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 age, releasing more electrons as they bounce off the sides of the tube. By the time they leave the end of the tube, they have created a cloud of 30 million electrons. A crossed grid of wires detects this electron signal and allows the position of the original x-ray to be determined with high precision. With this information, astronomers can create a finely detailed map of a cosmic x-ray source. The HRC will be especially useful for imaging hot matter in the remnants of exploded stars, in distant galaxies, and in clusters of galaxies and for identifying very faint sources.

The CXO CCD lmaging Spectrometer (ACIS) is the other focal plane instrument. As the name suggests, this instrument is an array of charged coupled devices (CCD’s) similar to those used in a camcorder. This instrument will be especially useful because it can make x-ray images and measure the energies of incoming x-rays. It will be the instrument of choice for studying the temperature variation across x-ray sources, such as vast clouds of hot gas in intergalactic space.

In addition to the focal plane instruments, CXO will have two sets of finely ruled gratings, which can be swung into position between the mirrors and the focal plane. These gratings change the direction of incoming x-rays by amounts that depend sensitively on their energies. When used with either the HRC or ACIS, they will allow for the precise determination of the energies of the x-rays. The grating spectrometers, as they are called, will be useful for studying the detailed energy spectrum of strong sources to determine the temperature and chemical composition.

The science instruments are mounted on the Science Instrument Module, which contains mechanisms to move the science instruments in and out of the focal plane. This module also has insulation for thermal control and electronics to control the operation of the science instruments via the communication, command, and data management systems of the spacecraft.

The science instruments will be controlled by commands transmitted from the Operations Control Center at the CXO Science Center in Cambridge, Massachusetts. A preplanned sequence of observations will be uplinked to CXO and stored in the on-board computer for later execution. Data collected by observations with CXO will be stored on a recorder for later transmission to the ground every 8 hours during the regularly scheduled Deep Space Network contacts. The data will then be transmitted to the Jet Propulsion Laboratory and then to the Operations Control Center for processing and analysis by scientists.