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{{Short description|Telescope designed to observe remote objects by detecting X-rays}}
{{Short description|Telescope designed to observe remote objects by detecting X-rays}}
{{More citations needed|date=November 2023}}
{{More citations needed|date=November 2023}}
{{Short description|Telescope designed to observe remote objects by detecting X-rays}}
[[File:IXOFlyby1Large.jpg|thumb|''[[International X-ray Observatory]] concept'']]
[[File:Chandra_artist_illustration.jpg|thumb|[[Chandra X-ray Observatory]], launched by NASA in 1999, is still operational as of 2024]]

An '''X-ray telescope''' ('''XRT''') is a [[telescope]] that is designed to observe remote objects in the [[X-ray]] spectrum. In order to get above the [[Earth's atmosphere]], which is opaque to X-rays, X-ray telescopes must be mounted on high altitude rockets, [[High-altitude balloon|balloons]] or [[Space telescope|artificial satellites]].
An '''X-ray telescope''' ('''XRT''') is a [[telescope]] that is designed to observe remote objects in the [[X-ray]] spectrum. X-rays are absorbed by the [[Earth's atmosphere]], so instruments to detect X-rays must be taken to high altitude by [[balloon]]s, [[sounding rocket]]s, and [[satellite]]s.


The basic elements of the telescope are the [[optics]] (focusing or [[collimating]]), that collects the [[radiation]] entering the telescope, and the [[X-ray detector|detector]], on which the radiation is collected and measured. A variety of different designs and technologies have been used for these elements.
The basic elements of the telescope are the [[optics]] (focusing or [[collimating]]), that collects the [[radiation]] entering the telescope, and the [[X-ray detector|detector]], on which the radiation is collected and measured. A variety of different designs and technologies have been used for these elements.


Many of the existing telescopes on satellites are compounded of multiple copies or variations of a detector-telescope system, whose capabilities add or complement each other and additional fixed or removable elements<ref>{{Cite web|title = Chandra :: About Chandra :: Science Instruments|url = http://chandra.si.edu/about/science_instruments.html|website = chandra.si.edu|access-date = 2016-02-19}}</ref><ref>{{Cite web|title = Instruments|url = http://sci.esa.int/xmm-newton/31281-instruments/|website = sci.esa.int|access-date = 2016-02-19}}</ref> (filters, spectrometers) that add functionalities to the instrument.
Many of the existing telescopes on satellites are compounded of multiple copies or variations of a detector-telescope system, whose capabilities add or complement each other and additional fixed or removable elements<ref>{{Cite web|title = Chandra :: About Chandra :: Science Instruments|url = http://chandra.si.edu/about/science_instruments.html|website = chandra.si.edu|access-date = 2016-02-19}}</ref><ref>{{Cite web|title = Instruments|url = http://sci.esa.int/xmm-newton/31281-instruments/|website = sci.esa.int|access-date = 2016-02-19}}</ref> (filters, spectrometers) that add functionalities to the instrument.

==History of X-ray telescopes==
{{See also|History of X-ray astronomy}}
The first X-ray telescope employing Wolter Type I grazing-incidence optics was employed in a rocket-borne experiment on October 15, 1963, 1605 UT at White Sands New Mexico using a Ball Brothers Corporation pointing control on an Aerobee 150 rocket to obtain the X-ray images of the Sun in the 8–20 angstrom region. The second flight was in 1965 at the same launch site.<ref>R. Giacconi et al., ApJ '''142''', 1274 (1965)</ref>

The [[Einstein Observatory]] (1978–1981), also known as HEAO-2, was the first orbiting X-ray observatory with a Wolter Type I telescope.<ref>R. Giacconi et al., ApJ '''230''',540 (1979)</ref> It obtained high-resolution X-ray images in the energy range from 0.1 to 4 keV of stars of all types, supernova remnants, galaxies, and clusters of galaxies. [[HEAO-1]] (1977–1979) and [[HEAO-3]] (1979–1981) were others in that series. Another large project was [[ROSAT]] (active from 1990 to 1999), which was a heavy X-ray space observatory with focusing X-ray optics.

The [[Chandra X-Ray Observatory]] was launched by NASA in 1999 and is operated for more than 25 years in a high elliptical orbit, returning thousands 0.5 arc-second images and high-resolution spectra of all kinds of astronomical objects in the energy range from 0.5 to 8.0 keV.


==Optics==
==Optics==
{{main|X-ray optics}}
{{main|X-ray optics}}
[[File:XRISM s X-ray mirror assembly.jpg|thumb|One of the mirrors of [[XRISM]] made of 203 foils]]
The most common methods used in X-ray optics are [[Wolter telescope|grazing incidence mirrors]] and [[Collimator|collimated apertures]].
The most common methods used in X-ray optics are [[Angle of incidence (optics)|grazing incidence mirrors]] and [[Collimator|collimated apertures]]. Only three geometries that use [[Angle of incidence (optics)|grazing incidence reflection]] of X-rays to produce X-ray images are known: [[Wolter telescope|Wolter system]], [[Kirkpatrick–Baez mirror|Kirkpatrick-Baez system]], and [[lobster-eye optics]].<ref name="WSPC">{{cite book |author1=Richard Willingale |editor1-last=Sternberg |editor1-first=Amiel |editor2-last=Burrows |editor2-first=David N |title=The WSPC Handbook of Astronomical Instrumentation: Volume 4: X-Ray Astronomical Instrumentation |date=July 2021 |publisher=World Scientific Publishing Co. Pte. Ltd. |isbn=978-981-4644-38-9 |pages=33-47, 85-106 |url=https://doi.org/10.1142/9446-vol4 |access-date=1 January 2024 |language=en |chapter=Lobster Eye Optics}}</ref>


