Blog to share figuring progress on yet-to-be-named telescope. I don’t name telescopes but I want to start! Artemis is taken.
Background: 3/8” pyrex slumped by Newport Glass Works in 1997 or 1998 per my order. Idea was to bond to ceramic honeycomb after giving up on ceramic mirrors due to glaze issues. Slumped glass had a lot of ripple and a poor symmetry, had to be ground both sides to get even. The convex side was ground against the ceramic honeycomb which already had nearly the same curve. Once these matched the glass was bonded to the ceramic using pitch. Pitch was powdered and sprinkled over ceramic, placed in oven (probably on lowest setting). When pitch was melted glass was laid on top and baked some more. Bond appeared to be bubble free.
Glass was then ground using tile-on-concrete tool. After fine grinding I built a mirror-o-matic (MOM) and started polishing, mirror on top (MOT). Details are lost, but suffice to say it polished out rapidly. First Ronchi test looked horrible. I put the project away, traded Howard B. the MOM for a Televue Panoptic 22mm, and put mirror making in the rearview while I raised kids and wrangled photons.
By 2016 I was hearing Mel talk about thin meniscus mirrors, and when he finished his 25” the message sank in. I had the right blank already! In late 2019 I put the assembly back into the oven and slid the glass of the ceramic. A Ronchi test showed that it was poor, but not nearly as horrible as when it was bonded. Mel had a look and I was committed to going back into the polishing game.
In late 2020 the time was finally right. Pandemic, work-from-home, lots of year-end vacation time, and a free machine from a friend. Onward!
January 2, 2022
A closer look at ronchigrams and artificial star, comparing edge and center support. Center support wins again. The question of grinding the back or not isn’t answered, but trending heavily toward “no.”
…sometime later that day…
January 1, 2022
As the mirrors are getting quite close to spherical, it’s time to evaluate in detail, as well as study the mount some more. An artificial star test on-axis is very helpful! But I really need help with my next step. Grind the backs or not? If so, how?
Will post ronchigrams too.
I decided I needed deeper, wider channels in my lap than I could get from the plastic needlepoint grids. So I made my own! Screenshots from Fusion360 show the details. I can print 1/4 grid on my Prusa. Any bigger mirror would require sectioning out radial zones. The channels are now 2.5mm deep x 3.0mm wide. Small, yes, but so easy to press and re-press.
December 27, 2021
Quick summary of past month:
Replaced two Mirror O Matic 1″ shafts and bearings. Used shafts with flats to allow set screws on pulleys to do their jobs without damaging the polished shaft surfaces. Ball bearings are seated in plywood mounts, ends of shafts are extended all the way to bottom of box for superior strength. No more squeaks, the turntable and eccentric are running super smoothly, and very little energy is lost. No more belts slipping either. Machine runs quietly and I can hear the glass/pitch/cerox interface sliding along like a whisper. I have increased weight on the stroke arm to 10 pounds and the machine is having no trouble. At 25 pounds weight the plywood box flexes a bit under the heavy load, so I’m keeping it light.
With silver on the mirrors I could see a slight amount of haze in the last inch of radius, and a few minor sleeks and scratches. Overall, I would be happy with how the surfaces appear if these mirrors were parabolized. Mirrors stripped and back into polishing. Lap needs to be re-channeled now. Next polishing run will push the central hump down to spherical by lengthening the stroke.
December 13, 2021
I have spent more time working on the machine than the mirrors. That old M.O.M. had some seriously worn-out bearings. Still does, but some of them have been replaced. Notably, the shaft for the eccentric drive is replaced, and instead of using a bronze sleeve bushing I went with precision ball bearing assemblies. I also got a longer the shaft so that it anchors at the bottom of the machine box, greatly increasing the load capability (2X at least.) I can now polish with added weight on the mirror! The turntable shaft is going next. Squeaky and stiff, same problems as the eccentric shaft, but since it’s load never shifts directions it is much less troublesome. At present the machine is polishing at the same quiet hush that it did in fine grinding when there were practically no loads. And no more slipping belts either!
