Astronomy Principles and Practice Fourth Edition.pdf

X-ray astronomy 385
Figure 23.4. An artist’s impression of the XMM-Newton x-ray telescope launched in December 1999 in prospect
of a 10-year mission. (By courtesy of ESA.)
been also been recently launched for extended high-energy x-ray observations of the Sun with high
spatial resolution.
As with all other kinds of telescope development, the game is to design instruments with larger
collection apertures for better flux gathering and improved angular resolution. It may be noted that
the current XEUS design project by ESA plans to increase the effective collection area over the early
mission of ROSAT by factors of ∼100 and ∼1000. In order to achieve this, a strategy needs to be
developed to assemble the telescopes in orbit. To establish the instruments, the XEUS system will
comprise two freeflyers—a mirror spacecraft and a detector spacecraft, separated so that the detector
is at the focus of the mirror. The plan is to launch the two vehicles together into a low Earth orbit and
to position them by an active orbit control and alignment system.
As the proposed aperture exceeds the capacity of available launch vehicles, the mirror spacecraft
will be designed to visit the International Space Station (ISS) to build it up with additional modules
that are launched separately. To achieve this, the detector vehicle will have the ability to dock with
the mirror spacecraft, after which the mirror spacecraft can dock with the ISS for refurbishment and
for the mirror aperture to be further increased, thus converting XEUS1 to XEUS2 with an additional
collection aperture some ×10 larger. The detector vehicle will be replaceable at any time by launching
a new version.
23.2.3 X-ray spectrometry
X-ray spectrometry or energy isolation can also be performed by using the crystal planes of materials
to act in the same way as a diffraction grating for the optical spectrum, the principle being that of the
Bragg spectrometer as sketched in figure 23.5. Interference is achieved according to the dimensions
of the lattice and the wavelengths of the incident x-rays.
Some of the incident radiation is scattered by the first layer of the crystal lattice—radiation
penetrating the crystal is scattered by successive layers. It can be readily seen from figure 23.5 that the
path difference for the radiation emerging from two adjacent layers is 2d sin θ g where d is the crystal
plane separation and θ g is the grazing angle.
For constructive interference to occur, the path lengths need to be equal to an integer number of
wavelengths, that is
mλ = 2d sin θ g
where m is an integer. The spectrum can, therefore, be readily scanned by altering the angle of θ g by