Q&A on telescope meniscus mirrors by Mel Bartels

What is a meniscus mirror?

A telescope meniscus mirror is a constant thickness mirror where the backside has the same curvature as the front side. My 16.25x3/8 inch mirror (image by Barbara Bajec), Larry Sharper's ultra-thin meniscus (image by Pierre Lemay)

Why a meniscus mirror?

No hogging out the curve; constant glass thickness; thinner faster larger mirrors

The meniscus shape allows us to have a constant thickness mirror from mirror edge to center. A large fast mirror requires a huge sagitta, or depth in the mirror's center. I've ground curves into common flat mirrors ending up with a measly 1/4 inch [6mm] or less of center thickness. That's concerning.

Slumping a flat mirror in a kiln over a precision refractory mold into a curved shape takes the place of rough grinding in a curve. The amount of glass removed grinding in the curve for a standard flat blank 30 inch [76cm] F2.7 is about 250 cubic inches. That's the same volume as a full thickness 12.5 inch [32cm] mirror blank. Imagine grinding down such a big chunk of glass until nothing remains!

To get this sagitta by grinding-> completely grind away this volume of glass->

John Wall, inventor of the Crayford focuser, ground a 48 inch diameter x 1 inch thick [122x2.5cm]. Perhaps if the blank had been slumped, he would have chosen a focal ratio faster than F8. Here the mirror is being polished with a sub-diameter tool.

My 30x5/8 inch [760x16mm] F2.7, ready for the night. It's a no-ladder scope.

Instant or near instant thermal equilibrium

Last night (Feb 1, 2023) I heard that the comet 2022 E3 ZTF was breaking up. High clouds, patchy snow and ice on the ground. I picked up my 16.25 inch [41cm] F2.9 scope that weighs 25 pounds [11kg], carried it outside, sitting it down in an open area. As soon as I uncovered the mirrors, I was observing at my highest possible magnification. No cooling period needed; good star images. I could have rolled out my 30 inch [76cm] F2.7 that weighs a 100 pounds [45kg] and sets up in five minutes, but there wasn't a clearing large enough. That's what large thin fast meniscus mirrored telescopes offer: lots of aperture, sit down no ladder eyepiece height, lightweight transport, quick setup with no cool down needed. And the comet breaking up? A false report from someone who's tracking failed.

Where did the meniscus mirror idea come from?

I first thought of meniscus mirrors when looking at slumped cellular mirrors circa the year 2000. The face plate is a constant thickness slumped piece of glass, typically 1 inch [2.5cm] thick, that is fused to the cellular structure. I reasoned that the slumped face plate sans cellular support structure might work as a mirror. I obtained a 13.2x1 inch [335x25mm] F3.0 slumped mirror blank. Not only was I able to parabolize the mirror to a high degree of accuracy, but the first light views through my ZipDob revealed a new world of visual observing.

Does the meniscus shape have an antecedent in the amateur telescope making world?

Yes, in the guise of optical windows for Maksutov telescopes, popularized in the amateur telescope making world by the Maksutov Club Circulars first appearing in 1957, and by the subsequent book, "Advanced Telescope Making Techniques Volume 1 Optics" by Allan Mackintosh, 1977. Lawrence Braymer's 1954 Questar telescope featured a meniscus primary mirror. Conical shaped mirrors have a long history and are found in modern Schmidt-Cassegrain amateur telescopes. Conical mirrors that are center stalk supported sag into a close paraboloidal approximate, making them worth serious consideration.

Do meniscus mirrors have to be thin?

No, a meniscus mirror can be any thickness.

  1. The thinner the lighter the mirror. The lighter the mirror, the lighter the telescope. The lighter the telescope, the more likely it will be used. The difference in weight is significant: my 16.25x3/8 inch [410x10mm] F2.9 weighs 7.5 pounds [3.5kg]. Conventional mirrors of the same size weigh 30 pounds [14kg] or more. This makes a difference while grinding, polishing and parabolizing the mirror too. And during subsequent handling.
  2. Thinner mirrors cool down more quickly and more evenly compared to standard mirrors that are thin in the center and thick at the edge. Though very large very thin meniscus mirrors are sensitive to temperature differences between their front and back sides.
  3. Thinner mirrors are harder to support during grinding, polishing and parabolizing and harder to support in the telescope. Harder, not impossibly difficult at all.

