The two identical X-ray CCD cameras on board Astro-D are known as the Solid-state Imaging Spectrometers (SIS) and are being provided by a hardware team from MIT, Osaka University and ISAS. Each CCD camera head is based around four 420 x 422 pixel MIT Lincoln Laboratory CCD chips abutted side by side, with four preamplifiers, front-side illuminated. The pixels measure 27 x 27 microns^2, and the resistivity is 6500 Ohm cm; measured system readout noise levels of 3-4 electrons RMS are characteristic of the flight instruments. When cooled to a modest temperature of -50 degrees Celsius, a FWHM energy resolution of 2 per cent at 5.9 keV has been achieved. Using an approximately 100 micron thick depletion layer, the entire energy range from 0.4-12 keV can be covered, allowing the spectroscopic study of high-ionization lines from oxygen through nickel. The measured quantum efficiency (QE) at 6 keV is 80 per cent. The field of view of each camera is 22 x 22 arcminutes.
Since the SIS will be the first X-ray CCD instrument in orbit, its resistance to radiation damage will be fully established as the mission progresses. Protons are responsible for the majority of radiation damage and the effects depend critically on the proton energies. According to current estimates, the charge transfer efficiency (CTE) of the CCDs may begin to deteriorate significantly after 2-3 years, causing the energy resolution to degrade. This degradation will be position dependent as events which are closer to the read-out will be less severely affected. To reduce the damage rate, the SIS cameras are equipped with a combined aluminium-polyethylene shield. Software will also be able to correct for the change in CTE to some extent, although a portion of the response changes are ``irreducible". For observations of bright sources, however, the original energy resolution should be retained throughout the mission with a determinable reduction in QE.
Various modes are available for observing a source with the SIS, the choice depending on source brightness, the extent of the source and the time resolution desired. For an observation, two concurrent modes have to be set: the clocking mode, which determines how the CCD is read out, and the data mode, which determines how the charge cloud created by a photon is described.