Thursday, 15 February 2007

Chandra X-ray Observatory

"I am aware of the usefulness of science to society and of the benefits society derives from it."

~Subrahmanyan Chandrasekhar~

Chandra X-ray Observatory
Chandra X-ray Observatory inside the Space Shuttle payload bay.jpg
Chandra X-ray Observatory and Inertial Upper Stage sit inside the payload bay on Space ShuttleColumbia mission STS-93
General information
NSSDC ID1999-040B
OrganizationNASA, SAO, CXC
Major contractorsTRW, Northrop Grumman
Launch date23 July 1999
Launched fromKennedy Space Center
Launch vehicleSpace Shuttle Columbia STS-93
Mission lengthplanned: 5 years[1]
elapsed: 12 years and 13 days
Mass4,790 kg (10,600 lb)
Orbit height
apogee 133,000 km (83,000 mi)

perigee 16,000 km (9,900 mi)
Orbit period64.2 hours
WavelengthX-ray (0.1 - 10 keV)
Diameter1.2 m (3.9 ft)
Collecting area0.04 m2 (0.43 sq ft) at 1 keV
Focal length10 m (33 ft)
Instruments
Websitechandra.harvard.edu


The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.

Chandra Observatory is the third of NASA's four Great Observatories. The first was Hubble Space Telescope; second the Compton Gamma Ray Observatory, launched in 1991; and last is the Spitzer Space Telescope. Prior to successful launch, the Chandra Observatory was known as AXAF, the Advanced X-ray Astrophysics Facility. 


AXAF was assembled and tested by TRW (now Northrop Grumman Aerospace Systems) in Redondo Beach, California. Chandra is sensitive to X-ray sources 100 times fainter than any previous X-ray telescope, due primarily to the high angular resolution of the Chandra mirrors.
Since the Earth's atmosphere absorbs the vast majority of X-rays, they are not detectable from Earth-based telescopes, requiring a space-based telescope to make these observations.

In 1976 the Chandra X-ray Observatory (called AXAF at the time) was proposed to NASA by Riccardo Giacconi and Harvey Tananbaum. Preliminary work began the following year at Marshall Space Flight Center (MSFC) and the Smithsonian Astrophysical Observatory (SAO). In the meantime, in 1978, NASA launched the first imaging X-ray telescope, Einstein (HEAO-2), into orbit. 

Work continued on the Chandra project through the 1980's and 1990's. In 1992, to reduce costs, the spacecraft was redesigned. Four of the twelve planned mirrors were eliminated, as were two of the six scientific instruments. Chandra's planned orbit was changed to an elliptical one, reaching one third of the way to the Moon's at its farthest point. This eliminated the possibility of improvement or repair by the space shuttle but put the observatory above the Earth's radiation belts for most of its orbit.



AXAF was renamed Chandra in 1998 and launched in 1999 by the shuttle Columbia (STS-93). At 22753 kg, it was the heaviest payload ever launched by the shuttle, a consequence of the two-stage Inertial Upper Stage booster rocket system needed to transport the spacecraft to its high orbit.
Chandra has been returning data since the month after it launched. It is operated by the SAO at the Chandra X-ray Center in Cambridge, Massachusetts, with assistance from MIT and Northrop Grumman Space Technology. The ACIS CCDs suffered particle damage during early radiation belt passages. To prevent further damage, the instrument is now removed from the telescope's focal plane during passages.

In 2004 Chandra celebrated its fifth year of operation.


Technical description

 

 

Unlike optical telescopes which possess simple aluminized parabolic surfaces (mirrors), X-ray telescopes generally use a Wolter telescope consisting of nested cylindrical paraboloid and hyperboloid surfaces coated with iridium or gold. X-ray photons would be absorbed by normal mirror surfaces, so mirrors with a low grazing angle are necessary to reflect them. Chandra uses four pairs of nested mirrors, together with their support structure, called the High Resolution Mirror Assembly (HRMA); the mirror substrate is 2 cm-thick glass, with the reflecting surface a 33 nm iridium coating, and the diameters are 65 cm, 87 cm, 99 cm and 123 cm.

The thick substrate and particularly careful polishing allowed a very precise optical surface, which is responsible for Chandra's unmatched resolution: between 80% and 95% of the incoming X-ray energy is focused into a one-arcsecond circle. However, the thickness of the substrates limit the proportion of the aperture which is filled, leading to the low collecting area compared to XMM-Newton.


Chandra's highly elliptical orbit allows it to observe continuously for up to 55 hours of its 65 hour orbital period. At its furthest orbital point from earth, Chandra is one of the furthest from earth earth-orbiting satellites. This orbit takes it beyond the geostationary satellites and beyond the outer Van Allen belt.




With an angular resolution of 0.5 arcsecond (2.4 µrad), Chandra possesses a resolution over 1000 times better than that of the first orbiting X-ray telescope.

Instruments



The Science Instrument Module (SIM) holds the two focal plane instruments, the Advanced CCD Imaging Spectrometer (ACIS) and the High Resolution Camera (HRC), moving whichever is called for into position during an observation.


ACIS consists of 10 CCD chips and provides images as well as spectral information of the object observed. It operates in the range of 0.2–10 keV. HRC has two micro-channel plate components and images over the range of 0.1–10 keV. It also has a time resolution of 16 microseconds. Both of these instruments can be used on their own or in conjunction with one of the observatory's two transmission gratings.


The transmission gratings, which swing into the optical path behind the mirrors, provide Chandra with high resolution spectroscopy. The High Energy Transmission Grating Spectrometer (HETGS) works over 0.4–10 keV and has a spectral resolution of 60–1000. The Low Energy Transmission Grating Spectrometer (LETGS) has a range of 0.09–3 keV and a resolution of 40–2000.


Discoveries


SN 2006gy (upper right) and its parent galaxy NGC 1260 (lower left) in false color as observed through the Chandra X-Ray Observatory.

In this image of PSR B1509-58, the lowest energy X-rays that Chandra detects are red, the medium range is green, and the most energetic ones are colored blue.
The data gathered by Chandra have greatly advanced the field of X-ray astronomy.

Informal Education


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1. Wikipedia

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