===Focusing mirrors===
===Focusing mirrors===
[[File:Pointing X-ray Eyes at our Resident Supermassive Black Hole.jpg|thumb|NuSTAR, has captured these first, focused views of the supermassive black hole at the heart of our galaxy in high-energy X-ray light.]]
The utilization of X-ray mirrors allows to focus the incident radiation on the detector plane. Different geometries (e.g. Kirkpartrick-Baez or [[lobster-eye optics|lobster-eye]]) have been suggested or employed, but almost the totality of existing telescopes employs some variation of the [[Wolter telescope|Wolter I design]].{{Citation needed|date=August 2019}} The limitations of this type of [[X-ray optics]] result in much narrower fields of view (typically <1 degree) than visible or UV telescopes.
The utilization of X-ray mirrors allows to focus the incident radiation on the detector plane. Different geometries (e.g. Kirkpartrick-Baez or [[lobster-eye optics|lobster-eye]]) have been suggested or employed, but almost the totality of existing telescopes employs some variation of the [[Wolter telescope|Wolter I design]].{{Citation needed|date=August 2019}} The limitations of this type of [[X-ray optics]] result in much narrower fields of view (typically <1 degree) than visible or UV telescopes.


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==Detectors==
==Detectors==
[[File:Ill-2 O3.jpg|thumb|X-rays start at ~0.008 nm and extend across the electromagnetic spectrum to ~8 nm, over which Earth's atmosphere is [[Opacity (optics)|opaque]].]]
{{main|X-ray astronomy detector}}Several technologies have been employed on detectors for X-ray telescopes, ranging from counters like Ionization chambers, [[geiger counters]] or [[scintillator]]s to imaging detectors like [[Charge-coupled device|CCDs]] or [[CMOS sensor|CMOS]] sensors.{{Citation needed|date=August 2019}} The use of micro-calorimeters, that offer the added capability of measuring with great accuracy the energy of the radiation, is planned for future missions.{{Citation needed|date=August 2019}}
Several technologies have been employed on detectors for X-ray telescopes, ranging from counters like ionization chambers, [[geiger counters]] or [[scintillator]]s to imaging detectors like [[Charge-coupled device|CCDs]] or [[CMOS sensor|CMOS]] sensors.{{Citation needed|date=August 2019}} The use of micro-calorimeters, that offer the added capability of measuring with great accuracy the energy of the radiation, is planned for future missions.{{Citation needed|date=August 2019}} X-ray detectors have been designed and configured primarily for energy and occasionally for wavelength detection using a variety of techniques usually limited to the technology of the time.


==Detection and imaging of X-rays==
==Missions employing X-ray telescopes==
[[File:Saturn comp.jpg|thumb|350px|[[Chandra]]'s image of [[Saturn]] (left) and [[Hubble Space Telescope|Hubble optical image]] of Saturn (right). Saturn's [[X-ray]] spectrum is similar to that of X-rays from the [[Sun]]. 14 April 2003]]
{{main|List of X-ray space telescopes}}
X-rays has a huge span in wavelength (~8&nbsp;nm - 8 pm), frequency (~50 PHz - 50 EHz) and energy (~0.12 - 120 keV). In terms of temperature, 1 eV = 11,604 K. Thus X-rays (0.12 to 120 keV) correspond to 1.39 × 10<sup>6</sup> to 1.39 × 10<sup>9</sup> K. From 10 to 0.1 nanometers (nm) (about 0.12 to 12 [[keV]]) they are classified as soft X-rays, and from 0.1&nbsp;nm to 0.01&nbsp;nm (about 12 to 120 keV) as hard X-rays.


Closer to the visible range of the electromagnetic spectrum is the [[ultraviolet]]. The draft ISO standard on determining solar [[irradiance]]s (ISO-DIS-21348)<ref name=ISO21348>{{Cite journal |title=ISO 21348 Process for Determining Solar Irradiances |journal = 36th Cospar Scientific Assembly|volume = 36|pages = 2621|url = http://www.spacewx.com/ISO_solar_standard.html|bibcode = 2006cosp...36.2621T|last1 = Tobiska|first1 = W|last2 = Nusinov|first2 = A|year = 2006}}</ref> describes the ultraviolet as ranging from ~10&nbsp;nm to ~400&nbsp;nm. That portion closest to X-rays is often referred to as the "extreme ultraviolet" ([[Extreme ultraviolet|EUV]] or XUV). When an EUV photon is absorbed, [[photoelectric effect|photoelectrons]] and [[secondary electrons]] are generated by [[ionization]], much like what happens when X-rays or electron beams are absorbed by matter.<ref name=Henke>{{cite journal |author=Henke BL |display-authors=etal |journal=Journal of Applied Physics|date=1977 |volume=48 |issue=5 |page=1852|doi=10.1063/1.323938 |title=0.1–10-keV X-ray induced electron emissions from solids—Models and secondary electron measurements|bibcode = 1977JAP....48.1852H }}</ref>
==History of X-ray telescopes==
{{See also|History of X-ray astronomy}}
{{unreferenced section|date=August 2019}}
The first X-ray telescope employing Wolter Type I grazing-incidence optics was employed in a rocket-borne experiment on October 15, 1963, 1605 UT at White Sands New Mexico using a Ball Brothers Corporation pointing control on an Aerobee 150 rocket to obtain the X-ray images of the Sun in the 8–20 angstrom region. The second flight was in 1965 at the same launch site (R. Giacconi et al., ApJ '''142''', 1274 (1965)).


