for hours at a time. The whole session was kept under the watchful eye of our Great Grinding Grand- father, a man who's been to 50 Stellafanes and has ground more mirrors than most of us will ever see in our lifetimes. Every astronomy club has at least one guy like this, and if you need a mental picture, just substitute the one in your club whenever I mention him here. The Great Old Grinder looked over my mirror after the first hour and pronounced that I was "pretty much average" in terms of rate and the quality of my work, which was fine by me. Two more people finished up during the second hour, but I wasn't so nervous anymore. My rate had increased steadily, and I had already registered 17.5 on the spherometer. In fact, I had already passed one person who had been there for both sessions. I stayed a little late, and actually overshot my f/8 target a little when all was said and done. I also passed another grinder by then. Next time, it's on to the 120 grit. As I was cleaning up my work station, I thought about something I once heard John Dobson say about the rough-grinding stage: "This is a Caveman's work. You eat well, you sleep well, and in between you work like hell."
    Working Like Hell: My Mirror Grinding Station
    Our Great Guru looked at my work, and said, "Say, you're pretty fast." Then, ominously, he waved the blank at me. "Next time we'll see if this is a half- assed job." Chapter Two: Grinding With the 120 Grit, June 11, 2000 My mirror is hyperbolic. In my zeal to achieve an f/8 curve so quickly, I ground out the center of the mirror far more than the required depth. I'm not alone in this; a couple of other grinders also have hyperbolic mirrors, although mine is the worst off. I work carefully, grinding center-over- center in short strokes to take down the edges. After two hours of this, though, there's not much progress. While grinding the blank, there's a huge air bubble in the center that's visible even from the next grinding station. The bubble shrinks in size as time goes on but never completely disappears. Through the din, I think I hear someone refer to me as "Air-Bubble Ed." At the end of the session, our Great Grinding Guru looks over the surface of the mirror with a loupe and says the 80 grit pits are all gone, the surface looks smooth, and that the focal length now sits at 45 inches. So what I have here, as of June 11th, is a quickly-manufactured, smooth- surfaced 6" f/7.5 hyperboloid. Chapter Three: Finishing the 120 Grit Stage, July 9, 2000 By now we have settled into a comfortable routine. We show up around 3PM on these Sundays, talk a bit amongst ourselves, take our direction from the Grinding Guru, and set off to work. Things have undergone a subtle change, however. There is more talk, more direction given than before, and somewhat less action. The Grinding Guy points out what seems like an endless stream of cautions as to what can go wrong: Clean your station thoroughly, don't wear long sleeves (grit can collect in them) don't hold the salt shaker over the mirror (you don't know where it's been) use less pressure, test after every few wets, etc, etc. As if to reinforce this, our Grinding Guy stays very close to the tables this time, issuing small corrections, evaluating our strokes, looking over our work. The message, subtle or not, is clear: you can goof off all you want during the rough grinding stage, but now things are starting to get a little more more serious. Pay attention. Through careful work (or perhaps blind luck) my blank returns from its hyperboloid and is now a rough sphere again. At around the same time, the 120 grit pits seem to have dissipated from the surface of the mirror. Not wanting to push my luck, I call myself finished on this stage and clean my workstation. We carefully spray off the tables outside afterwards. There are to be no 120 grits left over for the next phase. I look up and it starts to rain. I don't mind; it's been a remarkably cool summer, the coolest in recent memory. I have this to be thankful for. We could have been sweating in the barn these past few weeks. Chapter Four: The 240 Grit Stage, August 13, 2000 This has got to be the coolest summer on record. As the grit stages progress, we spend more and more time preparing, and less time actually grinding. The work area must be cleaned, dusted, vacuumed, and wetted down before each use. The tables are power-washed outside and wiped down with cloth before use as well. All of this takes time, of course, and the actual grinding part of these sessions is slowly diminishing.
