The case for gluing mirrors

Cell history

ATM cell designs — mine included — are stuck in time between Thomas Grubb's whiffletree invention first used in the Armagh Observatory's 15-inch¹ in 1835 and William Lassell's invention of astatic floatation levers for a 9-inch telescope in 1842².

The evolution of mirror cell design has been spurred by the need to reduce any astigmatism, or bending of the mirror induced by its underlying support cell. If a mirror's weight is not balanced among the support points, or if the tilt of the mirror during use induces unequal weight distribution, then the result is likely to be astigmatism.

The ATM approach

Most ATMs who build their own mirror cells almost always follow the design popularized by Dave Kriege and Richard Berry³. This design, endorsed by almost every Dobsonian vendor, uses a sling for edge support. Slings themselves have come under close scrutiny, with the design being improved to minimize unequal lateral support.

A few ATMs, however, attach their mirrors directly to their cells with silicone (RTV) pads, simplifying the mirror cell's design by eliminating edge supports altogether. And the results have generally been good. Others argue that mirrors should never be glued to their cells with RTV because a cell without lateral edge supports will also bend. Some have even justified their argument with a limited-case finite element analysis.⁴

Flotation systems

What's usually missing in the endless debate over RTV use is any mention of flotation cells. A whiffletree by itself doesn't make a flotation cell, even though many ATMs probably think so. What really provides flotation is a system of astatic, or balanced supports, employed in various forms by professional cell designers for nearly a century. So what's the principle of astatic support?

Astatic cells — how they work

The ideal cell holds the mirror so gravity won't distort its surface. To avoid this bending, an astatic cell "floats" the mirror against the force of gravity as the telescope changes its angle. To prevent any bending, however slight, the mirror's weight must be carried by reaction forces at each support point, which change with orientation.

It's as if the mirror was in a zero-gravity environment, or as if it was buoyed by forces felt if the mirror floated in a liquid of its own density, or as if air jets just held the mirror off each lateral and axial support point. A mirror then maintains its figure even if the telescope structure bends or flexes. This is true whether the mirror back is fixed to the cell or not: bending could occur in either case.

Professional solutions

Astatic cells were all passive until the advent of large, thin meniscus mirrors. The era of modern reflectors, from the G. W. Ritchey's early reflectors through the 200" Hale telescope used astatic levers attached to the back of the mirror. These were all passive with fixed points of support, usually at holes cast into the back of the mirror. The big telescopes made today employ active astatic actuators in their cells to counteract not only gravity, but deformation caused by wind or accidental telescope acceleration.

Mirrors made by the Steward Observatory Mirror Laboratory (SOML)⁵ at the University of Arizona exemplify the state of the art in cell design for large telescopes. They are found in the Phillips Lab 3.5m telescope, the MMT 6.5 conversion, the Magellan 6.5m telescopes and the 8.4m Large Binocular Telescope. All of these professional grade instruments use mirrors glued with RTV at their back plates near rib junctions to push-pull astatic actuators that provide both lateral and axial active support.

The SOML experts have written that these mirrors "could operate in principle with a passive support system, but...[the active support system] has the advantage of allowing lateral forces to be applied near the back plate rather than at the neutral surface of the mirror, because the overturning moment and deflections [read bending] can be compensated by the active axial supports." For that reason, these mirrors don't need edge supports. (SPIE Proceedings Volume 2199, Advanced Technology Optical Telescopes V, p. 251-253)⁶.

SOML also uses RTV to attach the lifting rig to their large 12 to 19 ton cast mirror blanks. They're obviously not concerned with the glue's lack of shear strength.

Will an ATM cell work without astatic levers?

So what's all this got to do with ATM cells? How could they work at all if none of them use astatic supports (except for few like Oliver Raua's⁷ telescope)?  My guess is that even today's "thin" mirrors have so much inherent stiffness that any distortion introduced by bending or overbending, even with a perfect mirror, is inconsequential because the wavefront error is dominated by seeing on those scales even in the best conditions. Bubble wrap, carpeting, whiffletrees: all seem to work.

So why worry? JMI has been delivering telescopes with glued whiffletrees for a long time, which by all accounts are anything but commercial rip-offs⁸ . I also believe some ATM cells are built without really eliminating lateral slop in the whiffletree, which could itself introduce significant bending. If you really think your mirror and seeing are so near perfection, perhaps you should redesign your whiffletree to include astatic levers. (While you're at it, you can toss those slings and use RTV!)

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Internet links

1. Armagh Observatory's Grubb telescope
2. Lassell's astatic levers
3. The Dobsonian Telescope: A Practical Manual for Making Large Aperture Telescopes, by D. Kriege and R. Berry, published by Willman-Bell
4. Robert Houdert's edge support calculator
5. Steward Observatory Mirror Laboratory
6. Abstracts: SPIE Proceedings Volume 2199
7. Oliver Raua's astatic cell
8. JMI's NGT reflector