X-ray mirrors 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 AXAF, which will be 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: BBXRT, ASCA and the upcoming ASTRO-E , which will be launched in the year 2000. 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. ASTRO-E will have 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.

• Sensitivity, especially at higher energies.
  

  Because of their thick substrates, thick-shell mirrors are limited in the number of nesting of shells. This limits the effective collecting areas, especially that at higher energies due to the larger angles of incidence. In contrast, the extensive nesting of thin foils greatly enhance the collecting area without making the mirror system unneccessarily big. In particular, many foils can be positioned in the inner part of the 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 a better 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 ASTRO-E weigh approximately 16 kg. The weight factor is further multiplied by using a light weight support system.

• 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.