    The work area, just before wet-down. Tables are outside, drying
    We've settled into a routine that's comfortable for us, and with this a degree of boredom has crept in as well. Things aren't going to get too interesting until we get to the pitch lap stage, which seems far, far away at this point. In fact, there are still three grit levels left, and it's been taking us two sessions for everyone to get through each level. Throw in the fact that we meet every two weeks, and this project could go well into winter at this pace. When I point this out to the group, I see a few shoulders sag. As a result, we've agreed to increase the length of these sessions. We intend to meet slightly more often as well. The goal on the upcoming Sundays is to get everyone through one entire grit level on one session. What began as a leisurely mirror-grinding exercise is turning into a race against time. Hopefully we have caught this soon enough. Chapter Five: The 30 Micron and 12 Micron Stages, August 20, 2000 This was the first of our double-length sessions. Four of us showed up early to get through these grit stages more quickly. I had stayed up all night with a group of friends evaluating the Tak FS152 and the AP155 and I was dragging. The prospect of grinding for 4-6 hours wasn't terribly appealing. But in a strange turn of events, I had a blast. These smaller grits look like baking soda or flour, and when you grind with them it is hard to believe they are having any effect at all. The noisy scraping sounds from the 80 grit stage have been replaced by occasional near-silent squeegee-like peeps. With the microscope, you can measure your progress through the grits. The pits slowly become smaller and smaller, and when they stop getting smaller you are ready to move on. It takes me two hours to get through the 30 micron grit, and I get my second wind. Instead of going home, I power-wash the table, break out the 12 micron grit, and start again. The 12 micron stuff feels like finely-ground, sifted flour, and again I can't believe the stuff has any effect on the glass. But it does. The microscope records my progress and it takes me a mere thirty minutes to finish up this stage. My mirror feels sooooo smooth now. The Grinding Guy takes note of my rapid progress (I am now two stages ahead of the last person) and remarks that I seem to be in a hurry to finish my mirror. As if on cue, I ask him what would happen if I were to simply go out and get the mirror aluminized right now, skipping the polishing stage altogether. What kind of images would it form? For the first (and possibly last) time since we have started this project, the Grinding Guru has no response. I am not sure if this is good or bad. I feel like I've accomplished a lot today. One more grit to go, and then it's onto to pouring the pitch lap. Many of you have already written in to tell me how much you hate the polishing stage. I'll see for myself in a few weeks... Chapter Six: Finishing up the Grits, September 10, 2000 If the 30 and 12 micron grits seemed finely-ground, the 5 micron stuff is even more so, almost like a white liquid. It's so fine that the only way I can use the stuff is to mix it up in a slurry in the water bottle. Then I just shake up the bottle and squirt the white liquid on the mirror. While grinding, it feels like it's not doing anything at all, but the microscope tells me otherwise - real progress is being made here, and in about 2 hours I'm done. The end of the grinding (or the beginning of the polishing, depending on your point of view) arrives uneventfully - for me. Others have not been so lucky. One member discovered a crack near the edge of his mirror. Cracks have a tendency to expand and propogate (think about the cracks you get in your windshield) so he went to the grindstone and ground down his mirror, past the crack. The problem is, he had to take so much off, the surface of the mirror is now shaped like a "D" rather than an "O". Another member lightly dropped his mirror on the tool while grinding - something I had done dozens of times myself to my own mirror - only to discover that he had chipped off a tablespoon-sized hunk right off the surface of his mirror. Ouch. He is examining his options right now and may or may not continue with the rest of us. Chapter Seven: The Pitch Lap, October 8, 2000 Pitch is a mysterious substance made up of rosin, bees-wax, turpentine, and "other stuff" that sits in a small cardboard container looking like brown glass. Pitch is not a solid, it's a liquid with a very high viscosity. Add a little heat and it turns into a fluid. The usual way to handle this is to heat the pitch in a small used coffee can and pour it onto your mirror or tool. But the Grinding Guy has other ideas, and his solution is simple and elegant. I wrap my tool with 2" wide masking tape, making a small reservoir for the pitch, which was crushed to pieces with a hammer. The whole thing gets tossed in a small oven and is heated until the pitch melts.
    Crushed pitch on the tool, ready to heat (mirror in back)
    Twenty minutes later the tool is taken out. The pitch is allowed to cool, and I add a small slurry of cerium oxide to the top that will act as the polishing compound. When everything is dry, I peel off the tape and the pitch/cerium oxide layers stay put. The pitch looks like chocolate pudding, while the cerium oxide layer looks a little like frozen chocolate milk on top. Together, they look like one of those double-layer Jell-O instant pudding mixes (Hmmm....when I get done uploading this segment, I think I'm going to get myself something to eat.)
    Soup's on! The melted pitch, 20 minutes later
    Facets are cut into the still-cooling pitch. I used the edge of a chopstick, but anything sharp and long would have done the job. After the facets are cut, the tool takes on the "brown squares" look that you have all seen in the textbooks. From here on, it works just like the grinding segments. You work the mirror and tool over each other and test periodically. Polishing takes a long time - I was told to figure 5 or 6 hours for this - and takes more effort as well. The pitch/cerium oxide don't exactly glide over the mirror. It feels a bit like pushing and pulling two tightly-sealed plungers against each other. My arms get tired after only a few minutes, and I have to take frequent breaks.
    The Grinding Guru takes a few strokes on my mirror "It's not bad," he says
    After a few hours of this, I get my first taste of what this work is all about. We take my mirror outside and reflect an image of the sun onto a piece of paper. I see a white round disc with clearly-imaged clouds rapidly moving across the surface. I'm thrilled in a way that I cannot describe. I go back to my polishing with renewed vigor. The end is in sight! Chapter Eight: Fine-Polishing and Testing, October 30, 2000 Polishing is hard work. The mirror and pitch/tool feel as if they're vacuum sealed against each other, and I find I have to use my upper arms to get them to move. Pitch is strange stuff. It's brittle and chips easily when cool, but glides and shapes itself like hard caramel when worked. After about an hour and a half, the channels I cut close up and the pitch lap has to be trimmed again. I'm pretty bad at trimming the lap - I made several chips in it. Fortunately, it doesn't matter once polishing begins again in earnest.
