16.25″ F/3.1 Meniscus Mirror

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!


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.

Close up view of mirror, pads, whiffle

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:

Day 51: 24:20 edge work

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.

Short video at https://drive.google.com/file/d/1kO85eIWb1mgwa_88hzlVXFLaJ7kJ5ZI9/view?usp=sharing

January 30, 2021

A big day, polishing and re-channeling lap. End of day/week summary:

January 29, 2021

January 28, 2021:

Tempted to dig another hole with a 10% stroke length, wider center, and more tapered petals. But this also slows the stroke (same period, less distance) so maybe not any faster. I think I will stay with the 25% stroke and taper the petals.

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

Central pedestal with yoga mat removed, just for clarity. Not tested this way.

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.

Pitch Lap sous-vide!
Tester on tripod. Much more convenient. And I like having the mirror tilted back. This will help when I want to put the mirror on the whiffle-tree in the mirror box. Might be able to get 45 degrees elevation with this!

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:

The negative of the above image is pressed into the pitch. Immerse lap in warm water (90F for me), place thick paper cutouts on pitch, place glass on top, add weight. Be patient.
Either not patient enough, or the lap wasn’t properly warmed, 1:00 to 3:00 positions aren’t pressed properly. But no matter, the lap did the job it was intended to do.

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