The XGS will be a dispersive spectrometer with an effective area of more than 1000 cm² from 0.3 to 1 keV. It will have a spectral resolving power of about 1250 over the full band. Two grating arrays will be mounted behind the optics. Both arrays will use the same CCD readout.

Two grating technologies are under investigation: a 'critical angle transmission' (CAT) grating, and an off-plane reflection grating.

Above: Schematic diagram of the transmission grating and off-axis reflection grating concepts. Credit: ESA
	Above: The tower configuration for the off-plane configuration. Credit: OP-XGS team Left: Camera assembly for the off-plane configuration. Credit: OP-XGS team
Schematic figures showing the (left) camera assembly for the transmission grating optics and (right) grating arrays behind the Silicon Pore Optics. Credit: CAT-XGS team

The CCD detector array, comprising 10 CCDs in a 780 mm long array, would be read out at 10Hz. Back-illuminated CCDs with high QE below 1 keV and thin optical blocking filters are foreseen. Energy resolution of 160 eV is required to separate the spatially overlapping spectral orders.

Technical Status

The XGS will employ standard CCD technology similar to that used for XMM-Newton. No specific technology development for the camera is envisaged for IXO.

The CAT gratings are constructed from very thin plates: 100 nm thin with 100 nm spacing. The large scale construction of these gratings has yet to be demonstrated.

X-ray Polarimeter (XPOL)