  We present our proposal for a small X-ray mission DIOS (Diffuse Intergalactic
Oxygen Surveyor), consisting of a 4-stage X-ray telescope and an array of TES
microcalorimeters, cooled with mechanical coolers, with a total weight of about
400 kg. The mission will perform survey observations of warm-hot intergalactic
medium using OVII and OVIII emission lines, with the energy coverage up to 1.5
keV. The wide field of view of about 50' diameter, superior energy resolution
close to 2 eV FWHM, and very low background will together enable us a wide
range of science for diffuse X-ray sources. We briefly describe the design of
the satellite, performance of the subsystems and the expected results.
