Astronomy Principles and Practice Fourth Edition.pdf

384 High energy instruments and other detectors
Figure 23.3. (a) A simple arrangement for an imaging x-ray telescope, (b) the simple imaging principle employed
in a nested x-ray telescope.
allow investigation of a significant sample of normal stars and galaxies. The instrument was able to
tackle the problem concerning the nature of the diffuse hard x-ray background with the result that it
was probably due to a large number of very faint, distant sources, the conclusion being a little more
definite for low-energy x-rays.
In the following two decades, satellite missions included EXOSAT, GINGA, ROSAT, ASCA
and BeppoSAX but with barely any improvement on spatial resolution. During this period, any
imaging was substantially worse by factors between 5 and 50 relative to most other wavelength
domains. The situation has now been redressed by Chandra W 23.1 launched by NASA in July 1999
and XMM-Newton W 23.2 launched by ESA in December 1999. Chandra has provided x-ray images
at sub-arc second resolution putting them on a par with pictures by ground-based optical and infrared
telescopes. XMM-Newton (see figure 23.4) with its large collection area and multiple nested mirrors is
particularly sensitive. Both systems carry grating spectrometers with specially developed CCD chips
as detectors. Individual platforms such as HESSI (High Energy Solar Spectrometry and Imaging) have