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X-ray Telescopes

The Need for Focusing Cosmic X-rays

Prior to the introduction of imaging optics into X-ray astronomy, the most sensitive X-ray instruments consisted of collimated detectors with large collecting areas. A large collecting area was required in order to obtain a sufficiently strong signal from the relatively weak X-ray sources, in the presence of a large background signal. Placing a collimator in front of a large-area detector restricted the size of the sky from which a signal was collected at any time, and thus reduced the background signal when the detector was pointed at a source. For very bright X-ray sources, this approach is still adequate, and can still lead to major scientific advances (thus the rationale behind the Rossi X-Ray Timing Explorer with its array of large area proportional counters). There are practical limitations on how large an array of proportional counters or how restricted a collimator one can construct. Thus, a collimated detector cannot detect any of the many thousands of weak X-ray sources that comprise the background as seen by proportional counters.

One concept for increasing the ability to detect weaker sources is the use of an X-ray telescope to create an image of a portion of the X-ray sky. In much the same way as an optical telescope increases the ability of the human eye to see faint stars, an X-ray telescope can in principle concentrate the light from an X-ray star onto a small portion of an electronic eye. If that electronic eye is able to record the location where the X-ray signal impinges upon it, then the effective background signal from the sky is reduced dramatically to just that amount coincident with the source location. Equally important, such an "imaging detector" can view several X-ray emitting objects simultaneously, or can create pictures of regions from which diffuse X-ray emission arises. While the appeal of an imaging X-ray system is obvious, the means by which one actually constructs an X-ray telescope required many years to develop after the birth of X-ray astronomy. This is due primarily to the tricks one must employ in order to bring a beam of X-rays to a focus.

Contributed by Rob Petre of the Laboratory for High-Energy Astrophysics, GSFC

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