X-ray Telescopes
Introduction
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 recently
launched
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.
Back to Learning Center...
Back to Home...
Any Suggestions, mail to: emrah@astroa.physics.metu.edu.tr