Wolter telescope
{{Short description|X-ray source magnifier}}
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A Wolter telescope is a telescope for X-rays that only uses grazing incidence optics – mirrors that reflect X-rays at very shallow angles.
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,{{cite book |article=X-Rays: Optical Elements |last=Spiller|first= E. |editor1-first=Craig|editor1-last=Hoffman|editor2-first=Ronald|editor2-last=Driggers|title=Encyclopedia of Optical Engineering |publisher=Taylor & Francis |year=2015 |doi=10.1081/E-EOE2 |isbn=9781439850992}} 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).{{cite journal |url=http://www.ias.ac.in/article/fulltext/reso/010/07/0008-0020 |first=Kulinder Pal |last=Singh |title=Techniques in X-ray Astronomy |journal=Resonance |volume=10 |issue=7 |date=July 2005 |pages=8–20 |doi=10.1007/BF02867103 |s2cid=118308910 |format=pdf}} 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.{{cite journal |title=Glancing incidence mirror systems as imaging optics for X-rays |last=Wolter |first=Hans |authorlink=Hans Wolter |journal=Annalen der Physik |volume=10 |pages=94 |year=1952 |ref=Wolter, Glancing Incidence Mirror Systems, 1952 |doi=10.1002/andp.19524450108 |bibcode = 1952AnP...445...94W }}{{cite journal |title=A generalized Schwarzschild mirror system for use at glancing incidence for X-ray imaging |last=Wolter |first=Hans |author-link=Hans Wolter |journal=Annalen der Physik |volume=10 |page=286 |year=1952 |ref=Wolter, Generalized Schwarzschild Mirror System, 1952 |doi=10.1002/andp.19524450410
|bibcode = 1952AnP...445..286W }} These are called Wolter telescopes of type I, II, and III.{{cite web |url=https://imagine.gsfc.nasa.gov/observatories/technology/xray_telescopes2.html |title=X-ray Telescopes - More Information |publisher=NASA Goddard Space Flight Center |date=11 Dec 2018 |access-date=19 June 2020}} Each has different advantages and disadvantages.{{cite web |first=Rob |last=Petre |url=https://imagine.gsfc.nasa.gov/observatories/technology/ |title=Technology for X-ray and Gamma-ray Detection |publisher=NASA |ref=Petre, X-ray Imaging Systems}}
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
References
{{reflist}}
- {{cite web |first=Arndt |last=Last
|url= http://x-ray-optics.eu/index.php/en/types-of-optics/reflecting-optics/curved-mirrors#Wolter_optics |title=Wolter-optics |access-date=21 Nov 2019}}
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