The 8,000-pound (3,600-kilogram) XMM observatory actually holds three telescopes. It combines an X-ray imaging telescope, an X-ray spectrometer and an optical imaging telescope, all aimed together to provide a range of information about the most energetic objects in the universe.

"X-rays are formed at very high temperatures -- millions of degrees -- so looking with X-ray eyes we will see a completely different universe than when you look with optical eyes," said Bert Brinkman, who is principal investigator for the observatory's Reflection Grating Spectrometer. Brinkman is an astronomer at the Space Research Organization of the Netherlands.

"[XMM and Chandra] will really work to complement each other, not in competition. It will be great to have them both up there."

The X-ray spectrometer, which is much more sophisticated than similar instruments, will allow astronomers to study the composition of very high-energy sources such as quasars and black holes.

Just as in optical spectroscopy, different elements absorb light at specific and unique points of the radiation spectrum. The light absorption bands appear as distinct lines in the rainbow of X-ray colors, acting as signatures that reveal a great deal of information to astronomers.

"If you compare the intensity of different spectral lines, you can deduce the temperature at which the X-rays were generated, or you can learn the density of the emitting gas, and learn which elements are in the plasma that is emitting the X-rays," Brinkman said.

Each one of XMM's three telescopes uses an elaborate array of 58 small mirrors arranged in a circle to provide the detection area of a much larger mirror.

The three XMM mirror modules are shown on the backside of the XMM service module. A full set of 58 mirror shells is visible on the left side. The two other modules are covered. (Photo courtesy of Dornier Satellitensysteme GmbH.)

XMM's forte will be in taking images of large, diffuse objects in the sky, such as galaxy clusters. Chandra's strength meanwhile is in getting sharp detail of very small targets.

While Chandra delivers high-resolution images of otherwise indistinguishable sources, it is limited in its ability to stand back and get the big picture.

"You get this weird situation where, because of the fact that the mirror on Chandra is so good, and they have such an excellent spatial resolution, the telescope is limited with any source in the sky which is a little bit extended. In that case, Chandra cannot get a good spectrum measurements," Bertman said.

The different capabilities will allow the two telescopes to work well together, said Wallace Tucker the science spokesman for the Chandra X-ray Observatory Center.

For instance, XMM might be used to take pictures of the large-scale structure of a galaxy cluster and to measure its composition, while Chandra could zoom in to reveal specific features in the center of the cluster.

In the past several months Chandra has already done this by focusing on objects that have already been imaged by the Hubble Space Telescope, producing high definition images.

"The two will really work to complement each other, not in competition," Tucker said. It will be great to have them both up there."