X-rays can be reflected off smooth metallic surfaces at very shallow
angles---grazing incidence. Such reflections are particularly efficient
for metals with high density, such as gold, platinum or iridium.
The reflection, similar to those radiations in the optical wavelengths,
is non-dispersive. Thus, X-ray mirrors
functioning at grazing incidence can focus X-rays over a broad energy
band, limited only by the critical angle of incidence beyond which the
reflectivity drops significantly. The critical angles are smaller
for x-ray of shorter wavelength.
In the present two-stage circular design of an X-ray telescope,
X-rays are reflected off two surfaces consecutively
at grazing angles. Instead of the Wolter Type parabolic-hyperbolic surfaces,
the Suzaku mirrors use a much simpler approximation of conical surfaces. This
simplification allows a more straightforward, and less expensive, fabrication
of these reflectors, especially for very thin substrates. Such simplification,
however, limits the resulting resolution of the telescope. A process of surface
replicaton is used to put very smooth surfaces on the conical substrates.
Another feature of the design is the use of thin aluminum foil as substrates. With
such thin and light foils, the reflecting surfaces can be extensively nested
in a compact fashion, providing a telescope with large throughput and small
weigh for the aperture.
Conventional X-Ray Mirrors
An X-ray mirror can be made of glass ceramics which is polished to give a
very smooth surface (with root-mean-square surface roughness of a few
Angstroms) and is coated with metal for X-ray reflection.
Several of such reflectors in
cylindrical layout can be nested to give a larger collecting area and thus
better sensitivities. These mirrors can be accurately ground to the precise
Wolter design and therefore gives sharp X-ray images of the order of
an arc-second. Examples of X-ray telescopes of this types includes
Chandra , which was launched
in 1999. However, due to the thick and massive mirror substrates, these
mirrors generally have limited collecting areas, especially at higher
energies, due to the limited nestings of shells allowed. These mirrors are
also quite heavy and take a lot of resources to fabricate. The combination
of these drive the production cost of such telescope high.
Foil X-Ray Mirrors
An alternative to the thick shell approach is to make a mirror with thin
foils. Such foil can be fabricated to a proper geometry and coated with
a smooth layer of high density metal. Examples of X-ray telescopes employing
this approach are:
and Suzaku ,
which was launched in 2005. Such thin-foil mirror system can
be extensively nested to greatly enhance the effective collecting area.
For example, in the case of ASCA, each of its 4 telescopes consists of
120 layers. Suzaku has about 180 layers in each of
the 5 telescopes.
In practice, however, the thin foils, of which the thickness is typically
about 170 micrometers, cannot be made to the precise Wolter geometry. A
cylindrical section of a cone is usually taken as an approximation.
This limits the
spatial resolution to a fraction of an arc-minute in theory and about
a minute of arc in the current practice.
In comparison with the thick-shell approach, the thin foil approach provides
many advantages in the following aspects.
Because of their thick substrates, thick-shell mirrors are limited in the number of
nesting of shells. This limits the effective collecting areas. In contrast, dense
nesting of thin foils greatly enhances
the collecting area without
making the mirror system unneccessarily big. In particular, many foils
can be placed in the inner part of a concentric mirror system. These foils
has a smaller angle to the optical axis, and allow smaller
angle of incidence of the incoming X-ray. Therefore, a thin foil telescope
can have high sensitivities at higher energy at which the critical
angle for reflection is small.
The thick shells make a fairly heavy
mirror system. In contrast, the thin foil mirror system is very light.
For example, each of the 4 ASCA telescopes weighs less than 10 kg, while
that of Suzaku weighs approximately 20 kg.
The conical approximation of the thin foils allow a relatively
straightforward process of fabrication.
The combination of the last two factors reduces the cost of production
of a thin-foil mirror system, as compared with a thick-shell system.