    Polishing the mirror Note the channels are still wide open
    This takes a long time. It takes over five hours to polish the mirror until its surface glows with a shiny sheen. The center polishes out first, the edges last. Watching the "frosty" look at the edges slowly go away was like watching paint dry. Every time I thought I was done, I'd have to go back for another half hour or more. Ugh. Testing A Foucault tester is a very simple device. A light shines through a slit, hits the mirror, and reflects back towards a knife-edge where you look. As you move the knife edge across the field of view, you magically see the surface of your mirror magnified many, many times over. Little hills, valleys, and zones become obvious. The test is so sensitive that a medium-sized cat walking near the tester or mirror will induce heat waves that obliterate the view for several seconds. We spend close to half an hour setting up the mirror and Foucault tester -everything has to be aligned perfectly, which is a major pain- and, in the darkened room, I get my first look at the condition of the mirror which I've been working on for over five months now. Slowly, with my heart in my throat, I slide the knife edge into place and take a peek.
    Judge, Jury, and Executioner: The Foucault Tester
    The surface looks smooth, and the edges are clean. There is no turned- down edge, the bugaboo of first-time mirror makers. The mirror is also not hyperbolic, which would be difficult or impossible to correct. The only aberration I can see is a half wave hill, or bump, in the center of the surface, which I am told is very easy to remove. Figuring A 6" f/8 sphere departs from a 6" f/8 paraboloid only very slightly. How slightly? One method suggested is to take seventy-two 3/4 length "W" strokes on your spherical mirror, and you should be close to a paraboloid. That's right - of the tens of thousands of strokes taken on the mirror over all these months, it comes down to a mere seventy-two strokes on the tiniest grit of all. This takes all of two to three minutes. We let the mirror cool for several hours and a reading is taken. The hill has been cut in half, so more fine figuring is necessary. I take another one hundred "W" strokes and leave until the following morning. The waiting is necessary to allow the mirror to fully cool. Even the heat from your hands, or the friction from the pitch lap, can throw off your results. We confirm this by looking at the mirror right after figuring. It's a mess. It "shows" a huge depression and severe astigmatism. But the following morning, the mirror looks completely different. Running the numbers through "Tex", the program developed around Jean Texereau's classic test methods shows the mirror deviating only .10-.13 waves O-P center to edge. There is a minor zone about 2.45 inches out from the center of about .10 waves. This zone is small in magnitude, but rather steep and far enough out to be of minor concern. We talk for a bit about this zone. It may be possible to fix it, but Sir Isaac's pesky laws come into play - for every action, there is an equal and opposite reaction. Taking out the zone may induce another aberration in another part of the mirror. Due to this, the fact that this is my first effort, and the apparent excellent condition of the mirror otherwise, I decide to leave well-enough alone. First Light With the Uncoated Mirror Early the next morning, I install the mirror in the cell of a discarded 6" f/8 Newtonian owned by the club. An uncoated piece of polished glass reflects 5%-10% of incoming light, so I'm effectively looking through a 1.5" - 2" telescope. It's enough. The star test reveals slight undercorrection, perhaps a bit more than the Foucault tester showed, but quite acceptable nonetheless (I was warned in advance to mentally double the wavefront error calculated by the Foucault tester to be on the safe side - this turned out to be good advice.) The last quarter moon looks pinpoint sharp and I'm thrilled like a kid on Christmas morning. I pump up the power on Jupiter and Saturn - silly considering the dim images - but the planets hold up even at 190X. I'm satisfied with the figure. Off to the aluminizer it goes! Aluminization, October 19, 2000 I visited Research Service in Seabrook, NH and dropped off my mirror for aluminization. Visible were about ten evaporation bell jars in various states. The aluminization process is simple in concept, but the equipment is extremely expensive.
    "Cool! Can I watch it work?" The Evaporation Tank
    These guys did a great job (I had my mirror the next day.) Research Service is run by Don and Steve Jaynes at (603)-474-9332. Much of their business is done in the institutional/commercial market; their work with amateur astronomers is done as a courtesy to us, at rock-bottom prices. If you do wind up using them for your own project, please keep this in mind and do your best to be a "low- maintenance" customer. We want these guys around for a long time.
    The completed mirror, October 20, 2000
    Like marathoners at the end of a long race, our ATM'ers are spread out all over the field - I'm finished, but one very careful member is still patiently removing grits at the 30 micron stage. For me, this may not be the end of the line, as I am considering building a Dob mount for the mirror and using the whole assembly as a star party telescope. If I decide to go ahead with the project, you'll be the first to know. Click Here to see the conclusion!
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