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, balloons or artificial 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 (filters, spectrometers) that add functionalities to the instrument.
The utilization of X-ray mirrors allows to focus the incident radiation on the detector plane.
Different geometries (e.g. Kirkpartick-Baez or Lobster-eye) have been suggested or employed, but almost the totality of existing telescopes employs some variation of the Wolter I design. 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.
The mirrors can be made of ceramic or metal foil 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. Using new multi-layered coated mirrors, the X-ray mirror for the NuSTAR telescope pushed this up to 79 keV light. To reflect at this level, glass layers were multi-coated with tungsten (W)/silicon (Si) or platinum (Pt)/silicon carbide(SiC).
While earlier X-ray telescopes were using simple collimating techniques (e.g. rotating collimators, wire collimators), the technology most currently used on present days employs coded aperture masks. This technique uses a flat aperture patterned grille in front of the detector. This design results less sensitive than focusing optics and imaging quality and identification of source position is much poorer, however it 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.
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. 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.
Missions employing X-ray telescopesEdit
Hard X-ray telescopeEdit
On board OSO 7 was a hard X-ray telescope. Its effective energy range: 7–550 keV, field of view (FOV) 6.5°, effective area ~64 cm2.
The Filin telescope carried aboard Salyut 4, consisted of four gas flow proportional counters, three of which had a total detection surface of 450 cm2 in the energy range 2–10 keV, and one of which has an effective surface of 37 cm2 for the range 0.2–2 keV. The FOV was limited by a slit collimator to 3° × 10° FWHM. The instrumentation included optical sensors mounted on the outside of the station together with the X-ray detectors. The power supply and measurement units were inside the station. Ground-based calibration of the detectors occurred along with in-flight operation in three modes: inertial orientation, orbital orientation, and survey. Data were collected in 4 energy channels: 2–3.1 keV, 3.1–5.9 keV, 5.9–9.6 keV, and 2–9.6 keV in the larger detectors. The smaller detector had discriminator levels set at 0.2 keV, 0.55 keV, and 0.95 keV.
The hard X-ray and low-energy gamma-ray SIGMA telescope covered the energy range 35–1300 keV, with an effective area of 800 cm2 and a maximum sensitivity field of view of ~5° × 5°. The maximum angular resolution was 15 arcmin. The energy resolution was 8% at 511 keV. Its imaging capabilities were derived from the association of a coded mask and a position sensitive detector based on the Anger camera principle.
ART-P X-ray telescopeEdit
The ART-P X-ray telescope covered the energy range 4 to 60 keV for imaging and 4 to 100 keV for spectroscopy and timing. There were four identical modules of the ART-P telescope, each consisting of a position sensitive multi-wire proportional counter (MWPC) together with a URA coded mask. Each module had an effective area of approximately 600 cm2, producing a FOV of 1.8° × 1.8°. The angular resolution was 5 arcmin; temporal and energy resolutions were 3.9 ms and 22% at 6 keV, respectively. The instrument achieved a sensitivity of 0.001 of the Crab nebula source (= 1 "mCrab") in an eight-hour exposure. The maximum time resolution was 4 ms.
Focusing X-ray telescopeEdit
The Broad Band X-ray Telescope (BBXRT) was flown on the Space Shuttle Columbia (STS-35) as part of the ASTRO-1 payload. BBXRT was the first focusing X-ray telescope operating over a broad energy range 0.3–12 keV with a moderate energy resolution (90 eV at 1 keV and 150 eV at 6 keV). The two Co-Aligned Telescopes with a segmented Si(Li) solid state spectrometer each (detector A and B) composite of five pixels. Total FOV 17.4´ diameter, Central pixel FOV 4´ diameter. Total area 765 cm2 at 1.5 keV, and 300 cm2 at 7 keV.
XRT on the Swift MIDEX missionEdit
The XRT on the Swift MIDEX mission (0.2–10 keV energy range) uses a Wolter I telescope to focus X-rays onto a thermoelectrically cooled CCD. It was designed to measure the fluxes, spectra, and lightcurves of Gamma-ray bursts (GRBs) and afterglows over a wide dynamic range covering more than 7 orders of magnitude in flux. The XRT can pinpoint GRBs to 5-arcsec accuracy within 10 seconds of target acquisition for a typical GRB and can study the X-ray counterparts of GRBs beginning 20–70 seconds from burst discovery and continuing for days to weeks.
The overall telescope length is 4.67 m with a focal length of 3.500 mm and a diameter of 0.51 m. The primary structural element is an aluminum optical bench interface flange at the front of the telescope that supports the forward and aft telescope tubes, the mirror module, the electron deflector, and the internal alignment monitor optics and camera, plus mounting points to the Swift observatory.
The 508 mm diameter telescope tube is made of graphite fiber/cyanate ester in two sections. The outer graphite fiber layup is designed to minimize the longitudinal coefficient of thermal expansion, whereas the inner composite tube is lined internally with an aluminum foil vapor barrier to guard against outgassing of water vapor or epoxy contaminants into the telescope interior. The telescope has a forward tube which encloses the mirrors and supports the door assembly and star trackers, and an aft tube which supports the focal plane camera and internal optical baffles.
The mirror module consists of 12 nested Wolter I grazing incidence mirrors held in place by front and rear spiders. The passively heated mirrors are gold-coated, electroformed nickel shells 600 mm long with diameters ranging from 191 to 300 mm.
The X-ray imager has an effective area of >120 cm2 at 1.15 keV, a field of view of 23.6 × 23.6 arcmin, and angular resolution (θ) of 18 arcsec at half-power diameter (HPD). The detection sensitivity is 2 × 10−14 erg cm−2s−1 in 104 s. The mirror point spread function (PSF) has a 15 arcsec HPD at the best on-axis focus (at 1.5 keV). The mirror is slightly defocused in the XRT to provide a more uniform PSF for the entire field of view hence the instrument PSF θ = 18 arcsec.
Normal incidence X-ray telescopeEdit
History of X-ray telescopesEdit
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 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 1990-1999), which was a heavy X-ray space observatory with focusing X-ray optics.
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.
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 44Ti in supernovae.
Gravity and Extreme Magnetism (GEMS) would have measured X-ray polarization but was canceled in 2012.
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