Development of the Four-stage X-ray telescope (FXT) for the DIOS mission

A Four-stage X-ray Telescope (FXT) has been developed as the best-fit optics for the Diffuse Intergalactic Oxygen Surveyor (DIOS) mission, a small satellite mission for mapping observations of the warm-hot intergalactic medium. The FXT mirrors are based on a conical approximation of the Wolter-I design, fabrication technique used in the Suzaku satellite. We made the second FXT demonstration model, in which we installed 4 sets of 4 stage mirrors with diameter of about 500 mm using alignment plate. Both optical and X-ray measurement were done to estimate FXT performance. Although angular resolution is two to three times worse than that of the requirement and the goal, the field of view and the effective area are consistent with expected performance derived by the ray tracing simulation.