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A Stellar Imagemaker

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

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“They had to make special structures just to build these mirrors,” Tucker told me. “They searched the world for grinding powders. Finally a guy in Tennessee developed a cerium oxide compound that was mixed with a tree sap extract from Switzerland.”

And delicate: touch the surface and grease from your fingertips could ruin it. Imagine not only building these mirrors but getting them fixed precisely in line, and so firmly that the shock of being hurled into space wouldn’t knock them a hair off kilter.

I studied a color photograph of Cassiopeia A, and it was hard to relate the picture to the first dots that appeared on the plate. Building the portrait is a laborious process, the ultimate pointillist art.

“We detect the photons one at a time and keep track of when they were found, where and how much energy was in them,” Tucker told me.

And what about the camera that records these amazing sights? There are two of them, a high resolution one, designed by Smithsonian scientists, with 69 million glass tubes in a grid for determining the exact position and arrival time of each x ray, and an imaging spectrometer, a special digital-like camera whose ten x-ray-sensitive chips contain a million pixels each to record the position and energy of the rays. Two special screening devices disperse the rays into a high-energy rainbow, like a spectroscope with thousands of distinct colors, to allow study of the chemistry of their celestial source.

“NASA’s Deep Space Network stations in Australia, Spain and California send us the data,” Tucker continued. “And we send back information saying where we want Chandra to look next, every 72 hours or so. The targets are selected by a peer review process.”

The flying observatory travels almost one-third of the way to the moon in an elliptical orbit ranging from 6,000 to 86,400 miles up as it orbits the Earth every 64 hours. On average its orbit is 200 times higher than the Hubble telescope’s.

There have been other x-ray telescopes, but Chandra can see objects that are 20 times fainter than anything they could detect.

Chandra’s resolving power is 0.5 arc seconds, which means that it could read the letters of a stop sign from 12 miles away. Or a newspaper headline one centimeter high at a distance of half a mile. On the other hand, it can observe x rays in gas clouds so wide that it takes light five million years to cross them. And it can study quasars whose light has taken ten billion years to get to us, so that we are seeing that many years into the past. I love stats.

As Edward Weiler, a top NASA administrator, put it: “History teaches us that whenever you develop a telescope ten times better than what came before, you will revolutionize astronomy. Chandra is poised to do just that.”


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