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The Question

(Submitted February 23, 1998)

I'm a student from Belgium. I'm writing a paper on applications of foil. Can you tell me why precisely you use foil for the making of the X-ray telescope. Thank you.

The Answer

Thank you for your question. The basic reason why we use foil for X-ray telescope mirrors is because X-rays only bounce at shallow angles. So the mirrors must deflect the X-rays just only a little from their path.

As you can imagine, this means that the mirrors must be shaped like a cylindrical tube.

The problem, however, with this comes in collecting area. If you have a tube shaped mirror, it will not collect very many X-rays.

The solution is to make many thin tube shaped mirrors and nest them. Put one inside the other. If they were not thin, it would be hard to put them inside each other --- hence the need for foil.

Pictures and other descriptions are at:

http://imagine.gsfc.nasa.gov/docs/science/how_l2/xray_telescopes.html

http://imagine.gsfc.nasa.gov/docs/introduction/xray_detectors.html

For more elementary information, try reading this:
http://heasarc.gsfc.nasa.gov/docs/xte/learning_center/universe/universe.html

Good luck,

Jonathan Keohane
for Ask a NASA scientist

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