Cross of Gold

X-rays images are difficult to make, since X-rays easily pass through most materials without focussing. To focus X-rays into an image of the sky, astronomers use special mirrors in which the X-rays skip off the mirror surface at grazing incidence, much like a rock skips off the surface of a lake. Most X-ray mirrors have been made of glass, which must be highly polished in order to focus accurately the incident X-rays. For example the best X-ray mirrors ever built are on the Chandra X-ray observatory. These mirrors are so smooth, it's as if the earth has been bulldozed so that the highest mountain is less than 2 meters high! But such mirrors are difficult to make and expensive to launch.

An alternative type of X-ray mirror has been constructed using, not glass, but aluminum foils coated with a thin layer of gold. Such foil mirrors are much lighter than glass mirrors and so are cheaper to use in X-ray satellite observatories. Because hundreds of thin foils can be used to create a single mirror, such mirrors can be much more efficient at gathering X-rays from astronomical sources. These mirrors however do not produce the exquisite images that can be obtained by mirrors like those on Chandra. The image above shows an X-ray star observed by the ASCA satellite, the first X-ray observatory to use thin foil X-ray mirrors. The thin foil mirrors produce the signature "cross" shape due to the the mirror support structures.

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Each week the HEASARC brings you new, exciting and beautiful images from X-ray and Gamma ray astronomy. Check back each week and be sure to check out the HEAPOW archive!

Page Author: Dr. Michael F. Corcoran
Last modified December 1, 2000