Development of a four-stage X-ray telescope for DIOS mission

The DIOS, Diffuse Intergalactic Oxygen Surveyor mission, is planned to observe diffuse soft X-rays, which might originate from warm-hot intergalactic medium as a target of missing baryon and as a probe of dark matter. To observe very weak diffuse soft X-rays, the FXT, four-stage X- ray telescope, is specially designed to have wide field of view and large effective area for the small satellite BIOS. It is confirmed that FXT meets basic requirements of the BIOS mission based on the ray tracing analysis. Al-foil based epoxy replication method is applied to the fabrication of element mirror of the FXT. Several tens of foil mirrors were fabricated and measured their performance. Mirror material combination was also reviewed from the view point of line observation other than oxygen.