Picture the solar system’s largest telescope, a telescope as long as the island of Manhattan, incorporating a lens the size of a football field: an instrument possessing the resolution to examine earth-like planets around neighboring stars light-years away. Now picture a paper airplane. Could the latter lead us to the former? The answer is: perhaps.
At the turn of the millenium, it was the brainchild of a group of innovative scientists at one of the world’s most secretive research facilities, the Lawrence Livermore National Laboratory in Livermore, California, located about 40 miles east of San Francisco. LLNL, or “The Lab,” as it is locally known, is run by the Department of Energy with a primary mission of studying things nuclear — both reactors and bombs. Within its brain trust, a lot of interesting and out-of-the-box scientific ideas get hatched and built in ancillary fields ranging from advanced computation to genomics. The Eyeglass is one such out-of-the-box idea — it would be about 22,000 miles out of the box, in geostationary Earth orbit.
The Eyeglass concept is based on a few simple, well-known facts about telescopes, namely: bigger is always better. Bigger gets you light-gathering power; larger telescopes can see dimmer objects because bigger apertures collect more light from dim objects. Bigger also gets you sharper images at high magnification. The fundamental limit on image sharpness, called the “diffraction limit,” depends on the diameter of the collecting aperture; larger apertures give sharper images. On Earth, for apertures above a few meters, atmospheric distortion limits the resolution of terrestrial telescopes, but in space, there is no atmospheric distortion and the achievable resolution rises steadily with size; if you build it, it will see. The largest optical telescope currently in space, NASA’s Hubble Space Telescope, has an aperture of 2.4 meters, which has allowed it to image some of the most distant objects in the known universe.
At the time it was launched, the Hubble Space Telescope was the most ambitious optical instrument in space and after a somewhat rocky start, it became one of the great success stories for NASA. An astronomical success leads inevitably to the question: what can you do for a follow-on? At Livermore, they don’t look for incremental improvements. Roderick Hyde and the Diffractive Optics Group at Livermore started with a nice, meaty goal: how can you put a telescope in space some FORTY times larger than the Hubble Space Telescope?
Well, the first thing you have to do is reexamine how you design a telescope. The Hubble telescope, like many others, uses a single very large, curved reflector machined to incredible precision as its primary light-gathering element. The design goal of Eyeglass was to expand the main element to a diameter of 100 meters — about a football field. Making a single reflector 100 meters in diameter with an optical precision of roughly 1/100 of the diameter of a hair was out of the question; even if you could make the reflector, how would you loft it into space?
Most high-performance telescopes, like the Hubble, are “reflective” telescopes. Their main optical element is a curved mirror. The big advantage of using a mirror is that it lets the telescope be fairly short, just a few times the diameter of the lens. (the Hubble, with a 2.4 meter diameter, is 13 meters long.) Most people’s conception of a telescope is a bit different than this: the common notion of a telescope is a long tube with lenses, not mirrors, at each end. This old-fashioned type of telescope is called “transmissive.” Transmissive telescopes are by their nature quite long, with a distance between the two lenses that is many times larger than the diameter of the main lens. A transmissive telescope with a 100 meter diameter lens would need to be thousands of meters long. This does not seem, upon first consideration, like a good thing.
But if you’re putting a telescope in space you can take advantage of one of the features of its destination: there’s a lot of space in space. When the nearest interfering object is forty thousand kilometers away, a distance of a kilometer or two doesn’t matter much. Even better, you don’t actually need to build a tube between the two lenses; you simply put your main lens into one orbit, and then put the other lens (and the camera and associated electronics) into another orbit a few kilometers away.
Another nice thing about long transmissive telescopes is that the mechanical tolerances required on the optics are much less than the requirements on a reflective telescope, permitting a thinner, lighter optical structure. In fact, the main lens of the Eyeglass could be as thin as a sheet of paper, if it were an unusual type of lens called a “diffractive lens.” A diffractive lens is a microscopic pattern of marks (or ridges) on a sheet of glass or plastic that effectively turns the surface into a hologram of a lens. In its simplest form, a diffractive lens consists of a series of circular grooves machined into the surface. (A simple version of such a lens, called a Fresnel lens, is commonly used in overhead projectors.) The Eyeglass would use as its main lens a 100 meter diameter diffractive lens, which could be a thin membrane. A thin plastic lens wouldn’t be very stiff or strong, of course, but in orbit, with almost all effects of gravity cancelled by orbital motion, the lens would experience free-fall conditions; little stiffness required.
