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!

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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