The distinction between X-rays and [[gamma ray]]s has changed in recent decades. Originally, the electromagnetic radiation emitted by [[X-ray tube]]s had a longer [[wavelength]] than the radiation emitted by [[radioactive]] [[atomic nucleus|nuclei]] (gamma rays).<ref name=Dendy>{{cite book |author=Dendy PP |author2=Heaton B |title=Physics for Diagnostic Radiology |publisher=[[CRC Press]] |date=1999|page=12 |url=https://books.google.com/books?id=1BTQvsQIs4wC&pg=PA12 |isbn= 978-0-7503-0591-4}}</ref> So older literature distinguished between X- and gamma radiation on the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10<sup>−11</sup> m, defined as gamma rays.<ref name=Hodgman>{{cite book |editor=Charles Hodgman |title=CRC Handbook of Chemistry and Physics |edition=44th |publisher=Chemical Rubber Co.|date=1961|page=2850}}</ref> However, as shorter wavelength continuous spectrum "X-ray" sources such as [[linear accelerator]]s and longer wavelength "gamma ray" emitters were discovered, the wavelength bands largely overlapped. The two types of radiation are now usually distinguished by their origin: X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the [[nucleus (atomic structure)|nucleus]].<ref name=Dendy/><ref name=Feynman>{{cite book |author=Feynman R |author2=Leighton R |author3=Sands M |title=The Feynman Lectures on Physics|volume=1|publisher=[[Addison-Wesley]] |date=1963 |pages=2–5 |isbn=978-0-201-02116-5}}</ref><ref name=LAnnunziata>{{cite book |author=L'Annunziata M |author2=Baradei M |title=Handbook of Radioactivity Analysis |publisher=[[Academic Press]] |date=2003 |page=58 |url=https://books.google.com/books?id=b519e10OPT0C&pg=PA58|isbn=978-0-12-436603-9}}</ref><ref name=Grupen>{{cite book |author=Grupen C |author2= Cowan G |author3=Eidelman SD |author4= Stroh T |title=Astroparticle Physics |publisher=Springer |date=2005 |page=109 |isbn= 978-3-540-25312-9}}</ref>
The [[Einstein Observatory]] (1978–1981), also known as HEAO-2, was the first orbiting X-ray observatory with a Wolter Type I telescope (R. Giacconi et al., ApJ '''230''',540 (1979)). It obtained high-resolution X-ray images in the energy range from 0.1 to 4 keV of stars of all types, supernova remnants, galaxies, and clusters of galaxies. [[HEAO-1]] (1977–1979) and
[[HEAO-3]] (1979–1981) were others in that series. Another large project was [[ROSAT]] (active from 1990 to 1999), which was a heavy X-ray space observatory with focusing X-ray optics.


Although the more energetic X-rays, [[photon]]s with an energy greater than 30 [[keV]] (4,800 [[Atto-|a]]J), can penetrate the [[Earth's atmosphere]] at least for distances of a few meters, the Earth's atmosphere is thick enough that virtually none are able to penetrate from outer space all the way to the Earth's surface. X-rays in the 0.5 to 5 keV (80 to 800 aJ) range, where most celestial sources give off the bulk of their energy, can be stopped by a few sheets of paper; 90% of the photons in a beam of 3 keV (480 aJ) X-rays are absorbed by traveling through just 10&nbsp;cm of air.
The [[Chandra X-Ray Observatory]] is among the recent satellite observatories launched by NASA, and by the Space Agencies of Europe, Japan, and Russia. Chandra has operated for more than 10 years in a high elliptical orbit, returning thousands 0.5 arc-second images and high-resolution spectra of all kinds of astronomical objects in the energy range from 0.5 to 8.0 keV. Many of the spectacular images from Chandra can be seen on the NASA/Goddard website.


===Proportional counters===
[[NuStar]] is one of the latest X-ray space telescopes, launched in June 2012. The telescope observes radiation in a high-energy range (3–79 keV), and with high resolution. NuStar is sensitive to the 68 and 78 keV signals from decay of <sup>44</sup>Ti in supernovae.
{{Main|Proportional counter}}
A [[proportional counter]] is a type of [[gaseous ionization detector]] that counts [[Charged particle|particle]]s of [[ionizing radiation]] and measures their energy. It works on the same principle as the [[Geiger counter|Geiger-Müller counter]], but uses a lower operating [[voltage]]. All X-ray proportional counters consist of a windowed gas cell.<ref name=PropCount>{{cite web |title=Proportional Counters |url=http://imagine.gsfc.nasa.gov/docs/science/how_l2/proportional.html}}</ref> Often this cell is subdivided into a number of low- and high-electric field regions by some arrangement of electrodes.


Proportional counters were used on [[EXOSAT]],<ref name=Hoff>{{cite journal |author=Hoff HA |title=Exosat - the new extrasolar X-ray observatory |journal=J. Br. Interplanet. Soc.|date=1983 |volume=36|page=363|bibcode=1983JBIS...36..363H}}</ref> on the US portion of the [[Apollo–Soyuz]] mission (July 1975), on French [[Granat#TOURNESOL|TOURNESOL]] instrument consisted.<ref name="NASA2007">{{cite web |url=http://heasarc.gsfc.nasa.gov/docs/granat/granat_about.html |title=Granat |publisher=[[NASA]] |access-date=2007-12-05}}</ref>
[[Gravity and Extreme Magnetism]] (GEMS) would have measured X-ray polarization but was canceled in 2012.

===X-ray monitor===
Monitoring generally means to be aware of the state of a system. A device that displays or sends a signal for displaying X-ray output from an X-ray generating source so as to be aware of the state of the source is referred to as an X-ray monitor in space applications.
On [[Apollo 15]] in orbit above the [[Moon]], for example, an X-ray monitor was used to follow the possible variation in solar X-ray intensity and spectral shape while mapping the lunar surface with respect to its chemical composition due to the production of [[X-ray fluorescence|secondary X-rays]].<ref name=Adler>{{cite journal |author=Adler I |author2=Gerard J |author3=Trombka J |author4=Schmadebeck R |author5=Lowman P |author6=Bodgett H |title=The Apollo 15 x-ray fluorescence experiment |journal=Proc Lunar Sci Conf|volume=2 |date=1972 |page=2157|bibcode=1972LPSC....3.2157A}}</ref>

The X-ray monitor of [[P78-1|Solwind]], designated NRL-608 or XMON, was a collaboration between the [[Naval Research Laboratory]] and [[Los Alamos National Laboratory]]. The monitor consisted of 2 collimated argon proportional counters.

