X-ray sensitive silicon charge-coupled devices (CCDs) are a key device for the X-ray astronomy. The low background and high energy resolution achieved with the XIS/Suzaku clearly show that the X-ray CCD will also play very important role in the ASTRO-H mission. Soft X-ray imaging system consists of an imaging mirror and a CCD camera (Soft X-ray Telescope (SXT-I) and Soft X-ray Imager (SXI)).
In order to cover the soft X-ray band below 10 keV, the SXI will use next generation Hamamatsu CCD chips witha thick depletion layer, low noise, and almost no cosmetic defects. The SXI features a large FOV and covers 35×35 arcmin2 region on the sky, complementing the smaller FOV but much higher spectral resolution of the SXS calorimeter. A mechanical cooler ensures a long operational life at -120 degree. The overall quantum efficiency and spectral resolution is better than the Suzaku XIS. The imaging mirror has a 6-m focal length, and a diameter no larger than 45 cm.