Dec 8, 1996

Unique telescope to open the X(-ray) Files







Artist's concept of AXAF in orbit., The nested mirrors are at center behind the dotted circles.

The finest set of mirrors ever built for X-ray astronomy has arrived at NASA's Marshall Space Flight Center for several weeks of calibration before being assembled into a telescope for launch in late 1998.

The High-Resolution Mirror Assembly (HRMA), as it is known, will be the heart of the Advanced X-ray Astrophysics Facility (AXAF) which is managed by Marshall Space Flight Center. HRMA was built by Eastman Kodak and Hughes Danbury Optical Systems. In 1997-98, they will be assembled by TRW Defense and Space Systems into the AXAF spacecraft. AXAF is designed to give astronomers as clear a view of the universe in X-rays as they now have in visible light through the Hubble Space Telescope.

Indeed, one of the Hubble's recent discoveries may move near the top of the list of things to do for AXAF. Hubble recently discovered that some quasars reside within quite ordinary galaxies. Quasars (quasi-stellar objects) are unusually energetic objects which emit up to 1,000 times as much energy as an entire galaxy, but from a volume about the size of our solar system.

More clues to what is happening inside quasars may lie in the X-rays emitted by the most violent forces in the universe.

Before AXAF can embark on that mission, though, its mirrors must be measured with great precision so astronomers will know the exact shape and quality of the mirrors. Then, once the telescope is in space, they will be able to tell when they discover unusual objects, and be able to measure exactly how unusual.

These measurements will be done in Marshall's X-ray Calibration Facility, the world's largest, over the next few weeks.

AXAF will use four sets of mirrors, each set nested inside the other, to focus X-rays by grazing incidence reflection, the same principle that makes sunlight glare off clear windshields. AXAF's smallest mirror - 63 cm (24.8 in.) in diameter - is larger than the biggest - 58 cm (22.8 in.) flown on the Einstein observatory (HEAO-2) in 1978-81.

Mapping the details of the mirror will start with an X-ray source pretty much like what a dentist uses to check your teeth. But that's next week's story.