Chandra X-ray Observatory
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 mass limit for white dwarf stars to become neutron stars.  "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 Space Technology) in Redondo Beach, California.
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.

# Discoveries
The data gathered by Chandra have greatly advanced the field of X-ray astronomy.
- The first light image, of supernova remnant Cassiopeia A, gave astronomers their first glimpse of the compact object at the center of the remnant, probably a neutron star or black hole. (Pavlov, et al, 2000)
- In the Crab Nebula, another supernova remnant, Chandra showed a never-before-seen ring around the central pulsar and jets that had only been partially seen by earlier telescopes. (Weisskopf, et al., 2000)
- The first X-ray emission was seen from the supermassive black hole, Sagittarius A*, at the center of the Milky Way. (Baganoff, et al, 2001)
- Chandra found much more cooler gas than expected spiralling into the center of the Andromeda Galaxy.
- Pressure fronts were observed in detail for the first time in Abell 2142, where clusters of galaxies are merging.
- The earliest images in X-rays of the shock wave of a supernova were taken of SN 1987A.
- Chandra showed for the first time the shadow of a small galaxy as it is being cannibalized by a larger one, in an image of Perseus A.
- A new type of black hole was discovered in galaxy M82, mid-mass objects purported to be the missing link between stellar-sized black holes and supermassive black holes. (Griffiths, et al., 2000)
- X-ray emission lines were associated for the first time with a gamma-ray burst, GRB 991216. (Piro, et al., 2000)
- High school students, using Chandra data, discovered a neutron star in supernova remnant IC 443.
- Observations by Chandra and BeppoSAX suggest that gamma-ray bursts occur in star-forming regions.
- Chandra data suggested that RX J1856.5-3754 and 3C58, previously thought to be pulsars, might be even denser objects: quark stars.  These results are still debated.
- TWA 5B, a brown dwarf, was seen orbiting a binary system of Sun-like stars.
- Nearly all stars on the main sequence are X-ray emitters. (Schmitt & Liefke, 2004)
- Sound waves from violent activity around a supermassive black hole were observed in the Perseus Cluster.
- The X-ray shadow of Titan was seen when it transitted the Crab Nebula.
- X-ray emissions from materials falling from a protoplanetary disc into a star. (Kastner, et al., 2004)
- Hubble constant measured to be 76.9 km/s/Mpc using Sunyaev-Zel'dovich effect.
- 2006 Chandra found strong evidence that dark matter exists by observing supercluster collision
- 2006 X-ray emitting loops, rings and filaments discovered around a supermassive black hole within Messier 87 imply the presence of pressure waves, shock waves and sound waves. The evolution of Messier 87 may have been dramatically affected.
- Observations of the Bullet cluster put limits on the cross-section of the self-interaction of dark matter.

# Technical description
Unlike optical telescopes which possess simple aluminized parabolic surfaces (mirrors), X-ray telescopes generally have 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 iridium mirrors, together with their support structure, called the High Resolution Mirror Assembly (HRMA).
Chandra's high elliptical orbit allows it to observe continuously for up to 55 hours of its 65 hour orbital period.
With an angular resolution of 0.5 arcsecond (2.4 rad), Chandra possesses a resolution over one thousand 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.

# History
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.