Stroke speed is 52 strokes per minute (26 RPM on eccentric drive). I’ve had it up much faster, around 110 strokes per min for fine grinding. But this is about as fast as I dare go in polish. I have a thing where I worry about breaking the mirrors. With everything mostly in tune I can let the machine run unattended for an hour, but I do check it every 15-30 minutes. Seems to need fresh slurry only every hour.
Fine grinding was completed by end of November, as noted earlier. I finished with 9 micron.
Pitch polishing has been stable, no changes to figure and both figures match: Hill in central 50%, no turned edges. Scratches have come and faded so I’m hopeful that with careful cleaning of glass and lap I can keep them in the category of faded and fading fast. It’s a dirty garage, and I won’t mind if there are just a few sleeks and scratches. But I want the pits to be completely gone. And in that department, I have found what appears to be an excellent situation: The backs of the mirrors are shiny and unground, so when I hold the mirrors up to a bright light I can easily see very fine haze on the front surface. Central 50% is totally free of haze and pits, outer couple of inches has just the faintest, sparsest scattering of tiny pits. Same for both mirrors. In the old days I would have called it done.
I hope to get all the pits out by the time the experimental silvering chems arrive. This weekend. Then I will silver both of these and use both types of silver. If nothing else to gain I’ll know just how well I did with polishing!
The lap had to be completely re-done about halfway through. I made the mistake of leaving off the small weight on the stroke arm, and sure enough it climbed out of the pusher, hit the foam pad on the mirror, then pushed everything off to the side. The pusher fell to the floor, the mirror stayed stuck on the lap, but the stroke arm spindle proceeded to chunk out pitch on the exposed part of the lap. I removed the pitch under water as hot as I could stand it, about 140F to start. Then, with a clean tool and hot melted pitch I poured a new lap and I challenged myself to NOT use a tape dam on the tool! It worked! I had to be very careful, and the pitch did stop a little short of the edge in some places, spilled over in only one small spot. After several warm pressings to get contact and facets the edge actually moved out a bit further. Might be too thin for parabolizing, but still seems around ¼ inch thick (and uniform.) Parabolization will be done with a spider lap anyway.
Sorry no photos or ronchigrams at the moment.
Meanwhile, think about this slogan that I never want to see around any telescopes:
“In Case of Mirror Break Glass”
November 28, 2021
So much learning! Polishing was producing a well-polished 70% and unpolished 90% to edge. Why? Fine grinding done incorrectly. I had completely forgotten about short strokes since grinding my last mirror way back in 2000 or 2001. With lots of help from various mentors, I was instructed to get my strokes below 30%, use the Sharpie test correctly, and my fine grits 15 micron on down were replenished with donations.
Machine is running at 182 strokes per minute in fine grinding! Super fast, but too fast for polishing. As configured for polishing the stroke rate is 30 strokes per minute, and I will increase it to around 60 very soon when the pulley arrives.
Procedure was to run 500, 25, 15, 9, Cerox. At each grit I did a Sharpie test to see what was happening. At 25 microns the ink always seemed to clear in the center first, but then I used hand pushing to shorten the strokes to <1 inch and increased the frequency to something like 300 per minute (as fast as I could). This worked like magic to grind down the ink in the edge!
For the 15 and 9 micron grit I varied the hand stroke length to find out what removed the ink uniformly and had great success on each mirror. West was then flash polished by hand, and East was flash polished with the machine at the best stroke length determined by hand. Hand polishing was faster due to the higher stroke frequency (~60/min vs. 30/min). Results:
East has a touch of astigmatism, West is perfectly stigmatic. I’ll bet the pushing tool is mostly to blame. I’ll need some help with that. Hand polishing is not going to work for me in the long run, although hand figuring isn’t out of the question.
Another huge finding is that 2-point edge support is working perfectly with these mirrors! The previous mirror was thinner and much more flexible. I reported that it needed central support. These mirrors do not seem to flex at all with the 2-point edge that others have found suitable! Thickness is 0.48 at center, where the old mirror was down to 0.32. I can now confidently evaluate astigmatism using the point source and beamsplitter, and I can correlate this to any bending I might see in the Ronchi test. The point source test is far more sensitive.