Do meniscus mirrors have to be fast?

No. I call mirrors with a focal ratio of F3 or below 'fast'. A meniscus mirror can be any focal ratio.

After the success of my first meniscus mirror, then the success of my second meniscus mirror, a 10.5 inch [267x19mm] F2.7, I chose to work with fast thin meniscus mirrors. I use three-fourths inch [19mm] thick plate glass, commonly available as glass table tops and quite inexpensive.

Fast mirrors favor wider fields. And maximize etendue. Results with these fast meniscus mirrors speak for themselves. Just a few of my many examples include my (re)discovery of the Pleiades Bubble, Integrated Flux Nebula (IFN) or galactic cirrus and tidal streams in far away galaxies.

How does your thin meniscus 25 inch [64cm] mirror compare to John Dobson's 24 inch [61cm] f6.5 porthole made fifty years ago?

My mirror differs little. Here is a comparison:

Who made the first thin meniscus mirror?

The first ultra-thin meniscus mirror that I saw was made by David Davis in 2005. His mirror really turned my head, a 16.25 x 3/8 inch [41x1cm] thick slumped F3. The mirror was supported on a bed of compliant 1/4 inch diameter heads that looked like marshmallows arranged in a grid. The regular figure with complete absence of astigmatism was astonishing. Here is David on the Oregon Star Party Telescope Walkabout.

Also check this out 30 inch that is 1/2 inch thick, made in 1991 by E. Arthaud of France.

Do I need a kiln?

Tom Otvos acquired a kiln, slumped a flat piece of glass, ground, polished and parabolized his mirror. Then built the telescope. So did David Davis. Monumental efforts for sure. You do not have to go that far. We've acquired 16 inch [41cm] slumped mirror blanks from DOTI.

What's so special about fast thin meniscus mirrors?

For more on etendue, see

Whoa - what's this strain in the polarization test?

Actual images from Rob Brown (16 inch - note that this image also shows stress birefringence induced by edge contact). This mirror is sufficiently annealed.

Aren't meniscus mirrors unconventional?

Sure they are.

Unconventional thinking

For me, meniscus mirrors are part of a larger unconventional strategy aiming for unconventional views through the eyepiece of ever more ergonomic and usable telescopes. Here is a partial list of unconventional features that I use.

A question of quality?

Do large diameter meniscus mirrors work?

Yes. The proof is in the views and ergonomics.

As a founding member of the Altaz Initiative group whose aim is to make 1-2 meter scopes an order of magnitude cheaper, I decided to investigate ever larger curved cellular mirror face plates. I reasoned that at some point, the thin meniscus mirror will prove impractical. What is the largest thin meniscus telescope mirror that can be made by an amateur using ordinary means?

The 16 inch was a joy to make, taking a couple three months. It just works. The 25 and 30 inch took much longer to make and are more temperamental when observing with their thermal issues. Nonetheless, these mirrors show me sights in the sky I would not have imagined.

What are your results?

I contend that the value of a telescope is in the observations made. While specs are important in making the scope, it is the view through the eyepiece or the value of the digital image that counts.

I have made countless drawings. You can find them here:

Comparing a meniscus to a flat back mirror

Ronchi unwrap tests of one of the 30x5/8 inch [760x16mm] F2.7 mirrors and my old 24x1 1/8 inch [610x29mm] F5.5. Both mirrors tested horizontally while supported by a 18 point back side flotation system and a sling for the edge support. The 24 inch's center thickness is 27/32 inches [21mm].

What about small meniscus mirrors?

The smallest meniscus mirror that I have made is the 10.5 inch [27cm]. I made a smaller fast mirror, a 6 inch [15cm] f2.8, from a standard thickness Pyrex blank. It makes little sense to slump smaller mirrors. The weight of a standard thickness blank is light enough and the sagitta or center depth is minimal.

