Nov 3, 2006

AXAF calibration


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Artist's concepts of AXAF in orbit (left) and
showing the mirrors at the heart of the telescope.


X-ray Telescope Challenges
Scientists and Engineers

Surprise is everywhere in the universe. Peer through a better microscope or telescope, and you discover details, subtleties, and even oddities that you might only suspect with that less sophisticated instrument.

That is true of X-ray astrophysics, a branch of astronomy that barely existed four decades ago. Now it is one of the most dynamic fields in science. X-rays are photons, like the light that we see, but at much higher energy and thus have much shorter wavelengths. Because they carry more energy, they are created by more energetic or violent events than what we see with the eye or optical telescopes.

The universe looks different in X-rays - "empty" space between stars and galaxies often is filled with superhot gas, like the galaxies below. We need to see the universe in X-rays (and radio, infrared, ultraviolet, and gamma rays as well as visible light) in order to understand what is happening out there. Restricting our view to one part of the spectrum is like listening to just the middle keys of a piano: you miss most of the music.

Earth's atmosphere completely absorbs X-rays, meaning that this is one of the youngest astronomical disciplines. It was impossible until we could fly instruments aboard rockets and satellites.



Abell 1367 optical
Abell 1367 X-ray


The differences in visible and X-ray views of the universe can be seen in these different pictures of the Abell 1367 cluster of galaxies. The optical view (left) shows isolated islands in space. An X-ray view (right) shows the void between filled with hot gas. The colors indicate intensity and do not correspond to colors that the human eye sees.


These X-ray instruments have revealed many surprises in our galaxy and beyond. Most of these discoveries have yielded one of the most precious things you can find in science, more questions. Answering those questions has challenged us to build larger, more precise X-ray telescopes.

The most sophisticated of these telescopes, the Advanced X-ray Astrophysics Facility (AXAF), is being prepared by Marshall Space Flight Center and a team led by TRW Defense and Space Group for launch in late 1998. With an X-ray telescope larger and more precise than any yet flown in space, AXAF will produce sharp, magnified images in X-rays to complement those made by the Hubble Space Telescope in visible light.

At least that's what we believe. Before we launch AXAF, before we finish assembling the telescope, we have to prove it. To do that, AXAF's high-resolution mirror assembly (HRMA) has been sent by its builders, Eastman Kodak Co. and Hughes Danbury Optical Systems, to Marshall for several months of tests in the X-ray Calibration Facility.

The tests will map the exact figure so astronomers know how sharp and clear an image it will produce - and how to correct for the telescope's minor imperfections.

Some images courtesy of the AXAF Science Center, Boston.

To get really detailed...

about AXAF, visit Marshall's X-ray Calibration Facility, the X-ray calibration test conductor, or the AXAF Science Center which includes an excellent science education outreach program.

Updated Feb. 4, 1997

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