…a few hours later, after polishing East on the machine and using two yoga mats for padding between pusher and glass (instead of one.)
Edge is now as reflective as center, no astigmatism visible, smooth. Ink marks still there, haze is not gone yet. Slight turned edge I will have to make sure goes away as the polishing proceeds. Figure matches West.
November 14, 2021
Haven’t posted for a while but not for lack of effort on the project. So many things to report!
Pads: I got the pads to stick better, but still lost a couple. I don’t approve.
Pitch: Thanks to several comments I now know how to prepare a lap much more cleanly and easily than before. I use a slow cooker set on low to melt the pitch which is slightly bubbly. No stirring required. I was able to pour a stream onto the center of the lap and have it come out at a uniform thickness (~1/4 inch) across the F/3 convex plaster tool. The tool was painted with thinned pitch first to aid adhesion. Channels were pressed using cold aluminum bar, 1/8 thick. Thicker would be better, channels closed too soon. Re-pressed using needlepoint plastic screen.
Machine: Hoo boy! I have been puzzled by how weakly this machine strokes, it can barely push the glass across the pitch with no added weight to the mirror. The belts were slipping, so I replaced them. I added as much tension as I could manage using the MOM springs. I built a lightweight spider gripper just so that the machine could move, see below.
Then after seeing that the belts were STILL slipping, I finally found the problem. The eccentric wheel bearing was extremely sticky! I couldn’t turn the wheel by hand when disconnected from the arm.
The bronze bearing was totally stuck and the whole assembly was turning in the plywood. Seconds after the above photo was taken a neighbor drove up in his old VW Bus and asked if I might have a bearing puller. Unfortunately I did not because I could have used it at that moment too, but he did have a can of WD-40 in his passenger seat and that did the trick for me!
It took me about an hour to re-assemble the machine into good working order, and now the stroke arm is finally stronger than my own arms!
You may have noticed that the mirror is back on the grinding tool. Yes, I decided that West needed to be re-ground to match East, because West simply wasn’t polishing past the 70% zone. Back to 25 micron today. Same for East too, because it had started to develop astigmatism again, and I think the whole reason was the wimpy machine and a very low stroke period to turntable RPM ratio. After the regrind I will install one of two new pulleys on the stroke drive to increase the stroke period from 11/min to around 30 or 60 per minute. Pulleys to arrive Weds.
And you certainly have noticed my messy garage. Ah life…I can’t hope to come out of this mirror process free of scratches, but I can do a better job. We’ll see…kinda glad I get another crack at fine grinding and polishing again, and I hope the newly refurbished machine does what I expect.
November 2, 2021:
I should add that if I can successfully polish East using pads I will then finish polishing West the same way. Then I will make a new pitch lap, a spider lap, for figuring. I still don’t know a good way to melt pitch without making a huge mess and I still don’t know how to cause it to stick to a plaster tool. Help! email@example.com
October 26, 2021:
Polishing underway with full-contact lap. Using fixed-post method, MOT, with 4.5″ offset. The polishing times are not accurate, nor are they exactly identical for each mirror. But they are close, within reason. The figures are roughly equivalent too, which is very encouraging for when I have to start correction. Furthermore, the mirrors seem to be substantially stiffer than the previous one, and with 0.425″ edge thickness (0.48″ center thickness) they have a distinct advantage over the 0.31″ thick (now broken) mirror. I can judge the stiffness with the on-axis point source tester (laser diode, beamsplitter, eyepiece) because it shows that Mirror East has just a little astigmatism that rotates with the mirror. The previous mirror bent like a potato chip and gave an image that otherwise didn’t make sense.
October 15, 2021
Grinding complete and polishing begun! Grinding was done in the sequence:
80: Tool on top, mirror “potted” into plaster support so that it would lie flat on the turntable. I did not worry about runout very much, because I am grinding this way only to get the mirrors flatter. Once the sag was reached I flipped to mirror on top (MOT) for a few wets just to get them close to spherical. Each mirror was ground for 1 wet and then switched to the other mirror. This continued throughout the grinding process.