How flexible are thin meniscus mirrors?

Quite flexible. The meniscus shape gives it an overall stiffness, but when considering bending and shear from a point to a nearby point, there is no difference compared to a flat backed mirror.

Here is a chart from my webpage on the 30 inch

 description   diameter   thickness   R/e   R^4/e^2   mirror cell design   edge support
my 6 inch F2.8 Pyrex 6 1.00 6 81 3 pts 2pt @90deg
my 8 inch F6 8 1.125 7 202 3 pts 3 pt
my 10.5 inch F2.7 meniscus 10.5 0.75 14 1351 3 pts 2pt @90deg
my 13.2 inch F3.0 meniscus 13.2 1 13 1897 3 pts 2pt @90deg
my 20.5 inch x 2 inch thick F4.8 20.5 2 10 2760 9 pts 2pt @90deg
my Dobsonian 16 inch F5 portholes 16 1 16 4096 carpet or 9 pts sling
my 24 inch F5.5 24 1.4 17 10580 18 pts sling
my 30 inch F4 Pyrex sheet glass 30 2 15 12656 18 pts sling
my 16.25 inch F2.9 meniscus 16.25 0.4 40 25600 6 pts sling
John Dobson's 24 inch 25.5 1 26 26427 18 pts sling
Steve Swayze's 40 inch F5 40 2 20 40000 27 pts sling
my 25 inch F2.6 meniscus 25 0.56 44 77160 9 rings 2pt @90deg
my 30 inch F2.7 meniscus 30 0.63 48 127551 18 pts sling or central hub

Does the backside of a meniscus mirror need to be ground?

Probably. There is often wedge (one side of the mirror is a few thousands of an inch [0.1mm] thicker than the other side). This can lead to low order astigmatism (the thinner portion of the mirror blank flexes a tiny bit more than the thicker portion).

We've also discovered that if the backside is ground, then it should be fine ground through at least 220 grit. See the discussion on stress in my 16 inch mirror log, steps #1 and #2.

How do you design the mirror cell?

Just like a standard mirror cell, except that the supports (either flotation or astatic) must take into account the curvature of the mirror's back. The edge support needs to be centered on the center of gravity. Any deviation from supporting the edge at the center of gravity results in noticably deformed star test images. A sling works well, with the proviso that the sling be well designed.

Note that the edge support goes through the meniscus mirror's center of gravity by virtue of touching the mirror's back edge for the 16, 25 and 30 inch mirrors. That is, the mirror's sagitta happens to equal the mirror's thickness.

As far the edge support goes, here is R.N.Wilson's analysis from his books, "Reflecting Telescope Optics I and II"

Do larger aperture have thermal issues?

Yes.

As anticipated with something new, there are surprises. Reality is not a perfect fit with speculation. Only by building do we know; only by iterating do we solve. As it turns out, the thinness of the mirrors is not a major obstacle, not even the extreme parabolization.

If considering a very large meniscus plate glass mirror, be prepared to wrestle with thermal issues. Since this is 'engineering', it is likely that this will be solved then standardized just as thermal issues with previous mirrors were.

I hear that you only use your thin meniscus mirrored telescopes at low magnifications, implying a lack of optical quality. Is this true?

No.

I use my telescopes at the maximum power afforded by my eyepiece collection. Less maximum magnification than an equivalent F5, but adequate.

Here are high magnification examples.

How many thin meniscus mirrors have you made?

Six, all F3 and faster and 1 inch and thinner.

Diameter (inches) Thickness (inches) Focal ratio Mirror weight (pounds) Mirror blank source
(two) 30 0.63 2.7 38 David Davis
25 0.56 2.6 25 Greg Wilhite
16.25 0.4 2.9 7 DOTI
13.2 1 3.0 12 Richard Schwartz
10.5 0.7 2.7 5 Richard Schwartz

Who else has made/makes meniscus mirrors?

