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X-ray telescope
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What is an X-ray telescope?
An X-ray telescope is an instrument designed to detect X-rays from outside Earth's atmosphere. Because the atmosphere absorbs X-rays, these telescopes must be placed at high altitudes via rockets or balloons, or in orbit. Unlike conventional optical telescopes, X-ray telescopes capture X-rays by bouncing them off a mirror at a very low angle, using a technique called grazing incidence.
The first X-ray telescope was the Apollo Telescope Mount, which studied the Sun from Skylab. The Chandra X-ray Observatory obtains high-resolution spectra and images of astronomical objects.
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How do X-ray telescopes differ from regular optical telescopes?
X-ray telescopes differ significantly from optical telescopes due to the properties of X-rays and the need to observe them from space. Here's a breakdown:
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Atmospheric Absorption Because Earth's atmosphere absorbs X-rays, X-ray telescopes must be placed at high altitudes via rockets, balloons, or satellites.
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Design Differences X-ray telescopes use grazing incidence mirrors, which capture X-rays by bouncing them off a mirror at a very low angle. This is unlike optical telescopes, which use lenses or mirrors to directly focus visible light.
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Detection X-ray telescopes use detectors like Geiger counters and scintillation counters, requiring large collection areas due to the weakness of celestial X-ray sources.
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Examples The Chandra X-ray Observatory is X-ray astronomy's equivalent to the Hubble Space Telescope.
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What kinds of objects or phenomena do X-ray telescopes observe in space?
X-ray telescopes observe various cosmic objects and phenomena by detecting radiation at X-ray wavelengths. Because Earth's atmosphere absorbs most X-rays, these telescopes are placed at high altitudes using balloons and spacecraft.
X-ray telescopes have identified X-rays emitted from most types of stars, supernova remnants, and binary stars (with one star likely a black hole). They have also detected a diffuse background of X-ray radiation from distant individual sources. With advanced instruments, X-ray telescopes continue to make discoveries concerning black holes, the evolution of stars and galaxies, and the nature of supernova remnants.
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Why can't X-ray telescopes be used on Earth's surface?
X-ray telescopes cannot operate on Earth's surface because Earth's atmosphere absorbs most X-rays. To detect X-rays from space, these telescopes must be carried to high altitudes via rockets or balloons, or placed in orbit outside the atmosphere.
Balloon-borne telescopes can detect the more penetrating (harder) X-rays, while those carried aloft by rockets or satellites detect softer radiation. Since the atmosphere strongly absorbs X-rays, astronomical observations in the X-ray region must be made from orbiting satellites.
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How do scientists focus and collect X-rays with these telescopes?
X-ray telescopes use a technique called grazing incidence to focus and collect X-rays. Unlike conventional optical telescopes, X-ray photons possess high energy and would pass through a standard mirror. To capture them, X-rays must be bounced off a mirror at a very low angle.
X-ray telescopes feature mirrors mounted with their surfaces nearly parallel to incoming X-rays. This enables the concentration of X-rays from cosmic objects into an image that can be electronically recorded.
Examples of X-ray telescopes include the Chandra X-ray Observatory, which has iridium mirrors with a 10-meter aperture, and the Röntgensatellit (ROSAT), which had two parallel grazing-incidence telescopes.
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What are some famous X-ray telescopes and important discoveries they have made?
X-ray telescopes detect X-rays from outside Earth's atmosphere using special mirrors and detectors. Some notable X-ray telescopes and their discoveries:
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Chandra X-ray Observatory: Since 1999, Chandra has been making high-resolution images of celestial X-ray sources. It found that starburst galaxies have a higher proportion of intermediate-size black holes and confirmed a supermassive black hole at the Milky Way's center. Chandra also provided direct evidence for dark matter by observing galaxy cluster mergers.
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XMM-Newton: Launched in 1999, this European Space Agency satellite discovered a new class of black holes and determined that neutron stars are likely composed of ordinary matter. It also identified a massive, distant galaxy cluster, offering insights into the universe's evolution.
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ROSAT: Launched in 1990, ROSAT detected X-rays from protostars, comets, and the Moon. It also observed the effects of dark matter on the intergalactic medium in a nearby galaxy cluster.
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X-ray telescope, instrument designed to detect and resolve X-rays from sources outside Earth’s atmosphere. Because of atmospheric absorption, X-ray telescopes must be carried to high altitudes by rockets or balloons or placed in orbit outside the atmosphere. Balloon-borne telescopes can detect the more penetrating (harder) X-rays, whereas those carried aloft by rockets or in satellites are used to detect softer radiation.
The design of this type of telescope must be radically different from that of a conventional optical telescope. Since X-ray photons have so much energy, they would pass right through the mirror of a standard reflector. X-rays must be bounced off a mirror at a very low angle if they are to be captured. This technique is referred to as grazing incidence. For this reason, the mirrors in X-ray telescopes are mounted with their surfaces only slightly off a parallel line with the incoming X-rays. Application of the grazing-incidence principle makes it possible to focus X-rays from a cosmic object into an image that can be recorded electronically.
Several types of X-ray detectors have been used, involving Geiger counters, proportional counters, and scintillation counters. These detectors require a large collecting area, because celestial X-ray sources are remote and therefore weak, and a high efficiency for detecting X-rays over the cosmic-ray-induced background radiation is needed.
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The first X-ray telescope was the Apollo Telescope Mount, which studied the Sun from on board the American space station Skylab. It was followed during the late 1970s by two High-Energy Astronomy Observatories (HEAOs), which explored cosmic X-ray sources. HEAO-1 mapped the X-ray sources with high sensitivity and high resolution. Some of the more interesting of these objects were studied in detail by HEAO-2 (named the Einstein Observatory).
The European X-ray Observatory Satellite (EXOSAT), developed by the European Space Agency, was capable of greater spectral resolution than the Einstein Observatory and was more sensitive to X-ray emissions at shorter wavelengths. EXOSAT remained in orbit from 1983 to 1986.
A much larger X-ray astronomy satellite was launched on June 1, 1990, as part of a cooperative program involving the United States, Germany, and the United Kingdom. This satellite, called the Röntgensatellit (ROSAT), had two parallel grazing-incidence telescopes. One of them, the X-ray telescope, bore many similarities to the equipment of the Einstein Observatory but had a larger geometric area and better mirror resolution. The other operated at extreme ultraviolet wavelengths. A position-sensitive proportional counter made it possible to survey the sky at X-ray wavelengths and produced a catalog of more than 150,000 sources with a positional accuracy of better than 30 arc seconds. A wide-field camera with a 5°-diameter field of view that operated with the extreme ultraviolet telescope was also part of the ROSAT instrument package. It produced an extended ultraviolet survey with arc minute source positions in this wavelength region, making it the first instrument with such capability. The ROSAT mirrors were gold-coated and permitted detailed examination of the sky from 5 to 124 angstroms. The ROSAT mission ended in February 1999.
X-ray astronomy has its equivalent of the Hubble Space Telescope in the Chandra X-ray Observatory. Chandra’s mirrors are made of iridium and have an aperture of 10 metres (33 feet). It can obtain high-resolution spectra and images of astronomical objects.
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