Astronomy Principles and Practice Fourth Edition.pdf

X-ray astronomy 383
Figure 23.2. (a) The principle of an x-ray modulation telescope, (b) the form of the output signal for a point
source as the telescope is panned.
of collimator is limited. For low energies, the x-rays may be reflected off the walls of the tubes, so
accepting radiation from a wider range of angles on the sky. For high energies, the radiation can pass
through the walls of the tube and any sense of directivity is completely lost.
A more sophisticated system is in the form of the modulation collimator. This comprises two
(or more) parallel gratings separated by a small gap. A schematic diagram of the device is depicted in
figure 23.2(a). Full intensity is passed when the x-ray source is on axis and also when it is at directions
θ givenby2d/s, 4d/s,...,etc. If the collimator is scanned across the sky, then a given source will
provide a sine-wave modulation as illustrated in figure 23.2(b). Other sources will provide similar
signals but with a phase according to their position in the sky. The addition of several gratings to the
system introduces modulations at other frequencies. By investigating the amplitudes and phases of the
various frequencies within the combined signal by Fourier analysis, the spatial image and positions
of the sources can be mapped. Orthogonal sets of gratings provide a means of determining the twodimensional
image distribution.
Direct imaging can also be achieved by using metallic reflection at grazing angles of incidence.
The cross section of a simple design is shown in figure 23.3(a). For 2 keV x-rays, reflection efficiencies
of the metal mirrors are the order of 50%. In order to prevent the image plane receiving radiations from
objects not on axis, a central occulting disc is required and a larger collection area can be achieved by
nesting a series of basic mirror systems as depicted in figure 23.3(b), the design being referred to as a
Woltjer I system.
Following the first detection of celestial x-ray sources by transient rocket platforms, major steps
were achieved by the first sky scanning instruments such as UHURU and Ariel V. With the launching
of the Einstein Observatory in 1978 with a grazing mirror telescope, the sensitivity was sufficient to