Development of Four-Stage X-ray Telescope (FXT) for DIOS mission

To search for warm-hot intergalactic medium (WHIM), a small satellite mission DIOS (Diffuse Intergalactic Oxygen Surveyer ) is planned and a specially designed four-reflection X-ray telescope (FXT) has been developed as the best fit optics to have a wide field of view and a large effective area. Based on the design of optics and mirror fabrication method developed for FXT, we made the quadrant model with ten nested four-stage mirrors. We describe the expected and the measured performance of this system.