The Constellation-X Observatory (HTXS or Con-X) was a mission concept for an X-ray space observatory to be operated by NASA; in 2008 it was merged with ESA and JAXA efforts in the same direction to produce the International X-ray Observatory project, announced on July 24, 2008.
The intention of the Con-X project was to provide enough X-ray collecting area to be able to feed a spectroscope of substantially higher resolution than the previous generation (XMM-Newton, Chandra and Suzaku) of space-based X-ray telescopes; this would allow the resolution of individual hot-spots at the event horizon of black holes, of warm intergalactic matter (by seeing absorption lines at various red-shifts superposed onto the spectra of background quasars) and of dynamics within galaxy clusters.
Technology for Con-X 
The project intended to have separate low-energy and high-energy X-ray telescopes, to work from 100eV to 40keV spectrum.
The collecting area requirements would have been achieved using a segmented-mirror technique based on slumping thin (400 µm) glass sheets onto mandrels, which avoids the handling problems of dealing with whole thin shells. Dispersive optics for the spectrometer were developed, as well as a microcalorimeter-array detector providing energy resolution per pixel of about 5eV.