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((Short description|Telescope designed to observe remote objects by detecting X-rays))
((More citations needed|date=November 2023))
[[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. 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.

Many X-ray telescopes on satellites are compounded of multiple small detector-telescope systems whose capabilities add up 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==
[[File:X-Ray_Explorer_Satellite.jpg|thumb|[[Uhuru (satellite)|Uhuru]] X-ray satellite]]
[[File:HEAO-2 Image of the Supernova Remnant Cassiopeia A Taken by the High Energy Astronomy Observatory 8003547.jpg|thumb|Photo of [[supernova]] remnant [[Cassiopeia A]], taken by the first imaging X-ray telescope, [[Einstein Observatory]].]]
((See also|X-ray astronomy|History of X-ray astronomy|List of space telescopes#X-ray))

X-ray telescopes were first used for astronomy to observe the Sun, which was the only source in the sky bright enough in X-rays for those early telescopes to detect. Because the Sun is so bright in X-rays, early X-ray telescopes could use a small focusing element and the X-rays would be detected with photographic film. The first X-ray picture of the Sun from a rocket-borne telescope was taken by John V. Lindsay of the NASA Goddard Space Flight Center and collaborators in 1963. The first orbiting X-ray telescope flew on [[Skylab]] in the early 1970s and recorded more than 35,000 full-disk images of the Sun over a 9-month period.<ref name="nasa-history">((cite web |title=A Brief History of X-ray Telescopes |url=https://imagine.gsfc.nasa.gov/observatories/history/xray/history.html |website=imagine.gsfc.nasa.gov |access-date=7 January 2024)) ((PD-notice))</ref>

First specialised X-ray satellite, [[Uhuru (satellite)|Uhuru]], was launched by NASA in 1970. It detected 339 X-ray sources in its 2.5-year lifetime.<ref name=overview>((cite journal |title=X-Ray Observatories - an overview ((!)) ScienceDirect Topics |journal=www.sciencedirect.com |url=https://www.sciencedirect.com/topics/physics-and-astronomy/x-ray-observatories |access-date=7 January 2024))</ref>

The [[Einstein Observatory]], launched in 1978, was the first imaging X-ray observatory. 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. Another large project was [[ROSAT]] (active from 1990 to 1999), which was a heavy X-ray space observatory with focusing X-ray optics, and European [[EXOSAT]].<ref name=overview/>

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. Chandra's resolution is about 50 times superior to that of ROSAT.<ref name="nasa-history"/>

==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|url=http://www.swpc.noaa.gov/sxi/index.html|title=GOES Solar X-ray Imager))</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]].

The 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]] making 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 on 27 July 2022 as a technology demonstrator for [[Einstein Probe]], launched on January 9, 2024, dedicated to time-domain [[high-energy astrophysics]].<ref>((Cite web|url=https://www.esa.int/ESA_Multimedia/Images/2024/01/Einstein_Probe_lifts_off_on_a_mission_to_monitor_the_X-ray_sky|title=Einstein Probe lifts off on a mission to monitor the X-ray sky|website=www.esa.int))</ref> The [[Space Variable Objects Monitor]] observatory scheduled for launch in June 2024 will be directed at studying the explosions of massive stars and analysis of [[gamma-ray bursts]].<ref name="SVOM Mission profile">((cite web |url=http://www.svom.fr/en/ |access-date=14 January 2024 |title=Svom ))</ref>

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.

==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 [[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===
[[File:xray_telescope_lens.svg|thumb|Focusing X-rays with glancing reflection]]

A simple parabolic mirror was originally proposed in 1960 by [[Riccardo Giacconi]] and [[Bruno Rossi]], the founders of extrasolar X-ray astronomy. This type of mirror is often used as the primary reflector in an optical telescope. However, images of off-axis objects would be severely blurred. The German physicist [[Hans Wolter]] showed in 1952 that the reflection off a combination of two elements, a paraboloid followed by a hyperboloid, would work far better for X-ray astronomy applications. Wolter described three different imaging configurations, the [[Wolter telescope|Types I, II, and III]]. The design most commonly used by X-ray astronomers is the Type I since it has the simplest mechanical configuration. In addition, the Type I design offers the possibility of nesting several telescopes inside one another, thereby increasing the useful reflecting area. The Wolter Type II is useful only as a narrow-field imager or as the optic for a dispersive spectrometer. The Wolter Type III has never been employed for X-ray astronomy.<ref>((cite web |title=X-ray Telescopes - More Information |url=https://imagine.gsfc.nasa.gov/observatories/technology/xray_telescopes2.html |website=imagine.gsfc.nasa.gov |access-date=7 January 2024)) ((PD-notice))</ref>

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.

The mirrors can be made of [[ceramic]] or [[metal foil]]<ref name="xraysMirror">((Cite web|url=https://science.gsfc.nasa.gov/sci|title=Sciences and Exploration Directorate|website=science.gsfc.nasa.gov))</ref> 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 X-ray Observatory|Chandra]] and [[XMM-Newton]] X-ray [[Space observatory|observatories]] was about 15 kilo-[[electronvolt]] (keV) light.<ref name="nustar1">((Cite web|url=http://www.nustar.caltech.edu/about-nustar/instrumentation/optics|archiveurl=https://web.archive.org/web/20101101113623/http://www.nustar.caltech.edu/about-nustar/instrumentation/optics|url-status=dead|title=NuStar: Instrumentation: Optics|archivedate=November 1, 2010))</ref> Using new multi-layered coated mirrors, the X-ray mirror for the [[NuSTAR]] telescope pushed this up to 79 keV light.<ref name="nustar1" /> To reflect at this level, glass layers were multi-coated with [[tungsten]] (W)/[[silicon]] (Si) or [[platinum]] (Pt)/[[silicon carbide]](SiC).<ref name="nustar1" />

===Collimating optics===
((main | coded aperture))
While earlier X-ray telescopes were using simple collimating techniques (e.g. rotating collimators, wire collimators),<ref>((Cite book|title = Exploring the X-ray Universe – Cambridge Books Online – Cambridge University Press|doi = 10.1017/cbo9780511781513|first = Frederick D.|last = Seward|first2 = Philip A.|last2 = Charles|year = 2010|isbn = 9780511781513))</ref> 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.

==Detection and imaging of X-rays==
[[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]].]]
[[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]]
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>

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>

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.

===Proportional counters===
((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|url=https://imagine.gsfc.nasa.gov/science/index.html|title=Science|website=imagine.gsfc.nasa.gov))</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), and on French [[Granat#TOURNESOL|TOURNESOL]] instrument.<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>

===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]] [[MWPC]]s, 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===
[[Transition-edge sensor]]s 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]]).

==See also==
* [[List of telescope types]]

==References==
((Reflist|30em))

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