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Description of the Chandra Mission

The Chandra X-ray Observatory is the U.S. follow-on to the Einstein Observatory. Chandra was formerly known as AXAF, the Advanced X-ray Astrophysics Facility, but renamed by NASA in December, 1998. The Chandra spacecraft carries a high resolution mirror, two imaging detectors (the HRC and ACIS), and two sets of transmission gratings. Important Chandra features are: an order of magnitude improvement in spatial resolution, good sensitivity from 0.1 to 10 keV, and the capability for high spectral resolution observations over most of this range. Chandra uses an elliptical high-earth orbit allowing uninterrupted observing intervals of more than 48 hours in length.

The Chandra telescope was designed to have three times the area of the Einstein mirror at low energies and to have considerable collecting area between 6 and 7 keV, the energy of iron lines emitted by many astrophysical sources. The mirror consists of four pairs of nested reflecting surfaces, arranged in the usual Wolter type 1 geometry. A combination of high resolution, large collecting area, and sensitivity to higher energy X-rays will make it possible for Chandra to study extremely faint sources, sometimes strongly absorbed, in crowded fields.

There are two focal plane instruments. One is a High Resolution Camera (HRC) . Smaller pore size, larger microchannel plate (MCP) dimensions, lower background, charged particle anticoincidence, and possible energy resolution are all advances over the HRI carried by Einstein and ROSAT. It will be used for high resolution imaging, fast timing measurements, and for observations requiring a combination of both.

The second instrument, the AXAF CCD Imaging Spectrometer (ACIS), is an array of charged coupled devices. A two-dimensional array of these small detectors will do simultaneous imaging and spectroscopy. Pictures of extended objects can be obtained along with spectral information from each element of the picture. The new device combines the spatial resolution of the Einstein HRI with the spectral resolution of the Einstein SSS, an order of magnitude improvement over the IPC in both respects. ASCA carries a similar CCD array but the mirror limits the spatial resolution to ~2 arcminutes.

There are two transmission grating spectrometers, formed by sets of gold gratings placed just behind the mirrors. One set is optimised for low energies (LETG) and the other for high energies (HETG). Spectral resolving powers (E/deltaE) in the range 100-2000 can be achieved with good efficiency. These produce spectra dispersed in space at the focal plane. Either the CCD array or the HRC can be used to record data.

 
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