120: Continued MOT until prior pits seemed gone. Used loupe.
220: Continued MOT until prior pits seemed gone. Used loupe. Big pits were obvious at first, and I eliminated all of them.
500: Same as above. No scratches! At this point I was able to let the machine run unattended for 15 minute intervals. The project picked up speed.
25 micron: Each mirror was run for 90 minutes in 30 minute intervals. Done in a day! A loupe reveals a fine ground surface that almost looks velvety. There are a few pits here and there, and I decided to move onto polishing with paper pads just to see what would happen. Video
Flash polish: The machine is too slow. I had to push glass manually at high frequency, 1/3 stroke, to get perceivable change in about 10 minutes. As a result the mirrors are not identical in their polish, but they match exactly in focal length (47.375″, target 48″, I’m happy) and the figures sure look the same. Video
Plan is to continue until fully polished out, using the machine, and possibly increasing the stroke frequency 5X. At this point I will report on my project at the 14th AltAz Initiative conference, October 20th.
September 6, 2021
I have received two new blanks toward the end of July, slumped soda-lime glass, 1/2 inch thick. Slumped curves are 92″ radius to yield a 46″ focal length. I expect to grind them back to 96″ radius to a target 48″ focal length.
Testing between crossed polarizers shows no obvious stress, however soda-lime glass surprises me. At this thickness I can make a thermal handprint and see it in crossed polarizers! (Very thin glass just doesn’t make a visible feature, I tried!)
A grinding tool was made from Denscal plaster (a Hydrostone equivalent) on September 6, with embedded hex tiles. Rough grinding on both blanks was done for less than 10 minutes each on the same tool. 90/60 grit was used. The tool rotates on the machine while hand stroking about 25% length. I will use hand grinding until the tool and mirrors get into very good contact. Then I will change to a Mirror-O-Matic and work my way down the grits. Some tiles are not in good contact yet, but overall this looks pretty nice and I’m sure most of the outliers will contact before the next grit.
July 4, 2021
TRAGEDY. I thought it would be a good night to finally get my mirror star tested, and instead I broke it. Yes, I included retention, no, it didn’t work despite prior tests.
“There’s nothing you can do while making a mirror that will lead to a life of woe.” – John Dobson
The project continues. I will build the telescope properly, while waiting for new blank to arrive. Too painful to report on details of how it broke. Maybe later.
June 19, 2021
June 18, 2021, Continued
June 18, 2021
Need to get 4″ secondary mirror on order to enable star testing!
Hole is diminishing and I am now attacking the 30%-80% zone.
June 16, 2021
June 10, 2021
It’s been a while since posting, but I’ve been working away at this mirror. The latest, in chronological order this time:
Not getting much change from spider lap, I pressed out the legs, mostly, and ended up with a star lap having good contact to 70% then tapering. So I went ahead as you see in the sequence above, digging a hole and working it smooth. After trying a variety of strokes I saw that W and CoC were most effective, bare-handed instead of using pusher. Then, CoC gave way to W, as shown below:
Also worth noting: I haven’t measured the mirror’s radius of curvature in quite a while. With all this parabolization it is likely to have changed a bit.
June 3, 2021
June 2, 2021
I added two more figuring sessions to the series, but now with stroke length all the way out to the kink, 83%. This is really working!
June 1, 2021
Two hypotheses turned into theorems! I think the next one is that I should increase stroke length to 13.4″ which will just touch kink to center. Hoping it will also smooth the nicely developing curve.
May 31, 2021
May 20, 2021
May 17, 2021
I believe I have whiffle-tree success! More details: I can’t take credit for the torsion idea. But I can’t remember who on Oregon Scope Werks suggested it. Please let me know. Pads are 3D printed PETG and conform to the spherical rear surface of the mirror.
April 27, 2021
Updated movie showing progress to sphere!
