X-Ray Optics

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 Einstein , ROSAT and 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

basic concept of a circular 2-reflection
x-ray telescope

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: BBXRT, ASCA 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.

Finished reflectors. Smooth surfaces are made with the replication process. Coated material is gold. In comparison with the thick-shell approach, the thin foil approach provides many advantages in the following aspects.

  • Sensitivity.

    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.

  • Weight.

    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.

  • Fabrication.

    The conical approximation of the thin foils allow a relatively straightforward process of fabrication.

  • Cost.

    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.

Mirror Lab Home ] [ Suzaku Mirror ] [ Suzaku ] [ X-Ray Astrophysics ] [ Astrophysics Science Division ]

NASA Logo, National Aeronautics and Space Administration