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Will they work?

The AXAF mirrors have been brought to Marshall not just to prove that they work as advertised, but to map them in detail.

In order for scientists to use the data from AXAF, they have to know some basic properties about the mirrors. One thing that is already know is that they are not absolutely perfect. That's impossible. Anything manmade will have minor deviations from perfection. Even the glue that binds the mirrors to the graphite epoxy truss pulls every so slightly and causes small imperfections in the mirror. These are within limits for the astronomy we want to do with AXAF -- but we need the numbers to remove these imperfections from the AXAF images.

But how can you be sure it's within limits?

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Marshall's X-ray Calibration Facility has been used since the 1970s to test and calibrate X-ray telescopes and other instruments designed to observe the sun and stars from suborbital rockets and satellites. An X-ray source shines down a half-kilometer-long tube (first picture) to instruments placed in a large vacuum chamber at the opposite end.

You put AXAF in front of point source and measure what comes out the other end. The devil, of course, is in the details, and they range from small crystals that will fine-tune the X-rays source, to special mechanisms to reshape the mirror much as it will when it is in space and gravity no longer deforms it.

Marshall's X-ray Calibration Facility is the world's largest and best. The vacuum chamber in which AXAF will sit is large enough to hold anything the Space Shuttle can launch (including modules for International Space Station; they use the chamber next). That's the receiving end of the facility where two rounds of tests will be run, first with the mirrors alone, then with the cameras and spectrometers that will take pictures and record the energies of X-ray sources (but that's another story).

The "sending" end of the facility is an X-ray source 518 meters (1/2 km) away. This distance is not to protect anyone from radiation - there's plenty of shielding for that - but to make it appear that the source is almost as far away as a star.

Half-kilometer is not the same as a light year, but for testing the mirror, it's good enough. Only a few of the X-rays will pass through a pinhole and travel down a vacuum tube to the test chamber. By then, the rays are virtually parallel to each other, just like light from a star.

One of the X-ray sources is not too different from what your dentist uses. An electrical filament boils off electrons (just like the back of a TV picture tube) which are magnetically focused on a target. When the electrons stop, they release their energy of motion as X-rays, and excite the target atoms to emit X-rays.

These then pass through one of several pairs of crystals to clean up and fine-tune the X-ray beam. We use the spaces between molecules and atoms in the crystals as a kind of gate. We tilt the crystals to the right angle and the crystal admits then bends X-rays with a wavelength about the same size as the gap. A second crystal helps clean and redirect the beam some more so the AXAF mirrors a kilometer away see X-rays of one "color."

After scientists finish calibrating AXAF's mirrors, they will mount the instruments behind the mirrors, much as they will be in flight, and calibrate their performance. But that's a story for next year.

Return to AXAF main story.


Updated Feb. 4, 1997

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Authors: Dave Dooling
Curator: Bryan Walls
NASA Official: John M. Horack