===Scintillation detector===
{{Main|Scintillator}}
[[File:SGCat24454-scint-gris.noirEtBlanc.jpg|thumb|Scintillation crystal surrounded by various scintillation detector assemblies]]

A scintillator is a material which exhibits the property of [[luminescence]]<ref name=Leo>{{cite book |author=Leo WR |date=1994 |title=Techniques for Nuclear and particle Physics Experiments |edition=2nd |publisher=Springer}}</ref> when excited by [[ionizing radiation]]. Luminescent materials, when struck by an incoming particle, such as an X-ray photon, absorb its energy and scintillate, i.e. reemit the absorbed energy in the form of a small flash of light, typically in the visible range.

The scintillation X-ray detector were used on [[Vela (satellite)|Vela 5A]] and its twin [[Vela (satellite)|Vela 5B]];<ref name=Conner>{{cite journal |author=Conner JP |author2=Evans WD |author3=Belian RD |title=The Recent Appearance of a New X-Ray Source in the Southern Sky |journal=Astrophys J|date=1969 |volume=157 |page=L157|doi=10.1086/180409 |bibcode=1969ApJ...157L.157C}}</ref> the X-ray telescope onboard [[OSO 4]] consisted of a single thin NaI(Tl) scintillation crystal plus phototube assembly enclosed in a CsI(Tl) anti-coincidence shield. [[OSO 5]] carried a CsI crystal scintillator. The central crystal was 0.635&nbsp;cm thick, had a sensitive area of 70&nbsp;cm<sup>2</sup>, and was viewed from behind by a pair of photomultiplier tubes.

The [[Granat#PHEBUS|PHEBUS]] had two independent detectors, each detector consisted of a bismuth germinate (BGO) crystal 78&nbsp;mm in [[diameter]] by 120&nbsp;mm thick.<ref name="NASA2007"/> The [[Granat#KONUS-B|KONUS-B]] instrument consisted of seven detectors distributed around the spacecraft that responded to [[photon]]s of 10&nbsp;keV to 8&nbsp;MeV energy. They consisted of [[Sodium iodide|NaI]](Tl) scintillator crystals 200&nbsp;mm in diameter by 50&nbsp;mm thick behind a [[Beryllium|Be]] entrance window. [[Kvant-1]] carried the HEXE, or High Energy X-ray Experiment, which employed a [[Phoswich Detector|phoswich]] of sodium iodide and caesium iodide.

===Modulation collimator===
In [[electronics]], [[modulation]] is the process of varying one waveform in relation to another waveform. With a 'modulation collimator' the amplitude (intensity) of the incoming X-rays is reduced by the presence of two or more 'diffraction gratings' of parallel wires that block or greatly reduce that portion of the signal incident upon the wires.

An [[Collimator#Neutron, X-ray and gamma ray collimators|X-ray collimator]] is a device that filters a stream of X-rays so that only those traveling parallel to a specified direction are allowed through.

[[Minoru Oda]], President of Tokyo University of Information Sciences, invented the modulation collimator, first used to identify the counterpart of Sco X-1 in 1966, which led to the most accurate positions for X-ray sources available, prior to the launch of X-ray imaging telescopes.<ref name=Cominsky>{{cite web |author=Cominsky L |author2= Inoue H |author3=Clark G |title=Minoru Oda (1923 - 2001) |url=http://www.aas.org/head/headnews/headnews.may01.html}}</ref>

[[SAS 3]] carried modulation collimators (2-11 keV) and Slat and Tube collimators (1 up to 60keV).<ref name=Smale>{{cite web |title=The Third Small Astronomy Satellite (SAS-3) |url=http://imagine.gsfc.nasa.gov/docs/sats_n_data/missions/sas3.html|archive-url=https://web.archive.org/web/20010417123458/http://imagine.gsfc.nasa.gov/docs/sats_n_data/missions/sas3.html|url-status=dead|archive-date=2001-04-17}}</ref>

On board the [[Granat]] Observatory were four [[Granat#WATCH|WATCH]] instruments that could localize bright sources in the 6 to 180 keV range to within 0.5° using a Rotation Modulation Collimator. Taken together, the instruments' three fields of view covered approximately 75% of the sky.<ref name="NASA2007"/>

The [[Reuven Ramaty High Energy Solar Spectroscopic Imager]] (RHESSI), Explorer 81, images solar flares from soft X-rays to gamma rays (~3 keV to ~20 MeV). Its imaging capability is based on a Fourier-transform technique using a set of 9 [[Rotational Modulation Collimator]]s.

===X-ray spectrometer===

[[OSO 8]] had on board a Graphite Crystal X-ray Spectrometer, with energy range of 2-8 keV, FOV 3°.

The [[Granat]] [[Granat#ART-S|ART-S X-ray spectrometer]] covered the energy range 3 to 100&nbsp;keV, FOV 2° × 2°. The instrument consisted of four detectors based on [[Spectroscopy|spectroscopic]] MWPCs, making an effective area of 2,400&nbsp;cm<sup>2</sup> at 10&nbsp;keV and 800&nbsp;cm<sup>2</sup> at 100&nbsp;keV. The time resolution was 200 [[microsecond]]s.<ref name="NASA2007"/>

The X-ray spectrometer aboard [[ISEE-3]] was designed to study both solar flares and cosmic gamma-ray bursts over the energy range 5-228 keV.nThe experiment consisted of 2 cylindrical X-ray detectors: a Xenon filled proportional counter covering 5-14 keV, and a NaI(Tl) scintillator covering 12-1250 keV.

===CCDs===
Most existing X-ray telescopes use [[Charge-coupled device|CCD]] detectors, similar to those in visible-light cameras. In visible-light, a single photon can produce a single electron of charge in a pixel, and an image is built up by accumulating many such charges from many photons during the exposure time. When an X-ray photon hits a CCD, it produces enough charge (hundreds to thousands of electrons, proportional to its energy) that the individual X-rays have their energies measured on read-out.

