Grazing incidence optics for X-ray Astronomy X-ray Optics

Abstract

Cosmic X-ray sources are usually very weak and their detection, therefore, needs large area telescopes to gather light and sensitive detectors to enhance quantum efficiency. Conventional telescopes for visible light use refracting or reflective optics which is impractical for X-ray wavelengths because photon energies are greater than the binding energies of the typical atomic electrons leading to a refractive index for X-rays being less than unity. Thus single surface reflectivity for near-normal incidence is negligible for X-rays. However, by Snell's Laws, total external reflection occurs and X-rays can be reflected from a surface up to a critical angle (usually about a degree for energies below 10 keV) given by cosine θ = n. This is known as the grazing angle. X-ray telescopes are made to exploit the grazing incidence from a set of co-axial and con-focal shells of paraboloidal and hyperboloidal mirrors. X-ray reflectors having high atomic number surfaces with low scattering are used to realize imaging capability for a telescope. I describe here various configurations required, and the various technologies used and their limitations, to make practical X-ray telescopes. A soft X-ray imaging telescope (SXT) using grazing incidence has been built at TIFR for ASTROSAT-an Indian Multiwavelength Satellite designed to cover a very broad band of X-rays, UV and optical. Astrosat is planned to be launched by a Polar Satellite Launch Vehicle in 2012 into a near-Earth Equatorial orbit. I will also describe the ongoing R & D for realizing telescopes for hard X-rays above 10 keV useful for both Astronomy and medical diagnostics.