Jerry NeIson, W. M. Keck Observatory
Atop Hawaii’s Mauna Kea, the world’s biggest telescope opened its eye last March and gazed across the cosmos. Eight years in the making, the W. M. Keck Observatory used its ten-meter primary mirror to capture the light of a quasar 13 billion light-years away and reveal new objects in the most distant galaxy astronomers have ever found.
Keck’s enormous reflector--a mosaic of mirrored segments that stretches more than 150 inches wider than its biggest working rival--has shattered a major barrier. Among telescope mirrors, size is paramount. Larger reflectors gather more light and resolve dimmer, more distant objects. But until Keck opened, astronomers had hit a glass ceiling in their climb toward giant mirrors of the traditional monolithic type. The obstacle was gravity. The broader the reflector, the thicker it must be to keep its precisely sculpted profile from warping under its own weight. Not only does a thick slab of astronomical glass come with an astronomical price tag, it requires an exorbitant support structure. At five meters across, the massive reflector of the Hale Telescope on California’s Mount Palomar was thought to represent the outer limit of conventional mirror making.
To push this limit, Jerry Nelson, an astrophysicist at Lawrence Berkeley Laboratory, proposed in 1977 the idea of a segmented mirror made of 36 thin glass hexagons. Each hexagon would be small and light enough to resist the tug of gravity. Nelson spent the next two years polishing the idea.
Although the segmented mirror relieved the weight problem, it came with its own burden of engineering challenges. For example, because the segments must fit together to form a bowl-shaped mirror, each would have to be polished down to an asymmetric profile, somewhat like the contour of a potato chip. Nelson also wanted to give the overall mirror as sharp a curvature as possible, so that it would fit into a compact domed enclosure. That demand only made each individual segment more difficult to polish.
Nelson and colleagues cast about for a solution. If you need something esoteric, someone long ago figured out how to solve your problem, says Nelson. And if you have enough engineers, someone will remember. In this case, that someone was Jacob Lubliner, a professor of civil engineering at the University of California at Berkeley. He and Nelson adapted a trick invented 60 years ago by a German engineer named Bernhard Schmidt.
The technique, called stressed-mirror polishing, involves warping a plate of glass by bending its edges, then imparting a simple spherical curvature to its face. When the glass is released, it snaps back into the desired aspherical shape. Nelson’s group spent more than two years calculating the profile of the segments and assembling the hardware to shape the mirrors.
Meanwhile, an equally demanding chore loomed. To hold the segments in alignment would require a fiendishly meticulous control system, able to stop one 880-pound slice of glass slipping away from another by any more than one one-thousandth the diameter of a human hair. All this while the entire mirror slowly swings to track heavenly objects that wheel overhead in the night sky.
The solution, which took six years to perfect, is an unprecedented marriage of precision sensing and motion control. Each segment has several electronic sensors around its rim and is supported by three precision motor-driven pistons. Each segment’s sensors monitor its relative position to its neighbors. When a segment shifts, the sensors alert a central computer, and twice every second the computer activates one or more of the pistons to nudge the segment back into position. So precise is the mechanism that it can correct a displacement of .2 millionths of an inch.
In 1984, Nelson demonstrated the control system on a single full- size reflector segment shaped with the stressed-mirror polishing technique. The following year, with a $70 million grant from the W. M. Keck Foundation, the University of California and Caltech joined forces to construct the $95 million telescope.
Today the observatory is mechanically complete, and its identical twin, Keck II, is under construction 90 yards away. Nelson, who serves as project scientist for the telescope, along with project manager Gerald Smith, is busy integrating the cameras and other instruments onto Keck’s mirror. So far, the telescope has lived up to its billing as a device that will double the viewing range of astronomers. And in Nelson’s eyes, the segmented mirror is a reflection of the future of astronomy.
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