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The Advanced X-ray Astrophysics Facility - Imaging (AXAF-I) is the next in a series of NASA astrophysical observatories that promises to be a significant advancement in the field of X-ray astrophysics for the late 1990's and for the first part of the twenty first century.

AXAF-I will have two state of the art X-ray imaging instruments at the focal plane of a grazing incidence X-ray mirror assembly that has very high angular resolution. Both focal plane instruments - the ACIS and the HRC (High Resolution Camera) - will be mounted to the Science Instrument Module (SIM). The SIM will have the capability of positioning either the ACIS or the HRC at the focus of the observatory X-ray optics. In addition, AXAF-I will also have two X-ray grating assemblies, the Low Energy Transmission Grating (LETG) and the High Energy Transmission Grating (HETG). Each grating assembly will be capable of being moved into the X-ray path between the X-ray optics and the focal plane instruments to provide high resolution spectral measurements of the dispersed spectra. One of the two AXAF-I focal plane instruments - the AXAF-I CCD Imaging Spectrometer (ACIS) - is presently being developed at the Massachusetts Institute of Technology Center for Space Research in conjunction with the Pennsylvania State University, with the support of MIT Lincoln Laboratory, and the Martin Marietta Aerospace Group.

The ACIS focal plane instrument is based on two arrays of large Charge Coupled Devices (CCDs) arranged in two configurations, as an imaging array and a spectroscopy array. The ACIS focal plane CCD imaging array will provide high resolution spectral and temporal photon-counting images of celestial X-ray objects. The ACIS CCD spectroscopy array, when used in conjunction with the HETG, will provide very high X-ray spectral resolution of a large variety of celestial X-ray sources.

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