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To achieve revolutionary science, Constellation-X will carry larger X-ray mirrors than any mission ever flow in space. With its giant mirrors, Con-X will catch X-rays from faint sources, and will more efficiently collect X-rays from bright objects.
But each individual X-ray "light particle," or photon, packs roughly 500 times more energy than a photon of visible light, meaning that conventional optics have difficulty bringing them to a focus. To get around this problem, X-ray astronomers build cylindrical mirrors nested in a tube-shaped support structure, so the photons bounce off the mirrors at grazing angles—like skipping stones across water.
 | | Fabrication of the Chandra mirror. |
Chandra's mirrors were built as a series of four concentric shells inside a tube 33 feet long. Each mirror is about 1.5 inches (40 millimeters) thick. The mirrors and the assembly holding them together collectively weighs 1 ton (900 kilograms). Chandra itself weighed 5.3 tons (4,800 kilograms) before going into orbit, making it one of the heaviest payloads ever launched by NASA's Space Shuttle. Overall, Chandra's mirrors had 124 square inches (800 square centimeters) of collecting area.
Con-X, by contrast, will have 32 to 43 square feet (30,000 to 40,000 square centimeters) of collecting area, about 100 times greater than Chandra's at medium-energy X-ray wavelengths, where most X-ray sources shine at their brightest. Instead of four nested shells of mirrors, it will have 230. All of this is great news for X-ray astronomers, but if Con-X used the same mirror technology as Chandra, the spacecraft would be so heavy and cumbersome that it would never get off the ground.
Fortunately, Con-X scientists and engineers have already solved this problem. They have developed a new technique for fabricating X-ray mirrors, making the mission possible and affordable. The maximum weight of the mirrors will be only 550 pounds (250 kilograms). In other words, Con-X will have vastly larger mirrors than Chandra, but they will weigh much less.
 | | HEFT nested glass mirrors |
To reduce weight, Con-X's mirrors will be only 0.015 inch (0.4 millimeter) thick, not much thicker than a sheet of paper. Yet despite the thinness of the mirrors, the glass will be strong enough to withstand the tremendous shaking and roar of an Atlas V rocket launch into space.
The mirrors will come in 12,000 individual panels that will later be fitted together. Many of the panels will have identical shapes and sizes, which minimizes the difficulty and cost of the manufacturing process.
Scientists are currently perfecting mass-production techniques to reduce costs by a factor of 50 with respect to Chandra. They wrap a sheet of glass partway around a large drum-shaped cast known as a mandrel. Next, they heat the glass for eight hours to 1,100 degrees F (600 degrees C), and then gradual cool it over 20 hours back to room temperature. With this heating and cooling, the mirror segments soften and sag until they assume the curvature of the mandrel.
Work is underway to develop techniques to mount the mirrors so that they maintain a perfect alignment during and after launch.
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