Ion Beam Figuring of X-Ray Mirrors

X-ray astronomy has made significant contributions towards understanding the evolution of the universe. X-ray telescopes must be operated in space, due to absorption of X-rays by the earth’s atmosphere.  As the objects being studied have a low delivery of radiation (a few photons per hour), every single photon is of particular interest. To increase collection efficiency, hundreds or thousands of mirrors are nested concentrically, as in a Wolter telescope (see Fig. 1). To ensure good image quality, the surface geometry of the reflective X-ray mirrors must be as close to ideal as possible. Any deviation between the ideal mirror geometry and the real mirror geometry is critical for imaging quality.

The basic shape of the mirrors is made from silicon blocks which at first are brought into shape by mechanical machining. After that, chemical mechanical polishing (CMP) is applied to the reflecting side. However, due to the rectangular shape of the mirrors and their curvature, the residual error of CMP processing is still several hundred nanometers in range. This residual error can be corrected by means of ion beam figuring (IBF).

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