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A Stellar Imagemaker

Smithsonian and NASA's Chandra x-ray observatory sheds new light on the mysteries of the universe

Chandra’s high-resolution images surely will give us new insights into black holes, which are space entities so dense that nothing that ventures close can escape their gravity, not even light. Chandra’s ability to examine particles up to the last millisecond before they are sucked out of sight will enable astronomers to study the theory of gravity under the most extreme conditions.

Smithsonian’s Chandra X-ray Center operates the space-based observatory under contract with NASA’s Marshall Space Flight Center in Alabama. On my visit to the Smithsonian center in Cambridge, I needed a lot of help. (Got a D in physics in prep school.) Wallace Tucker, astrophysicist and Chandra spokesman, was able to talk me in as much as anyone could.

X rays are at the short end of the light-wave spectrum. Optical telescopes can deal with stars radiating tens of thousands of degrees of heat, but x-ray telescopes (Smithsonian, July 1998) can observe gaseous objects up to several hundred million degrees.

A wave with such fantastically high energy is extremely difficult to focus or direct. If you put a conventional telescope in front of it, the wave is simply absorbed.

But, I interrupted, what about my x-rays at the hospital? Ah, replied Tucker, those pictures are just shadows. The bones being denser than the flesh, they make a deeper shadow as the x rays pass through your whole body.

“Besides,” he added, “we’re talking about much longer distances and finer images. Like looking at a dime from four miles away.”

The solution to directing the waves was to design a mirror that would reflect the rays at an extremely shallow angle so that they would bounce off, like skipping stones on water, instead of being absorbed. Then they could be directed onto an electronic detector, stored and later transmitted to the Chandra center.

Whereas optical telescope mirrors are dishes that focus the faint beams from space, Chandra’s mirrors are barrel-shaped. Four pairs of them are nested like Russian dolls to provide a larger area for the x rays to hit.

It was not a new idea. Hans Wolter did the basic design work, a geometrical invention on paper, in Germany in 1952. In the 1970s Riccardo Giacconi successfully adapted the principle to x-ray astronomy. Giacconi moved on to other conquests in the 1980s, notably to direct work on the Hubble Space Telescope, but his team carried on here. Of course a large number of brilliant people created Chandra, but I don’t think it is too much to say that the person responsible for the unique mirrors, the world’s great expert on their design, is Leon van Speybroeck, officially the Chandra Telescope Scientist, an MIT graduate from Wichita, Kansas, who has been with the Smithsonian since the early 1970s.

“Giacconi had the idea in the 1960s,” noted Tucker, “but NASA was skeptical. The Chandra mirrors are a high point of Leon’s career.” We are talking about a mirror so smooth that if it were the state of Colorado, Pikes Peak would be less than an inch high. We are talking about smoothness to within a few atoms, smoothness that is virtually mathematical in its perfection. The mirrors are two to four feet in diameter, nearly three feet long and weigh more than a ton.


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