===Microcalorimeters===
[[Calorimeter#X-ray microcalorimeter|Microcalorimeters]] can only detect X-rays one photon at a time (but can measure the energy of each).

===Transition edge sensors===
TES devices are the next step in microcalorimetry. In essence they are super-conducting metals kept as close as possible to their transition temperature. This is the temperature at which these metals become super-conductors and their resistance drops to zero. These transition temperatures are usually just a few degrees above absolute zero (usually less than 10 [[Kelvin|K]]).

==X-ray space telescopes==
{{main|List of space telescopes#X-ray}}
[[File:X-Ray_Explorer_Satellite.jpg|thumb|Uhuru X-ray satellite]]
An X-ray space telescope is a type of [[space telescope]] used to study X-ray emissions from celestial objects. First specialised X-ray space telescope, [[Uhuru (satellite)|Uhuru]], was launched by NASA in 1970.

===Active X-ray observatory satellites===
Satellites in use today include ESA's [[XMM-Newton|XMM-Newton observatory]] (low to mid energy X-rays 0.1-15 keV), [[NASA]]'s [[Swift Gamma-Ray Burst Mission|Swift]] observatory, [[Chandra X-ray Observatory|Chandra]] observatory and [[IXPE]] telescope. [[JAXA]] has launched the [[XRISM]] telescope, while [[ISRO]] has launched [[Aditya-L1]] and [[XPoSat]].

The [[GOES 14]] spacecraft carries on board a Solar X-ray Imager to monitor the Sun's X-rays for the early detection of solar flares, coronal mass ejections, and other phenomena that impact the geospace environment.<ref name=NOAAGOES>{{cite web |title=GOES Solar X-ray Imager |url=http://www.swpc.noaa.gov/sxi/index.html}}</ref> It was launched into orbit on June 27, 2009, at 22:51 GMT from [[Cape Canaveral Air Force Station Space Launch Complex 37|Space Launch Complex 37B]] at the [[Cape Canaveral Air Force Station]].

Chinese [[Hard X-ray Modulation Telescope]] was launched on June 15, 2017 to observe black holes, neutron stars, active galactic nuclei and other phenomena based on their X-ray and gamma-ray emissions.<ref name="hxmt">{{Cite news|title=China launches X-ray telescope via Long March 4B|url=https://www.nasaspaceflight.com/2017/06/china-x-ray-telescope-long-march-4b/|publisher=[[NASASpaceFlight.com]]|author=Rui C. Barbosa|date=14 June 2017|access-date=23 August 2020}}</ref>

The [[Lobster-Eye X-ray Satellite]] was launched on 25 July 2020 by [[CNSA]]. it is the first in-orbit telescope to utilize the [[lobster-eye optics|lobster-eye]] imaging technology of ultra-large field of view imaging to search for dark matter signals in the x-ray energy range.<ref name="lobster-eye">{{Cite news|title=Launch of the world's first soft X-ray satellite with 'Lobster-Eye' imaging technology|url=https://copernical.com/news-public/item/18179-2020-07-27-15-55-20|publisher=copernical.com|date=26 July 2020 |access-date=23 August 2020}}</ref> [[Lobster Eye Imager for Astronomy]] was launched in 2022 as a technology demonstrator for planned [[Einstein Probe]].

A soft X-ray solar imaging telescope is on board the [[Geostationary Operational Environmental Satellite|GOES-13]] weather satellite launched using a [[Delta IV]] from [[Cape Canaveral]] LC37B on May 24, 2006.<ref name=WadeGOES>{{cite web |author=Wade M |title=Chronology - Quarter 2 2006 |url=http://www.astronautix.com/chrono/20062.htm|archive-url=https://web.archive.org/web/20030902042509/http://www.astronautix.com/chrono/20062.htm|url-status=dead|archive-date=September 2, 2003}}</ref> However, there have been no GOES 13 SXI images since December 2006.

The Russian-German [[Spektr-RG]] carries the [[eROSITA]] telescope array as well as the [[ART-XC]] telescope. It was launched by [[Roscosmos]] on 13 July 2019 from [[Baikonur Cosmodrome|Baikonur]] and began collecting data in October 2019.


==See also==
==See also==
* [[List of telescope types]]
* [[List of telescope types]]
* [[List of X-ray space telescopes]]
* [[X-ray astronomy]]
* [[Wolter telescope]]: A type of X-ray telescope built with glancing incidence mirrors.
* [[X-ray astronomy detector]]


==References==
==References==
{{Reflist|30em}}
{{Reflist|30em}}

==External links==
*{{cite journal |doi=10.1107/S090904950200376X |author=Kamijo N |author2=Suzuki Y |author3=Awaji M |title=Hard X-ray microbeam experiments with a sputtered-sliced Fresnel zone plate and its applications |journal=J Synchrotron Radiat |volume=9 |issue=Pt 3 |pages=182–6 |date=May 2002 |pmid=11972376 |display-authors=etal|doi-access=free }}
*[http://www.cxro.lbl.gov/BL612/index.php?content=research.html Scientific applications of soft x-ray microscopy]


{{X-ray science}}
{{X-ray science}}

Revision as of 08:25, 7 January 2024

Chandra X-ray Observatory, launched by NASA in 1999, is still operational as of 2024

An X-ray telescope (XRT) is a telescope that is designed to observe remote objects in the X-ray spectrum. X-rays are absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and satellites.

The basic elements of the telescope are the optics (focusing or collimating), that collects the radiation entering the telescope, and the detector, on which the radiation is collected and measured. A variety of different designs and technologies have been used for these elements.

Many of the existing telescopes on satellites are compounded of multiple copies or variations of a detector-telescope system, whose capabilities add or complement each other and additional fixed or removable elements[1][2] (filters, spectrometers) that add functionalities to the instrument.

