 | | HXT diagram. Schematic of the Constellation-X Hard X-ray Telescope instrumentation. |
Chandra, XMM-Newton, and other X-ray observatories have been tremendous scientific and engineering successes, and have met all of their goals. But despite astronomers’ ever-advancing knowledge about the X-ray universe, previous and current satellites have not been designed to study the highest-energy X-rays, also know as hard X-rays.
Constellation-X will open this window by carrying a Hard X-ray Telescope (HXT). This instrument will study the portion of the spectrum where monster black holes, containing millions or even billions of times the Sun's mass, radiate most of their energy. The HXT will simultaneously observe many objects with Con-X's main telescope, the Soft X-ray Telescope. The combined observations will yield unprecedented information about an object’s entire X-ray spectrum, which guarantees major discoveries.
Hard X-rays pack so powerful a punch that they have to be reflected at extremely shallow angles, like skipping a flat stone across a pond where the thrower releases the rock just inches above the water. As a result, the X-rays come to a focus over 130 feet (40 meters) from where the X-rays are reflected. With this focal length, a hard X-ray telescope tube would have to be the size of a medium-size commercial airliner, which would make the spacecraft far too big and heavy to launch into space.
But scientists and engineers have largely solved this problem, and a hard X-ray telescope has been successfully tested on a high-altitude balloon flight. Two teams have proposed designs for Con-X's HXT, and an open competition will determine which one actually flies on the mission.
Both designs rely on multi-layered coatings. Conventional X-ray mirrors are made of a glass known as zerodur, and coated with nickel, gold, or iridium. But scientists discovered years ago that X-ray mirrors coated with interspersed layers of tungsten and silicon can reflect X-rays across a wider range of angles, shortening the telescope's focal length. This makes the telescope smaller, lighter, and affordable.
One of the HXT designs is similar to the mirror design of Con-X's main telescope, which will collect low-energy (soft) X-rays. But instead of having 240 shells of glass, the HXT will have 75 to 100. The technical challenge is to produce many segments of glass and fit them together perfectly to make sure the mirror maintains the correct shape.
 | | CZT detector array. |
The other design will use mirrors made of nickel rather than glass. Instead of fitting many segments together to form a shell, each shell is made in one piece. Since nickel is a denser material than glass, the shells have to be extremely thin, roughly the thickness of a sheet of paper.
The HXT will have its own detector, but the final design has not yet been selected. In all likelihood, the detector will operate more like a conventional detector with individual picture elements (pixels) detecting the hard X-rays. Most of the technology for the HXT detector has already been successfully tested. Overall, no great technological leaps are required for any aspect of the HXT.
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