Wolter telescope

A Wolter telescope is a telescope for X-rays that only uses grazing incidence optics – mirrors that reflect X-rays at very shallow angles.

Wolter telescopes of types I, II and III (top to bottom).

Problems with conventional telescope designs

Conventional telescope designs require reflection or refraction in a manner that does not work well for X-rays. Visible light optical systems use either lenses or mirrors aligned for nearly normal incidence – that is, the light waves travel nearly perpendicular to the reflecting or refracting surface. Conventional mirror telescopes work poorly with X-rays, since X-rays that strike mirror surfaces nearly perpendicularly are either transmitted or absorbed – not reflected.

Lenses for visible light are made of transparent materials with an index of refraction substantially different from 1, but all known X-ray-transparent materials have index of refraction essentially the same as 1,^[1] so a long series of X-ray lenses, known as compound refractive lenses, are required in order to achieve focusing without significant attenuation.

X-ray mirror telescope design

X-ray mirrors can be built, but only if the angle from the plane of reflection is very low (typically 10 arc-minutes to 2 degrees).^[2] These are called glancing (or grazing) incidence mirrors. In 1952, Hans Wolter outlined three ways a telescope could be built using only this kind of mirror.^[3][4] These are called Wolter telescopes of type I, II, and III.^[5] Each has different advantages and disadvantages.^[6]

Wolter's key innovation was that by using two mirrors it is possible to create a telescope with a usably wide field of view. In contrast, a grazing incidence telescope with just one parabolic mirror could focus X-rays, but only very close to the centre of the field of view. The rest of the image would suffer from extreme coma.

See also

• List of telescope types
• Nuclear Spectroscopic Telescope Array (NuSTAR) (2012+)
• Swift Gamma-Ray Burst Mission Contains a Wolter Type-I X-ray telescope (2004+)
• Chandra X-ray Observatory Orbiting observatory using a Wolter X-ray telescope. (1999+)
• XMM-Newton Orbiting X-ray observatory using a Wolter Type-I X-ray telescope. (1999+)
• ROSAT Orbiting X-ray observatory (1990-1999)
• eROSITA Orbiting X-ray observatory using Wolter Type-I X-ray telescope on board Spektr-RG (SRG) (2019+)
• ART-XC Orbiting X-ray observatory using Wolter Type-I X-ray telescope on board Spektr-RG (SRG)(2019+)
• ATHENA (2031+)
• Neutron microscope
• Hans Wolter