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AnX-ray telescope (XRT) is atelescope that is designed to observe remote objects in theX-ray spectrum. X-rays are absorbed by theEarth's atmosphere, so instruments to detect X-rays must be taken to high altitude byballoons,sounding rockets, andsatellites.
The basic elements of the telescope are theoptics (focusing orcollimating), that collects theradiation entering the telescope, and thedetector, 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[1][2] (filters, spectrometers) that add functionalities to the instrument.
X-ray telescopes were first used for astronomy to observe theSun, 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 NASAGoddard Space Flight Center and collaborators in 1963. The first orbiting X-ray telescope flew onSkylab in the early 1970s and recorded more than 35,000 full-disk images of the Sun over a 9-month period.[3]
First specialised X-ray satellite,Uhuru, was launched byNASA in 1970. It detected 339 X-ray sources in its 2.5-year lifetime.[4]
TheEinstein 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 wasROSAT (active from 1990 to 1999), which was a heavy X-ray space observatory with focusing X-ray optics, and EuropeanEXOSAT.[4]
TheChandra 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.[3]
Satellites in use today includeESA'sXMM-Newton observatory (low to mid energy X-rays 0.1-15 keV),NASA'sSwift observatory,Chandra observatory andIXPE telescope.JAXA has launched theXRISM telescope, whileISRO has launchedAditya-L1 andXPoSat.
TheGOES 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.[5] It was launched into orbit on June 27, 2009, at 22:51 GMT fromSpace Launch Complex 37B at theCape Canaveral Air Force Station.
The ChineseHard 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.[6]
TheLobster-Eye X-ray Satellite was launched on 25 July 2020 byCNSA making it is the first in-orbit telescope to utilize thelobster-eye imaging technology of ultra-large field of view imaging to search for dark matter signals in the x-ray energy range.[7]Lobster Eye Imager for Astronomy was launched on 27 July 2022 as a technology demonstrator forEinstein Probe, launched on January 9, 2024, dedicated to time-domainhigh-energy astrophysics.[8] TheSpace Variable Objects Monitor observatory launched on 22 June 2024 is directed towards studying the explosions of massive stars and analysis ofgamma-ray bursts.[9]
Asoft X-ray solar imaging telescope is on board theGOES-13 weather satellite launched using aDelta IV fromCape Canaveral LC37B on May 24, 2006.[10] However, there have been no GOES 13 SXI images since December 2006.
The Russian-GermanSpektr-RG carries theeROSITA telescope array as well as theART-XC telescope. It was launched byRoscosmos on 13 July 2019 fromBaikonur and began collecting data in October 2019.
The most common methods used in X-ray optics aregrazing incidence mirrors andcollimated apertures. Only three geometries that usegrazing incidence reflection of X-rays to produce X-ray images are known:Wolter system,Kirkpatrick-Baez system, andlobster-eye optics.[11]
A simple parabolic mirror was originally proposed in 1960 byRiccardo Giacconi andBruno 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 physicistHans 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, theTypes 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.[12]
With respect to collimated optics, focusing optics allow:
The mirrors can be made ofceramic ormetal foil[13] coated with a thin layer of a reflective material (typicallygold oriridium). Mirrors based on this construction work on the basis oftotal 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 withChandra andXMM-Newton X-rayobservatories was about 15 kilo-electronvolt (keV) light.[14] Using new multi-layered coated mirrors, the X-ray mirror for theNuSTAR telescope pushed this up to 79 keV light.[14] To reflect at this level, glass layers were multi-coated withtungsten (W)/silicon (Si) orplatinum (Pt)/silicon carbide(SiC).[14]
While earlier X-ray telescopes were using simple collimating techniques (e.g. rotating collimators, wire collimators),[15] 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 largerfield 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.
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 million to 1.39 billion K. From 10 to 0.1 nanometers (nm) (about 0.12 to 12keV) 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 theultraviolet. The draft ISO standard on determining solarirradiances (ISO-DIS-21348)[16] 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 andsecondary electrons are generated byionization, much like what happens when X-rays or electron beams are absorbed by matter.[17]
The distinction between X-rays andgamma rays has changed in recent decades. Originally, the electromagnetic radiation emitted byX-ray tubes had a longerwavelength than the radiation emitted byradioactivenuclei (gamma rays).[18] 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.[19] However, as shorter wavelength continuous spectrum "X-ray" sources such aslinear 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 thenucleus.[18][20][21][22]
Although the more energetic X-rays,photons with an energy greater than 30keV (4,800aJ), can penetrate theEarth'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.
Aproportional counter is a type ofgaseous ionization detector that countsparticles ofionizing radiation and measures their energy. It works on the same principle as theGeiger-Müller counter, but uses a lower operatingvoltage. All X-ray proportional counters consist of a windowed gas cell.[23] Often this cell is subdivided into a number of low- and high-electric field regions by some arrangement of electrodes.
Proportional counters were used onEXOSAT,[24] on the US portion of theApollo–Soyuz mission (July 1975), and on FrenchTOURNESOL instrument.[25]
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. OnApollo 15 in orbit above theMoon, 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 ofsecondary X-rays.[26]
The X-ray monitor ofSolwind, designated NRL-608 or XMON, was a collaboration between theNaval Research Laboratory andLos Alamos National Laboratory. The monitor consisted of 2 collimated argon proportional counters.
A scintillator is a material which exhibits the property ofluminescence[27] when excited byionizing 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 onVela 5A and its twinVela 5B;[28] the X-ray telescope onboardOSO 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.
ThePHEBUS had two independent detectors, each detector consisted of a bismuth germinate (BGO) crystal 78 mm indiameter by 120 mm thick.[25] TheKONUS-B instrument consisted of seven detectors distributed around the spacecraft that responded tophotons of 10 keV to 8 MeV energy. They consisted ofNaI(Tl) scintillator crystals 200 mm in diameter by 50 mm thick behind aBe entrance window.Kvant-1 carried the HEXE, or High Energy X-ray Experiment, which employed aphoswich of sodium iodide and caesium iodide.
Inelectronics,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.
AnX-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.[29]
SAS 3 carried modulation collimators (2-11 keV) and Slat and Tube collimators (1 up to 60keV).[30]
On board theGranat Observatory were fourWATCH 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.[25]
TheReuven 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 9Rotational Modulation Collimators.
OSO 8 had on board a Graphite Crystal X-ray Spectrometer, with energy range of 2-8 keV, FOV 3°.
TheGranatART-S X-ray spectrometer covered the energy range 3 to 100 keV, FOV 2° × 2°. The instrument consisted of four detectors based onspectroscopicMWPCs, making an effective area of 2,400 cm2 at 10 keV and 800 cm2 at 100 keV. The time resolution was 200microseconds.[25]
The X-ray spectrometer aboardISEE-3 was designed to study both solar flares and cosmic gamma-ray bursts over the energy range 5-228 keV. The 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.
Most existing X-ray telescopes useCCD 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 can only detect X-rays one photon at a time (but can measure the energy of each).
Transition-edge sensors 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 10K).
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