History of X-ray telescopes

See also: History of X-ray astronomy

The first X-ray telescope employing Wolter Type I grazing-incidence optics was employed in a rocket-borne experiment on October 15, 1963, 1605 UT at White Sands New Mexico using a Ball Brothers Corporation pointing control on an Aerobee 150 rocket to obtain the X-ray images of the Sun in the 8–20 angstrom region. The second flight was in 1965 at the same launch site.[3]

The Einstein Observatory (1978–1981), also known as HEAO-2, was the first orbiting X-ray observatory with a Wolter Type I telescope.[4] It obtained high-resolution X-ray images in the energy range from 0.1 to 4 keV of stars of all types, supernova remnants, galaxies, and clusters of galaxies. HEAO-1 (1977–1979) and HEAO-3 (1979–1981) were others in that series. Another large project was ROSAT (active from 1990 to 1999), which was a heavy X-ray space observatory with focusing X-ray optics.

The Chandra X-Ray Observatory was launched by NASA in 1999 and is operated for more than 25 years in a high elliptical orbit, returning thousands 0.5 arc-second images and high-resolution spectra of all kinds of astronomical objects in the energy range from 0.5 to 8.0 keV.

Optics

Main article: X-ray optics
One of the mirrors of XRISM made of 203 foils

The most common methods used in X-ray optics are grazing incidence mirrors and collimated apertures. Only three geometries that use grazing incidence reflection of X-rays to produce X-ray images are known: Wolter system, Kirkpatrick-Baez system, and lobster-eye optics.[5]

Focusing mirrors

The utilization of X-ray mirrors allows to focus the incident radiation on the detector plane. Different geometries (e.g. Kirkpartrick-Baez or lobster-eye) have been suggested or employed, but almost the totality of existing telescopes employs some variation of the Wolter I design.[citation needed] The limitations of this type of X-ray optics result in much narrower fields of view (typically <1 degree) than visible or UV telescopes.

With respect to collimated optics, focusing optics allow:

  • a high resolution imaging
  • a high telescope sensitivity: since radiation is focused on a small area, Signal-to-noise ratio is much higher for this kind of instruments.
Focusing X-rays with glancing reflection

The mirrors can be made of ceramic or metal foil[6] coated with a thin layer of a reflective material (typically gold or iridium). Mirrors based on this construction work on the basis of total reflection of light at grazing incidence.

This technology is limited in energy range by the inverse relation between critical angle for total reflection and radiation energy. The limit in the early 2000s with Chandra and XMM-Newton X-ray observatories was about 15 kilo-electronvolt (keV) light.[7] Using new multi-layered coated mirrors, the X-ray mirror for the NuSTAR telescope pushed this up to 79 keV light.[7] To reflect at this level, glass layers were multi-coated with tungsten (W)/silicon (Si) or platinum (Pt)/silicon carbide(SiC).[7]

Collimating optics

Main article: coded aperture

While earlier X-ray telescopes were using simple collimating techniques (e.g. rotating collimators, wire collimators),[8] the technology most used in the present day employs coded aperture masks. This technique uses a flat aperture patterned grille in front of the detector. This design gives results that are less sensitive than focusing optics; also the imaging quality and identification of source position is much poorer. Though this design offers a larger field of view and can be employed at higher energies, where grazing incidence optics become ineffective. Also the imaging is not direct, but the image is rather reconstructed by post-processing of the signal.

Detectors

X-rays start at ~0.008 nm and extend across the electromagnetic spectrum to ~8 nm, over which Earth's atmosphere is opaque.

Several technologies have been employed on detectors for X-ray telescopes, ranging from counters like ionization chambers, geiger counters or scintillators to imaging detectors like CCDs or CMOS sensors.[citation needed] The use of micro-calorimeters, that offer the added capability of measuring with great accuracy the energy of the radiation, is planned for future missions.[citation needed] X-ray detectors have been designed and configured primarily for energy and occasionally for wavelength detection using a variety of techniques usually limited to the technology of the time.

Detection and imaging of X-rays

Chandra's image of Saturn (left) and Hubble optical image of Saturn (right). Saturn's X-ray spectrum is similar to that of X-rays from the Sun. 14 April 2003

X-rays has a huge span in wavelength (~8 nm - 8 pm), frequency (~50 PHz - 50 EHz) and energy (~0.12 - 120 keV). In terms of temperature, 1 eV = 11,604 K. Thus X-rays (0.12 to 120 keV) correspond to 1.39 × 106 to 1.39 × 109 K. From 10 to 0.1 nanometers (nm) (about 0.12 to 12 keV) they are classified as soft X-rays, and from 0.1 nm to 0.01 nm (about 12 to 120 keV) as hard X-rays.

Closer to the visible range of the electromagnetic spectrum is the ultraviolet. The draft ISO standard on determining solar irradiances (ISO-DIS-21348)[9] describes the ultraviolet as ranging from ~10 nm to ~400 nm. That portion closest to X-rays is often referred to as the "extreme ultraviolet" (EUV or XUV). When an EUV photon is absorbed, photoelectrons and secondary electrons are generated by ionization, much like what happens when X-rays or electron beams are absorbed by matter.[10]

The distinction between X-rays and gamma rays has changed in recent decades. Originally, the electromagnetic radiation emitted by X-ray tubes had a longer wavelength than the radiation emitted by radioactive nuclei (gamma rays).[11] So older literature distinguished between X- and gamma radiation on the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10−11 m, defined as gamma rays.[12] However, as shorter wavelength continuous spectrum "X-ray" sources such as linear accelerators and longer wavelength "gamma ray" emitters were discovered, the wavelength bands largely overlapped. The two types of radiation are now usually distinguished by their origin: X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.[11][13][14][15]

Although the more energetic X-rays, photons with an energy greater than 30 keV (4,800 aJ), can penetrate the Earth's atmosphere at least for distances of a few meters, the Earth's atmosphere is thick enough that virtually none are able to penetrate from outer space all the way to the Earth's surface. X-rays in the 0.5 to 5 keV (80 to 800 aJ) range, where most celestial sources give off the bulk of their energy, can be stopped by a few sheets of paper; 90% of the photons in a beam of 3 keV (480 aJ) X-rays are absorbed by traveling through just 10 cm of air.

