X-ray telescope
X-ray telescopes are instruments for receiving and measuring the X-rays coming from space . A distinction is made between imaging telescopes , which can examine the areal intensity distribution of X-ray sources , and instruments without an optical imaging system.
The X-ray astronomy is a very young field of research. Astronomical X-ray sources are v. a. the solar corona , young or very hot stars (over 100,000 °), supernova explosions, active galaxy nuclei and high- energy synchrotron radiation in strong magnetic fields. They cannot be observed from the ground because the atmosphere in this energy range of electromagnetic radiation is not permeable. That is why X-ray telescopes are mostly used in research satellites or space observatories. Early measurements in the 1960s were also carried out with the help of ballistic missile flights ( sounding rockets ).
Imaging instruments
The construction of an X-ray telescope is made difficult by the special features of the X-ray optics . In particular, there is a lack of materials that reflect or refract X-rays such as visible light . Almost all materials - including metals - absorb X-ray photons on vertical or steep impact and the refractive index is very close to 1, which makes optical imaging with lenses impossible.
The problem can be compared to the task of redirecting the hail of bullets from a rifle in a certain direction. If the balls hit a metal plate vertically, it will be dented or perforated. Only when the balls rub flat against the metal will they be deflected a little to the side.
Imaging X-ray telescopes therefore focus the incident radiation by reflecting it off curved metal mirrors that are shaped as parts of a paraboloid or hyperboloid (see picture). However, the X-rays must be completely grazing on their surfaces, which are often additionally coated. H. far from the paraboloid apex. If the angle of incidence is greater than about 3 °, the very high-energy radiation passes through the material. The telescopes are therefore very elongated tubes, very different from the bowl-shaped radio telescopes . Not only can you image the X-ray sources, you can also measure radiation intensities and spectra .
First X-ray telescopes
The X-ray telescope of the Uhuru satellite , which was launched in 1970, was a sheet of lead with many parallel holes running through it, so that only radiation from a certain direction could reach the radiation detector . This device was not a telescope in the literal sense of the word, but only a collimator that made the detector used directionally sensitive.
In later X-ray telescopes, elongated, paraboloidal mirror arrangements were and are used, which are called Wolter telescopes after their inventor Hans Wolter (1952) . Here, the effect of total reflection of X-rays at grazing incidence (2–3 °) on metal mirrors is used to achieve a magnification effect like that of light telescopes . In order to increase the incidence of radiation, which is only annular, reflectors of different diameters are pushed into one another coaxially. The first such telescope was launched in 1977, the High Energy Astronomy Observatory 1 (HEAO-1), followed by two other HEAO satellites until 1979.
Current X-ray telescopes
The most modern Wolter telescopes work with triple or quadruple nested ("nested") mirror systems which are only slightly curved in the longitudinal direction and whose coaxial tubes appear almost like elongated cylinders . In some designs, the second reflection does not take place on the adjacent surface, but on the outside of the inner tube, or vice versa: on the outside of a paraboloid and the inside of an ellipsoid mirror.
Today's X-ray astronomy is dominated by two space telescopes : the Chandra X-ray Observatory launched by NASA in 1999 , named after Subrahmanyan Chandrasekhar , and the European XMM-Newton ( X-ray Multi-Mirror ). Both were started at the end of 1999 and are still in operation (as of April 2018). The telescopes are 13 or 10 meters long and have a resolution of 1 or 5 arc seconds due to their extremely high precision . XMM-Newton makes up for the slightly lower resolution with a much higher sensitivity. The orbits are elongated ellipses with apogee 80,000 and 115,000 km away from the earth. XMM can cooperate with the gamma telescope Integral , launched in 2003 , which also has optical and X-ray monitors for identifying gamma-ray flashes. NuSTAR has been delivering images in the high-energy X-ray range since 2012 .
See also
- X-ray satellite
- Swift (satellite) , HETE
- High Energy Astronomy Observatory 2 ("Einstein")
- X-ray Evolving Universe Spectroscopy (ESA project)
Web links
literature
- Helmut Zimmermann and Alfred Weigert : Lexicon of Astronomy . Spektrum-Verlag, Heidelberg 1999
- J. Bennett, M. Donahue, N. Schneider, M. Voith: Astronomie (Chapter 6.4) , Ed. Harald Lesch, 5th edition, Pearson-Studienverlag, Munich-Boston-Harlow-Sydney-Madrid 2010
- Stars and Space : Perspectives in X-ray Astronomy . Special 3/2003 "The hot cosmos", Heidelberg 2003.