Testing the spherical mirror at center of curvature with a point source will allow me to really evaluate astigmatism induced by the mount. At present I believe I can say that the mirror has no inherent astigmatism but depending on how I let go of it when it seats into the test stand it may have anywhere from zero to “total potato chip.” (and I mean the Kettle kind, not Pringles.) As seen above it is resting against the pushing tool with foam pad and two edge contacts roughly at the 5 o’clock and 7 o’clock positions. The mirror is tilted at roughly 25 degrees from the horizon (a valid use-case!) I can move the edge contacts anywhere and see some change, but the biggest effects come from re-seating the mirror on the foam pad. A total crapshoot. See January 10-15 for details on the laser-based point source.
April 25, 2021
April 24, 2021
April 22, 2021: Spider lap reduces bulge smoothly! Subtle, which is good!
April 17, 2021
Edge is gone! (retraction: hill NOT reducing)
April 12, 2021
I haven’t posted as I work the turned edge problem. But I am keeping copious notes as always. Instead of bore you, here’s a video:
And here is where my mirror is at 24 hours of polishing:
April 3, 2021
March 31-April 2, 2021
March 30, 2021
Note of concern: Working the mirror by hand allows me to observe contact with the lap. I have seen the mirror flex under pressure by watching the contact come and go where I press. I figure this is good if I press in the right place, which for now is accented pressure at the edge. While I have previously assumed uniform pressure when using the foam mat and pushing tool, I can no longer. Maybe it doesn’t matter, as long as the results are consistent. But it worries me about how to get full contact when pressing. Maybe the answer is to not press at all, but rather use gravity, warm water bath, and time, where the mirror just sits on the lap. (All 5 pounds of it.)
March 26, 2021
March 25, 2021
A technique change seems not to have accelerated progress. Video. Ref. How to Make a Telescope, Texereau, Vol 2, p. 92. Fig 42 item 6, turned down edge. Except I have mirror on top.
March 24, 2021
March 23, 2021
Putting some real labor into eliminating the turned edge. See video.
March 21, 2021
Going the wrong way. Reverse petal lap or back to sphere?
Day 33: March 20, 2021
Day 32: March 17, 2021
Day 31: March 13, 2021
A full month of days of work now! But they are short days, just an hour or two most times. Like today, including working on this logbook:
I was really disappointed to see that the center is flattening and not deepening, but I am reassured that this is the least of my worries. Get the edge to behave first.
Day 30: March 12, 2021
March 10, 2021
PS: I did another null lens test, but now the mirror is so close to parabolic that the null test doesn’t show me anything useful. Stand-induced astigmatism dominates, the lines are straighter but narrower at top, wider at bottom.
March 6, 2021
March 4, 2021
After putting a lot of effort into rebuilding the lap from essentially full-contact with pressed petals to a true spider lap, 8 days of tinkering and hard work, then only about 45 minutes polishing 1/3 strokes, the mirror appears to be within about a wave of a parabola! It is full of kinks, but they should be smooth-able.
One thing I did not do was to put a contact gap at the 85% zone to manage the kink there. I was hoping that the bigger problem was just getting a general shape to form. I think that was the right call! But the last wave might be the longest wave. I will now be simultaneously correcting and smoothing, and I can hear Mel saying, “don’t chase the kinks.”
Plan now is to press out gaps corresponding to kinks and run combination short and 1/3 strokes
February 24, 2021
February 23, 2021
Time to get help from the experts. Sending email to OSW. Change petals or more of the same?
February 21, 2021
After overnight repress of petal lap, another 30 minutes polishing. First time using Mel’s Ronchi Matching Test on this project!
February 20, 2021
February 19, 2021
February 7, 2021
I used the new pitch lap calculator, but I didn’t apply it to the zone correctly. Not sure what I should have done, but clearly not run a small lap tangent to the inner edge of the hole.
February 6, 2021
The tool on top experiment has yielded positive results!
After I posted this I got caught up with email and found that Mel had just updated his pitch lap calculator to do sub-diameter tools! Playing with that now.