Proportional counters

Main article: Proportional counter

A proportional counter is a type of gaseous ionization detector that counts particles of ionizing radiation and measures their energy. It works on the same principle as the Geiger-Müller counter, but uses a lower operating voltage. All X-ray proportional counters consist of a windowed gas cell.[16] Often this cell is subdivided into a number of low- and high-electric field regions by some arrangement of electrodes.

Proportional counters were used on EXOSAT,[17] on the US portion of the Apollo–Soyuz mission (July 1975), on French TOURNESOL instrument consisted.[18]

X-ray monitor

Monitoring generally means to be aware of the state of a system. A device that displays or sends a signal for displaying X-ray output from an X-ray generating source so as to be aware of the state of the source is referred to as an X-ray monitor in space applications. On Apollo 15 in orbit above the Moon, for example, an X-ray monitor was used to follow the possible variation in solar X-ray intensity and spectral shape while mapping the lunar surface with respect to its chemical composition due to the production of secondary X-rays.[19]

The X-ray monitor of Solwind, designated NRL-608 or XMON, was a collaboration between the Naval Research Laboratory and Los Alamos National Laboratory. The monitor consisted of 2 collimated argon proportional counters.

Scintillation detector

Main article: Scintillator
Scintillation crystal surrounded by various scintillation detector assemblies

A scintillator is a material which exhibits the property of luminescence[20] when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, such as an X-ray photon, absorb its energy and scintillate, i.e. reemit the absorbed energy in the form of a small flash of light, typically in the visible range.

The scintillation X-ray detector were used on Vela 5A and its twin Vela 5B;[21] the X-ray telescope onboard OSO 4 consisted of a single thin NaI(Tl) scintillation crystal plus phototube assembly enclosed in a CsI(Tl) anti-coincidence shield. OSO 5 carried a CsI crystal scintillator. The central crystal was 0.635 cm thick, had a sensitive area of 70 cm2, and was viewed from behind by a pair of photomultiplier tubes.

The PHEBUS had two independent detectors, each detector consisted of a bismuth germinate (BGO) crystal 78 mm in diameter by 120 mm thick.[18] The KONUS-B instrument consisted of seven detectors distributed around the spacecraft that responded to photons of 10 keV to 8 MeV energy. They consisted of NaI(Tl) scintillator crystals 200 mm in diameter by 50 mm thick behind a Be entrance window. Kvant-1 carried the HEXE, or High Energy X-ray Experiment, which employed a phoswich of sodium iodide and caesium iodide.

Modulation collimator

In electronics, modulation is the process of varying one waveform in relation to another waveform. With a 'modulation collimator' the amplitude (intensity) of the incoming X-rays is reduced by the presence of two or more 'diffraction gratings' of parallel wires that block or greatly reduce that portion of the signal incident upon the wires.

An X-ray collimator is a device that filters a stream of X-rays so that only those traveling parallel to a specified direction are allowed through.

Minoru Oda, President of Tokyo University of Information Sciences, invented the modulation collimator, first used to identify the counterpart of Sco X-1 in 1966, which led to the most accurate positions for X-ray sources available, prior to the launch of X-ray imaging telescopes.[22]

SAS 3 carried modulation collimators (2-11 keV) and Slat and Tube collimators (1 up to 60keV).[23]

On board the Granat Observatory were four WATCH instruments that could localize bright sources in the 6 to 180 keV range to within 0.5° using a Rotation Modulation Collimator. Taken together, the instruments' three fields of view covered approximately 75% of the sky.[18]

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), Explorer 81, images solar flares from soft X-rays to gamma rays (~3 keV to ~20 MeV). Its imaging capability is based on a Fourier-transform technique using a set of 9 Rotational Modulation Collimators.

X-ray spectrometer

OSO 8 had on board a Graphite Crystal X-ray Spectrometer, with energy range of 2-8 keV, FOV 3°.

The Granat ART-S X-ray spectrometer covered the energy range 3 to 100 keV, FOV 2° × 2°. The instrument consisted of four detectors based on spectroscopic MWPCs, making an effective area of 2,400 cm2 at 10 keV and 800 cm2 at 100 keV. The time resolution was 200 microseconds.[18]

The X-ray spectrometer aboard ISEE-3 was designed to study both solar flares and cosmic gamma-ray bursts over the energy range 5-228 keV.nThe experiment consisted of 2 cylindrical X-ray detectors: a Xenon filled proportional counter covering 5-14 keV, and a NaI(Tl) scintillator covering 12-1250 keV.

CCDs

Most existing X-ray telescopes use CCD detectors, similar to those in visible-light cameras. In visible-light, a single photon can produce a single electron of charge in a pixel, and an image is built up by accumulating many such charges from many photons during the exposure time. When an X-ray photon hits a CCD, it produces enough charge (hundreds to thousands of electrons, proportional to its energy) that the individual X-rays have their energies measured on read-out.

Microcalorimeters

Microcalorimeters can only detect X-rays one photon at a time (but can measure the energy of each).

Transition edge sensors

TES devices are the next step in microcalorimetry. In essence they are super-conducting metals kept as close as possible to their transition temperature. This is the temperature at which these metals become super-conductors and their resistance drops to zero. These transition temperatures are usually just a few degrees above absolute zero (usually less than 10 K).

X-ray space telescopes

Main article: List of space telescopes § X-ray
Uhuru X-ray satellite

An X-ray space telescope is a type of space telescope used to study X-ray emissions from celestial objects. First specialised X-ray space telescope, Uhuru, was launched by NASA in 1970.