Once you’ve solved the many problems in building a 100-meter diameter thin plastic lens for space (and there are a lot), you still face a big problem: How do you get it up there in good shape? All currently available space vehicles, whether shuttle or heavy-lift rocket, have about the same payload space: about 4 meters diameter and about 10 meters long, give or take a bit. This means that a 100-meter sheet of plastic is going to have to get crumpled, folded, or otherwise stuffed into the tube of the rocket, like a sleeping bag going into a stuff sack.
And that’s a bad thing, because although transmissive lenses have looser tolerances than reflective elements (and diffractive lenses have, in some ways, even looser tolerances), one thing they can’t tolerate is being crumpled up. Any undesired fold, wrinkle, or buckle in the surface of the Eyeglass diffractive lens would irreparably damage the optical performance,. The only way such a surface was going to go into a rocket would be if it were collapsed into a smaller shape along a precise, controlled set of creases, whose locations and structure could be chosen in such a way as not to degrade the optical performance.
Hmmm…flexible planar surface, lots of folds…sounds like origami. Well, Rod and his engineers thought so, too, and so they did what all good engineers do when presented with a new problem: they did a literature search. It turns out that origami has been to space before. Back in the 1980s, an origami-based folding solar panel designed by Koryo Miura flew on a Japanese satellite. While Miura’s solar structure was not applicable to the Eyeglass, Rod and his team thought (correctly, as it turned out) that there might be other origami structures that might just work.
And there were. Rod’s research led him to my own work in computational origami, and a phone conversation revealed the happy coincidence that I lived just 5 miles away from LLNL. I paid him and his team a visit, where he laid out the story of Eyeglass pretty much as you’ve just read it, and we were off. Over the next few months, I met with the Eyeglass team several times and adapted several different structures from origami usage to telescope application. We needed a structure that was radially symmetric (so that it could be spin-stabilized), that collapsed on a finite number of creases, that ultimately fit within a cylindrical form factor (i.e., a rocket). After considering several different configurations, the team settled on an origami structure, which we called the “Umbrella” structure after its resemblance (in the furled state) to a collapsible umbrella, that was scalable, had mass-producible parts, and folded from a large flat disk down to a much smaller flanged cylinder.
Of course, origami is the tiniest tip of the iceberg of engineering that needed to be carried out to develop lenses, hinges, assembly and alignment, support, et cetera. Many design issues arise only after you get into the nitty-gritty of building something real, and so the Livermore team set out to do exactly that. With funding from LLNL and DARPA, over the course of a year, the Eyeglass team built a 5-meter prototype to test out the folding structure and the ability to build and assemble the individual facets of the lens into a complete folding structure.
The figure at the top of the page shows the completed 5-meter prototype (behind the author). The lens was tested by hanging it in the rig shown and firing an expanded laser beam at it from about 100 meters away, then examining the focal spot at a similar distance on the other side. Although the first prototype was not built with diffraction-limited components to reduce cost, it worked and performed as predicted. The next phase of work at LLNL was to be further analysis and evaluating alternate segmenting strategies, as well as looking at scaling to even larger sizes. Anyone for a kilometer?
I say future work "was to be" because subsequent work was not funded—at least, not as far as I know. And why make that qualification? Well, there's two directions such a telescope can point: it can look up; and it can look down. Those folks who were interested in the down-looking versions certainly wouldn't be telling the local origami consultant about any further development. But next time you look into the night sky, if you see a twinkle that doesn't seeem to be in the right place, look hard: it just might be a bit of origami looking back at you!*
For further description of the Eyeglass, see the LLNL Diffractive Optics Group web page.
*That's a joke. You wouldn't actually be able to see it.