February 4, 2021
It’s time to try Tool On Top! I’ve been working that tool for a few days and after some brief trials, today was the first real session. I really had to study the results before I was convinced I had done the right thing! Could I have done it better? Certainly. Suggestions welcome.
January 30, 2021
A big day, polishing and re-channeling lap. End of day/week summary:
January 29, 2021
January 28, 2021:
January 25, 2021:
January 24, 2021:
January 23, 2021:
Here’s the progress since I deliberately started parabolization attempts:
I think maybe best to go back to W, MOT, by hand.
January 18, 2021:
What is consistent is that polishing the center is flattening the curve, not making it deeper. Or, I am looking at the curves backwards! But I thought I understood these things from testing a couple of other mirrors!
January 17, 2021:
January 16, 2021: Parabolization begins
Video of new polishing stroke on petal lap: https://photos.google.com/photo/AF1QipPGt2xi5MM1fLVLiM1zCHhisGlZaccXkWa7Ei62
January 15, 2021: ASTIGMATISM GONE!
Tip for removing astigmatism, from Jan Vangastel. I did this for about 10 minutes with a 6″ pad, but only after I was absolutely certain about the axes of the “saddle” I was about to destroy.
I can’t claim that I knew what I was doing. The stand astigmatism is so bad that I am guessing as to the mirror’s astigmatism. But after doing this quick polishing run, Dale Eason suggested I support the mirror by its center of gravity. I went about this as shown below (not how Dale envisioned it, but still gets the job done):
Step 1: Place mirror on pad with central pedestal underneath, get it about level to Earth
Step 2: Set up laser/beamsplitter/eyepiece assembly 100.5″ above mirror and find return spot. Remind yourself why you wanted a short focus scope! Imagine if this was F/5 or even F/4!
A few minutes with the W stroke tells me something about what I am about to encounter. It will be slow going, if I am lucky the kink will disappear on its own, as will the hole but that’s under the secondary, and the turned edge will smooth into the parabola. But likely I will return to sphere a couple of times using the machine. Next step is to press petals into lap.
January 10, 2021
January 9, 2021
Summary: Still learning about astigmatism. Clearly there is something in the test stand causing asymmetric bands. Rotating the mirror reveals changes between 45 and 90 positions. Zero and 45 seem to match. Noticing a kink around 70%. What to do?
January 7, 2021
Summary: Pressed out the petals to prepare for parabolization and to investigate astigmatism with new learning. 1hour at about half-speed. Set of 6 images shows primarily tester-induced astigmatism (source is below image). However, rotation of mirror indicates some change. I’m not too worried, more polishing to come before going for the parabola. Hole has reduced substantially! Now well within secondary shadow.
Also, here’s a short video of the machine running at full speed: https://drive.google.com/file/d/1ZXXakab2ECrKJQ_Z0u53l04Yce9H0kgr/view?usp=sharing
January 2, 2021
I’ll begin by simply pasting my powerpoint notes one slide at a time. I hope this is legible.
Jan 2, 2021: The new motor arrived today! Happy new year!
Another hour of polishing tonight. Results:
Now that the machine is polishing at a nice rate, it’s a good time to review, especially for clarity. Here is where I started, with asymmetry induced by bonding, polishing, and releasing. (In fact, the asymmetry was worse BEFORE releasing, but that was 20 years ago, no digital camera.)
Here is where I got with slow polishing, 6 cycles/minute, for about 40 hours. A petal lap helped some.
And finally, after just 2 hours with 8X faster stroking rate (~50 cycles/minute) and a petal lap:
I’m very glad I got a variable speed motor. https://makermotor.com/variable-speed/50-rpm/ I can see that when I get down to the last fraction of a wave of parabolization I will want to be going slower.
Petal lap for last 2 hours:
Goal: remove hole, continue on to parabola. I’m OK with doing both at once because there isn’t a turned edge and the process is going the right direction.
Also, I will be using a null lens later on! Stay tuned. Next post after I have appreciable progress. – Rob Brown, Jan 2, 2020