Active X-ray observatory satellites

Satellites in use today include ESA's XMM-Newton observatory (low to mid energy X-rays 0.1-15 keV), NASA's Swift observatory, Chandra observatory and IXPE telescope. JAXA has launched the XRISM telescope, while ISRO has launched Aditya-L1 and XPoSat.

The GOES 14 spacecraft carries on board a Solar X-ray Imager to monitor the Sun's X-rays for the early detection of solar flares, coronal mass ejections, and other phenomena that impact the geospace environment.[24] It was launched into orbit on June 27, 2009, at 22:51 GMT from Space Launch Complex 37B at the Cape Canaveral Air Force Station.

Chinese Hard X-ray Modulation Telescope was launched on June 15, 2017 to observe black holes, neutron stars, active galactic nuclei and other phenomena based on their X-ray and gamma-ray emissions.[25]

The Lobster-Eye X-ray Satellite was launched on 25 July 2020 by CNSA. it is the first in-orbit telescope to utilize the lobster-eye imaging technology of ultra-large field of view imaging to search for dark matter signals in the x-ray energy range.[26] Lobster Eye Imager for Astronomy was launched in 2022 as a technology demonstrator for planned Einstein Probe.

A soft X-ray solar imaging telescope is on board the GOES-13 weather satellite launched using a Delta IV from Cape Canaveral LC37B on May 24, 2006.[27] However, there have been no GOES 13 SXI images since December 2006.

The Russian-German Spektr-RG carries the eROSITA telescope array as well as the ART-XC telescope. It was launched by Roscosmos on 13 July 2019 from Baikonur and began collecting data in October 2019.

See also

References

  1. ^ "Chandra :: About Chandra :: Science Instruments". chandra.si.edu. Retrieved 2016-02-19.
  2. ^ "Instruments". sci.esa.int. Retrieved 2016-02-19.
  3. ^ R. Giacconi et al., ApJ 142, 1274 (1965)
  4. ^ R. Giacconi et al., ApJ 230,540 (1979)
  5. ^ Richard Willingale (July 2021). "Lobster Eye Optics". In Sternberg, Amiel; Burrows, David N (eds.). The WSPC Handbook of Astronomical Instrumentation: Volume 4: X-Ray Astronomical Instrumentation. World Scientific Publishing Co. Pte. Ltd. pp. 33–47, 85–106. ISBN 978-981-4644-38-9. Retrieved 1 January 2024.
  6. ^ "Mirror Laboratory".
  7. ^ a b c NuStar: Instrumentation: Optics Archived November 1, 2010, at the Wayback Machine
  8. ^ Seward, Frederick D.; Charles, Philip A. (2010). Exploring the X-ray Universe – Cambridge Books Online – Cambridge University Press. doi:10.1017/cbo9780511781513. ISBN 9780511781513.
  9. ^ Tobiska, W; Nusinov, A (2006). "ISO 21348 Process for Determining Solar Irradiances". 36th Cospar Scientific Assembly. 36: 2621. Bibcode:2006cosp...36.2621T.
  10. ^ Henke BL; et al. (1977). "0.1–10-keV X-ray induced electron emissions from solids—Models and secondary electron measurements". Journal of Applied Physics. 48 (5): 1852. Bibcode:1977JAP....48.1852H. doi:10.1063/1.323938.
  11. ^ a b Dendy PP; Heaton B (1999). Physics for Diagnostic Radiology. CRC Press. p. 12. ISBN 978-0-7503-0591-4.
  12. ^ Charles Hodgman, ed. (1961). CRC Handbook of Chemistry and Physics (44th ed.). Chemical Rubber Co. p. 2850.
  13. ^ Feynman R; Leighton R; Sands M (1963). The Feynman Lectures on Physics. Vol. 1. Addison-Wesley. pp. 2–5. ISBN 978-0-201-02116-5.
  14. ^ L'Annunziata M; Baradei M (2003). Handbook of Radioactivity Analysis. Academic Press. p. 58. ISBN 978-0-12-436603-9.
  15. ^ Grupen C; Cowan G; Eidelman SD; Stroh T (2005). Astroparticle Physics. Springer. p. 109. ISBN 978-3-540-25312-9.
  16. ^ "Proportional Counters".
  17. ^ Hoff HA (1983). "Exosat - the new extrasolar X-ray observatory". J. Br. Interplanet. Soc. 36: 363. Bibcode:1983JBIS...36..363H.
  18. ^ a b c d "Granat". NASA. Retrieved 2007-12-05.
  19. ^ Adler I; Gerard J; Trombka J; Schmadebeck R; Lowman P; Bodgett H (1972). "The Apollo 15 x-ray fluorescence experiment". Proc Lunar Sci Conf. 2: 2157. Bibcode:1972LPSC....3.2157A.
  20. ^ Leo WR (1994). Techniques for Nuclear and particle Physics Experiments (2nd ed.). Springer.
  21. ^ Conner JP; Evans WD; Belian RD (1969). "The Recent Appearance of a New X-Ray Source in the Southern Sky". Astrophys J. 157: L157. Bibcode:1969ApJ...157L.157C. doi:10.1086/180409.
  22. ^ Cominsky L; Inoue H; Clark G. "Minoru Oda (1923 - 2001)".
  23. ^ "The Third Small Astronomy Satellite (SAS-3)". Archived from the original on 2001-04-17.
  24. ^ "GOES Solar X-ray Imager".
  25. ^ Rui C. Barbosa (14 June 2017). "China launches X-ray telescope via Long March 4B". NASASpaceFlight.com. Retrieved 23 August 2020.
  26. ^ "Launch of the world's first soft X-ray satellite with 'Lobster-Eye' imaging technology". copernical.com. 26 July 2020. Retrieved 23 August 2020.
  27. ^ Wade M. "Chronology - Quarter 2 2006". Archived from the original on September 2, 2003.
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