Check out this 30 inch that is 1/2 inch thick, made in 1991 by E. Arthaud of France. Also see here from here.

Also see Rik ter horst's work here (16 inch) and here (24 inch).

Tom Otvos slumped and parabolized a thin 14 inch F2.6.

BVCTek has made/makes thin meniscus mirrors using laminated glass layers. Chris Fuld's 41 inch is an example.

Norm Fullum makes thin meniscus mirrors. See the posting describing the mirror and scope.

How difficult is it to parabolize large fast mirrors?

It is very difficult: lots of waves of correction, lots of mirror area, highly flexible glass.

Here are the issues I faced.

  1. Learning how to make large pitch laps
  2. Thin mirrors flexing while polishing and on the test stand
  3. Thermal effects during polishing (the heat of polishing overcorrects plate glass)
  4. Zones: their interpretation and how to deal with them

I invented a new way of parabolizing large mirrors. Mirror on top of a full sized spider tool. And wrote a software calculator to help me shape the lap. Check out problem #10 onward in my log of making the 25 inch mirror. And my pitch lap calculator.

Testing the mirrors is also quite the challenge. I refined my Matching Ronchi test to detect subtle variations in the Ronchi band positions. And I finish parabolizing using the star test. See the mirror logs. Particularly instructive are the seven star test parabolizing sessions that I used to finish the 16 inch mirror.

After the difficult challenge of the 25 inch and the two 30 inchers, the 16 inch was a joy to make. It took me 4.5 hrs of parabolizing time to finish the mirror (the 2nd time around!).

Making two 30 inch mirrors at the same time (what I did to one, I did to the other, regardless of effect) proved that most of the time working a mirror is spent figuring out how to make that specific mirror: its diameter, thickness, and focal ratio. Once a particular mirror size is pioneered, subsequent similar mirrors go relatively quickly.

Are thin meniscus mirrors ready for the mainstream?

Sizes up to 20 inch are fairly well understood; sizes to 30 inches can experience temperamental thermal issues and need to be ventilated properly.

What size should be my first meniscus mirror project?

Somewhere between 10 and 16 inches [25 to 41cm].

The 10 inch and 13 inch mirrors fine ground, polished and parabolized like other mirrors. I used a mild parabolizing lap. The Matching Ronchi got me to snap focus. I used the high power star test to finish the parabolizing. In lieu of the star test the Bath IF can be used. The form factor is small: relatively small physical size and lightweight. The views are eye popping: rich wide fields, serious planetary detail at high magnification.

The 16.25 inch mirror [41cm] started 0.5 inches thick and ended at 0.4 inches thick [12mm to 10mm] due to fine grinding the front and grinding the back regular. The thinness was not an issue; in fact, I used the thinness to my advantage to preferentially polish a high zone by pressing down harder on the mirror's back over the high zone. I encountered thermal effects during parabolizing but not any in the sealed tube assembly while observing during dropping nighttime temperatures. The scope gives steady well corrected high power star images at high power (20x per inch of aperture) as soon as I carry the scope outside. The widest angle views are 1.5 to 2 two degrees.

The 16 inch appears to be the sweet spot. After all, it was the aperture chosen for the first standardized Dobsonian telescope design. With a fast thin meniscus, the form factor is shrunk to an 8 inch [20cm] scope.

Here's a snap of the 16.25, 10.5 and 30+6 inch [41, 27, 76+15 cm] scopes.

Pivoting to Night Vision with home silvering

Sometimes when pursuing something new, an unexpected opportunity comes along in a quite different direction. Pivoting. Night Vision devices on home silvered (not aluminized) very fast mirrors give mind-blowing views. Here is an image of the Pillars of Creation in M16 that matches the view through the 30 inch equipped with a NV device.

Here is my drawing of the Pelican Nebula with my 30 inch followed by an image through Howard Banich's 30 inch, the second 30 inch mirror that I made, that I grabbed from the Wikipedia page then reprocessed it to look like the view through the NV device. Image attribution: By Urmymuse - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=107860278

Retrospective

The good:

The bad:

The ugly:


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